does anyone know the specifics of boosting compression ratios. whats the advantage of higher compression?

 

Higher compression = more power

Mill your heads down, that's the best way to do it without a rebuild I think....

Now if you want to do a rebuild, you can mill the heads down a little and use different pistons, I'd shoot for about 10 1/2:1 then you can still use forced induction should you choose to do so later on.

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[This message has been edited by 92GTA (edited April 23, 2001).]

 

I think the rule of thumb is for every 1 point increase or decrease in compression, you gain or lose 4% in power.

 

change pistons, change heads to some with a smaller chamber ,mill the heads, but you're limited on how much you can remove.

 

I dont like the idea of milling heads, thats like a last resort IMO. Every time you take material off the head it leaves more of a tendency for the head to warp. Some heads are designed with thick decks so you can mill them alot, but factory heads arent. I'd stick with stock or change pistons.

 

thanks for all the responses, so changing my LG4 (85 trans am purchased last week) to flat top pistons should give me the 9.5 to 1 compression that my L69 has or do i have to change the cam too? and aluminum heads with smaller chambers will improve this even further?

 

Yea pistons are the best way to go on an lg4.
Lg4's have dished pistons that cut back on compression "why god why ". But The lg4 stock heads are 58cc and great for building compression. One advantage of having a low compression engine that you might want to consider before beffing up is octane. With low compression engines you can run cheap gas with no ill effects. The higher compression you go the more octane you need and that =$$$$$ at the pump. SSC

 

so the pistons will put me at 9.5 to 1 is it possible to get an LG4 to say 10.0 to 1 or even 10.5 to 1?
on a different note how much horsepower will full length headers (i think hooker sells them), flowmaster 80 series w/gutted cat and edelbrock rpm intake and open element k&n free up? the stock motor only has about 150 hp, and i'm wondering how hard it is to get another 50 hp out of it.

 

Whoa just a minute. Just... whoa.

The compression ratio of an engine needs to be tied closely with the camshaft specifications in order to realize the gains that people generally talk about. The only true studies which I have ever seen in print were thorough comparisons of compression ratio changes on an engine combined with camshaft changes in order to realize the maximum performance gain. The test engine used in the study was assembled with a given compression ratio, then a series of camshafts were tried in there until the best power was produced.

The horsepower gain is really a combination of the longer duration numbers of a given camshaft design together with the higher compression ratios assembled together to match that particular camshaft. You can't just change your compression ratio and magically obtain significant increases in horsepower.

A slight adjustment in the compression ratio on an otherwise stock motor MAY net you some more power when combined with the correct tuning afterwards, but don't expect to see tremendous gains unless you are ready to pop a new cam in there to go with it.

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D speaks truth. No matter how much you try to compress that mixture, You gotta make sure you can get it all in there first! But the principal is the Same.


A mixture, when compressed, explodes more violently.

So compress it more, and you get more explosion, In laymans terms. More Power.


Sooo.... Its one way to Make more power. Like Anything, Its nO magic secret to high HP, but just a part of the picture.


As far as running low comp So you can run lower octane w/o Ill effects, This only applies if you consider a loss Of potential Power an ill effect or not

If you plan On changing pistons In your lg4, Im gonna ask the age old Question Of Why ?

Just get a damn 350, only a few hundred bucks more in the long run, and Much more potential


Unless, Of course, You are a retard Like me Who likes to build The underdogs



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not a good idea. forced induction engines should not be high compression. a good compression if you will be using a blower or turbo is around 8-8.5:1

remember, forced induction drastically increases compression. why build a 10.5:1 engine if you can only run 1psi of boost, which would put you well into 11+:1

no, not good. 8-8.5:1 will take 6-7 psi of boost without too much trouble.

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good i see i have some educated responses here, i'm starting to see the picture here but what is the optimal combination for good compression boost since i'm not running forced induction or plan on it. why do i want to work on a 305, it's what the car came with and i'm not buying another crate engine in the near future, so i will work on this for the time being (at least i didn't ask how to make it a stroker)ha ha ha

 

About the octane issue - my engine has the stock 8.5:1 CR. I run 87 all the time and it never detonates. Helps alot with gas so freakin expensive...

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I went with a two valve relief piston and raised my c/r to 9.75 to 1.

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Look at the profile for what ya should do get 200 hp outta your lg4.

cat back, high flow cat, headers, tune up that carb a little (check to make sure for full throttle, adjust air valve, etc)add a carb spacer (yes 1 inch will just fit with out rubbing) and away ya go with a 15.1 sec car.

if ya really want to solve the problem with the lg4, throw a mild cam in it, and put headers and an exhauston it, and you'll get a lot more power.

you dont really wanna increase the compression ration on an lg4 by milling... if your gonna tear into the motor that much, make sure ya do a cam when swappin out flat top pistons. make it 9.5 one, and put a good ol cam in there, port them heads while your at it, add headers and a holley carb, and you;'ve got yoruself a 300hp lg4!

(even though its notreally an lg4) the weakest link of an lg4 is the cam- they are small, low lift, short duration.puttin a cam in an lg4 is a serious upgrade for that motor. The easiest thing you can do is free up the exhaust though, wheni put the cat back on, the thing felt like it had a new motor in it


Steve

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