compression ratio comparison

[futten]

can someone tell me how much horsepower difference there between 10:1 compression and 12:1 compressin in a otherwise stock engine i have been told 30 hp by one person and 70 hp by another engine in question a N/A engine 1980 3.8

 

[bison]

There's too many things that could effect the result. My guess with all else the same and not detonating is. 6-7%. 70hp not going to happen

 

[earlbrown]

and stop listening to both of those people.

I've found that anyone that says such overly simplified statements as fact don't know enough to trust.

 

[futten]

and stop listening to both of those people.

I've found that anyone that says such overly simplified statements as fact don't know enough to trust.

well the person who said 70 is a local engine build with 20 years experience with building high performance v8s and works on methanol and nitro engines but the thing is most people in here may have built N/A engines but not with the same setup aka 12:1 compression ported iron heads modified and ported edelbrock performer intake a and the cam specs as in the picture ive added equal length headers etc

 

[cruzn57]

if he is such an "expert" .(local builder)
why come on here to ask?

 

[futten]

because he hasent buildt any buick v6`s but knows about em and has had a gnx and i dont know if that really makes a difference

 

[bison]

well the person who said 70 is a local engine build with 20 years experience with building high performance v8s and works on methanol and nitro engines but the thing is most people in here may have built N/A engines but not with the same setup aka 12:1 compression ported iron heads modified and ported edelbrock performer intake a and the cam specs as in the picture ive added equal length headers etc

The displacement and engine speed plays way more of a role than the CR. The returns of increased CR diminish the further it's increased. Going from 8:1 to 10:1 might give 8-10%. I've seen a few super stock applications that will thoroughly embarrass some much more modified engines. The best engines have large bores and really good heads. The Buick 3.8 has a relatively small bore from a performance standpoint and the cam journal and cam location is not ideal for anything high rpm. Additionally the stock heads limit the rpm potential a bunch. Trying to get enough spring on there to get 7000rpm or more reliably is not easy and port volume is limited. That's probably more like what rpm it would take to see a 70hp difference on a stock stroke 3.8 NA Buick by raising the CR from 10:1 to 12:1. Filling the cylinder is of prime importance when it comes to power. If you had 1.5hp/ci and 250ci at 10:1 I'm not too convinced you'd have 445hp by only bumping CR 2 points and this is an example where rpm would be very high and show a lot more gain than a lower rpm application.

 

[futten]

ok well to add to the info on my engine is ported iron heads edelbrock intake equal length headers (if N/A) stock valves max rpm 6500 and that cam in the file i added forged crank stronger rods and forged pistons

 

[turbo89]

ok well to add to the info on my engine is ported iron heads edelbrock intake equal length headers (if N/A) stock valves max rpm 6500 and that cam in the file i added forged crank stronger rods and forged pistons

The dynamic compression is more important than the static ... if all else is equal... I cant see adding 2 points of CR making 70 hp .. 30 is more like it..

now if we are making cam changes in conjunction with the CR bump ... I can see 70hp

 

[bison]

ok well to add to the info on my engine is ported iron heads edelbrock intake equal length headers (if N/A) stock valves max rpm 6500 and that cam in the file i added forged crank stronger rods and forged pistons

I'd be happy with 1.2hp per ci. Anything that can be done to increase the intake valve size will help a lot.

 

[futten]

but i dont see going from 1.7 to 1.77 and 1.5 to 1.55 on the valves really helps much

 

[bison]

but i dont see going from 1.7 to 1.77 and 1.5 to 1.55 on the valves really helps much

Going to an aftermarket valve will help because of the shape of the valve. I doubt it's easier to go larger than 1.77 intake unless you are going to relocate the valve and that would be a lot of work.

 

[futten]

down and back cut valves and a proper 3 angle valve seat helps more then a 0.070" and 0.050" increase in diameter

 

[turbo89]

down and back cut valves and a proper 3 angle valve seat helps more then a 0.070" and 0.050" increase in diameter

that and pulling weight off the valve

 

[futten]

what ya mean?

 

[turbo89]

what ya mean?

more in the valve seat.. the valve itself.. and weight in the valve than in the diameter

 

[futten]

well yeah hollow valves for example would be nice

 

[turbo89]

well yeah hollow valves for example would be nice

Im referring to something more economical that does the same function... gotta think out of the box !

 

[futten]

by doing what?

 

[fuzzyGN]

Adding 1 point of compression is worth about 4% but as Bison stated as you go higher the number decreases. From 8:1 to 10:1 I bet it would be close to an 8% gain. Going from 10 to 12 my guess it would be close to 6% increase. So also as others have stated the stock 3.8 bore is going to be the killer. The bigger 4.1L bore has many advantages. With your app though, the bigger valves would help but only marginally compared to the bigger bore 4.1. The valves are already somewhat shrouded and increasing the diameter makes it worse. A lighter valve will help much more (than bigger) because it will be easier to rev and much less taxing on the valvetrain components. A light stable valvetrain is a happy valvetrain. So, the heads, valve size and bore is all set and can only be slightly improved upon the next place to make power would be compression. Go for it. I have a tad over 13:1 in my N/A engine.