High Compression and Compression Ratios

When do you use High Compression? Understand, and make the decisions for compression ratio on your engine before you buy anything. Know what your end goals are because compression can limit you when it comes to what fuels you can use. Compression ratio is one of many decisions that should be taken into account in order to create the best combination of parts for the application of the machine.

Leave questions below, or suggestions for other topics you would like to hear about!

Cheers!

[jessicaembers]

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.

[ktmarvin]

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!

[chippyjohn]

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.

[brettwhitelaw]

Super great video, you did an excellent job explaining everything

[terryenyart]

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😊

[RickyBobby]

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.

[hectorortega]

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

[arlowicks]

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

[donkeytoots]

Youre kinda right with all this. First though, CR = (Swept Volume + Combustion Chamber volume) / Combustion Chamber volume = (Bore^2/4 x stroke + Head volume - piston protrusion) / (Head volume - piston protrusion). You didn't make it clear that combustion chamber volume is the denominator in that equation.
Second, you're kinda right about increasing pressure differential, but the change in volume per cycle, swept volume, is constant regardless of CR, and the 'spingyness' of the extra air at low CRs is such a marginal effect.
Really, the benefits of a higher CR is only improved thermal efficiency from higher combustion pressures. To an extent you also get squish turbulence from lower head clearances but the main benefit of this is knock resistance, and going to higher CR pushes you up closer toward the knock limit anyway.
And all these effects pale into insignificance as soon as you go forced induction.

[xaviergonzalez]

New subscriber bro! Nice and very informative video!

[peterbryant]

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

[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?

[josecuevas]

Their 4 Types "Compression.
●Static Compression
●Cranking Compression
●Running Compression
● Dynamic Compression

•Do your homework on the 4 types of "Engine Compression".

[Topsiekku]

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

[jrsgarage]

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 😳

[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 😂 .

[bossstrick]

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

[daleryan]

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...

[MrTAKIS]

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??

[Lexaturbo]

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?