Higher compression, better? Why?

[Ed]

We've all heard that the higher the compression in an engine the more HP you will have. I was wondering how this works. And also what is too high for a street car?

[LanceM]

the fuel mix is compressed more allowing for more efficency from the fuel, once you hit 10:1 you are probably done as far as pump gas goes. Really the HP gain by just bumping compression is small, it again is part of a performance package. Cam overlap and duration plays a big part in how high you can go on pump gas too, again part of the package.

Lance

[Victor Laury]

Compression ratio is a static mathmatical figure based on bore, stroke and combustion chamber vollume. The actual pressure of the A/F mixture in the combustion chamber varies in any given engine build, at different RPM's. As the engine RPM's reach a certain level, less A/F is drawn in due to frictional forces and the duration of the intake valve opening. With our "Pre Variable Valve Timing" engines, optimal output becomes a balancing act. We can build a "high performance engine", but We'll be only making an improvement to a certain RPM range. We build a 10.5:1 engine but we use a cam grind with an overlap. This negates some of that compression ratio in the lower RPM's, but at higher valve speeds, The longer duration intake dwell will increase combustion chamber pressure closer to optimal.

Dam, I know that there's a real engine builder out there who can describe it better!

[halz]

I was wondering how this works. And also what is too high for a street car?

Your engine is a heat pump and heat is created by burning a mixture of fuel and air in a small space. As the misture burns its heat (and therefore pressure) increases. This expanding force drives the piston down. If you burn the same amount of fuel/air mixture in a smaller space then a higher relative pressure is produced and this is the basic effect of increasing compression.

Of course, for all of the reasons mentioned earlier it is not as simple or straightforward as this. Increases in compression typically demand corresponding changes in valve and ignition timing. Significant increases in compression will require non-standard fuels to be used - fuel that won't simply ignite under compresion in the absence of a spark.

If compresion is raised high enough to require a specific non-pump fuel then I would say that it is too high for street use.

[26th-Z]

Victor, you hit most all the basics as I understand it all. Most everyone I talk with refers to a 10:1 ratio which I think that is a little high for street use. And then, fuel is going to be the limiting factor. 10:1 is going to require at least 98 octane for an engine like the L24. I think no matter how you acheive a higher compression ratio, its the whole package of cam, valves, etc. that is going to relate to horse power increase.

[Zrush]

We build a 10.5:1 engine but we use a cam grind with an overlap.

What is "cam grind with an overlap"?

Vicky

[26th-Z]

The profile of each cam lobe is ground into a shape that keeps one valve open just long enough for the other valve to open so that at one point both intake and exhaust valves are actually open.

[Ed]

Interesting, I'm starting to wonder if my engine is going to have too much compression. It's a F54 block with the flattops. N42 head shaved .040. I don't want to be dependent on non-pump fuel.

[Gav240z]

Just use a thicker head gasket if your concerned. You will probably want to cc the head and measure how much the pistons come up above the block to work out the comp ratio from there.

[Dan Baldwin]

10:1 is NO PROBLEM on a stock-cammed L-series engine. I ran my 3.1 liter at ~10.3:1 for years with the stock cam, and 34 deg max ignition advance, on 91-93 octane pump with no problems.

Maximizing CR WILL noticeably increase performance and economy, even with a stock cam. Going from ~8.5:1 to 10:1 should give on the order of 10% more torque EVERYWHERE. Maximizing CR is the 2nd most important thing to do (1st being maximize displacement) for NA performance.

Ed, with a stock cam and a .040" shaved N42, you should be A-OK with the 2mm gasket like Gav says. Check head cc and calculate CR before assembly.

[Gav240z]

Do you wanna hear the scary part I'm gonna be running ~11:1 compression on the new motor I'm building. However this shouldn't be a problem on the 98 octane we can get here in Australia

[tehghey]

Aren't there also octane additives that can be added to the gas to increase the octane rating?

One more thing, I've heard 2 different version of octane "math". One guy I know says that you can mix for example 50% 93 octane with 50% 89 octane to end up with 91 octane fuel in the tank. Another guy I talked to about it said that nope that doesn't work, mixing higher octane gas with lower gas won't do you any good and you'll still end up with the lower grade. The first way makes sense to me since from what I understand octane is just an additive to the fuel so they should mix. ANyone know which is correct?