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MG MGB Technical - High (?) compression readings

I have a 3-main MGB engine which a previous owner supposedly had rebuilt. He has passed away, so I don’t have records of exactly what was done. As a partial check I tested the compression. I got 182 plus or minus 2 in each cylinder. This seems high, as I had expected the compression of an 1798 cc engine to be around 160. What does this high reading suggest about the engine’s rebuild?

Thanks in advance for your thoughts!! Mike Whalen
M.E. Whalen

Two possibilities come to mind
1, an over bore with larger swept volume or
2, a head skimmed to increase CR and hence increase the compression pressure.

whip the head off to confirm which or both.

Best of....
MGmike
M McAndrew

Mike another thing that can give a high reading, is too much oil added to the cylinders on a "wet" test.
Even a few CCs too much can make a difference. 182 is still not too much for the road and there nice and even. Denis
Denis4

Another reason for a highre reading could be advanced camshaft timing. The earlier closing of the inlet valve would be enough to do this. Certainly with race cams the effect is the other way, with compression readings being lowered.
However, a good B should deliver 175psi. 182 corresponds to a bit over 9:1 and is the sort of figure you'd get with a fresh engine that had had both head and block refaced.
Having said all that, gauges can very too!
Paul Walbran

Mike,

I found the following on an MG site (http://www.mgexp.com/article/specification.html):

It states that compression for an 18G engine should be around 160psi, while the 9:1 CR 18V engine should generate 170psi. I would imagine your engine is the standard early 18G variation with a CR of 8,8:1.


Go to youtube and look at John Twists videos #46 How to Check Compression; #49-Diagnosis of Faulty Compression; and #50 Determining the cause of Bad Compression.

What I find a little confusing is that dong the calculation for a good 8,8:1 18G engine: 8,8 x 14,6psi should give less than 130psi, not 160pos. Maybe it has to do with the heat of the engine raising the induced air to more than the nominal 14,6psi (~1bar).

Brian
Brian McIlvenna

I have an 18G engine in my MGA roadster - compression is180- 185 across the engine (mildly worked engine, replacement alloy cylinder head). I would not be concerned with your readings - enjoy.
Mike
Mike Ellsmore

Ive got an 18ga in my MGA 175 across the board. I have to run super in my car. I would think 182 woud be ok!

Run it n hight test and as Mike Ellsmore says....enjoy!
Steven Devine

Brian, you are exactly right. 8.8 x 14.6 is a calculation based on a constant temperature situation. Compression testing is basically adiabatic (no heat transfer from the gas). As the piston rises on the compression stroke, some of the energy used in compression the gas is converted to heat, so the temperature rises. (A universal phenomenon: think compressors have fins to disapate heat, plunger type tyre pumps heat up while being used etc). The approximate rule of thumb that results from this is compression reading = 20xCR. (Noting my comment above about variable gauges - mainly due to the length or diameter of the tube to the gauge)

BTW The 18G engine had the same CR as 18V, 8.8:1, so the different figures quoted reveal a different tolerance of leakage, or it could just be the difference between cold and hot engine, or WOT or not. Oversize bores raise it about 0.1 per 20 thou oversize IIRC (sorry, too late here to be bother calculating!)
Paul Walbran

"BTW The 18G engine had the same CR as 18V, 8.8:1"

Not according to my Workshop Manual or Haynes, which specify 9.0:1 for the 18V.
Paul Hunt

Sounds fine for an 18G engine.

The only way to tell for sure what your compression ratio is, would be to do what I do on the race engines - and it is much easier if the engine is on a stand so you can rotate it to have the plug hole uppermost.

Just set the engine at TDC and use a Burette to measure the volume of chamber, any dish in the piston, head gasket volume - everything that people usually have to ballpark. I've found it to be very accurate.
Bill Spohn

It depends on which part of the manual you are reading. The specification pages in the factory manual list 8.8 as 18G, and then with each revamp only the changes are listed - and no change in CR is shown. 9.0:1 is shown on the tuning data pages.
But it's all somewhat academic, as by this age most engines have had the block and/or the head trued up at least once, and are probably oversize (though there are still some virgin engines about) all of which result in a rise of CR.
Paul Walbran

"9.0:1 is shown on the tuning data pages."

Exactly.
Paul Hunt

This thread was discussed between 02/11/2014 and 06/11/2014

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