Auburn 652 compression

  • Maxie
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06 Jul 2004 00:11 #1913 by Maxie
Maxie created the topic: Auburn 652 compression
We did a compression test on our 652 engine this afternoon but I am not able to find what the compression should be.

Help,
Ron

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06 Jul 2004 16:39 #1914 by PushnFords
PushnFords replied the topic:
The general rule of thumb is 10-15% difference from highest reading to lowest. Any readings below 70 indicate a bad cylinder that will probably foul plug regularly and cause a miss. A worn engine that still runs good.....maybe 90ish. Modern engines seal a lot tighter. Last test I did was in a '91 S10 and all cylinders were 190-195.

What readings did you find?

Derek

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06 Jul 2004 19:04 #1916 by Auburn/Cord Parts
Auburn/Cord Parts replied the topic: Auburn Compression
Ron-

Somewhere around 105 lbs. is normal on an engine in good condition. The main thing is to have all cylinders within 10%. If any are 70 lbs. or lower, they likely won't fire or be almost a dead cylinder.

Stan

Auburn/Cord Parts, Inc. P.O. Box 547 1400 N. "A" St. Wellington, KS 67152 (620) 326-7751 This email address is being protected from spambots. You need JavaScript enabled to view it.

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07 Jul 2004 12:57 #1920 by balinwire
balinwire replied the topic: Disconnect coil before checking
If there is a low compression reading in any cylinder it could be one of three reasons, Rings and bore, valves or head gasket.

To check rings squirt some 30-weight oil into the offending cylinder spark plughole. Then crank the engine over a few times. Then recheck the readings.

If the reading comes up, it indicates the oil sealed the rings. If the reading is still low the valves are leaking.

If two of the cylinders are leaking side by side and the others are not it may be a cylinder head gasket.

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07 Jul 2004 21:24 #1925 by Maxie
Maxie replied the topic: 652 compression
I suspect that I need a valve job but don't want to do one before Auburn. We're getting the six to Auburn for the "Year of the Sixes" (or bust.)

The compression is <80, 91, 90, 100, 98 & 90.

I have felt there was at least one weak cylinder for some time but have avoided the job. Maybe this fall when Texas gets cool (after Auburn) I'll take it down and do a valve job or overhaul...

Thank you all for your input.

See you in Auburn,
Ron

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04 Aug 2004 01:45 #2033 by balinwire
balinwire replied the topic:
Hi R&S

The cylinder chamber combustion volumes vary as much as 20 lbs because it is impossible to make all the chambers the exact same size.

An overhead valve engine would have less variation between the cylinders because of smooth machined surfaces than a cast flathead would have.

Here is a good chart on the approximate relationship between compression ratio and compression at cranking speeds.

Ratio Pressure
6.0 110 lbs.
6.5 120
7.5 130
7.0 140
8.0 150
9.0 160

Gauge design factors and carbon deposits will raise or lower combustion chamber pressures. Also gauges are not perfectly accurate and can vary as much as five pounds.

To determine compression ratio, determine the volume of the combustion chamber at top dead center?say this volume is 10 cubic inches for example and the piston displacement is 70 cubic inches, the compression ratio is 8-1.

What?s happening is you are dividing the total cylinder volume by the volume left in the top of the chamber at top dead center. Smaller volume would increase pressures and change the ratio.

Pressures would also be increased with carbon buildup. In a 10 to 1 engine only one third of a cubic inch variance in a cylinder, would change pressures eight pounds, plus or minus.

If it starts and does not foul the plugs or smokes I would not worry too much about pressures, although chamber pressures can affect high speed performance and economy.

Peace, balin?

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