Just as the title states: Is there an accepted ratio on compression and horsepower?

If you took a stock motor and increased compression by decking the block, milling the heads and/or swapping to a thinner head gasket, is there an accepted ratio that says for every 1/2pt of compression you can expect an increase of X HP?

Just curious, as I just replaced my headgasket. The heads were milled .005 and I used the Impala gaskets. Wondering what .025 reduction will do to my compression and HP (if anything). Last dyno was 287rwhp before headers and tune, if that helps.

Thomas.

 

When I was talking to AI about my motor build I was originally shooting for 11.4:1 compression. The pistons I had planned to order would actually lower my compression to around 11.0:1. I asked AI how it would affect horsepower and below is thier answer:

Typically if we change the compression alone (independent of quench) then you'd see ~6-9hp difference for .4:1 lower compression. That of course is an approximate difference in the peak values, not average.

Not sure if that helps but it is something...

TOM B

 

that sounds about right to me, thats why I decided 10.8 was good enough. no point in stressing out over a few HP andworring about detonation.
the quench is just as important and you took the biggest part of that diff. from the quench. so that will imiprove throttle response, eng. efficiency (more power/gas used), and sp. knock sensitivity. so yeah it might even be a noticible difference with just swapping the head gaskets. but you prolly wont notice that little unless you bracket race your car and are consistant.

 

There's a chart in David Vizard's book "How to Build horsepower SBC" forgot the exact title.
-B

Basicly the higher you go the less gain you'll see.

 

Thanks guys. I'm not so sure just a gasket change with a .005 shave would provide a significant gain, just wondering how much it would provide. If stock compression is 10.5:1, what would my compression be after getting .025 closer?

When the heads were at the machine shop, I had them gasket match them and the intake, back cut the valves, and do a bit of work to the bowls. I'm sure this will net a bigger gain than the compression increase or any gains in quench effeciency.

Thomas.

 

this should help, its a sticky at the top of the forum

https://www.camaroz28.com/calcs/index.shtml

 

David Vizard has a nice chart in one of his books (even though the legend is printed backwards). If you don't find what you are looking for, I can scan it and email it to you.

Rich

 

When I did an estimate using the calcs to see what my compression would be with stock everything minus switching the stock head gasket for a mr gasket .026" head gasket it came out to a hair under 11:1.

 

Thanks for the link dangalla, there is a nifty calculator there which estimates HP change based on current HP and compression ratio change.

That would be great Rich, thanks a lot! I'll PM you an email address to send it to.

Thomas.

 

Super Chevy magazine had an article a while back with back-to-back dyno pulls testing the impact of compression increases. To run their test SC merely changed the head gasket, and in some cases had multiple gaskets installed to obtain the desired combustion volume. After reviewing results, it was found that for every unit increase in compression ratio(for example, from 10:1 to 11:1), there would be roughly a additional 20hp.

 

the calculator on this site show a decent increase of a few HP by increasing CR...
https://www.camaroz28.com/calcs/crchange.html

 

I read (I believe in muscle mustangs and fast fords) that every point increase in compression is a 4% increase in power. If you have a 100 hp engine and go from 9:1 to 10:1 you're not going to gain 20hp... that would be a 20% gain!

A stock LT1 starting at 275 hp going from 10.4:1 to 11:1 should gain approximately 6.6 hp.
4% * .6 points of compression * 275 = 6.6

 

The lower the CR to start with, the more the gain per point of increase. That makes sense when you consider that going from 8:1 to 9:1 is a 12.5% increase but going form 11:1 to 12:1 is only a 9% change, and so on. And the estimates are only theory - they do not account for detonation, changes in CC shape, and so on. They are most accurate in the mid ranges. At very low CR larger than expected increases are seen and less at very high. Anyway, I did get a request to scan the chart, so I will do so and either email it as requested or post a link.

Rich

 

Here it is:

 

Thanks Rich.

I assume the chart numbers are percentage of increase while the axis numbers are compression ratio. If so, assuming the stock CR is 10.5:1, I dynoed 287RWHP with that CR, and trimming .025 will net me close to 11:1, I can expect an increase of about 1%, or 2.8RWHP?

If I'm reading that correctly, then I don't see what all the arguing is about when it comes to using different head gaskets?

Thanks again for posting up the chart,
Thomas.