Compression test on Brumfield 5.9 to 1 head

JimK1930A

JimK1930A

Senior Member
Advice & experience needed.
Today my mechanic friend is coming over with his compression tester, and leak down tester to test my Model A with a Brumfield 5.9 to 1 compression.
What numbers or measurements should I expect to see with each cylinder?
I will write them down as we test to share this information with you later today.
Thank you in advance.
Jim
 
My newly rebuilt engine with a Brumfield 5.9 head measured 59.9 lbs. of compression in the cylinders. It was bored .06 over standard. Hope this helps...........
 
Jim,

I see this all the time, it is a valid question, but depends on many factors.
While yes, the static CR will show some increase of cranking numbers, it is not the "final word" in testing. Starter condition, battery condition, even the valve lash will contribute to the final result, including the accuracy of the gauge.

The cranking numbers will tell you the relative condition of each cylinder. Even on a fresh motor just seated rings, there will be a difference in each cylinder. And this is the real information you need.

The "leak down" tester was designed for aircraft, piston engines. It is a safe way to test a motor and not be influenced by the other factors. Properly done it is the best indication of the condition of the engine.

If you are testing to diagnose a problem, it will show very quickly.
If you are testing for curiosity, good, record the differences between cylinders and log for future use.

Now, for reference, these are my findings;
OHV, 10:1 CR about 175 cranking pressure.
OHV, 13:1 CR about 250 cranking pressure.

All the best, John
 
I recently tested my engine with a Brumfield head. I tested it cold and have a 12 V system, so it likely cranks faster than a 6V system. Cylinders 1, 2 and 3 all tested close to each other, which was about 105 psi. From another article I found on the barn, that sounded like it was within the "normal range" for this head. My cylinder 4 was lower though, which was 75-80. I warmed up the engine and put a small amount of oil in the cylinder through the spark plug hole and got 90 psi.

I use to own an aircraft and still have my differential leak down tester. On an aircraft the norm was to supply 80 psi on the input and see how much of a differential or leak was on the guage going out. A good aircraft engine would read in the 70s.

With an aircraft it was easy, you needed to bring each cylinder up to top dead center on the compression stroke. You could feel the compression with your finger as you moved the prop and then held the prop at top dead center while you tested that cylinder. I tried to do that with the Model A but finding each top dead center per cylinder on compression stroke takes a bit more work. Easy to do number one which I did. The number seemed pretty good, based on 80 psi input but I don't remember what it was now.

Hope this helps. Frank
 
A couple comments, if I may...
"Oldman - NorCal" -
59.9 psi seems quite low for a Brumfield 5.9 head mounted on a healthy engine. Even a stock 4.2 CR head with a worn camshaft can produce about this amount, if not more. I see that you live in the Gold Country of California. Is that a high elevation? If so, that would help explain your low compression readings with a standard 5.9:1 CR Brumfield head. Altitude's affect on compression readings is approximately one pound (1 p.s.i.) of air lost for every 2.343 feet above sea level. Even assuming a modest 10% difference, those 59.9 psi findings are still too low. Are you certain your rings have seated properly? Did the gauge fit snugly in each spark plug hole? How confident are you that the gauge reads correctly? All these questions and suggestions made by the guys so far should be taken into consideration.
At the risk of insulting you - which is NOT my intention at all - allow me to present an excellent short explanation of how to make a compression check. These guidelines were originally offered by Larry Brumfield in past discussions on "Ahooga" and are just as valid over here on "Fordbarn". If Larry doesn't know how to correctly run a compression check, then WHO DOES??? :) Here are Larry's recommendations to achieve the best compression readings, "lifted" in toto (but giving author credit) so that I don't misquote:
"The following is the correct way to perform a compression test. Omitting these steps will reduce the accuracy of the results, possibly significantly. The engine must be in the state that it sees when running and the air must be allowed to flow through.
1. Make sure that the battery is a good battery and is fully charged. Make sure that the crankcase oil is of the correct viscosity.
2. Start the engine and operate it for a minimum of 30 minutes or until the engine has reached full operating temperature. Turn the engine off and remove the spark plugs.
3. Set the carb plates, throttle and choke, in the wide open position and fix them where they can’t move during the test, very important.
4. Install a compression gauge in cylinder # 1.
5. With the ignition off, crank the engine at least 5 pumping strokes and record the highest reading indicated. Note the approximate number of compression strokes required to obtain the highest reading.
6. Repeat the check on each cylinder cranking the engine with approximately the same number of compression strokes."

I offer Larry's advice not just to "oldman - Nor-Cal", but to everyone reading this. It doesn't hurt to re-read these suggestions from time to time to ensure we are all performing a compression check properly.
Marshall
 
I would agree that compression at 59 lbs is low.
"Marshall" had mentioned how to do the compression test.
I would add one further and Do a "Wet test" to see if the compression comes up to validate rings. add about teaspoon of oil to each cylinder after doing initial test and then see if oil make a difference when running test again. You are looking for consistency when you're doing a compression test as 10% off on one cylinder will be an issue. If wet test doesn't make a difference than rings are fine.
"John Neilson" mentioned leak down test; Valve clearance and valve seat continuity are another issue which also cause low compression.

These heads being at 5.9-1 vs stock 4.2- 1 would wake up any engine. Without pictures one can only speculate.
 
Yes, adding a little oil to each cylinder after the initial benchmark readings have been established and checking the results again will tell us if the rings are seated. If the results the second time go up significantly, then the rings are suspect. If the results stay the same or only rise a negligible amount, the rings are o.k. It won't help tell us if the valves are burned or worn, however. If the rings check o.k., the valves would be the most likely guilty party for low readings, assuming the camshaft has ANY lobes left on it and there are no cracked pistons.
Marshall
 
JohnLaVoy said:
Is it ready for Loveland?
Click to expand...

Marshall V. Daut said:
A couple comments, if I may...
"Oldman - NorCal" -
59.9 psi seems quite low for a Brumfield 5.9 head mounted on a healthy engine. Even a stock 4.2 CR head with a worn camshaft can produce about this amount, if not more. I see that you live in the Gold Country of California. Is that a high elevation? If so, that would help explain your low compression readings with a standard 5.9:1 CR Brumfield head. Altitude's affect on compression readings is approximately one pound (1 p.s.i.) of air lost for every 2.343 feet above sea level. Even assuming a modest 10% difference, those 59.9 psi findings are still too low. Are you certain your rings have seated properly? Did the gauge fit snugly in each spark plug hole? How confident are you that the gauge reads correctly? All these questions and suggestions made by the guys so far should be taken into consideration.
At the risk of insulting you - which is NOT my intention at all - allow me to present an excellent short explanation of how to make a compression check. These guidelines were originally offered by Larry Brumfield in past discussions on "Ahooga" and are just as valid over here on "Fordbarn". If Larry doesn't know how to correctly run a compression check, then WHO DOES??? :) Here are Larry's recommendations to achieve the best compression readings, "lifted" in toto (but giving author credit) so that I don't misquote:
"The following is the correct way to perform a compression test. Omitting these steps will reduce the accuracy of the results, possibly significantly. The engine must be in the state that it sees when running and the air must be allowed to flow through.
1. Make sure that the battery is a good battery and is fully charged. Make sure that the crankcase oil is of the correct viscosity.
2. Start the engine and operate it for a minimum of 30 minutes or until the engine has reached full operating temperature. Turn the engine off and remove the spark plugs.
3. Set the carb plates, throttle and choke, in the wide open position and fix them where they can’t move during the test, very important.
4. Install a compression gauge in cylinder # 1.
5. With the ignition off, crank the engine at least 5 pumping strokes and record the highest reading indicated. Note the approximate number of compression strokes required to obtain the highest reading.
6. Repeat the check on each cylinder cranking the engine with approximately the same number of compression strokes."

I offer Larry's advice not just to "oldman - Nor-Cal", but to everyone reading this. It doesn't hurt to re-read these suggestions from time to time to ensure we are all performing a compression check properly.
Marshall
Click to expand...

OK, I obviously don't know what I'm talking about regarding 59.9 lbs of compression since I know that the engine rebuilder didn't start the engine prior to delivering it to me. I can tell you that I now have 250 miles on it and it seems to run great with plenty of power and I intend to drive it to the MAFCA convention. Yes, John............Trail Ridge Road ready!!
 
A ball park approximation of the compression pressure is to multiply the advertised compression ratio by 14.7 psi. However, 14.7 is the atmospheric pressure at sea level, so you mountain folks will have less than 14.7.

Doing the math indicates your gauge readings should be a minimum of 87 psig, and the maximum should be considerably greater, perhaps over 100 psig.
 
Question on compression check.
Please forgive my ignorance.
At the end of Larry's second point it states to remove the plugs. Are all the plug still out when each cylinder is checked. If so, isn't some compression lost through the 3 open spark plug holes?
Neil
 
All plugs need to be out, and no compression is lost because each cylinder compresses it's own air into the head and gauge.
 
Tom's got the answer to Neil's question. Each cylinder should be independent of the other three in terms of compression, unless there is a breach between the paired cylinders (1& 2 and 3 & 4). In such a case, compression would leak between the two cylinders. This would definitely be reflected in the psi readings.
With all the spark plugs removed, the engine will turn over much faster than usual. This won't increase the psi readings, but it's kinda fun to hear the engine spin over almost twice as fast as when it's under compression! It's as if a 12 volt battery were juicing up the starter. :)
Marshall
 
Let me add this to my comment in post #16 above: If two adjacent cylinders (1 & 2 or 3 & 4) indicate low compression pressures on the gauge and squirting oil in does not increase the compression, then the head gasket may be leaking between the cylinders. This leak is what I meant by a "breach" between two paired cylinders.
But, then, I guess you all knew that, didn't you?
Marshall
 
Marshall V. Daut said:
Let me add this to my comment in post #16 above: If two adjacent cylinders (1 & 2 or 3 & 4) indicate low compression pressures on the gauge and squirting oil in does not increase the compression, then the head gasket may be leaking between the cylinders. This leak is what I meant by a "breach" between two paired cylinders.
But, then, I guess you all knew that, didn't you?
Marshall
Click to expand...

I got fooled once by that. It was a rare exception to the rule when I bought my second 1950 Studebaker Commander in 1969. I got it for only $50 because it needed engine work. I checked the compression and got 100-100-100-100-40-40. I removed the head expecting to find the gasket blown between 5 and 6, but it was two burned exhaust valves. I replace all 6 exhaust valves for a total cost of $7.60. Yep, that was still when we still had the good old days. :)
 
You guy's need to get in the '70s......1970 that is..
I haven't used my compression tester since I bought my leak-down tester
in about 1973.


Here's the deal with the compression test vs the leak-down..

Compression test variables..
1. How many times was the engine turned over too test each hole..
2. Are ALL the spark plugs out of the holes?...
3. The throttle,...open or closed?...
4. What kind of cam grind,..stock or modified?.....this is a big deal..
5. Valve lash?....
6. High compression pistons , low compression , or stock?...
7. Cylinder head,...high compression or stock?...
8. Is the bore and stroke stock or has it been changed?...
9. Extreme,...is it blown, turbo charged, normally aspirated?...


Leak-down test...
1. You pump air from the air compressor(100psi-125psi), threw the leak-down tool into the cylinder...
2. It checks for leaks in,...the cooling system, down the bore(rings), both valves...
3. It gives you , in percentage, the condition of each hole,....period!

A good engine will leak-down between 3 percent and 10 percent,...depending on the type of rings.
This is on a GOOD race engine. Street engine,..about the same up too 15 percent,..rings again.
If it leaks 30-50 percent,...it has a big hole somewhere!

Have a good day..
 
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