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MG MGA - Cylinder Compression

During a recent troubleshooting session, I found compression in each cylinder to be 160 psi to 163 psi. I was elated that they were all so close to each other but 160 seems very high to me. I was expecting readings in the 120 -125 range. Suspect that my new compression tester is out of calibration.

The engine was bored .040 over with low compression pistons installed 10 years ago (less than 2500 miles on the rebuild). The head was replaced at the same time and I suspect that it was milled but have no first hand knowledge of that.

What compression are you guys (and gals) seeing on your 1600's?

Don
D. R. Carlberg

140-145 is generally good.
125-135 is functional.
110-120 os low, but it still runs.
Under 100 on one or more cylinders will probably misfire on startup, then run with reduced power.
175 or so is high compressionm likely to have flat top pistons or a severely milled head.
Barney Gaylord

Don't know what was done to my engine. It was rebuilt as part of a High School automotive class by the students before I acquired the car but compression is about 185 psi.

FWIW One more data point.

Larry
58A 1500
Larry Hallanger

Yikes!
Barney Gaylord


My +40 MGB race engine 1840cc with:

35cc cylinder head. 60 Thou removed I think.
Fat 2cc gasket
9.0:1 pistons, which don't quite come out of the bore.
90 thou removed from the block.

Pushes 180 PSI and over.
But due to the extreme overlap on the cam 45/85 it's hard to achieve that.

I've never really worked out the Compression ratio, but I would guess it's about 11:1 or 11.5:1, it's only ever raced on Avgas.


From Here you can check out Stage 6 here and measure up the thickness of you head, even although it's an MGA to MGB comparison you could roughly work out how much has been removed.

http://www.1978mgmidget.com/Special_Tuning_for_the_MGB.pdf
Hope this helps.
<MARK>
Mark Hester

Larry: Buy a new gauge your is broken. If it was really that high pump gas would not be an option. But it would run.
fer sure dude

Don. Barney's experience is in line with mine. However, there are still a couple of things to consider. Certain "performance cams", as Mark mentions, do not operate efficiently at lower rpms, such as when being turned over with the starter. Thus, they give lower compression readings than the same basic engine set up would give with a less radical cam. This might be why Larry is getting such high readings. (Or, it may be that his gauge is off. They are not precison instruments until you get to the expensive ones. They do provide sufficient accuracy to give you a general impression of the compression and, more importantly, the balance between cylinders. I have two gauges and, if I get a low readings across the board, use the second gauge to confirm the accuracy of the first. Generally, the gauges are within about 10 psi of each other. if not, check the readings with a third gauge and get rid of the one which is going bad.)

Compression readings are also influenced by the valve clearances which should be set before making the test. If you have one or more valves tighter or looser than the rest, it can affect the pressure readings for the affected cylinders.

New rings, which have not bedded in, and it can take several thousands of miles with an engine running modern oils, can demonstrate lower compression readings than might be expected. So can an older engine which has a stuck ring(s) on one or more cylinders.

A compression check is easy to perform, especially with the relatively low cost of a good quality compression gauge. However, the standard compression check is only part of the test series. Depending on the readings and what, if any, perceived problems are, a running compression check and a leak down check might also be in order.

In your case, the readings are very consistant and within the normal range. I would suspect a well maintained engine that has been properly broken in and should perform well if the ignition and fuel systems are tuned and operating correctly.

Les
Les Bengtson

Thanks for all the information. I always thought that an 8.3 compression ratio would lead to a compression of 122.5 psi (14.7 psi atmospheric * 8.3 compression ratio). Using that same formula, 160 psi compression would equate to a compression ratio of 10.88 which is why I belive my gage may be reading high. As I said in my original post, I was most happy that all four cylinders had similar compression.

My engine has the standard cam and standard pistons (.040 oversize). I was fortunate to have Al Joniec (Ford factory racer for 10 years from 1962 - 1972) go through my ignition system and set the valve clearances right after my rebuild. Al is well recognized in the Ford Mustang community and at least 3 of his race cars have been restored by collectors. It was an absolute joy to watch him go through the distributor and set the timing.

Don Carlberg
D. R. Carlberg

D.R.

In addition to other things, such as filling the cylinder more than 100% due to valve duration, the increased pressure is due to an increase in temperature of the gas when it is compressed.
John

Comression ratio is rather simplistic and not directly proportional to compression test numbers. The simple CR normally stated is the full displacement of the piston plus the compression chamber volume, divided by the compression chamber volume.

The compression chamber includes the volume in the head, volume in the head gasket opening, and volume above the piston to the top of the block. Add volume of dish in top of piston, or subtract volume of crown on top of piston.

Then you need to know the cam timing numbers and valve clearance so you can know at what point in the stroke both valves are closed. From the piston displacement, subtract the volume at bottom of stroke when both valves are open. The remainder of the stroke gives the volume available for compression at cranking speed (including the combustion chamber volume).

When you know the volume available for compression, and the total chamber volume. Then you have to apply the formula for Adiabatic compression of air (look it up).

Because of the amount of valve overlap, more radical cams will give lower compression readings at cranking speed, which is the primary reason that a more radical cam likes a higher compression ratio.
Barney Gaylord

Barney. Well said. I would also point out that the theoretical compression ratio, based on the formula for total volume minus swept volume equals unswept volume, then, total volume divided by unswept volume equals the compression ratio, is an interesting theoretical abstract which is only absolutely true for a few thousand miles of engine life.

When the engine has been rebuilt, the valves are sealing at 100% efficiency, the piston rings are seated and the vacuum the movement is generating is at 100% efficiency, the theory is reality.

When the cylinder bores wear a little, and the valves do not seat perfectly, we begin to diverge from the ideal as it no longer describes what is actually happening. The cylinder bores wear, the rings do not seat perfectly, there is not perfect seating of the valves. All of this means that, as a vacuum pump and compression machine, the engine is not working as well as it does in theory. As an engine ages, its "effective compression ratio" declines against its "theoretical compression ratio" because the vacuum pump becomes less capable of sucking in the fuel/air mixture and sealing it in the unswept volume (combustion chamber).

Thus, the compression check is one, simple, test of engine efficiency. Consistancy between cylinders is more of a goal than any absolute compression psi. A balanced engine is what is most desirable, with all the the cylinders reading less than 10% difference.

Beyond that, further tests should be performed to see why the cylinders may not be in balance. A compression test is a quick go-no go test. If the engine shows a bad compression test, further testing should be done to determine where the problem is. My 68 BGT shows about 120 psi on all four cylinders (less than 5 psi difference). The lower than standard compression indicates that it is getting old and will need to be rebuilt. The consistant compression indicates that, although old, it is in good condition with no major problems and that I can drive it until I have time to build up a new engine for it. It is still capable of getting me where I need to go and still passes the required emissions testing. This is a good thing.

Les
Les Bengtson

Just a small reminder. At higher altitudes the gauges read lower than at sea level.
R J Brown

Don,
I noticed that you wrote that someone set your valves right after the rebuild. I'm sure you probably know but you should check the vave clearances about every 500 miles for the first 2000 miles and then per workshop manuel afterward.
J.H. Hall

This thread was discussed between 12/07/2006 and 18/07/2006

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