Method for Relative Compression Testing to get PSI DATA!!

[@KillerZero259]

I love it when math and theory come into play and then perform the tests to validate the calculated numbers. Golden info

[@KillerZero259]

Damn, guys, 5 years later lol

[@DjeDrie]

That why theoretical knowledge is important too

[@KillerZero259]

@@DjeDrie Damn, 9 years later tho.

[@mtberguy]

I have heard about relative compression test for 2 years now and kinda wrote it off bc it didn't give me the in depth answers I always wanted. Over the last 2 weeks, I have been trying to figure out ways to take this test from looking at the peaks to see a low cylinder to something more concrete than it just looks lower than the rest. Thanks for going total nerd on this. I hadn't even thought about using this way. Great job. You took a basic test took it to another level that will make my life easier(removing intakes to get compression values on a weak cylinder) and make me feel like the smart guy in the room.

[@JD-526]

Please do more using a scope you have just proved to me I can learn how to correctly use a scope unlike the other channels I watch, that I wont, that you have already mentioned. great job like all your other videos.

[@maycobarrionuevo1297]

Wow! To me this si so awesome I can't even believe that someone would give this knowledge away for nothing. Thank You very much for sharing.

[@SchrodingersBox]

Glad you enjoyed it!

[@Cavalier_Steve]

This method will be so handy for you especially those hard to hard to reach spark plugs, I enjoyed the maths part (not my strong point) I went in and started to work out the next cylinder to try and imprint this equation in my mind. By the way I hope me commenting is not a pain.

[@eugeniojimenez4140]

Awesome video. What a clear way to explain this. It is TODAY!!! that I have come to understand how important solving for "x" can be. Sad that we go to school with excellent teachers and we are never explained why it is important to learn the content. Thank you for the lesson. It is just extremely useful.

[@DENicholsAutoBravado]

You lucky man you. My standard for accurate compression testing the old fashion way is to hook up a slow charger to the battery after the first cylinder compression test. When voltage on a battery equals what it did on the first test, then I do another compression test and on down the line. My first compression test showed a pattern of less compression on each cylinder. Having heard the obvious that the engine had less to fire on, all the hints were there for how I did the testing in the future.

[@DENicholsAutoBravado]

Plus, I rested the first cylinder and got even less than when I tested the last cylinder. To me this proved that the PSI was also relative to battery condition. So, knowing how it is relative to each other is far more important than the number itself.

[@DENicholsAutoBravado]

Schrodingers Box I was so excited about how this technique turned out that I didn't get a point across. If the voltage on your battery wasn't the same then you were lucky that the test cylinder tested very close to what the math showed. Of course, with this technique you turn over the engine much less and having a battery run down is less of a worry. And to be overly scientific. I believe the amperage of the battery would be more important than the voltage, but voltage is all I can see without fancier equipment. lol

[@SchrodingersBox]

DE Nichols Wow you know what- I never thought about it but that's a fantastic point about less battery draw!! Indeed after doing traditional compression testing especially on a V8, the weakened battery definitely can affect those last readings. Using this method you are using the exact same battery power for each cylinder. Nice thinking bro!!!

[@DENicholsAutoBravado]

Schrodingers Box I'm all smiles buddy.

[@robmut1357]

What a cool idea! Not sure if someone mentioned this already but you could use the other channel to monitor the +12 volts and factor in any changes there to your equation. I think that would give you more accurate results, especially if the battery was a bit weak.

[@davidodea6189]

When you get a scope and start getting serious you suddenly realize that the engineers never gave a thought to ease of access to sensor connectors...seems most are buried away in tight spots under manifolds and other obstructions or the plugs are jammed close to a ground.(don't short VREF with the T pin, oh dear ). Keep up the good work Matt, nice one again..

[@SchrodingersBox]

David ODea Yeah I don't bother with digging out sensors that much- I just poke holes in the wire or measure from the nearest connector.

[@davidbellmechanical]

I have made a pressure transducer to connect to my scope, and have found that to be most useful for gathering similar data. The thing of interest to me is that I get lower than anticipated comp values, due to the fact that the pressure transducer does not have a schrader valve in it, so you only measure the "single piston up compression" A very clever mechanic I know is only ever interested in this first piston up value. I notice on your video that this value is about 100 psi for your car, although it pumps up to 170 after a few strokes. Thought this might be of interest

[@SchrodingersBox]

+David Bell Many people do use the first compression stroke data, which I do. I also consider the total number of compression strokes needed to get maximum compression- it's a good indicator of cam/crank timing.

[@Speedstack21]

True story, I used to skip my math class in school to stay in the auto shop longer. .. watching this video made me wished I hadn't. Thanks for my remedial math lesson!

[@SchrodingersBox]

lol true story- in high school I used to do the math for the "shop hall" kids to keep from getting beat up lol

[@SchrodingersBox]

***** Lol- believe it or not- and I am not making this up- I swear it's true- the guy who picked on me the most in school ended up working at the car wash where I took my Trans Am each week. He washed my car every week for over a year lol!!!!

[@turtlezed]

Great vid mate, very useful information for a comprehensive test, i may well start using this regularly for those hard to reach cylinder jobs.

[@joseburgos5106]

Man, that's a super way to explain this. Keep them coming. I am about to buy a scope and this way I understand it better. If you can recommend one for a DIY on my own cars and a good book other to learn how to play with this the easy way that would be super. Again, thank you for putting this together. Good class.

[@SchrodingersBox]

There are so many out there. as long as you get one with 2 channels, that’s all that matters. I have seen some hanteks just around $200 that will work very well.

[@joseburgos5106]

Thanks, I have see that one yes. Got to explore/read it more, I have been trying to play with the free download PICO software... Thank you!

[@andrewwmacfadyen6958]

Keep up trying to educate us petrolheads. ;-). Andy from Scotland

[@jgeorges3061]

hi matt, what an awesome way to check the engine compression it is really cool thing what this tool can do hope you will show more things that this tool can do and how it works it is really joy to watch ur videos thank you and cheers.

[@SchrodingersBox]

Thanks Georgie- but to remain true to the vast majority of my viewers who do not and likely will not have a scope, I'll generally keep it on the shelf for videos. But in real life- I totally use it on almost every diagnosis now!!

[@jeffhesse8415]

Excellent video. This is a quick test compared to removing the upper intake plenum to get to back plugs on some engines.

[@SchrodingersBox]

EXACTLY!!!! This is absolutely the most amazingly useful part of this test- doing a compression test in minutes and at no cost vs in hours and at expense of gaskets. Plus, it allows for accurate "hot engine" testing as opposed to engine cooling by time intake is removed on those models!!

[@jeffhesse8415]

Schrodingers Box . Thanks for what you do.

[@whitetiger8652]

Enjoying the scope videos and looking forward to more of them.

[@SchrodingersBox]

+White Tiger Good! Just wait til next video!

[@macknumber9]

Matt is there anything that you HAVEN'T covered?!?! This video is EXACTLY What I have been looking for

[@SchrodingersBox]

glad to hear! i try to be comprehensive and thorough.

[@macknumber9]

@@SchrodingersBox anything you recommend as far as an amp clamp and scope for this without breaking the bank but also without making it too difficult on myself to read and comprehend the test results...

[@SchrodingersBox]

you can use any DC amp clamp and I have seen many for under $100 but as for the scope- I know several viewers have made lots of recommendations but I can’t really recommend on. i have tried several and honestly any of them under $300 I really don’t like too much. I really like my used vantage pro which is just around that range.

[@simoncrane8267]

Hi Matt. Loving the minutiae, as always. Gotta say, that was a complicated way of calculating the actual PSI of each cylinder. Far simpler, I think, is to divide the 170 psi, by 22.4, taken from the hard reading on cyl 3, giving you a value of 7.589 (call it 7.59) and then using that value multiplied by the individual sope readings for each cylinder (ie cyl 1: 22.6 x 7.59 = 171.534psi - Cyl 8: 23.07 x 7.59 = 175.101psi etc) Can`t fault your maths sir, but it becomes clear that you`re a scientist, who uses cross multiplication a lot more frequently than the majority of people, even the hallowed 2%`ers. I think the 100 psi minimum reading figure is the figure that the vehicle will run at, and still comply with emissions laws, heard it somewhere, but it makes sense I guess. Nice work, keep it up, congrats on 25000 subs, impressive. Carry on....

[@sailorbob74133]

Calculating PSI per amp and then just multiplying that by each amp reading is definitely the way to go. One divide and seven multiplies vs. 14 multiplies and seven divisions.

[@webslinger2011]

This technique would have been fantastic working on diesels. Friend co-worker had to remove glow plugs on every cylinder on a car to test comp and one plug was in a very tight situation(couldn't put the gauge in since it was a very tight fit). Wish I was still working there though (immigration issues). People there were nice and worked as a team. Plus free pizza's on your birthday!

[@francogalloso2167]

Awesome video. Thanks. Scannerdanner and you are my favorite teachers:)

[@SchrodingersBox]

Love Paul- he's way better than I am lol!

[@ateshnarayan9769]

Very good explanation i have ever heard. Great

[@randalldalton5475]

Love the algebra! Awesome brain exercise! I love your videos....

[@SchrodingersBox]

Randall Dalton I know you do lol, thanks man.

[@hxc7273]

Assuming the relationship between cylinder pressure and start current is linear. Although the pressures are so close that a linear approximation is probably adequate. It would be obvious if there was an outlier.

[@carlosmatusmolina3002]

Hi matt, thx for this video. I was looking for something like this and I can see you have 84k subscribers!!!!!

[@SchrodingersBox]

Yes, I'll never get to 1M subs with material like this, but I also never thought I'd cross 50K either

[@TriggerTravels]

I wonder about the accuracy from a bit of a lower psi cylinder to a bit higher... As scanner Danner says it can show an exaggeration in the numbers because the starter moves a bit faster on a cylinder with less compression thus drawing less amperage. When the starter begins to go to the next cylinder it is moving faster than it would normally due to the previous cylinder drawing less current. So the following cylinder after a cylinder with a bit lower psi actually reads like it has slightly higher compression, but in actuality it's appearance is skewed by the previous cylinder. In any case, good stuff..... Nerd on 🤘

[@ScoutsOut]

PLEASE!!!! more lap scope lol another great video thanks again

[@JoseFernandez-zt5xs]

Hello I just wanted to say that I was a parts changer but I am still an idiot and thanks for your teaching.

[@SchrodingersBox]

Hahahaha. no no, once you recognize there’s a better way and you take the effort to learn it you are not an idiot. An idiot is someone who keeps doing things wrong.

[@diezukunftistelektrisch]

Hi Matt, Congratulations on reaching 25k subscribers!!!

[@SchrodingersBox]

Oh thanks Ha Ha I didn't even notice lol!!

[@lizard944]

+Michael Pracht Which, as of today, 12 May 16 is almost 50K

[@scientist100]

he has over 105k.

[@turtlezed]

@ 19/01/2019 i'm looking at 125 thousand subs, keep on doing what you do Matt. :¬)

[@merkur8777]

There is only one thing I disagree with in this video and that is the inference that 100 PSI is acceptable because it is the stated minimum. Stated minimums are quite often incorrect as to engine health. Any engine other than an air cooled VW (very low compression ratios) that has 100 PSI has a problem. That aside, I consider this video outstanding from a tech standpoint and explained very, very well. Kudos.

[@SchrodingersBox]

+Merkur87 What evidence do you have to substantiate that? Compression specs are very strictly relative.

[@merkur8777]

+Schrodingers Box Forty plus years as a mechanic with thousands of compression tests under my belt on just about every make of car along with motorcycles and aircraft. Look at it as a track record and averages so to speak. Not that it really matters but I hold multiple ASE and factory certs in engine building and also hold an FAA A & P license. The rule of thumb on compression pressures is 20 X the compression ratio with various factors at play. Please don't take my comment as being critical. I thoroughly enjoy your video and find it refreshing that you actually reference that all cylinders being equal on a scan tool could mean they're all equally bad. Most simply accept that being the same means they're all good. Hats off to you.

[@SchrodingersBox]

+Merkur87 Thata not evidence. I mean like actual specification sources. Experience and certification is an appeal to authority logic fallacy. An actual source with specifications is necessary.

[@rbbbjl1]

That was clever. When I first started to watch u sometime ago I thought u to be haughty. Maybe I confused it with confidence. At any rate I come to respect u more and I like your sense of humor, p.s. I even contributed some cash to the cause!

[@SchrodingersBox]

+rbbbjl1 Well maybe your assessment of me was correct the first time but I'll still keep the cash lol! Seriously though, thanks for the support and glad you found this channel.

[@josephtucci3666]

Matt great job my friend! But may I add? For the DIY on this channel you know the kind who think out side the box a old school vantage is a great tool to own.. $150 on EBAY well worth it IMHO It is like having a 600 HP motor and nor tranny! Thanks man

[@SchrodingersBox]

Joseph Tucci Yes you're right, I really should have pointed out options for inexpensive scopes that can do this kind of stuff. I think most people are under the impression you need a vantage pro to do this and you can easily do this with a scope 1/5 the price.

[@nickayivor8432]

What do you call INTELLECTUAL AND PRODIGY and SUBSTANTIAL add one more KNOWLEDGEABLE the answer is Schrodingers Box My teacher Outstanding tutorial great video start to finish loved it 👍 👏 thank you very much helpful information video Take care and have a great Evening PRODIGY Schrodingers Box From Nick Ayivor from London England UK 🇬🇧

[@J.T672]

Really interesting way of using maths with hard evidence . Nice 1 Keep it up .

[@merkur8777]

One thing I should have added to my prior comment is that a mechanic working on flat rate is not going to go through all of this for the obvious reason; it would kill the paycheck. A little mental "close enough" is all it would get.

[@lauronoriega3083]

Great video!! Can I make a request though? You said you'll do a video on the evap system at some point and I can't wait for that!!! But before that, how about a video on air conditioning??? I have looked everywhere for good information on ac systems and I cannot find any decent stuff!!! Every thing I find is just weak or useless!!! I happen to know the theory of how ac systems work, but I'm sure there are many people on this channel that don't and would like to learn about it. I personally would benefit from seeing you apply the scientific approach in trouble shooting an ac system.

[@SchrodingersBox]

One on AC is coming soon, hang on.

[@area46241]

And the ninety eight percenter said to the two percenter "Dude I need your help over here for a minute" then mr boss overheard and said" hey don't you have work to do?" at which point all 100 percent got back to work strategering on solving their new problem of how to satify both bosses...wifey and bossy boss. Good science for Applied Science of Automotive Technology...you'ld be a perfect classroom teacher at wyo tech...

[@everardocamacho5063]

thank you Mateo great video, I feel like going back to school for mathematics, cheers from ontario calif.

[@rhkips]

Haha, you're makin' me miss my Pontiac... Would have loved to come out there and meet'cha at the strip. :D That... Okay, just wow! I had completely discounted this method of calculating in-cylinder pressures, because I always thought the amperage readings had too much potential for variance. Nice! Thanks for brushing up my algebra skills, too! (Math and I just do not get along...)

[@SchrodingersBox]

rhkips Lol, yeah I experimented with this quite a number of times and I have to say- the amperage readings seem as consistent as a compression gauge. I thought the same as you but definitely I am convinced, this is as accurate as the gauge.

[@techsavantlove]

The linearity won't have to be perfect. If it is just fairly "close", then you can call a bad cylinder. Or, at the very least, it would give one some comfort in deciding that a physical test of a given cylinder (and a reference cylinder) is needed. Boyle's Gas law says that the pressure and volume of a gas in a vessel will be exact inverses of each other, assuming temperature is held constant. So, if the pressure in a one liter vessel is 100 psi, and the vessel is expanded to be 2 liters, the pressure will go to 50 psi. Conversely, the physical force needed to compress a gas to 1 liter would be twice the force required to compress it to 2 liters. So that takes care of the physical pressure vs volume relationship. The only other place to examine the linearity would be the relationship between force and current in the starter motor. I.E. if the starter motor has to draw 20 amps to compress one cylinder to 100 psi, but draws about 15 amps to compress it to 75 psi. I don't know the answer to that, I suspect there will be some non-linearity but not a lot. One factor might be the inertia of the vehicle crankshaft and flexplate assembly. And then what is going on inside the starter motor. I don't know any way to determine that without empirical testing. So yeah, next time you see a car that you think has one or more really low cylinders, that would be a great time to test the linearity, as you mentioned someplace below. Good vid. Fascinating. The amount of time and effort saved in doing something that gives results that are even "reasonably close" to a physical test..... wow.

[@DENicholsAutoBravado]

Yes. This video was REALLY good. Too bad I have to wait on affording an oscilloscope. You covered this measurement and it's inaccuracies perfectly. Of course internal engine momentum cut into it's accuracy. I'm feeling good that I thought of that during the video too.

[@andrewwmacfadyen6958]

A handy method that you can do as part of a normal service routine also especially for transverse V6 cars where the rear cylinders are hard to get at, however if one cylinder looked poor I would go straight for an old fashioned compression test and/or a cylinder leakdown test on the cylinders with the highest and lowest readings.

[@RC-Heli835]

Hey man this is awesome! Id been wondering how folks were doing compression tests like this.

[@SchrodingersBox]

It's awesome that you think it's awesome. I do love this stuff- so much better than videos on changing brake pads lol.

[@RC-Heli835]

Schrodingers Box And this is the Vantage Pro that you picked up a couple weeks ago right? Definitely ahead of break pad vids but some folks my need to know about those as well as drum brakes. I have seen some brake drums from hell. Whatever you do don't take the emergency brake cable loose to change a set of rear brake drum pads. You might have covered that in one of your vids but I missed it. I'm like how hard could it be to change brake drum pads? Let me at em. I screwed up and took the emergency cable loose from the r rear but I didn't take the left side off after doing the right side and it went about 1.5 hours faster. LOL! Hard lesson learned I guess.

[@DJDevon3]

100 PSI min holy crap are you sure that's right? My I4 minimum is 120 and my V6 minimum is 140. Amazing video. This is a gem.

[@SchrodingersBox]

***** Yeah it's right- 3 separate sources verified the same (alldata, Helms, and Haynes). I couldn't believe it either lol.

[@geraldhinson]

Cool video. Super simple math too. I wondered about the "is the relationship between amps and pressure linear" question that others mentioned. I am also curious how consistent the amperage readings are per cylinder on repeated startups - are they as consistent as repeated compression test readings? The Mazda B3000 truck I work on in my down time will be a superb test for this as it has significant compression issues and cylinders that differed significantly on a cold leak-down test. You keep motivating me to get a scope!

[@SchrodingersBox]

Gerald Hinson Yes agreed with you and the others- to really test this method we need a specimen where one or more cylinders is significantly below that of at least one other and see that the proportional difference is still accurately represented through this extrapolation method.

[@jasonthetalker7253]

Only if you where my teacher back in school. Thank you

[@1supratech]

Very informative video. Thanks for posting!

[@webslinger2011]

I have a hantek cc-65 low amp probe. It measures up to 65A? Can I use it for relative comp test on both petrol and diesel cars? I always saw vids of people using a high amp probe not a low amp one.

[@DENicholsAutoBravado]

With how hard it is to get to some spark plugs, I sure look forward to doing it this way instead!

[@SchrodingersBox]

Totally bro- just changing plugs on this thing takes over an hour- the idea of doing a full compression test in just a few minutes is awesome!!

[@ozzstars_cars]

As a 2% I say bring on more scope action!! This was a fantastic video, algebra has never been my favorite but in this case I really appreciated it. I don't think you went full out nerdy though......next time do the long math, no calc LOL. Awesome stuff...thanks for making the video man

[@SchrodingersBox]

Ozzstar Lol thanks man... I will definitely be using the scope more but only because some of the videos I am working on really need the scope to show the concepts.

[@JOHNPHUFNAGEL]

that was pretty cool I will have to remember that

[@bethreagan6923]

first and foremost I love this channel this is my wifes log on just fyi I am a "professional" with very limited training but have a lot of hands on experience that being said ive been messing with a scope for a lil bit now and one of the things I believe I noticed is an inherent spike on the next cylinder behind any cylinder that's misfiring due to what I believe is a freewheel effect on the previous cylinder am I in left field here??? one again keep up the good work mike

[@SchrodingersBox]

Beth Reagan You are correct indeed! I have also noticed this! In fact, this same thing happens on a cylinder that is next in firing order after one that has no or very low compression. I also attributed it to a "freewheel" effect as you described.

[@dksob1981]

I found it easier to find out how many PSI was equal to an amp, and then multiply that by the results of each cylinder. I.E. 170(PSI)/22.4(amps) = 7.59..... That means 1 amp is equal to 7.59 psi: 7.59 x 23.07 = 175 psi, 7.59 x 22.6 = 172psi, 7.59 x 24.5 = 186psi and so on..... Now, these number are only accurate for your vehicle since we have a baseline, but the theory applies to any.....

[@SchrodingersBox]

That's pretty good way of doing it too. Yes Either way, you still need a baseline because the conversion factor will be unique to each vehicle. But definitely like it!

[@findvoltage]

That was so cool, Now all I gotta do is buy a vantage pro. I'd like to see more of what that tool does, and the vantage pro.........(joking). Thanks Matt. That was great stuff.

[@SchrodingersBox]

This exact test can actually be done with any lab scope, even a very basic inexoensive one. I was actually using the most minimal features and settings this scope is capable of. No need for a vantage pro- or even a vantage, actually!

[@Janisg616]

Did you mention that cylinders won't fire at the same order as they are numbered? The same with compression. Cilinder order in relative compression will not be 1, 2, 3, 4 but more likely the order will be like 1-3-4-2 depends on the car.

[@SchrodingersBox]

If people didn't know what a firing order was I would think they wouldn't be doing a relative compression rest.

[@elora2525]

The old BMW DIS testers did this automatically. You put the transducer and do a relative compression test and it will give you a report in BAR. To be honest I have not done it in many years. Direct measurements are faster

[@mtrltoolman]

great video , i am thinking about an used snap on modis ultra in the future :)

[@SchrodingersBox]

toolman talal That would be a great thing. The modis is excellent!

[@noelmatute5977]

good video teacher best

[@trainer2018]

Good video Matt, nice to see you have a scope now. You will enjoy the Vantage Pro, a pretty capable scope. I have a original Vantage that I use and it is still handy. You can buy those on ebay for around 100 dollars or so, very attainable for the diyer and it has a lot of the same controls so viewers who purchase one can follow along. Plus people can see the value in having a graphing dvom/scope. Now that you have your scope, come over to Autonerdz on the general public area and get registered, there are a lot guys on there that use the Vantage Pro and can give assistance and tips if you have any questions. Did you get secondary adapter set for the Vantage Pro? Some people don't like it I heard but it does give you valuable info in a graph format of Firing and Spark Kv and burn time. Enjoy your new toy.

[@SchrodingersBox]

busjockey1 Thanks man. I did get the secondary set but haven't even touched it yet. I'll check out auto needs! sounds like my kinda hangout!!!

[@trainer2018]

www.autonerdz.com

[@techsavantlove]

Schrodingers Box thats autonerdz

[@SchrodingersBox]

douglas lee Yeah my iPad autocorrect again lol... I did look at the site a bit, it is very good!

[@lolzlarkin3059]

...and the student becomes the master.. lol. This certainly seems like more maths than any mechanic I know is going to do.

[@SchrodingersBox]

Certainly more than 98% will do lol.

[@Turbotomass]

lolz larkin If you simply take a compression reading on one cylinder and your lowest amp draw is close you can assume every cylinder isn't low. Even getting a good number from one cylinder you can pretty much say all cylinders are not low if the peaks are close. I'd be concerned on a low power complaint. Still great for misfires. excellent video. love the math to get actual psi. even with a pressure transducer with actual pressure readings this is faster.

[@lolzlarkin3059]

Turbotomass i imagine most people would use this to see if one cylinder is low, for speed. and use a compression tester for actual psi readings if they believe more then 1 to be low. I'd wanted to try this since i saw scanner danner do it, but like many people, i am scopeless. :'(

[@Turbotomass]

lolz larkin A scope can be had for cheap.

[@metalmaniac375]

lolz larkin Ironically many mechanics will say they "don't fool with electronics or math"... Really??? how do you pull that off on cars now days? I quit going to mechanics several years back for this and other reasons. For the most part, I found them useless to put it kindly. Another great video here effectively putting this channel heads and shoulders above the others!

[@jeffbonilla5600]

Thanks a lot for sharing your knowledge! #Respect

[@jnarez1]

You know how you use sertain sensors and what not to test other things, do you think you could use relative compression to test a clogged exhaust?

[@SchrodingersBox]

no because a clogged exhaust affects all cylinders equally.

[@ajsanchez1960]

thanks for the info. Excellent!!

[@victorguardado1857]

can you tell me the part number for your inductive probe that you are using on the second channel.

[@SchrodingersBox]

+victor guardado No inductive probe- I directly measured voltage at the injector harness.

[@Barnekkid]

Why not just divide out the left side of the equation and then multiply the result by the amperage reading for each cylinder? No need to cross multiply and no need to move the unknown value to the left side. It's a little easier this way. Sorry, I'm a former math major. But just the same, this is good information, thanks for sharing it.

[@SchrodingersBox]

When showing a formula you need to show how it was derived so people can understand it. Whatever way you do it- a formula, a calculation, whatever it is best to demonstrate how it was derived.

[@justaguy1576]

Instead of using the algebra, why not just take the 170psi and divide it by the 22.4amps used. Then you know for every amp consumed, you have 7.589 pounds of pressure (per square inch). Now you can multiply every other cylinder by 7.589 to determine what the compression is.

[@schrodingersboxquantummeka2008]

+Patrick Curran that's algebra too bro. it's same formula.

[@justaguy1576]

+Schrodinger's Box Quantum Mekanix Thanks...I am in my 50s and that cross multiplying stuff just confuses me...lol...

[@schrodingersboxquantummeka2008]

+Patrick Curran Lol what you said is basically what u get after cross multiplying lol. I figured people would like to see how the equation is derived. but basically "X" is still the same thing

[@Autofixpals]

+Patrick Curran i was just about to post that same thing.

[@Autofixpals]

+Schrodinger's Box Quantum Mekanix I am by no stretch of imagination a mathematician. So you kind of lost me when you started with the algebra. I was saying to myself ohhh he is just going to divide the PSI by the amps than figure out what each amp represents in terms of PSI... then you went into the algebra i was like "WTF" but i do realize it's the same thing and might be easier for someone who practices algebra on the daily basis.. But anyway what i am trying to say is that's an awesome method. thanks for sharing.

[@christheother9088]

Wondering if you could replace a spark plug with a regulator to get current readings at various arbitrary TDC pressures. Probably awfully hard on a regulator to hammer away on it like that, even briefly. Actually as I write this I remember playing with an electronic pressure sensor with an Arduino to control pneumatics. THAT would probably work as I recall it's range seemed about right.

[@SchrodingersBox]

***** There are pressure transducers for this but it would be no more advantageous or time saving than using just a traditional gauge.

[@70corwin]

The pressure transducers goes way past the limitations of a gauge. Haven't you seen scanner danner and people do running compression tests with a pressure transducer? Very cool stuff ,especially when you have a 4 trace scope. Even seen danner zoom in on a fuel pressure reading using a pressure transducer to check injector flow. I don't have a gauge left,they all have been replaced by transducers!! You need to keep making videos with the scope!! Its become a must have tool especially at the level of your videos! DIY guys can pick up used vantages on ebay all day long!

[@SchrodingersBox]

I haven't seen Danner do that- I will definitely check that out!

[@geekdiymechanic4539]

I might have missed it. Did you connect to positive cable to battery or positive cable to starter? Thanks for sharing

[@SchrodingersBox]

+GeekDIY Mechanic starter cable.

[@justinc.2246]

Wow! That is awsome, thanks.

[@jasonpanus8246]

Any idea on how to properly do compression test on a vehicle with electronic throttle actuators, will they got to WOT while cranking.... and what is the best method for keeping the vehicle from starting?

[@SchrodingersBox]

No difference whatseoever. just diable fuel and do test as usual.

[@renahomer3939]

hi. I would like to know if it is possible to use the smoke test to accurately determine where compression low is. I am asking because I saw a video showing that it was possible. Please reply when ever you have time. BTW I believe in your approach to these problems. thanks in advance.

[@SchrodingersBox]

+rena homer Theoretically you could- by putting smoke in the cylinder you Might be able to see the smoke leaving through the leak however the thing is usually you need very high pressure to find a compression leak. If the leak is really bad the smoke test would work. But on many leaks you need fairly high pressure (Well over the 20psi or so from a smoke tester) and thus a leakdown tester is far more reliable. If I did a smoke test and it was positive, that's great but if it was negative I would have to do a leakdown test to be sure and since the leakdown test tells you where the leak is anyway I would just do it first.

[@rheidtech]

Good stuff. What if one rod had a tight bearing and you were smokin crack while eating captain crunch. Would we have to break out the complex polynomials?

[@SchrodingersBox]

R HEIDEMAN I would hope for more cap'n crunch!! I love that stuff but it kind of eats away at the roof of your mouth

[@rheidtech]

I'll add with the crunchberries. (Stainless single ring mixing bowl and serving spoon method)

[@rheidtech]

Schrodingers Box yeah it does....specially after a whole box. Man alive!

[@michaelherbert8213]

Schrodingers Box you should try breading your chicken cutlets with captain crunch, seriously.

[@rheidtech]

Michael Herbert writin that one down. Lol. Thx

[@billwilliams6338]

SCHRODINGERS BOX, ,,, when one or more pistons are NOT giving any compression. What would be the problems on the cars engine? Do you have to replace the piston or the actual engine block is cracked inside that piston cylinder and have to replace the whole engine block?

[@SchrodingersBox]

The variables are ring sealing, valve sealing, or compromise of the combustion chamber. Determine which is the one via leakdown test.

[@mustafaabukhaled6911]

Exellence thank u for agood work big like for u

[@gusino34]

hello. any videos on can bus advance diagnosis?

[@edwardjames6070]

another great video.

[@fabioresende1331]

Hi, can i use the voltage drop in this test, since i don't have the current probe? Do you think i can get this kind of precision? Nice video!

[@SchrodingersBox]

Yes you can but its not nearly the resolution as with Amp Clamp.

[@fabioresende1331]

Schrodingers Box thank you!

[@WarpedinSpace]

Top on content, the rapid fire delivery is a little intense, makes me feel like I'm gonna blow a gasket.

[@slimeslimgamerz3043]

you are a cool geek.thanks

[@SchrodingersBox]

Amy Browning All geeks are cool!

[@michaelherbert8213]

NERD ALERT, hahaha, I'm just kidding man. Awesome video. You did a great job explaining it. I have a 2002 Z28 that I endlessly put money into so that when I bring it to the track I break something, lol. This method is much easier and faster than a standard compression test, and as you've proved, its accurate. I would actually like to see your vantage in future videos. I would also love to see some track footage, maybe even some data pids too if you you record them. Thanks for your time and effort in making your videos.

[@SchrodingersBox]

I take data all the time at the track but the problem with posting performance related material on youtube is you will get tons of CFU grads (that would be Chatroom and Forum University) who think they are John Lingenfelter despite never experiencing a second gear scratch.

[@grzegorz16100]

Did your vantage pro come with low amp probe?? Do you know if I can use any other probes with my scope?

[@SchrodingersBox]

Gregory May Yes it did- and 2 pressure transducers, 2 inductive spark wire probes, and inductive coil pickups.

[@grzegorz16100]

Schrodingers Box You got better deal than me. I didn't know how much all other accessories cost.

[@JD-526]

How can I get a copy of that macro.

[@wbfog]

Great video, but what am I missing? You are using a 40 amp probe on a starter circuit? I tried this with a 100 amp probe but couldn't get a reading because my circuit was pulling around 135 amps. I had to use a higher amp probe to get a reading.

[@ovwok]

marc wischkaemper he is not taking into consideration the probe setting. But it's a relative using a ratio.

[@hddm3]

cant wait to get one

[@SchrodingersBox]

Dont wait! Easily one of the best things i ever bought!

[@benjaminruiz6931]

great Job

[@glennallen7129]

Hi Matt! You may not see this but how or what tool do you hook up to the ignition to find out which cylinder is who?

[@SchrodingersBox]

Hi Glenn. There are lots of ways to do it including not even using a tool. The trick is to get a signal on a known coil and then, given the firing order of the engine, you can determine each cylinder you see on the scope by counting firing order from the known. You can use an inductive ignition probe, a capacitive clamp, you could prove a terminal for voltage or use an amp clamp, or you could even remove one spark plug from a cylinder and the missing peak will be that cylinder.

[@glennallen7129]

Schrodingers Box what did you use in this particular video? Also on COP design, how do you get a signal? Tapping into the trigger wire with back probe or pierce it?

[@SchrodingersBox]

In this video I used a inductive spark tester- simply because I was interested strictly in signal, not a diagnostic spark pattern. In the case of a COP you have three options. One is to backprobe or pierce to get voltage. The second way is the use an inductive amp probe and loop the signal or ground wire. A third way (and the way I do it now since I bought one) is to use a special COP probe that you simply touch to the coil and it gives a spark pattern much like a capacitive probe.

[@glennallen7129]

Schrodingers Box the reason why I’m asking you this is because I have a vantage ultra scope which is similar to your vantage pro. What would be the cheapest/easiest route for to get for picking up the ignition signal to sync with the relative compression test? Maybe something that could do either wire on plug and coil on plug? Also do you use an adapter for your COP probe? I know that our vantage only takes banana.

[@SchrodingersBox]

The cheapest way by far would be to just probe the signal wire voltage. All you need is a backprobe- that’s it. Nothing to buy- you can use a sewing needle.

[@mrgalea100]

How can I differentiate if oil is coming from valve seals or turbo seals

[@SchrodingersBox]

Valve seals will not leak oil into induction tubes

[@eurokid83]

I can't believe how much you paid for that scope. I never find deals like that.

[@SchrodingersBox]

Chester Wardan Lol- yeah when opportunity presents itself, you have to take advantage... or add vantage lol.

[@eurokid83]

lol, I see what you did there. I have the same low amp probe, I use it with my Fluke 87V for diagnosing parasitic draw on batteries quite a bit. It's actually one of the most accurate low amp probes on the market. I use it with my scope as well.

[@MrJcardimen]

Good stuff!

[@SchrodingersBox]

Ain't it though!! Its really awesome to see there is a subset of the population that likes this stuff. For me this is really awesome- if I can do a compression test on an 8 cylinder car without getting my fingers dirty, that's a major win as far as I'm concerned!!

[@vieczurable]

Ha! I got interesting case, where all cylinders have almost same even compression (+/-2%) but all are under desired value (35psi less) so I suspect incorrect timing.

[@SchrodingersBox]

correct. that would be far more likely than equivalent wear on each cylinder.

[@vieczurable]

@@SchrodingersBox So I don't trust computer throwing no dtc as apparently it is within safe limit. Either incorrectly installed belt by a tooth or two or faulty one of vvt hubs due to intermittent nature of underpowered rides (sometimes engine sounds like exhaust opens too early and throttle is much less responsive).

[@SchrodingersBox]

I usually check alignment by superimposing the cam and crank signals using a scope but without a scope you will have to just look at the timing marks and see if they are misaligned.

[@vieczurable]

@@SchrodingersBox I have both cams variable, so I will have to lock them from the other side and ensure sprockets were/are aligned as spring loaded - just waiting for a locking tool to arrive and a free weekend. I am even going for a fun to check crank tdc with a tdc wheel while being there..

[@vieczurable]

I heard that test with use of a gauge always shows lower than real pressure because gauge and hose add up extra space for air. Is it always a true?

[@christopheroliveriii3985]

LMAO, now its time to get back to my basic bitch stuff. I have to say, excellent math class.

[@matutenoel4833]

good video amigo exselente

[@paulmvn5431]

X = ([known pressure]/[value relative to known pressure])*[value relative to X]

[@rrmech11]

Thanks Matt

[@SchrodingersBox]

my pleasure! I love this stuff lol

[@mike333h]

Me dumb.... Lol you make this shit look fun

[@SchrodingersBox]

+mike333h Lol it is fun but it's really about spending time learning this stuff. Just a year ago I would have had no idea in the world what this video is even talking about-- now it is my main if not exclusive method for compression testing. It's all about putting the wrench down and realizing the highest level of fixing cars- or anything- is done with your brain not your hands.

[@Sandbag1300]

Interesting.

[@dopiaza2006]

But is the current directly proportional to cylinder pressure?

[@SchrodingersBox]

Yes. It has to be. the higher the pressure the slower the starter will turn and thus the higher amperage.

[@dopiaza2006]

@@SchrodingersBox I agree, it is proportional, but is it directly proportional? As in, will twice the compression result in twice the current? And while you're here - if you don't mind... How does this sound? Got low power on cylinder 2 but worried it might be low compression. Cranking sounds ok to me? kzbin.info/www/bejne/kKW2lplrep2tqq8

[@mattt4183]

@@dopiaza2006 An engine with no compression still draws starter current so current can't be directly proportional to pressure.

[@dopiaza2006]

@@mattt4183 Good point. I guess you'd need to baseline the current with no compression and take that off any reading to see if it really is from that point. And to answer the question about the video I linked about - it has 2 bent conrods!

[@hddm3]

thanks man

[@andrewthompsonuk1]

One problem in your control. You chose cylinder 1 that was almost the same as the reference. You should have chosen number 6 as well. Otherwise you are making an assumption that current is linear with compression. Anyway I enjoyed the video.

[@SchrodingersBox]

Yeah I thought about that too, but having established at the start of the video that there is less than a 15% difference between any cylinders, I think this car is not ideal to really test the extrapolation. If I had a car with maybe 40% compression in one cylinder that would really be a better opportunity to show the extrapolation is truly linear and accurate.

[@bmich833]

Just in case your in third grade................Hilarious!