Compression Test figures

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Jan 28, 2011 | 05:40 AM

briaineo

Thread Starter

Racer

Joined: Nov 2005

Posts: 340

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From: Dublin

Compression Test figures

Would anyone know a ball park figure for cylinder compression on a '78 305.

Going to be doing the test properly over the weekend, tested one cylinder cold and got 120

this sound relatively low?

Jan 28, 2011 | 06:22 AM

DKDunn04

Racer

Joined: Jun 2010

Posts: 251

Likes: 1

From: Edwardsville IL

Im no expert on these engines, but for a 305 with emission controlled heads and lower compression that sounds about right.

Jan 28, 2011 | 07:24 AM

RIO68

Burning Brakes

Joined: Feb 2009

Posts: 780

Likes: 1

From: Shelby NC

What's considered high or low and as compared to what?? Often thought about it--I used to be told less than 90 and no more than a variation of 20 or so between cylinders????

Jan 28, 2011 | 08:29 AM

RonR80

Melting Slicks

Joined: Apr 2008

Posts: 2,625

Likes: 242

From: Russell Ontario

St. Jude Donor '09

I agree on the variation, but I thought it should be a little higher 145-160 range . of course this depends on the mileage and condition of it all, when you did the comp. test did you have the throttle fully opened. Ron

Jan 28, 2011 | 01:45 PM

68-427Rich

Burning Brakes

Joined: Mar 2010

Posts: 1,079

Likes: 23

From: KY

Tie the carburetor wide open. Don't worry so much about the number. You really want to see consistent numbers on all cylinders.

Jan 29, 2011 | 03:56 AM

WESCH

Melting Slicks

Joined: Feb 2001

Posts: 3,330

Likes: 13

From: Europe , Luxembourg

Hi

The actual pressure reading does not only depend on the engine static compression ( 8:1 or 10:1 ) , but also on the cam used.

The compression check is mainly to check for pressure differences between the cylinders.

Rgds. Günther