compression

About 6 to 1 unless you get a really high one that were made in the 60's. A racing engine may be 12 to 1 or higher, maybe 16 to 1. That requires a special fuel.

There is more to head design than the compression ratio. It requires clearance so that the valves can breathe and so that the cylinder is not choked off. The squish area is also important to provide turbulence for better combustion.
 
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I'd like to see the figures on this also. Someone must know the compression readings for the 5.5 and 6.1 heads.
 
6.1 Xs 14.7 = 90 lbs compression reading.

5.5 Xs 14.7 = 80 lbs compression reading

4.2 Xs 14.7 = 60 lbs compression reading

Or divide the compression reading by 14.7 for the compression ratio

The above is at sea level, good engine, accurate gauge and is a rough guide
 
That way of calculating the CR assumes standard bore. On one of my cars fitted with a 5.5 head, I get 115 or there abouts on each cylinder. By Flame's method, that is 7.8:1. Not bad from a 5.5:1 head on a motor 0.080" oversized bores.
 
Remember that compression causes the air to heat up, increasing the reading. The simple calculation from atmospheric pressure assumes isothermal (fixed temperature) conditions whereas the compression test has adiabatic conditions (temperature rises with compression)
This has been discussed before. I think Jim Brierly quoted an approximate conversion factor to get a closer calculated result.
My 5.5 to1 head on a 125 thou overbore reads 100 psi on my test gauge, Wide open throttle and hot engine. Closed throttle reads about 10 psi less from memory.
Some say WOT makes no difference but that is not my experience on any engine I have tested
SAJ in NZ
 
Thats why a compression test is only relative to all four holes, the absolute number doesn't matter. Too many contributing factors to influence it.
What does matter is that all holes should be relatively the same.
As a rule of thumb, 10% difference is acceptable between all four.

Last year at Bonneville, I knew there was a problem with one, the pumping compression was about 125 psi when it should have been about 230psi. Removed intake and exhaust and looked into ports, one valve guide broken off, that was the end for me. And in case your thinking to ask, that pass was over 170 mph.

J
 
170 mph? And that's in a Modal A?
You must have that new Saturn V triple booster motor...Heck ya, it's after 10 AM, I'm going to start the rumor.
 
Keith,

Ya! start the rumor!!
No, the car is a belly tank style lakester, wheels exposed.
But, the motor is a "B" block, stk deck height, cam and crank in stk locations.

John
 
For a smooth running engine that will pull, what johnneilson says about even compressions being more important than what figure you read is right. Lots of miles were driven on original motors with readings that were in the 65 psi range. That's very low by today's standards but very forgiving of inexperienced drivers and we've all been that.
I can scare myself with 40 hp. I don't need an even bigger scare!!
 
johnneilson said:
Keith,

Ya! start the rumor!!
No, the car is a belly tank style lakester, wheels exposed.
But, the motor is a "B" block, stk deck height, cam and crank in stk locations.

John
Click to expand...

Driving anything at 170mph is insane, you are a much braver man then I, I would pay to see it, but I would never do it... I tip my hat to you Sir.
 

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burner31 said:
Driving anything at 170mph is insane, you are a much braver man then I, I would pay to see it, but I would never do it... I tip my hat to you Sir.
Click to expand...

Keith,

every job is easy with the proper tool, I sure as hell would not take a real Model "A" to the salt though some do.

Stay safe, Buddy, J
 
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