How important is compression?

[IsaacP]

I have read a lot of forums lately and want some opinions. I hear a lot of guys talking about compression and some say you can't make any "real" power with a low compression engine, but I have also read that for any given engine, an increase in compression by one point only adds roughly 3-5% more power which isn't much. And then you hear guys say that air flow is more important than compression and they say if you have good flowing heads and cam you can make good power. I guess my question is, if increasing compression only adds 3-5% more power, why do guys talk like compression is so important? Which side is right?

 

[biomedtechguy]

@IQ52 certainly has commented quite a few times about CR and power output, and with his experience and accolades, if Jim LaRoy says it-believe it!
He has built many low compression engines that make HUGE power.
I'll add that if you have too much compression, detonation will cause more harm by far than good, and trying to compensate for too high a CR by running high octane fuel or additives is expensive, and reducing timing in an attempt to prevent pinging can erase any potential power advantage of a higher CR.
Play it safe!

 

[451Mopar]

The 3-5% more power is peak power not overall power. The correct compression helps boost the entire torque curve, but really helps the lower RPM torque numbers.

 

[steve340]

Having enough compression to get the job done is vitally important.
But way too much compression is a real problem also.
Knowing what you want from the engine is also important and this will map out what compression ratio you need and the parts to use.
A race engine is a completely different animal to a primarily street engine.

 

[Bill Monk]

I agree with the above statements. If it's a street engine, build it to run on pump gas. In a drag race engine, you're only concerned about 1/4 mile at a time and running 112 leaded fuel or higher is acceptable to get all you can get out of the engine.

 

[BSB67]

Everything that you've said is probably accurate, and I don't necessarily see a conflict in what you've posted. But looking solely at CR in a vacuum from the rest of the engine build is probably not a great idea either.

So apparently people feel differently about the value of 5% more hp. If you're building a motor, why would you leave that on the table? Personally, I would not. On the other hand if you have a solid, reasonably fresh short block that's ready to go that is one point down on CR, I would not go through it for the sole purpose of increasing CR.

 

[miller]

Yup. And, don't forget CR can be 'adjusted' by the thickness of the head gaskets.

 

[Lefty71]

Compression ratio is not, in itself, the defining factor in an engine, but it sure is the most talked about, and certainly the first thing a seller boasts!

 

[BeepBeepRR]

Sounds to me like the OP might have an RV engine. When you hear people talk bad about the compression ratio of a 440 its usually an RV engine. Those were what? 8.12:1.

 

[Cranky]

Having a good running engine imo hinges on a lot of things including good combustion chamber design, decent compression along with good quench/squish, cam......and on and on. It's a total package that needs to be on point and not just one thing. One question I have about lower compression engines is how efficient can it be? I've ever had a low compression engine that got really good fuel mileage vs one that had more compression along with a good combination of what I mentioned in my first sentence. 8-1 engines with pistons that are .100+ in the hole AND using open chamber heads with composition gaskets (.040 thick) do not make a very good base for a good build imo. Had a few of those and a couple ran pretty dang good but they were also gas hogs and liked to ping once the temps went over 180 a bit.

 

[Darter6]

Also look up static compression vs. dynamic compression. And quoted compression figures have a range factor. Is 8 to 1 a real figure ? When we were doing allot of Slant Six racing some engines checked out to be about 7.2 to 1 compared to the factory rating of 8.4 to 1.
Hughes Engines claim to have specific cam profiles to make horsepower with a low compression engine.

 

[Cranky]

Also look up static compression vs. dynamic compression. And quoted compression figures have a range factor. Is 8 to 1 a real figure ? When we were doing allot of Slant Six racing some engines checked out to be about 7.2 to 1 compared to the factory rating of 8.4 to 1.
Hughes Engines claim to have specific cam profiles to make horsepower with a low compression engine.

I know there are many cams out there that are supposed to raise cylinder pressure in low compression engines and have been down that road. Not saying it's a bad way to go but rather have higher compression with good quench to start with.....

 

[Darter6]

Yeah, that's why I used the word "claim" True that if you are rebuilding go with a quality piston with all the good stuff.

Hughes Engines claim to have specific cam profiles to make horsepower with a low compression engine.

 

[Cranky]

Had a 400 that ran pretty good in a 3/4 ton pickup. Bought a wrecked truck years back that was a very nice truck. Almost cried when I saw it lol. The guy said it had forged pistons etc etc and after getting it home and pulling the heads, it did have all that he said. Wasn't concerned if it didn't since the price was cheap enough and it had everything I wanted for my old but not wrecked beater. Ending up having a beater with a nice interior and drive train that surprised a lot of people trying to brush me aside on the way back and forth to work lol

 

[dvw]

Compression increases power. Going from 7.5/8.0 to 9.0-9.5 is big. 9.5 to 11.0, still helps but gains but less. 11.0-15.0 more gains but they diminish as the ratio increases. Proper cam sizing will definately help. But don't kid yourself. If you have the means to obtain good fuel I'd never run less than 15-1. That being said if money is limited. Go for good heads, correct cam, and intake on a reasonable condition stock short block. It will make more power than a fresh short block with stock heads.
Doug

 

[6PKRTSE]

All true above. I know I left some on the table with my 528" Hemi build but it is still 100% streetable as only a 10.5:1 pump gas motor. More camshaft, more rocker arm ratio and 15:1 would have limited me to just a 1/4 or 1/8 drag car. I like to enjoy my cars for both street and strip. Good thing for me I can literally walk to PAC Springs from my house so wearing out some valve springs on the street is no big deal.

 

[IsaacP]

Sounds to me like the OP might have an RV engine. When you hear people talk bad about the compression ratio of a 440 its usually an RV engine. Those were what? 8.12:1.

Yes I do have an RV 440. The reason for the question is, I'm wondering what I can expect from my engine with aluminum heads and cam, without rebuilding the short block to increase the compression to 9.5-10:1. A lot of people say you can't make any power with a low compression engine

 

[Lefty71]

Starting out with a very low compression engine is a horse of a different color! What was the original application and year?? And what head gasket is in place now, still steel shim, or aftermarket blue?

 

[IsaacP]

Starting out with a very low compression engine is a horse of a different color! What was the original application and year?? And what head gasket is in place now, still steel shim, or aftermarket blue?

It came out of a 1978 motorhome, it has the stock head gaskets. I put on an edelbrock performer RPM intake, edelbrock 650 carb, flotech headers. Next is edelbrock heads and a cam

 

[BeepBeepRR]

Those engines are 8:1 engines. The only way you can squeeze a little out of them is with heads, head gaskets. Depending on the CC of the heads and thickness of the gaskets you can bring that up a little. I was told not to use Steel shim gaskets if I went with Aluminum heads. I had my 452 heads rebuilt and some port work done. I also did some speed pro forged pistons. My RV engine is at 9:64:1 I could have used a thinner head gasket and brought it up to 10:1