Guess my engine compression ratio.

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Jul 17, 2002 | 12:58 PM

scorp508

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From: Boston, MA

Guess my engine compression ratio.

Since my engine now has about 6.5k miles on it after rebuidling it, who wants to guess my CR since I never figured it out?

GM D-Port heads, stock combustion chamber
Felpro 1010 Head Gaskest
Stock Pistons
Stock Block

Felpro 1010
Chevrolet: 262-400, .039 in. compressed thickness, 4.166 in. gasket bore, 8.9 cc compressed volume, head gasket

Anything else needed to know?

Jul 17, 2002 | 01:09 PM

Monty

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From: Park Ridge IL

Re: Guess my engine compression ratio. (scorp508)

What's your displacement again (bore and stroke)? flattop pistons I assume?

Jul 17, 2002 | 11:14 PM

scorp508

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From: Boston, MA

Quote:

What's your displacement again (bore and stroke)? flattop pistons I assume?

Stock L98, 4.00" bore, 3.50" stroke.

The pistons each have 4 reliefs in them, I think they are 1cc each.

Jul 17, 2002 | 11:39 PM

SloRvette

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From: Holliston MA

Cruise-In VII Veteran

Re: (scorp508)

What was the stock chamber size ? What was the stock head gasket thickness ? What was the stock compression ratio ? Those pistons are dished.

Jul 18, 2002 | 12:37 AM

L79vette

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Re: (SloRvette)

No fair asking all these questions, the man said to GUESS the compression ratio.

I'll take 4:1, that way I own all the low numbers in the pool.

How much to enter?

Jul 18, 2002 | 12:50 AM

scorp508

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From: Boston, MA

Re: (SloRvette)

Quote:

What was the stock chamber size ? What was the stock head gasket thickness ? What was the stock compression ratio ? Those pistons are dished.

Chamber : 58cc
Stock Gasket : .051 (GM P/N is 12557236)
Stock CR: 9.5:1

Hold on, looked up the P/N....

For Corvette production aluminum cylinder heads. Compressed thickness is .051". This gasket's stainless steel fire rings prevent galvanic action between the aluminum head and cast iron block. (This gasket is used on 350 HO engines.) (Can be used on cast iron or aluminum heads.)

[Modified by scorp508, 11:57 PM 7/17/2002]

Jul 18, 2002 | 01:06 AM

CFI-EFI

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From: The Top of Utah

Re: Guess my engine compression ratio. (scorp508)

That is a BIG dish in the pistons. I can only assume the photo was taken with the piston at TDC, if so you have a TON of deck clearence. Based on a visual, even with the 58cc heads (if that is what they really measure) I speculate 10.00:1, maximum. Despite the OE specs, with that deck clearence and piston dish, you never had near 9.5:1. Just one man's opinion.

[Modified by CFI-EFI, 11:10 PM 7/17/2002]

Jul 18, 2002 | 01:41 AM

scorp508

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From: Boston, MA

Yes, very close to TDC if not exactly at it.

Here are 4 closeups of the pistons.

#1 after cleaning

#1 after cleaning

#2 after cleaning

# 4 before cleaning, thought the shadows might help show the surface shape.

[Modified by scorp508, 12:42 AM 7/18/2002]

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Jul 18, 2002 | 10:26 AM

SloRvette

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From: Holliston MA

Cruise-In VII Veteran

Re: (scorp508)

9.75 based on a 10cc dish and .025 deck height. I don't think you had anywhere near 9.5:1 stock though. :( Figure about .25 increase from where you were.

Jul 18, 2002 | 11:44 AM

#10

scorp508

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From: Boston, MA

Looks like I was wrong (again). My manual says 9.0:1 stock. Sorry about that.

Jul 22, 2002 | 01:02 PM

#11

MikeC

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From: Union Ontario

Re: (scorp508)

The dish in that piston is around 20CC's They look like the ones in my engine and when I calculated the dish in the piston it was 22CC's. And if your piston is at or around TDC(which I don't think it is) Your deck clearance is way too much for a performance engine of any kind. You'd be lucky if you have a 9:1 motor even with the 58CC heads.

Mine with 76CC heads and steel shim head gasket(3.4CC) is 8.1:1

[Modified by MikeC, 12:03 PM 7/22/2002]