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Compression ratio calculator comment

 
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Machine82'
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Joined: 03 Mar 2003
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Location: Omaha, NE
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PostPosted: Fri Aug 31, 2007 2:13 pm    Post subject: Compression ratio calculator comment Reply with quote

Although the compression ratio calculator on this site seems to be very thorough, it's missing one thing. It does not take into account that the diameter of the piston above the top ring land is smaller than the skirt diameter because of heat expansion I'm guessing. By calculating this volume I found another 1.1 cc's using an after market Pontiac 400 piston. Though it's only 1cc, it dropped my compression ratio by 0.12.

Not a criticism, only an observation. If I have missed something, please let me know.

Mike

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af2
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1933 Willys Coupe

PostPosted: Fri Aug 31, 2007 3:48 pm    Post subject: Reply with quote

Mike, I look at it as guide more than 100%. To be 100% be prepared to pull the head at least 3 times on the build using Vaseline and colored alky to measure static through the plug. .12 isn't too far off to make a decision.
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Machine82'
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PostPosted: Sat Sep 01, 2007 3:26 pm    Post subject: Reply with quote

Very true, and a good point. The reason I am so concerned with the compression ratio is because Pontiac's can be prone to detonation. I observed a recently built 400 Pontiac that had the same compression (10.5:1) and even the same heads I plan to use. The engine was detonating so bad it had to be run with 106 octane fuel. I'm guessing it's because of poor combustion chamber design. Or maybe whoever came up with the compression ratio made a mistake. Either way, I'm still maybe a week out from assembling my Pontiac and I am curious to see what the burette measurements come out to.

Thanks for the comment.


Mike

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af2
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1933 Willys Coupe

PostPosted: Mon Sep 03, 2007 1:14 am    Post subject: Reply with quote

Machine82' wrote:
I'm guessing it's because of poor combustion chamber design. Or maybe whoever came up with the compression ratio made a mistake.

Thanks for the comment.
Mike


The chamber design could be better as any out there but I feel the compression is beyond what pump gas will allow.
A good friend ran a 455 and had to use 1/2 AV GAS when racing at 36* total. I never got a chance to calculate compression but I believe to this day it was probably around 11:1 with iron heads. I used to think it should have ran better but I wasn't involved until he asked me? Still ran 1/2 AV GAS. Tried every thing! Still beat him with a 331 Chevy because of the tire fryer he had! Laughing
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dynoman14
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Joined: 15 Jul 2008
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Location: Statham , GA
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PostPosted: Fri Jul 18, 2008 2:03 pm    Post subject: Pontiac Heads Reply with quote

Mike,
We ran an 11:1 400" Pontiac on 93 octane pump gas and it made almost 600 RWHP was not simple or easy but after about 100 pulls or so we had it ciphered out. Car ran 11.10 at 142 with a 2.10 60ft. Was a great car til it was totalled in a street accident.

The key is in the valve job, big nail head intake small tulip exhaust ( sink the exhaust seat w/7* + radius ), Carburetor ( 830 ADB with extended boosters ) and metering blocks ( big primary powervalve windows and enlarged PV channels ), ignition system and spark plugs ( 25+ and surface gap plugs ) and eliminating spark knock on the chassis dyno ( 50cc primary pump shot ).

I is a recipe I have used many times to achieve the same end product.

Dynoman14
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SMOKEmUP
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PostPosted: Tue Jul 22, 2008 11:03 am    Post subject: Reply with quote

Yes it does, it uses the top right land height and piston to wall clearance to calculate the volume above the top ring.
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