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MG MGB Technical - New or rebuilt compression reading?

What would be expected as the compression reading on a new or rebuilt 18v engine? I'd like to have that info to compare to what I get on my 73B Roadster.

Thanks,
BH
BH Davis

After going through my manual several times I finally found this in the specifications (should have seen it the first time through...........doh!).

It says 170 lbs/sq.in. As such, if I'm getting around 115 to 120 lbs/sq.in. on my 73B roadster 18v engine, is that a serious depletion of power? All those who have extensive MG and general mechanical experience who have driven the car comment on it running really well and seeming to have plenty of power.

Note that all polution controls was removed before I owned the car.

Thanks,
BH
BH Davis

It is hard to compare these data, because it differs from one engine to another. What is more important is that the readings from the different chambers are the same. A healthy engine will give the same values for all the chambers, an unhealthy gives different readings.
But the standard values are about 9 for a 18v engine, which is the compression ratio of a new/rebuild engine (9:1). Other engine had less, 18GD/18GF had 8.8:1. Original american from 1972 had 8:1.

Good luck
R Karthaus

Please forgive my ignorance on this, but how does a ration of 8:1 etc. translate to lb/sq.in. pressure guage readings?

If I'm getting 115 to 120 lb/sq.in. on all 4 cylinders they are pretty even I would think. I just don't understand the 8:1 ratio. Is it the 8x the # of sq.inches of the cylinder head surface?

BH
BH

170 is a bit high, a compression ratio of 11.5:1.
120 is about 8.2:1. You could get a higher ratio by rebuilding your engine, and more power. But if your car has an original american engine from 1972 or younger, then your engine is fine.
R Karthaus

It's the ratio of compression to atmospheric compression. The conversion is about: 1 pound/square inch = 0.068 045 957 atmosphere [standard]
R Karthaus

But like I said above. It's not really the value you get, but how even they are. Your engine should be fine with the even results.

Other things that are an indication of a worn engine are, oil consumption and a blue smoke plume from your exhaust when you go full throttle or when you decellerate on your engine.
R Karthaus

I could be wrong, but I believe that the 18V engines are considered to be low compression or 8.0:1 when stock and should blow about 120PSI when in good working condition. Does the engine number have an "L" for low compression at the end? Stock 73 engine is 18V672-Z-L.

Darrell
Darrell

BH. On my rebuilt engines I get readings of between 150 psi and 170 psi depending on how the engines have been rebuilt. My reprint of the factory workshop manual shows the factory specification as being 130 psi on the low compression engines sold in the US.

But, compression readings are not an exact science. There is a range of acceptable figures and consistency counts more than actual pressure readings. If you do a compression check once a year, keep track of the readings, and evaluate how they may be changing, you are using the compression test in an effective manner. It will allow you to see changes which have taken place and decide if anything needs to be done about the changes. That is the main value of a compression test.

The second value of a compression test is when there is a noted problem. It allows you to test the cylinders and see if the problem can be traced to a single cylinder (hole in piston, broken valve, broken rings?) or whether it affects adjacent cylinders (head gasket problem?). So many variables involved that a compression check is only one part of the diagnostic process. It is a useful tool, but do not get too hung up on the readings unless they are wildly inconsistent. I got eight years use out of an 18GF, high compression, engine which showed 110 psi for all four cylinders when I purchased the car. It was my primary daily driver and saw several tens of thousands of miles of use during that period. Consistency is more important that exact numbers.

Les
Les Bengtson

Les,

Thank you......much appreicated.

BH
BH Davis

I did a compression check this moring. Here are the results:

#1.....114 psi
#2.....111 psi
#3.....104 psi
#4.....110 psi

Again, 1973 mgb roadster 18v enging with polution control devices removed.

I have recently adjusted the valves and the engine seems to be running really well with plenty of power.

It this range from 104 to 114 within acceptable limits in terms of "looking for problems"?

These results are comparable to what I got when I first bought the car 2+ years ago. I've only driven it 2 to 3 thousand miles in the interim.

Thanks,
BH
BH Davis

If oil pressure is good, the engine doesn't use lot's of oil and it doesn't smoke it's probably OK as is.

Clifton
Clifton Gordon

Clifton,

Thanks.

Yes, oil pressure is fine and it doesn't appear to use any oil (other then the typical oil coating on the bottom of the engine). Again, I only go a 1000 or so miles in a season so far, but the oil level always reads full.

Thanks,
BH
BH Davis

BH. Part of your problem is lack of use. Engines will do better being run 10K miles per year than 1K miles. Take the car out an enjoy driving it.

As to your compression readings, they are in order, all being within 10 percent of each other. Worth while keeping an eye on them over time. The lower reading might indicate a valve or seat that is going bad. Might also indicate a head gasket that is beginning to fail. It might also mean that the car simply needs to be driven more. (Sticky rings due to lack of engine run time.) Yes, keep an eye on the compression over time. Remember that your figures are not laboratory grade--never taken under exactly the same conditions and there can be up to five psi difference between otherwise accurate gauges. But, the compression figures you have are acceptable.

As Clifton notes, oil use is another limiting factor. Change the oil, making sure you refill it to the "max" mark after having let the engine sit over night, allowing all of the oil to drain down into the sump. Then, take the car for a drive of 100 miles or so. Check the oil after it has had a chance to fully drain down into the sump. Make a few longer trips and keep an eye on how much oil is used. Once you have some form of base line established, you can use the MGB on longer trips. Very good touring car and touring is very good to keep the engine in good condition.

Les
Les Bengtson

Les,

Thanks.......and a timely comment on touring.

I took the car on it's first 200 mile round trip a week ago Friday and then about a 150 mile trip this past week. I've had it just over 2 years and don't bring it out once the snow flys, and then leave it parked until they sweep the streets in the spring.

It took me the better part of 2 summers working on it to get comfortable enough with the car and my knowledge of the car to go that far. I had a 67 about 35 years ago on which I did a lot of engine work, but it had been a long time between MGs.

I plan a third similar trip this week......that is if I get the rear axle bearings, seals etc. back together properly today as you can see in my other post!

Thanks again for the advice.

BH
BH Davis

All the above is good advise.
What put my mind at ease was a leak down tester http://autos.yahoo.com/maintain/repairqa/engine/ques057_1.html

Well worth the money and answers all the questions the compression tester does not.

Bruce
Bruce Mills

This thread was discussed between 23/08/2007 and 27/08/2007

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