Changing compression ratio, need my math checked.

Reply Subscribe

Thread Tools

Search this Thread

Oct 3, 2001 | 10:50 AM

TransAm525

Thread Starter

Racer

Joined: May 2000

Posts: 432

Likes: 0

From: Daytona Beach, FL, USA

Changing compression ratio, need my math checked.

Still waiting on the Jet Hot coated Hooker headers for my side pipe exhaust system. While I have the exhaust system removed, I will also be replacing all the upper gaskets. I would also like to mill, hot tank, and cc my heads to give me something in the 10:1 to 10.5:1 (max) compression ratio. Here is the data and numbers I am working with:

Intake manifold material: aluminum (Crossfire injection)
Thermostat opening temperature: 180 degrees
Head numbers: 3998997 (light-weight castings for 305/350)
Combustion chamber volume: 76cc
Engine bore: 4.00 (standard, has not been bored over)
Engine stroke: 3.48 (has not been stroked)
Head gasket thickness: .015 inches (Felpro embossed steel shim gasket)
Head gasket bore: 4.100 inches
Deck height: stock (unsure of exact number)

I estimated about a +4 cc's for the valve reliefs on the pistons and estimated the deck height at .025 inches. Using an online calculator, the math states I will need to mill the heads to 68 cc's to achieve a 10.05:1 compression ratio and about 64 cc's for 10.5x:1 compression ratio. Now for the question. Is my math correct, and if so is there enough material on my 76 cc heads to mill them to a max of 62.5 cc's? Also, is the data I'm using for the valve reliefs and deck height about right?

Oct 3, 2001 | 10:55 AM

TransAm525

Thread Starter

Racer

Joined: May 2000

Posts: 432

Likes: 0

From: Daytona Beach, FL, USA

Re: Changing compression ratio, need my math checked. (TransAm525)

Correction: The last sentence should state to a max of 64 cc's, not 62.5.

Oct 3, 2001 | 12:28 PM

gkull

Team Owner

Joined: Apr 1999

Posts: 21,953

Likes: 1,445

From: Reno Nevada

2024 C3 of the Year Finalist- Modified

Re: Changing compression ratio, need my math checked. (TransAm525)

http://www.angelfire.com/fl/procrastination/motor.html

This is the place I use to calc. compression. I am not a machinist, but I wouldn't weaken a head by machining it to get compression. You can buy quality heads in all kinds of cc's

True flat tops like KB's with 2 valve reliefs are rated at - 4 cc's

Oct 3, 2001 | 03:11 PM

Corvettedez

Intermediate

Joined: Aug 2001

Posts: 48

Likes: 0

From: Tampa FL

Re: Changing compression ratio, need my math checked. (TransAm525)

On a SBC cylinder head you would mill about 10cc for every 1 point of compression.
.042" = 6.5cc
.007"/cc
I'm not sure how acurate that calculator on Angelfire is, I put in all of the info from my engine and it is way off. It's saying 10.185:1. I used a compression tester on my engine and came up with 9.25:1.

/Dez

Oct 3, 2001 | 03:52 PM

AD2VET

Melting Slicks

Joined: Aug 2000

Posts: 2,964

Likes: 0

From: Huntsville AL

CI 2-3-4-5-6-7-8-9-10 Vet

St. Jude Donor '09-'10

Re: Changing compression ratio, need my math checked. (TransAm525)

Dustin, you might want to consider aluminum heads instead. It's my understanding that because of the better heat transfer of the aluminum you can get away with the higher compression ratios, but with the cast iron heads you will probably encounter detonation.

Johnny

Oct 3, 2001 | 05:36 PM

gkull

Team Owner

Joined: Apr 1999

Posts: 21,953

Likes: 1,445

From: Reno Nevada

2024 C3 of the Year Finalist- Modified

Re: Changing compression ratio, need my math checked. (TransAm525)

Dustin. I ran the site I listed for you.

i just put in -6 for valve reliefs and the gasket 4.10 gap

deck and gasket = +.040 .015+.025

76=8.95
68=9.72
64=10.16 which is good for 92 octane with correct cam and timing curve

Oct 4, 2001 | 10:48 AM

TransAm525

Thread Starter

Racer

Joined: May 2000

Posts: 432

Likes: 0

From: Daytona Beach, FL, USA

Re: Changing compression ratio, need my math checked. (gkull)

I checked the calculations on two different compression ratio calculators. I also tried the one on Anglefire, which I too found was way off. I plan on changing over to a quality set of AFR heads later, when I swap my manifold for the Offenhauser Cross Ram with two 4-bbl carbs, but for now I just want to play with what I have without dumping large amounts of money into porting, etc. The milling is only $65 plus the cost of cc'ing. Now that I know how much I need to mill off, do I have enough material to turn a 76 cc head into a 68-64 cc head?