High Compression and Compression Ratios

[@jessicaembers924]

I bought a 2,021 MX-5 with a 2.0 liter engine that came stock with a 13:1 compression ratio. I'm quite pleased with the car.

[@Birb_of_Judge]

Yeah, thats mazda skyactive for ya. They have very high compression for a non diesel

[@ktmarvin300]

Very interesting video! I was already quite familiar with the benefits of increasing the compression ratio on the power-stroke but the increased suction is a new aspect to me. Thanks for bringing that up!

[@chippyjohn1]

Pre ignition is before the spark plug fires, post ignition (detonation) is after. Having a higher compression is also good for higher rpm. By using a much higher compression you can throttle the inlet air at lower rpm so you don't get pre ignition but due to poorer breathing at higher rpm, you can bring the efficiency up. If the fuel is stable at 12:1 with 100% VE at lower revs, you can run a 15:1 to one but slightly restrict inlet air. As the engine rpm increases and VE drops to say 80% you can allow the throttle to open all the way. This is a simple way of making power liner and is why most engines are now drive by wire and higher compression on lower RON petrol. 15 times 80% is the same as 12:1. This has been done in aircraft engines even before WW2 to increase efficiency and power at higher altitudes with fixed rpm.

[@juliennacer8871]

Why isnt this done then?

[@chippyjohn1]

@@juliennacer8871 It is done in cars with drive by wire for some time now hence the higher static compression ratios

[@blakefelice6625]

I've always heard ping, pre-ignition, and detonation used interchangeably. I don't think post-ignition is even a thing. In fact, modern fuel injected vehicles adjust idle rpm by (first) retarding timing. So in effect, causing "post ignition," So unless you're talking about ignition so delayed that the exhaust valve is already open, which your heat has bled off so much, and your plug is no longer firing unless you've managed to set your timing WAY off, I don't think it's an actual problem.

[@hectorortega9131]

Cylinder Cranking Compression must be kept under 200psi, otherwise you risk hitting Pre ignition with pump gas.

[@SweatyFatGuy]

That depends entirely on the fuel and how resistant or prone to ping/knock it is.

[@hectorortega9131]

@@SweatyFatGuy Nope, Im Not talking about the Fuel Type here, im talking about how late the Intake valve closes after bottom dead center, on much advance or retard is on the Camshaft when installed..

[@SweatyFatGuy]

@@hectorortega9131 yep cam timing events are important, they can change lots of things including cylinder pressure, but 200 psi is a limit for gasoline, not ethanol or methanol. It might actually work rather well with methane and hydrogen too in getting more range from those two particular fuels. Its like saying 9.5:1 is the compression limit, which is true for many engines on pump gas, but not for ethanol and the other fuels mentioned above. Different fuels have different properties you can exploit for more power.

[@hectorortega9131]

@@SweatyFatGuy correct.

[@stuartclements9290]

NA or boosted? I've run much higher NA. Cranking compression is a lot different to actual running compression. Many things come into play.

[@RickyBobby615]

Building compression with your head will make more HP. Milling, sinking valves and small base circle cam to accommodate for smaller CC. Also moving top ring closer to top of piston. High domes kill combustion swirl, cylinder filling during valve overlap and flame travel.

[@automotivation2527]

I agree with this 👍

[@brettwhitelaw2597]

Super great video, you did an excellent job explaining everything

[@automotivation2527]

Thank you 🙏

[@josecuevas8602]

Their 4 Types "Compression. ●Static Compression ●Cranking Compression ●Running Compression ● Dynamic Compression •Do your homework on the 4 types of "Engine Compression".

[@muftifaizan1964]

What’s the range for running compression on a high comp 12.5:1 motor on stock cam

[@muftifaizan1964]

With cranking compression 280psi

[@terryenyart5838]

Thank you, I never really thought about the pressure differential from a CR standpoint. I always looked at valve opening points & exhaust header primary tube length to maximize scavenging, but interesting point. Love all engines that run strong & long😊

[@Topsiekku]

Why doesn't fuel quality directly tell you the actual octane number in your country? Elsewhere it is like that.

[@SidAmadeusRyan]

I wonder if I add 410 cc/ min fuel injector with 102 mm throttle body and 12:5:1/ 13:1 compression ratio into a 986 boxter 🤔Mid engine application , with ECU and TCU upgrades 🤔 beats me , I can't buy it's just a Time passing speculation 😂 .

[@GrandPitoVic]

That engine is gorgeous

[@jrsgarage3244]

Dr.Bob, explains 5stroke , At low speeds the Piston is sucking on the intake valve at high speeds the exhaust is scavenging the cylinder ❓I told him I kind of get it 😳

[@daleryan7882]

Intresting video , but i was abit confused at first until i watched it two more times. Where i got lost was the part about the piston creating vacuum on the exhaust stroke..but then, i realized that you were referring to the scavenging part..which causes vacuum at the intake side. Obviously the valve overlap allows velocity to help the recharging into the cylinder. Asfar as the higher compression factor , i believe the benifet is only at top dead compression by causing higher cylinder pressure at ignition but after the power stroke, the high top piston has no more effect until compressed top dead center. Also i want to mention my l79 327/350 hp engine in my 1965 corvette. The guy i bought it from rebuilt the motor to original spec which has 11to1 pistons. I am surprised how much power that motor has , the car is so good on fuel. Is that because of the high compression? I should mention that i put a 600 cfm holley on it and replaced the cam to a comp cam h268..the car had a 750 dp when i got it which i stupidly misdiagnosed it with a bad camshaft before i realised it had the wrong fuel pump and carb...

[@bossstrick]

So basically the pistons you decide to use in your shortblock will decide your compression ratio ?

[@rtkl13]

Although a decent video for high compression ratio applications, it gets a whole lot deeper than here. Cylinder head design, cast or aluminum, spark plug location, fuel type, cam profile, rpm range, timing to name a few. Do your homework and be specific for what you want from your engine or you could be that guy who talks numbers, parts, and all the money you spent while your loading the car up on the trailer

[@automotivation2527]

Very true! These videos are short and to the point of the topic. There is a plethora of considerations for designing and building. Thanks for coming by the channel!

[@peterbryant3584]

Great content great job explaining how high comp work I actually doing one right know and everyone is like u should of when low comp but I dont want to much boost uknow great video

[@arlowicks9359]

This was a really great vid.. Goodluck with the channel!!

[@JoshsCarCorner]

I have an LS2, and I bumped the compression from 11:1 to 12:1. I would have gone higher, but I wanted to make sure I could run 91 octane if I just had to get home. Regarding what you said about increased intake vacuum with the higher compression, how much increase in volumetric efficiency do you think you get from going up one point in compression?

[@automotivation2527]

Not a HUGE amount. Plus, every motor is different. The only way to really tell is to datalog before and after. Read the gps (grams per second) from the MAF pid and compare. Again you have to take into account anything else that may be effecting VE that was changed. Not unusual to see an increase of 3-5% though.

[@Airman..]

Hopefully you won't see higher ECTs as compression means more heat, especially on a big displacement engine

[@xaviergonzalez5828]

New subscriber bro! Nice and very informative video!

[@Lexaturbo36]

Hi bro. Please tell me. My engine has a compression ratio of 10.5:1. I want to install a gt3582r turbocharger. And I'm going to fill up with 100 octane gasoline. Which ignition angle should I use? And what is the maximum boost I can use?

[@nickolasedmunds]

My favorite engine 🎉. Damn durable

[@eugenegutierrez5106]

My cx5 came with skyactve G engine. 14:1 compression but it’s sluggish. Any ideas why that is.?

[@MrTAKIS1984]

Do you know if a b6 engine can handle over 10:1 compr ratio? Stock is 9.4:1. Do we need any extra modsto keep it safe increasing compr ratio??

[@Reaper_f30]

I have a question, so i have bmw 2018 f30 320i lic b48 motor engine block B48B20A 11:0.1 compression ratio bm3 stage 1 CAI MST CHARGE PIPE catless dp on way and i am going to buy a turbo inlet pipe aswell as xhpflashtool to go stage 3 I only use 99 ron fuel, my question is once i put in forged wossner pistons of the same stock ratio, will my car handle more boost safer? did i make a mistake, should i have bought a 330i b48B20B with a cp of 10:2.1 or if im going all out, pistons, turbo etc my choice was fine?

[@automotivation2527]

What you are doing is fine, it you can raise the ceiling of power switching to E85. This requires more fuel volume due to the required A/F ratio.

[@mhh3]

Question: do you need higher octane fuel after this change? NA engine. 10:1 vs 11:1 as an example.

[@automotivation2527]

Not usually. However, it is possible to move i to a “knock limited” engine with these changes

[@JerZeyFiveOh]

This is Rambo_Nate bro! I have a question for you? What’s the difference between dynamic and static compression? A good example is Gen 3 coyote engines that have high static compression but doesn’t the dynamic compression changes when VVT system kicks in when certain fuel is present? I talked to a Ford engineer and he said the engine can run on 87 octane.

[@automotivation2527]

Dynamic compression is based on VE (volumetric efficiency). Static compression is calculated by the difference in volume between BDC and TDC. Its a hard number, meaning that doesn’t change. Now, if the cylinder can only fill itself to 80% with air before the intake valve is shut, then you are at 80% VE. The dynamic compression will not be as high as static because you never saw 100% cylinder volume. If you are boosted however, you can cram more than 100% cylinder volume, making your VE up in the 110-120% range and higher. Dynamic compression ratio at this point would be higher than static. Could be 16:1! Which is why octane limits boost so quickly. The coyote example is just stating how VVT is changing how well the engine is breathing, hence bringing VE closer to 100%. The engine can run 87 because the conditions of the tune and engine are not beyond that octane rating. The next video will be on E85 and the different fuels! So we will dive deeper into all this!

[@JerZeyFiveOh]

@@automotivation2527 Great explanation! I have to reread that again for a better understanding.

[@brandonbaxter7548]

If I switch from Stock Piston into a High Compression Piston, do I need to shift fron Regular to Premium Gasoline aswell?

[@automotivation2527]

I would anyway, but in actuality it will depend on the specific situation. Ive ALWAYS tuned a car on premium fuel at minimum

[@bloodspartan300]

What do you mean the rings are cut for a track application? Is it a single ring or a dual ring design?

[@automotivation2527]

Ring gaps are cut a little wider than stock, but not quite as wide as nitrous or boost setup

[@RAY-iq2hb]

Can a high compression that tuned on e85 and still spray nitrous? Would that messed up the mixture?

[@chiefdenis]

All nitrous does is cool the intake air so that ignition can be advanced and more fuel can be added, it also has the side effect of increasing the density of air molecules since cooler air is more dense

[@automotivation2527]

@@chiefdenis The biggest thing nitrous does is it is 80% oxygen molecules, so that is where you are adding the extra fuel in. It is crazy amounts of O compared to atmosphere

[@gii1655]

Its better to have super charger to get more air in that smaller chainber

[@johndomine1574]

You replacing the 2.3 with a Nissan engine?

[@automotivation2527]

No, this is a 1.8 Mazda engine 😊

[@gregleenyit]

Basically the tighter the squeeze, the bigger the bang, the better the suck

[@danielslocum7169]

as i understand it, the suction is created by the exhaust exiting the cylinder which pulls on the just beginning to open intake valve which is called the scavenging effect. so apparently......a higher compression ratio results in higher cylinder pressure even after combustion takes place,thus making the exhaust exit faster and create more efficient scavenging [according to what this video is saying]. is this true? does a higher compression ratio create more cylinder pressure after combustion takes place? i kind of doubt it honestly; is there something wrong with my logic here? anyone???

[@automotivation2527]

The scavenging effect is still there. Then once the exhaust valve closes, the multiplier of the compression ratio keeps a higher level of vacuum during the intake stroke. On compression, the cylinder pressure is going to be higher at that rotation degree than it was before because 1) higher comp ratio = more pressure and 2) slightly better charge. Once the charge is ignited you should see peak cylinder pressures higher than before. If you want to see all the answers you would need to see a data log on a cylinder pressure sensor compared on the same motor across two different static compression ratios.

[@danielslocum7169]

@@automotivation2527 so higher compression creates more vacuum during the intake stroke. not saying you are wrong,but iv never heard that before. if you are correct about that..... then you have provided very interesting and valuable information. can anyone else out there confirm or discredit this?????

[@beingquincy316]

I've got an Rb25neo non turbo but the ring on cylinder 5 seems to be well going its not broken yet is is possible to swap across the turbo component onto my non turbo head and block and run it? (The rb25neo det is kinda hard to get here but theres an availability of the parts)

[@automotivation2527]

This would take a little more research. I would check out what the factory ring gaps are for that engine, then compare them to the turbo engine.

[@beingquincy316]

Thanks for responding i've spent quite a while researching since then

[@daviddominguez1466]

Are u going to build a motor for an audi a4 2.0t

[@automotivation2527]

Next motor I’m going to build is a 4.6L Ford V8. Quad cam

[@gordonflash8976]

Your camera movement is totally annoying and distracting, your topic is great.

[@joe-hp4nk]

You're over reacting, just get your valve events right and a good free flow exhaust to start the intake flow and physics will take care of the rest.

[@automotivation2527]

Why so serious?

[@joe-hp4nk]

Just commenting, take a pill.@@automotivation2527

[@automotivation2527]

@@joe-hp4nk its good vibes bruv. Its just an info vid 🙃

[@joe-hp4nk]

It's just a comment, carry on.@@automotivation2527

[@mcanderson0]

a 9 min way to say something thats shown with maybe 4 or 5 rather straight-forward equations.

[@automotivation2527]

It takes the 9 minutes to explain the 4-5 simple equations. The video is for people who don’t quite have a grasp on engine concepts. Its also an introduction to the concept. Chill.

[@Manuqtix.Manuqtix]

Dude you lost me? I wish you could draw explain this on a board or paper? I don’t understand what you mean by pressure differences

[@spiroskatsikas]

U don't know what Ur talking about, high compression allows the cylinder to remove more exhaust and volume and the inlet get more clean air charge because there's less volume at top dead centre . It's more efficient getting a little more air in and more exhaust out ,and the best shape is a Sphere using domes isn't the best but is a compromise

[@automotivation2527]

Lol

[@SweatyFatGuy]

yeah, ok. Thats why my 8:1 455 has considerably less vacuum than my 13:1 455 does, and the pump gas engine makes a lot less power... because it only scavenges a little better. Perhaps Spiros, you should build more engines instead of just going with what someone said one time. Everything in an engine is compromise. You must drive around with tiny engines in Europe, where a 2.8L is considered huge.

[@TheDeadMan3848]

Ouch.. lots of sharp edges on those pistons. 💥

[@SweatyFatGuy]

Something the small engine guys might not notice is an improvement in fuel mileage with increased compression. Smaller gains are from smaller engines. Bore diameter and chamber shape play a big part in ping/knock, larger bores are more prone than smaller bores and chamber shape can cause the flame front issue you talked about. When you have a fuel that can withstand very high compression ratios, you can work it much harder and get more torque across the entire RPM range. More torque means it moves the vehicle easier, with less throttle angle, less air, and therefore uses less fuel as a result. Its something I have been utilizing with my Pontiac 455s for the last 30 years. Make as much torque as possible everywhere in the RPM range, which allows tall gears, and you use less fuel despite it being a large engine. Just so happens that Pontiac engines make lots of torque even with low compression, due to things like runner length, engine displacement, and port volume / shape which all create a high velocity in the port. Increasing static compression affects all of those except displacement in adding to the velocity of the port. This has led me to trying and achieving interesting things. Running 87 octane with an 8.8:1 455 made enough torque NA to push a 4100lb 1970 GTO into the 12.60s with a 2.93 gear, yes two nine three. Not a typo. With a 2004R with a .67 OD and a Qjet that has small primaries that increase velocity, that car averaged 18-19mpg highway and got a best of 20 following a semi down I-40 across Tennessee. Plenty of power, it would smoke the 275 65 15 tires at anything under 45mph easily. Whack it from idle and it boiled them like Bob Glidden without holding the brakes. Then in 2007 I switched to E85 and that 8:1 engine made even more power. In 2010 I completed a new 455 build with 13:1 SCR to take advantage of E85. Thats the highest compression I could get without a dome piston by milling the heads, adding .040" stroke, and zero deck. The power difference was huge, had to be very careful getting moving from stoplights or tickets would ensue. I didn't take it to a track to get an ET, but it ran like my 11.40 Firebird that had an 11.3:1 455 in it that used to run on VP red and Sunoco 110. The interesting part was with one of my modified Qjets it got 20mpg mixed city/highway mileage, provided I did not drive it like a 17 year old who stole dad's car. Everyone says ethanol gets less mileage because it has fewer BTUs in it, but that is leaving a lot out of the picture. BTU is only measuring how long it takes a given fuel to heat one pound of water one degree. It measures nothing about flame travel, burn rate, latent heat of vaporization, spark knock resistance, ease of vaporization, or oxygen content. Yet when you raise compression over 12:1 and every other parameter stays the same, like displacement, cam timing, cfm, etc, you get the same mileage from ethanol that the lower compression engine would get on pump gas. Go even higher, like 22:1 you find in a diesel, and you pick up even more mileage, its not linear though and it varies depending on engine design and where it makes its power. Ethanol makes the most power between idle and 4000rpm, with smaller gains as you go up in RPM. Now if you run ethanol in a low compression engine, particularly the same way you do with gasoline. i.e. 160 thermostat, everything as cold as possible, including cold air and cold fuel, you will get lower mileage than gasoline. You are not utilizing the properties of ethanol with low compression and cold air/coolant, and the added oxygen content in ethanol is why you need to run more of it to avoid lean conditions. It will make more power than gasoline even in low compression engines. If you think like you do when tuning gasoline, ethanol is not going to work well for you. It takes a different mindset. With gasoline, lean makes more power, and rich makes less. With ethanol rich makes more power lean makes less. Gasoline as a propensity to ignite when it feels like it due to heat and pressure, ethanol waits for the spark to show up and is very resistant to the diesel cycle. Most people tune E85 and higher % ethanol fuels like they do with gasoline, and they tend to lean them out trying to get more power. Best power on E85 is around 7.5:1 AFR, best mileage is between 8.5:1 and 10:1 AFR. Also you can give it more advance if the engine likes it, even with boost, but you need to add fuel with the advance. If you go beyond 7.5:1 it will use more fuel, but not lose any power until you get closer to 5:1 AFR. So you can go pig rich to be safe and it will keep grunting out the torque. The cooling effect of ethanol as it vaporizes can be utilized in race engines as well as daily drivers, and we all know that cooler intake charge means more power, more oxygen means more power and less its affected by atmospheric conditions. If you feed an ethanol engine hot air rather than cold air, the fuel vaporizes even faster and still cools the intake charge. If you heat the ethanol in the fuel rails (don't send it back to the tank though) it will instantly vaporize when it hits the relative vacuum of the intake port, thus cooling the intake charge even more. I have done this with LS engines and my Pontiacs. Placing the injectors where a carb would be, such as with the TBI systems sold by FiTech and Holley improves the cooling effect over MPFI which has the injectors close to the valves. There is power to be had there, and mileage as well. One thing you need to consider if you are running E85 to E100, a hotter thermostat. A 160 or no T stat will milkshake the oil. Condensation from heat cycles is not boiled out of the oil, nor is fuel that slips past the rings. Water does not lubricate worth a damn, and ethanol dilutes oil same as gasoline does, so if you can keep the oil temps over 212F it will boil out both the water and the excess unburned fuel. Hotter oil reduces windage and drag in the engine too, just like a thinner oil does. Running an engine cold like with a 160F T stat will not allow the oil to get hot enough no matter how far you drive it. Heat is your friend with ethanol fuel, not your enemy like gasoline. Compression makes more power across the entire RPM range of the engine, its always there, doesn't need a belt, extensive plumbing, or a refill, but it does need a good quality fuel. When utilized well, compression makes a more efficient engine, the quality of the fuel and the properties of said fuel will dictate how much compression you can run in any given engine design. The engine design itself also plays a part, bore size, chamber shape, etc... Something interesting about my Pontiacs, since they make all their power under 6000rpm, you can crank lots of advance into them and pick up ET and MPH when you run a 2.41-3.55 final drive gear. The engine stays in its torque band which is from idle to 5000 longer with a tall gear, a short gear like a 4.10-5.13 moves it through the RPM range too quickly and doesn't utilize the torque long enough. A 455 with a 4.10 gear will always be slower than the same engine with a 3.08 gear... which makes zero sense to small engine guys. Where do you want the most advance? Under 4000rpm. Retarding the advance above 6000 picks up power, but if your engine never goes above 5800, its not helping you. That 11.40 Firebird I talked about earlier, it wanted 52 degrees total timing on VP red and Sunoco 110. Back it down to 36-38 where the SBCs like it and it slowed to the mid to low 13s with no other changes. The balancer did not slip and I verified its accuracy multiple times. The engine just wanted that much timing. I slowed it to 12.0s by putting a 4.10 gear in it, that made it get to 6000rpm around 900ft so for the last 420ft it was not accelerating. The 1/8th mile slowed as well as the 1/4, and the 60ft remained the same. Those heads on another 455 (in my 65 GTO) wants 44 or so on E85. Small engines that are not producing 500+ ftlbs from 1900 to 5500rpm will not react the same to a highway gear. If you have a supercharged engine, you will need less final drive and it will be more like a Pontiac 455 than the small engine you have under it. So may fun things about engines and fuels. I simply adore high compression, it sounds good and feels good as I am shoved into my seat and watch the chevys and fords struggle with pump gas handicaps. :)