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Old 10-07-2020, 10:53 AM   #1
DBSHELTON
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Default Compression Test

A few weeks ago I installed a 6:1 head and checked the valve lash. Runs great, what a difference! Checked the compression yesterday 102# 92# 92# 102# Is that about right for a .100" overbore engine? Last year with stock head: 66# 66# 70# 69# Both times I checked cold with all 4 plugs out.
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Old 10-07-2020, 01:10 PM   #2
Patrick L.
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Default Re: Compression Test

How many times have you retightened the cylinder heads ? It usually takes about 3 times. The center cylinders seem a bit low and has me concerned about the gasket. But, the numbers seem good to me. They should within 10%.

Compression should be done with all the sparklers out.
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Old 10-07-2020, 02:05 PM   #3
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Default Re: Compression Test

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Originally Posted by DBSHELTON View Post
A few weeks ago I installed a 6:1 head and checked the valve lash. Runs great, what a difference! Checked the compression yesterday 102# 92# 92# 102# Is that about right for a .100" overbore engine? Last year with stock head: 66# 66# 70# 69# Both times I checked cold with all 4 plugs out.
Hey DB, I am curious, have you made any other mods to your engine besides the 6:1 head? Everything else stock? How was your head when you got it, did it require and machining to get it true (flat)? I have been told of some untrue heads right from the supplier. Thanks
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Old 10-07-2020, 06:27 PM   #4
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Default Re: Compression Test

No other mods to my engine. I guess I must have gotten a good one, since I did check the head with a straightedge and feeler gauge and it measures very flat.
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Old 10-08-2020, 08:12 AM   #5
Bob Bidonde
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Default Re: Compression Test

The 0.100" over bore raises your engine's displacement to 211 cubic inches. A 6:1 compression ratio should produce about 90+ psig, so the compression readings you have are reasonable.
However, the difference between 100 psig and 92 psig is 8% which is a bit too much. Moreover, the variation in pressures between cylinders before and after the new head are inconsistent. The old head had a 6% variation which is borderline, but the high and low differences are not at the same cylinders as with the new head.
Repeat the compression test. Make sure you have all of the spark plugs out, and the carburetor throttle is wide open.
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Old 10-08-2020, 09:51 AM   #6
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Try a redo w/a hot engine, plugs out, WOT.
Did you check combustion chambers volume of the new head before you put it on?
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Old 10-08-2020, 11:28 AM   #7
Jim Brierley
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I've always figured compression in psi by a rule of thumb of: advertised ratio x 14.7 + 14.7 = 102.9. (14.7 is sea-level atmospheric pressure) You readings of 102 indicate exactly that.
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Old 10-08-2020, 02:53 PM   #8
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Jim, shouldn't the answer be 104.37, instead of 102.9?
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Old 10-09-2020, 10:44 AM   #9
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Jim, shouldn't the answer be 104.37, instead of 102.9?
He calculated using 6 instead of 6.1
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Old 10-09-2020, 11:24 AM   #10
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Well, boy, the readings need to be exactly 104.3 or something surely is drastically wrong ! [ smiley face ]
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Old 10-09-2020, 11:49 AM   #11
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My head is starting to hurt!
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Old 10-09-2020, 01:14 PM   #12
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I want one of those gauges that reads that accurately and consistently.
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Old 10-09-2020, 01:20 PM   #13
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Yep, I'd like one those gauges that you set exactly where you want the numbers to be.
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Old 10-17-2020, 02:33 PM   #14
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Default Re: Compression Test

Tested again. Hot with all plugs out and throttle wide open.
2 times today. Got same numbers.
Cyl.1 118# Cyl.2 110# Cyl.3 108# Cyl.4 120#
I think I am OK.
I bought the compression gauge from JC Whitney over 48 years ago
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Old 10-17-2020, 03:28 PM   #15
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Aaah Oh, you're over the 104# numbers, something is drastically wrong ! [ smiley face]


Actually I think you're OK too. There are all within 10%. Thats fine.
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Old 10-17-2020, 07:15 PM   #16
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I also put a B carb on it. sure does run good!
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Old 10-17-2020, 09:30 PM   #17
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Default Re: Compression Test

Quote:
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Jim, shouldn't the answer be 104.37, instead of 102.9?
the head is a 6 to 1 ratio , not a 6.1 .
Bill
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Old 10-18-2020, 05:25 AM   #18
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the head is a 6 to 1 ratio , not a 6.1 .
Bill

Yes, but with an overbore the compression rises a bit.
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Old 10-18-2020, 08:28 AM   #19
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Default Re: Compression Test

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Yes, but with an overbore the compression rises a bit.
Yes , the overbore will raise the compression but my answer was to OLD31 who figured the compression ratio at 6.1 instead of 6 to 1 ,
Bill
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