[wolf]

Originally Posted by Tom Vaught

You said 10.5 and Jack said 14 so the difference is 3.5 points of compression.

It is a known fact that every point of compression will get you 3% more power.

3.5 X 3% = 10.5%

500 X .105 = 52.5

[Stan Weiss]

Originally Posted by Tom Vaught

You said 10.5 and Jack said 14 so the difference is 3.5 points of compression.

It is a known fact that every point of compression will get you 3% more power.

Originally Posted by wolf

3.5 X 3% = 10.5%

500 X .105 = 52.5

Sorry, I missed that. But I also do not believe that. Going from 8.5:1 to 9.5:1 gives you more HP increase than going from 13.5:1 to 14.5:1 will.

Stan

[stickboy]

I am at 11 to 1 and I run 91-93 octane with 30 degrees timing. The chamber is the key I think. No pinging, no running hot, all seems ok. I was a little worried but my cylinder head guy said it would be ok. Good vacuum and street manners even with Victor intake, he said it needs compression to pull on the large CSA port/manifold.

[wolf]

Originally Posted by Stan Weiss

Going from 8.5:1 to 9.5:1 gives you more HP increase than going from 13.5:1 to 14.5:1 will.

Agreed. The 3% rule is a linear approximation.

[BruceWilkie]

Originally Posted by Stan Weiss

Sorry, I missed that. But I also do not believe that. Going from 8.5:1 to 9.5:1 gives you more HP increase than going from 13.5:1 to 14.5:1 will.

Stan

On a percentage basis agree, its not linear. Going from 9.5 to 10.5 compression gives a greater percentage increase than going from 10.5 to 11.5 compression.

Percentage amount diminishes as compression increases.

[Overkillphil]

Or is it calculated like compounding interest?

[Garry Sr]

I just ordered a set of 72cc D-port Edelbrocks. Do you guys think the Kauffmans would have been a better choice? This are unported and going on a 461.

[lust4speed]

Here's a copy of the graph in H.O.'s book. I come up with .8 (8/10) percent drop in horsepower lowering the compression from 11:1 to 10:3:1. On a 500 HP engine, that would only be 4 horsepower.

Realize it's the race section, but a quick comment on what we have observed on aluminum headed engines in our club. Those running 10.3:1 or less haven't had a problem with the SoCal excuse for 91 octane. One member was having problems and on tear down inspection we found his pistons .010" out of the hole - which raised his compression to a true 10.6:1 when everything was figured in. Granted, the problem showed itself in the Summer heat and some mild freeway congestion - and not in a quick blast down the strip.

Maybe the most interesting thing wasn't that 10.6 was over the threshold for our crap gas, but that a really good quench area wasn't enough to offset the little extra compression.

[Tom Vaught]

Originally Posted by Stan Weiss

Sorry, I missed that. But I also do not believe that. Going from 8.5:1 to 9.5:1 gives you more HP increase than going from 13.5:1 to 14.5:1 will.

Stan

He did not ask about going from 8.5 to 9.5.

If you look at the slope difference between 10.5 and 14 (Jack's comment) vs your comparison of 8.5 to 9.5 the slope is different and the change is different.

I was speaking in relation to the change between 10.5 and 14 (which you can look at on the HO Graph) is on average 3% improvement per point.

Do you see the difference Stan?

Tom V.

[Stan Weiss]

Originally Posted by Tom Vaught

He did not ask about going from 8.5 to 9.5.

If you look at the slope difference between 10.5 and 14 (Jack's comment) vs your comparison of 8.5 to 9.5 the slope is different and the change is different.

I was speaking in relation to the change between 10.5 and 14 (which you can look at on the HO Graph) is on average 3% improvement per point.

Do you see the difference Stan?

Tom V.

Tom,
I see that graph, maybe I do not understand what it says. But what it tells me is at 10:1 I have a 4% power lose and at 11:0 I have a 3% power lose. In my math 4% - 3% = 1% difference in power lose with the 1 point increase in compression ratio

Stan