A question of compression ...

[68RustBucket]

I was wondering what factors in an engine determine compression, and how to increase it if necessary.

I've got a 68 289, all stock right now but planning some upgrades. Ported heads, 280H competition cam, edelbrock performer rpm intake, edelbrock 600cfm carb, try-y's bla bla bla. the compression on the stock engine is something like 9.0:1 maybe? (i recall reading it here a bunch of times and can't remember). I'd like to know how the different amounts of compression affect performance of an engine.

 

[67GTA-FB429]

More compression means higher octane gas....more $$$.

To get more compression, you can deck the block and/or heads, you can use different pistons.

Why do you want to increase compression though??

 

[taylor4g63]

I think he's just asking IF he raises his compression say to 10:1 how much power would he gain? I remember reading it somewhere, but i honestly can't remember. Hopefully somebody else will know.

 

[SuperDave]

To increase the compression ratio in these days of declining gasoline quality may create more problems than you might want.

 

[67GTA-FB429]

I just read on a webpage that 1 point of compression will yield about 4% HP gain.

 

[imshaggy2000]

Personaly I think upping the compression beyond 10:1 on a carbed street car is a pretty dumb idea. Lots of cars today like my mom's honda prelude have 11:1 compression or more even, but they require precise computer control to run corectly with premium pump gas. I can even put the cheap stuff in it because the computer automaticaly retards the timing so it wont knock. For a performance street car I would build an engine with 9.0:1 - 9.5:1 compression.

 

[brianj5600]

The cam has enough duration that it needs a little more compression. Comp cams recommends atleast 9.5 to 1. The first thing you need to do is figure out what you have, but I would not go past 10 to 1.

 

[rbohm]

things that affect the compression ratio are, combustion chamber size, pitson dish or dome size, how far down from the top of the cylinder the piston is, stroke length, and to an extent cylinder bore. sa to why you would want to increase compression, power and efficiency are the reason. there is a point of diminishing returns, and you need to keep in mind the octane rating of the fuel available to you and the valve timing. often times cam companies recomend a certain minimum compression ratio to help maximize efficiency with the particular cam grind. cams with high overlap will help an engine tolerate lower octane with higher compression, and a cam with lower than normal overlap will cause cylinder pressure to rise too high for the octane available even though you might have say 8:1 compression. cylinder pressure the engine sees is the eky to efficiency with a selected octane rated fuel.

 

['69Stang]

I run 10.75:1 compression with the stock iron heads and have my ignition set to 18 degrees of advance. No problems. Of course, I do use 93 octane gas. For a 280H cam I would go for 10.0 -10.25:1 compression. I am not sure how detonation resistant the combustion chambers are in the stock 289 heads. Essentially, figure out what type of pistons you have - domed, flat-top, or dished then decide what combustion chamber size you can safely achieve based on your cam spec's and the valve placement and you should be good. A good idea is to make sure that the combustion chambers are polished smooth in the heads so that there are no "hot spots" that could contribute to the dreaded "ping".

 

[68RustBucket]

thanks guys. all that helps out a lot. i know i'm not going to be using this as a daily driver. probably more of a weekend car, probably on historic insurance if i can manage. i've got a small airport near me, so i have airplane fuel (110+ octane, $3+/gallon)available to me if i may desire it in small quantities mixed with normal gas. since i won't drive it so very often, gas prices aren't going to be the biggest deal.

i wasn't asking that question in hopes to increase compression. i pretty much wanted to know what the advantages of differences in compression are, and if i should try to increase it or not

 

[rbohm]

ok let us look at this mathmatically. take two engines, one 8:1 compression and one 10:1 compression. we will also round off atmospheric presssure to 15 psi for the sake of simplifying the math, as well as assuming 100% volumetric efficiency. on the compession stroke of each engine we take 15psi and multiply it by the compression ratio, thus we get 8x15=120 and 10x15=150. when the spark lights off the cylinder, cylinder pressure rises by a factor of ten at its peak pressure, thus we have 10x120=1200 and 10x150=1500. at this point we subtract the gross cylinder pressure by the initial compression pressure to get the net cylinder pressure, thus we get 1200-120=1080 and 1500-150=1350. the net difference in cylinder pressure between the 10:1 compression engine and the 8:1 compression engine is 1350-1080=270psi. that is an extra 270psi per cylinder extra work on the piston by bumping your compression ratio by 2 points. there is a point of diminishing returns where the added cylinder pressure becomes detrimental to added performance, and that point depends on many factor. but you get the idea as to how compression ratio converts to compression pressure, which then converts to work.