Compression PSI

roccaas

roccaas

Well-known member
So I finally put the Compression Gauge with the custom fitting for Model A sparkplugs holes to use.

All 4 cylinders hit exactly 60 psi.

Is this within Specification??


Not to put TOO fine a point on it, I've NEVER had any engine match compression exactly between 4,6, or 8 cylinders, but I want to really know if it should be higher, and while 60 psi is good, do I have a valve problem and as such I should be seeing something higher?

In the mean time I'm NOT MESSING WITH IT because the engine runs fine!!

Worried for nothing??
 
How did you do the test? If you didn't prop the throttle open, you could probably get a higher reading with it open. Let the engine turn 4 or more good revs and see what you get.
 
Doesn't the engine run good ? 60# sounds just fine to me. The key is to take note of the very first pulse of the gauge and the last after the engine has spun over about half dozen times. They should all be within about 10% of one another.
 
Pulled 4 plugs. Turned 4 full turns (amazing to reach across an engine and push on a metal rod that fires the starter.

1st turn = 60 psi.
6th or more = 60 psi.

Throttle was closed, coil disconnected, gas gauge shut down, Tilly bowl dry.

This all came about from a Samba guy (VW Bus) who had 3 of 4 exhaust valves that looked like the back end of a Wookie the night after some Romulan Ale, so he suggested a vac test.

I really don't want to pay for a leakdown test, and the car runs fine - so I dont' want to mess TOO much with it, and 60 psi sounds good, but "The Book Of Les" does not give a figure.

Is there a better way to do this to get better answers?

I really don't want to pull the valves/cover, etc. cause then the problems will mount and I'll wish I'd just left a well running car ALONE!!
 
Sounds as though the only bit to change is to try it with the throttle wide open - lets more air into the intake manifold. Since your 1st and 6th turn readings are identical, that is fine.
Was there a reason for doing the test - other than that ugly sounding Wookie stuff, or idle curiousity?
 
According to the specs posted on Mac's Auto Parts page:

Compression Pressure: A6050A head, 76 PSI
A6050B head, 110 PSI
B6050 head, 90 PSI

We never worried too much about the compression to new specs. Of course I do not have much experience with these engines, I am thinking from working on an inline 6 w/overhead valves. The key is cylinder to cylinder being within 10 or 15 % of each other, and it seems you are right on the mark there.
 
roccaas said:
Pulled 4 plugs. Turned 4 full turns (amazing to reach across an engine and push on a metal rod that fires the starter.

1st turn = 60 psi.
6th or more = 60 psi.

Throttle was closed, coil disconnected, gas gauge shut down, Tilly bowl dry.

This all came about from a Samba guy (VW Bus) who had 3 of 4 exhaust valves that looked like the back end of a Wookie the night after some Romulan Ale, so he suggested a vac test.

I really don't want to pay for a leakdown test, and the car runs fine - so I dont' want to mess TOO much with it, and 60 psi sounds good, but "The Book Of Les" does not give a figure.

Is there a better way to do this to get better answers?

I really don't want to pull the valves/cover, etc. cause then the problems will mount and I'll wish I'd just left a well running car ALONE!!
Click to expand...

Yes, just keep asking until someone gives you the answer you were hoping for.:D LOL

Kidding aside, my engine has 50-50-50-50 and has good power. It's well worn and a bit noisy, but I'll just keep going with it until I rebuild my original engine with counterweights. You're doing fine with 60 lbs. and I'd just keep using it until it has a real problem.:)
 
roccaas said:
Pulled 4 plugs. Turned 4 full turns (amazing to reach across an engine and push on a metal rod that fires the starter.

1st turn = 60 psi.
6th or more = 60 psi.

Throttle was closed, coil disconnected, gas gauge shut down, Tilly bowl dry.

This all came about from a Samba guy (VW Bus) who had 3 of 4 exhaust valves that looked like the back end of a Wookie the night after some Romulan Ale, so he suggested a vac test.

I really don't want to pay for a leakdown test, and the car runs fine - so I dont' want to mess TOO much with it, and 60 psi sounds good, but "The Book Of Les" does not give a figure.

Is there a better way to do this to get better answers?

I really don't want to pull the valves/cover, etc. cause then the problems will mount and I'll wish I'd just left a well running car ALONE!!
Click to expand...
Always check compression with the throttle wide open.
 
While I agree with James that proper way to test compression is with the throttle open, I would say that your numbers of 60-60-60-60 are very good for a stock A with a stock head.

I suspect that if you did the test again with the engine hot and with the throttle open, you would probable get slightly higher readings. But I would not bother. These are good.

And you said it yourself: the car runs fine. Just go drive it!
 
QGolden said:
Wow roccaas, that is one fine looking car in your avatar.
Click to expand...

Not mine. GT Alumni Association 'Reck, 31(?) Deluxe Roadster pictured in front of Atlanta History Museum. Not the "Real" 'Reck - '30 Sports Coupe, but I've always likes the pic.
 
James Rogers said:
Always check compression with the throttle wide open.
Click to expand...

Why?

I am about to do a compression test and don't understand why the throttle must be wide open.

And do I shut the fuel valve off first? If not, am I not just introducing a lot of raw fuel into the cylinders and diluting the oil?

And why must the engine be hot?
 
Mad Mac said:
Why?

I am about to do a compression test and don't understand why the throttle must be wide open.

And do I shut the fuel valve off first? If not, am I not just introducing a lot of raw fuel into the cylinders and diluting the oil?

And why must the engine be hot?
Click to expand...

You open the throttle because you want to measure compression vs. atmospheric pressure. Cranking with the throttle closed will present a partial vacuum, yielding an erroneously low reading.

If the throttle is wide open, cranking will not provide sufficient flow velocity through the carb venturi to draw fuel. With the throttle closed, cranking does draw raw fuel from the idle port. The idle port delivers nothing when the butterfly is open.

Hot gives you a better indication of how the rings perform when heated and expanded to their normal operating position. They almost always leak more when cold. Since you run hot, that's the compression reading that matters most.
 
Thanks for your very clear and helpful answers :)

My compression tester has a rubber nozzle to insert into the spark plug hole but I feel it would be much better to screw something in there to get a more airtight fit. One of the above posts mentioned an adapter for that purpose. Where can I get one from?
 
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Most suppliers have them.... but you need the type of tester that has a flexable hose that threads into a modern type plug hole to thread into the adapter.....

http://www.snydersantiqueauto.com/3799

A12405ADP-8019.jpg


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Ran Hot compression test with throttle wide open.

65 lbs. across all four cylinders.

I found the gauge on ePay some time ago, some gentleman was making custom A Model compression gauges.
 
My engine guru says the test of rings is if you get 75% of the final number on the first stroke. Less than that and the rings are starting to wear. Final number should come with 3 strokes. More than that and the rings are starting to wear.
So long as the numbers are within 5 psi, drive it 'til it smokes.
 
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