Engine Math : Dynamic Stroke

[@kenretherford3519]

I know this video is old but, this isn't an accurate representation. Installed centerline is critical but, actual valve duration is key to finding dynamic compression. I.C.L. is the point of rotation when the intake valve is full open. The cylinder doesn't start building compression untill the valve contacts the seat. Dynamic compression is calculated from this point. Static compression is calculated from B.D.C. a cam with the same I.C.L. but a longer duration will yield a lower dynamic C.R. So Installed centerline will not give you accurate dynamic compression.

[@PetesGarage]

Well, you have to start somewhere.

[@kenretherford3519]

@@PetesGarage Yes indeed you have to start somewhere but, with all due respect, this video is drastically inaccurate. If someone is building a high compression engine and tries to design a cam using this principal, they will detonate their engine. You don't find dynamic compression using the cam specs alone. You need head volume, head gasket thickness, head gasket bore, Piston relief volume, Piston bore size, rod stroke, (rod length is arbitrary) and finally, valve closing angle. Duration, overlap and installed centerline angle are not necessary for calculating dynamic compression. They are critical for any engine build and to find the correct angle of actual valve closing but not needed in this aspect. All I'm saying is this video is Misleading and will not give beginners any type of "starting point" other than what not to do. I wish I could explain it in a manner easy to understand but I don't have time to write a novel.

[@PetesGarage]

If you have to write a novel, it's not easy to understand. You do realize I did not make up these formulas. This is the basic information you would need to do the calculations. There are hundreds of websites with more detailed calculators.

[@kenretherford3519]

@@PetesGarage Ok, it was not my intent to spark a debate but let's do a little challenge. I am currently building a 410 C.I. Ford Clevor (Windsor block, Cleveland heads) bore is 4.040, stroke is 4.000. 5.780 connecting rod. According to your calculations head volume is irrelevant but for the sake of argument, head volume is 68cc. I'm looking for a decent idle but with good high R.P.M. power. Using your calculations, design me a cam and tell me what the specs are and what my dynamic compression will be with your cam. Or just pick out an off the shelf cam and use those specs.

[@PetesGarage]

Comp Cams 35-000-B .579/.600 gross lift 242/248 @.050 LSA 110 Dynamic 9.25:1

[@silasmarner7586]

For those who want to buy/borrow/use the right calculator, that's not a "T-84" as verbalized on this video but a "TI-84" which means a "Texas Instruments model TI-84", TI slso standing for "Texas Instruments" I don't say this for criticizing the video,I say it for clarifying for those who need to locate or purchase or borrow this calculator. And also it's most precise to say "Five significant digits to the right of the decimal point". He also said "times" later in the discussion when he meant "plus" but "plus" was in the written formula. So it's all right and all cool Those calculators are still for sale; readily available; and just a bit over a hunnert bucks... And thanks for the video! Subscribed and liked!

[@PetesGarage]

+Silas Marner Thanks for the correction. I tend to speak quickly and since I don't have a script I just wing it. I was going to re-shoot the video when I heard I did not hear myself say TI, but the point is whichever calculator you choose to make sure it's in degrees. I still have an old TI graphing calculator from when I took calculus 20 years ago.

[@Dr_Xyzt]

I made a really extensive spreadsheet. I'm gonna do a video on it. Essentially, it intersects the limitations of the camshaft and the piston over a length of time to approximate my volumetric efficiency.

[@PetesGarage]

Awesome....share it with everyone

[@chubbs5188]

As always, fantastic work. You really make things simple, and interesting... I also wanted to share the conversion for us excel users. I use excel 2007 and the easiest way to convert native excel from radians to degrees is this: incorrect cell formula ex.: =SIN(72) This yields .253823, which is incorrect. By modifying the formula by adding *PI()/180 the conversion to degree occurs. correct cell formula ex.: =SIN(72*PI()/180) This yields .951057, which is correct. Again Pete, thank you for an outstanding job. I'm in the planning stages of a build on a 4.6 3V and you're really making it a lot easier and fun! Cheers

[@PetesGarage]

+Skip Abrams Thanks Skip!! That is awesome. I was going to put that in, but the whole conversion thing would have been more confusing. Thank you for sharing that

[@ludi2shoes]

Thanks Pete that took me back to the good old days!

[@PetesGarage]

+Milo Abel Ah the good old days....I still have my T square and drafting pencils

[@robertnailling613]

Nice job Pete, you have a great camera presence. Thank you for the honorable mention and I hope I didn't create more problems. Bob

[@PetesGarage]

+Robert Nailling You were very important in clearing this up. Thank you so much my friend!

[@carywhatzizname4060]

Thanks for sharing the DCR formula. I know I saved it somewhere, but I forgot where. One thing, though. As best I remember, the angle the formula calls for is the point where the intake closes after bottom dead center, not the intake center line. Since the advertised duration figures are not always published accurately, I used the 0.050 lift duration to figure a closing point based on the intake centerline and the duration.

[@PetesGarage]

Thanks Cary

[@carywhatzizname4060]

@@PetesGarage You're certainly welcome. I found the formula in my notes finally. There's a little more to it, but I'll only share it if you, or any of you, are interested. I don't want to bore you. :D

[@davidivancic9951]

Wish I could give you more than one thumbs up for clearing up the calculations. Thanks.

[@PetesGarage]

+David Ivancic I get by with a little help from my friends

[@michaelovitch]

Does it take in consideration the fact you can fill up the engine entirely,or even more, with intake air pulses ?

[@PetesGarage]

Yes

[@kenretherford3519]

Dynamic compression doesn't take air pressure or velocity into account. There are actually three "types" of compression, static compression, which is calculated by the full stroke compared to head (quench) volume at T.D.C., dynamic compression, which is calculated at degrees of valve closing compared to head volume, and "active" compression, which cannot be accurately calculated as the Dynamics are constantly changing with R.P.M., air velocity and intake/exhaust pressure pulses. Changes as simple as valve timing or open height, intake manifold or exhaust header change will change the active compression characteristics.

[@listenact]

Was this in response to a previous video? Not totally clear on the context here...

[@PetesGarage]

+Daniel Dempsey I had a previous video on cylinder heads where I talked about combustion chamber volume. I did the calculations in that video and some people found it confusing so I made this video and linked to that video. I know it;s out of order but I'll be back to engine building in the next video

[@antoniovarela6713]

love it, excellent video. watched a lot of them so far. thank you for doing this Pete.

[@PetesGarage]

+Antonio Varela Thank you Antonio....just wanted to clear up some confusion

[@jimnoneother6317]

I'm in trouble for next year.. The only thing that looked familiar was your shirt lol. Still, interesting video. I'm sure it will help a lot of people

[@PetesGarage]

+Jim Noneother Math is like anything to be good, practice, practice, practice

[@Q80100]

Thanks for your video but there is correction, in 0:37sec not exhaust open is intake valve close, when piston travel to compression the intake valve still open that will loss some compression

[@PetesGarage]

If the intake valve is still open on the compression stroke there will be major problems.

[@Q80100]

If the intake duration for example 60 degrees ABDC close what thats mean ? the intake valve will closed completely at 240 degrees after BDC that is what meant , for that the compression dynamic will vary depending on intake valve close not exhaust valve Check this table photobucket.com/gallery/user/gp_fillmore/media/cGF0aDpUZWNobmljYWwgSW1hZ2VzL0R5bmFtaWNjb21wUmF0aW9fenBzOTg0ZDg2ZGIuanBn/?ref=

[@kenretherford3519]

@@Q80100 You are correct. Valve overlap means at the top of the exhaust stroke, both exhaust and intake valves are open. As exhaust air escapes, the velocity creates a vacuum which pulls intake air in. As the Piston moves down on the cylinder, the exhaust valve closes, the intake valve continues to open. At the very top of valve travel, it is at it's "installed centerline" (Piston #1 only). The valve then begins to close. As the Piston passes bottom dead center (B.D.C.), the valve is still open. The velocity of air rushing in as the Piston travels back up begins to pack the air and compress it. AFTER BOTTOM DEAD CENTER, the intake valve meets the seat and closes. This is where dynamic compression starts. The faster the intake velocity, the higher your "active" compression will be.

[@Dr_Xyzt]

Is that the cam card from your cobra engine? Near 47 degrees abdc valve event with near 30 degrees of overlap at .050. I'm teaching myself to design headers. I'm treating them like a manometer, and the cylinder in overlap is what would normally be the u-tube on the manometer. I just need to set up the harmonics.

[@PetesGarage]

It is

[@InvincibleExtremes]

very cool. definately more to an engine than just raw displacement

[@PetesGarage]

Yeah....there's a little more to everything

[@MrBob58o]

I figured it out. ICA is wrong. "Installed CenterLine Angle" doesn't make sense. I was wondering where 72 came from. Intake Valve Closing. That is what is important with dynamic compression.

[@PetesGarage]

This is not an easy formula to execute. It will change based on the calculator you use, and will really be off if you use Excel. If you are having trouble with it there are many online dynamic stroke calculators available.

[@MrBob58o]

Still Installed Centerline Angle is not correct. Doesn't make any sense. It has to be Intake Closing

[@Djm9393]

Can you explain on a video how to find top dead center pete?

[@PetesGarage]

+djm93 I sure can. I'm making a video now and I'll include that....great idea!

[@jimjim2119]

Hi, and to begin, I am not a mechanic. But, enjoy the videos. May you briefly explain RD, RR, PR1 AND PR2? Thank you.

[@PetesGarage]

RD = Rod horizontal Displacement in inches RR = Rod Distance in inches below crank CL PR1 = Piston Rise from RR in inches on crank CL. PR2 = Piston Rise from crank CL

[@greglaroche1753]

Thank you for your great videos. Does dynamic stroke mean that total dispalacement of the engine is less too? Shuold we think of dynamic displacement to get a more realistic size of an engine?

[@PetesGarage]

Dynamic stroke is how much volume there is after all the valves have closed. ABDC some volume is lost as the piston comes back up because the valves are open a little.

[@wiggymccrackin8240]

really appreciate these videos. not everyone can afford college.

[@PetesGarage]

+Wiggy McCrackin It's all free to learn my friend. Thanks for watching.

[@midnighttutor]

If the cam angle for the one you showed is 106, cosine goes negative. How does that affect the overall calculation?

[@PetesGarage]

You'd have to so the calculations to see what you come up with. It should come out ok

[@joedirt9397]

If the cam card shows the intake centerline being 106 degrees then why is 72 degrees is entered as the ICA?

[@PetesGarage]

I'll have to go back and watch

[@monaromadness2539]

great video,... but your not taking into account the duration of the cam and the intake separation angle. Both of these things will have an affect on your compression ratio

[@PetesGarage]

The calculations do take that into account.

[@MrBob58o]

How do they take duration into account? They are not anywhere in calculations. Why do you show us 106 degrees, then use 72 degrees. Plus trig functions are available on calculator on any PC or phone. Just set it to Scientific mode. No need for a $120 calculator.

[@flinch622]

Dynamic compression is all about swept volume - not full travel, but after the intake closes. The cam card shows intake "closed" [.050" lift left] 47 degrees ABDC. You can follow some other engine builders tools that compensate for the lag between that moment and actually building pressure by just adding 15 degrees to the cam card number to get a pretty good number to start volume calculation, but profiles do vary - it would matter to somebody taking a build to the edge of detonation in the race world. That likely puts piston location in the bore at 62 degrees ABDC for this cam example and calculating that volume up to TDC. Whether you find the position with math or a degree wheel on the engine, then measure to remaining piston to deck height doesn't matter for most of us. Quench area and combustion chamber volume is dead space, but affects calculations because they are part of total volume. Hope that helps, Bob... I too felt a little fuzzy trying to follow this, even though the material is thoughtfully presented. I think this video was to supplement another made previously, but I can't find it anymore. The title is the tip off: Dynamic Stroke - which is the hard part to math out for figuring out dynamic compression.

[@Moparmaga-1]

@@MrBob58o you should make your own video

[@truckladders4104]

Is there a calculation to account for boost pressure as it relates to combustion /cylinder pressure? Hypothetically if you knew that at 30% overdrive your supercharger produced 20 lbs of boost at 5000 engine rpm how would you calculate that into dynamic compression? Great Video nice to see advanced thinking I enjoy all your videos very factual Thanks

[@PetesGarage]

+Jeff Pitman There are boost calculators online. The formulas for that are extremely complex.

[@jonathanbosco8458]

Thanks Pete for your excellent video, I have not had algebra/ math classes since 1984. What level of math do I need to tune diesel engines in the future? Thank you Sir

[@PetesGarage]

+jonathan bosco I'm not really sure. I guess it depends on what you're doing. There are so many engine calculators online you can get away with knowing zero math.

[@jonathanbosco8458]

Thanks Pete, Will take Algebra one on line this summer. Go for the PHD

[@PetesGarage]

That's awesome! Keep on learning and so will I

[@naveedaka]

sir...is 7:1 dcr ok..??

[@PetesGarage]

That sounds pretty low

[@Moparmaga-1]

Wow very cool

[@PetesGarage]

Thanks Korie

[@MeaHeaR]

Pete at 0:37, Don't you mean "Inlet Valvé" NOT Exhaust Valvé ????

[@PetesGarage]

Yes

[@danielnichols3914]

Keep rocking Pete! 😅

[@PetesGarage]

Every day!

[@MeaHeaR]

What Does R.D., R.R., PR1 unt PR2 ACTUALLY Stand for ¿¿¿

[@PetesGarage]

RD = Rod horizontal Displacement in inches RR = Rod Distance in inches below crank CL PR1 = Piston Rise from RR in inches on crank CL. PR2 = Piston Rise from crank CL

[@MeaHeaR]

@@PetesGarage Thanks Pete

[@EgoToxicBand]

Hey, I may be missing something. I tried to create an Excel sheet using your formulas and got frustrated when I couldn't reach your answer. Upon backing through the equation step-by-step I found that when I isolated (1.95520x1.75) my answer came up with 3.42160 instead of 3.70520 .... this changes the final DST to 3.07840 I think. Please correct me if I'm wrong. Cheers these videos are awesome!

[@PetesGarage]

Did you convert to radians?

[@EgoToxicBand]

+Petes Garage yeah it's all good, I read some other comments and Silas mentioned some confusion with DST being PR2xHalfSt or PR2+HalfSt. I changed it to plus and came to your answer. Thanks for replying, these videos give me confidence in starting my engine building journey!

[@PetesGarage]

Awesome!

[@alynromero8729]

Great Video! Keep it up!

[@PetesGarage]

Thanks Alyn

[@Palmerjct]

Hey. What are the odds that you would run my numbers if I send them to you. I don’t feel confident in my answers. Where did you get 72 degrees from the cam card? That’s what’s throwing me off. My card says 39 but I’ve read that you don’t actually use that number. You do a math equation. When I did the equation on your card, I did get 72. Is that what you did I could really use some help

[@PetesGarage]

..

[@chevy6299]

Good stuff.

[@brianmusick4363]

Thanks Pete!

[@PetesGarage]

+Brian Musick Thanks Brian

[@yasuke1075]

What is RD RR PR1 PR2

[@PetesGarage]

Those are variables needed to figure out the rest of the equations

[@naveedaka]

how high should i aim for...??

[@PetesGarage]

9.5:1

[@naveedaka]

Pete's Garage ...but sir if 9.5:1 should be my dynamic comp ratio ...then how high would be my static comp ratio...becoz i have a late int closing..95°abdc

[@PetesGarage]

What engine?

[@naveedaka]

Pete's Garage 222cc ...single....

[@PetesGarage]

That's a small hobby engine. The same rules don't directly apply

[@bwappliance7485]

NerdCore. Love it.

[@PetesGarage]

+B&W Appliance Gotta know where the numbers come from. I'm old school

[@danielnichols3914]

Totally bitchn' vid. Keep bringing the tech side of things; very crucial to understanding mechanics. Keep rocks' Pete!

[@PetesGarage]

I'm working on it....thanks Daniel

[@formenchangkongchannel1742]

Ica =intake center angel or Intake close angel

[@PetesGarage]

Installed centerline angle

[@Moparmaga-1]

Theres neither of those

[@kenretherford3519]

I.C.A. or I.C.L. is installed centerline. You will not find intake closing angle on a cam card or specs. You will find advertised valve duration @ .050 or actual valve duration at either.004 or .006. To calculate valve closing angle you MUST degree your cam and find it yourself. Don't try to figure half of the total duration, as most cam lobes are asymmetrical and will give you an inaccurate number.

[@tjhamer3]

Thanks for the vid

[@PetesGarage]

+tjhamer3 Thanks for watching

[@carlosavila8155]

L!O!S!T!....but interesting video. At this point I will simply take my vehicle problem to Petes Garage!!!! But great math for vehicle engineering.

[@PetesGarage]

+Carlos Avila LOL...the math is not as important as understanding dynamic stroke reduces the compression ratio

[@hectorortega9131]

Ok, after watching this video, I had to make an appointment with my psychiatrist.. there, i said it...

[@PetesGarage]

He won't understand it either

[@hectorortega9131]

@@PetesGarage 👍 🤣

[@rednecktech9485]

You may have had your calculator set to radians and not degrees.

[@PetesGarage]

That is the issue

[@ronniebuicks5569]

stand up job!

[@PetesGarage]

Thanks my friend

[@flytoday]

windows has a scientific calculator

[@silasmarner7586]

+flyright They're good no doubt but I always like having a physical one in my hands... not right or wrong but dangit I like the keys in my hands.

[@PetesGarage]

+flyright I know, just make sure it's in degrees

[@flytoday]

+Silas Marner you can also get them for android and IOS check the app store

[@silasmarner7586]

Thanks flyright!

[@patw52pb1]

Microsoft Mathematics is available as a free download from Microsoft and is a very capable advanced math calculator. www.microsoft.com/en-us/download/details.aspx?id=15702