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.

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.

Re: Compression Test
 

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

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.

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.

Re: Compression Test
 

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?

Re: Compression Test
 

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.

Re: Compression Test
 

Jim, shouldn't the answer be 104.37, instead of 102.9?

Re: Compression Test
 

He calculated using 6 instead of 6.1

Re: Compression Test
 

Well, boy, the readings need to be exactly 104.3 or something surely is drastically wrong ! [ smiley face ]

Re: Compression Test
 

My head is starting to hurt!

Re: Compression Test
 

I want one of those gauges that reads that accurately and consistently.

Re: Compression Test
 

Yep, I'd like one those gauges that you set exactly where you want the numbers to be.

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

Re: Compression Test
 

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.

Re: Compression Test
 

I also put a B carb on it. sure does run good!

Re: Compression Test
 

the head is a 6 to 1 ratio , not a 6.1 .
Bill

Re: Compression Test
 

Yes, but with an overbore the compression rises a bit.

Re: Compression Test
 

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