how to raise compression???

32342 Views 18 Replies 11 Participants Last post by techinspector1, Jan 27, 2007

53belair

Discussion Starter · Dec 29, 2006

im asking this question for my brother
he wants to know how to raise the compression in his chevy 350(hes keeping his in a chevy vega)

1 - 19 of 19 Posts

killerformula

· #2 · Dec 29, 2006

well, this is probably a question for hotrodding basics but I'm not an engine mod. Its pretty easy to do. Any way you can reduce the amount of compressed space at TDC. This includes lowering the combustion chamber volume for the head (different heads) decking the block, changing pistons or using a thinner head gasket.

This question spurs a multitude of secondary questions too. Why do you want to raise the compression, what is the goal, and what are the engine's specs (cam, intake, runner size etc).

K

Blazin72

· #3 · Dec 29, 2006

How much are you looking to raise it and how much does the engine have now?

If you're only looking to raise it a few tenths then you could have the heads milled or go to a slightly thinner head gasket. If you're looking to raise it a point or more then you're going to want to look into either going to a head with a smaller combustion chamber and/or going to a higher compression piston. It really depends on what you have now...

53belair

Discussion Starter · #4 · Dec 29, 2006

its a stock 350 out of a truck,i think
something we've always been taught was that higher compression offers more power,never questioned it really
ive seen on here several times that 305 heads raise the compression im not sure if this is true
hes looking for more power,would heads off a 99 chevy truck(its a 350) work on a 86 350?

killerformula

· #5 · Dec 29, 2006

if your goal is to "go faster" you probably would be better off just swapping in a steeper set of rear end gears. Compression alone is not really going to give you big net gains. Compression in conjunction with a motor that is designed for it (cam, intake, carb, exhaust) is essential, however.

K

53belair

Discussion Starter · #6 · Dec 29, 2006

how much of a power gain(bottom end)will there be?

rreruption

· #7 · Dec 29, 2006

I would swap heads,maybe to some later style Vortecs.It depends what compression and cc heads you have now.
If you swap heads,roller rockers,a mid range cam like a 262H XE or 268H XE,a good double roller timing chain,Performer intake,and carb,and a good ignition.You could see some nice gains.Maybe a good 330-360hp.

Arns85GT

· #8 · Dec 29, 2006

4 degrees of cam advance?

For the SBC guys, would adding 4 degrees on the cam advance boost the compression some?

Arn

rreruption

· #9 · Dec 29, 2006

no I dont think so.You could do it and it would add a little more low end though.But if I was going to that point and was looking for more power,I would also replace cam + lifters.You dont necessarily need a comp cam.If money is an issue,lots of companys like summit offer a good price.Really swapping heads is the easiest way to raise compression.But you gotta know what you have before you start.And so you know where to go.What casting number is on these heads?Im betting the heads you have now are an 76cc,and your compression is around 8.5:1.

steve t

· #10 · Dec 29, 2006

headers for more hp

FordMan1951

· #11 · Dec 30, 2006

Just do the math FIRST. I had a 221 SBF that I had polished the ports on and when it dropped a valve and ruined the block I thru together a 289 short block wih 13.5:1 forged pistons. Turns out the 221 heads had a lot smaller compustion chamber. My ratio wound up around 18:1. Installed two head gaskets per side and dropped it down where it would crank and run. Man that motor sounded sweet. :mwink: After the change, nobody would race me.

playsinthedirt

· #12 · Jan 24, 2007

how much horse power you think you were getting from that 289? im researching for my future builds and i think 289 might be good start. how expensive?

rreruption

· #13 · Jan 25, 2007

It depends on what is done with it.Most would say a strong 289 is worth 335hp.

GSX-R

· #14 · Jan 25, 2007 (Edited)

If your'e interested in the "theorical" gain of raising the volumetric ratio, you can try the otto cycle calculator there :

http://members.aol.com/engware/calc3.htm

Try to only change the compression ratio and appreciate the difference of the cycle efficiency (make a simple ratio of the both %). It will give you an gross idea of how much you can expect to increase your torque/power if you don't exprience knocking (and that's the main limiting factor of raising CR) and if additionnaly your block supports more mechanical constraints.

For instance, increasing the CR from 9:1 to 11:1 can potentially increase the power of 5.5% for the same BSFC.

Actually, even if we except knocking, the raise in torque (and thus in power) is less than the theory's one because of increased walls heat transfer and more engine friction.

If your're basically looking for more efficiency, you have to stay just below the knocking phenomenon without any knocking detector late ignition acting.

Hope it helps.

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xntrik

· #15 · Jan 25, 2007 (Edited)

Arns85GT said:
For the SBC guys, would adding 4 degrees on the cam advance boost the compression some?

Arn

Static compression cannot change.
Dynamic compression will increase a little. Probably a new timing set will help just as much by reducing wear induced cam retard.

xntrik

· #16 · Jan 25, 2007

53belair said:
its a stock 350 out of a truck,i think
something we've always been taught was that higher compression offers more power,never questioned it really
ive seen on here several times that 305 heads raise the compression im not sure if this is true
hes looking for more power,would heads off a 99 chevy truck(its a 350) work on a 86 350?

Each full number of compression raises power by about 3 %.

Basicly you want to increase chamber quench by installing proper heads and piston combination and acheive a compression of about 9/1 for 87 octane.

Raising compression with poor heads or poor quench is not a proper solution.

Arns85GT

· #17 · Jan 26, 2007

thanks

Thanks for the explanation.

Now I just have to get some info on dynamic compression.

My current project BTW is to advance the cam timing to bring my power band down a few hundred rpm. The added compression is a secondary benefit if it can be had.

Arn

xntrik

· #18 · Jan 26, 2007

Arns85GT said:
Thanks for the explanation.

Now I just have to get some info on dynamic compression.

My current project BTW is to advance the cam timing to bring my power band down a few hundred rpm. The added compression is a secondary benefit if it can be had.

Arn

Be sure and check the valve to piston clearance. When you advance a cam the intake clearance decreases. The opening intake valve is chasing the descending piston.

techinspector1

· #19 · Jan 27, 2007

Arns85GT said:
For the SBC guys, would adding 4 degrees on the cam advance boost the compression some?

Arn

It will increase the dynamic compression ratio because it closes the intake valve earlier and traps more of the mixture on the compression stroke. The motor will have more low end torque, but will peak early and not have as much top end. To get something, you have to give up something. Pay attention to xntrik's post about valve to piston clearance when installing a cam either advanced or retarded from the grinder's specs.

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