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1962 948cc MM - Compression Test
Posted: Mon Mar 11, 2024 4:46 pm
by Klooid
Hi Everyone,
I have a 1962 948cc MM which starts and idles very well and does not belch any smoke from the exhaust.
However, the power starts to diminish fast when going up a hill, and hence I just carried out a compression test (after the engine was warned up) and I have had the following results:
Cylinder No. 1 - Dry and Throttle Open = 125 psi and with a squirt of Oil and Throttle Open = 130 psi
Cylinder No. 2 - Dry and Throttle Open = 119 psi and with a squirt of Oil and Throttle Open = 135 psi
Cylinder No. 3 - Dry and Throttle Open = 110 psi and with a squirt of Oil and Throttle Open = 129 psi
Cylinder No. 4 - Dry and Throttle Open = 125 psi and with a squirt of Oil and Throttle Open = 141 psi
In looking through this forum, these results appear to be on the low side, but before I go delving into a drastic full engine re-build, what do you think would be the primary cause of the compression loss with either Cylinder Head, Cylinder Bores, or both of these?
Thanks.
Re: 1962 948cc MM - Compression Test
Posted: Mon Mar 11, 2024 5:01 pm
by philthehill
They are on the low side but if the engine is not using oil I would be inclined to run the engine a bit longer before considering an engine overhaul.
Whilst the 948cc engine is good it is not a power house.
Are you comparing the current engine power output with what you have experienced in the past?
I would suggest engine wear as the primary cause.
Re: 1962 948cc MM - Compression Test
Posted: Mon Mar 11, 2024 5:18 pm
by Klooid
Hi Phil,
Thanks, and unfortunately, I have nothing to compare the power output with, as I have only had my MM for just over 18 months now, hence my only experience is that when I go up hill, I am having to use lower gears, and the trail of cars behind me.
However, I do like your inclination to run the engine a bit longer, as I'd rather spend some time on the bodywork if possible.
Thanks again.
Re: 1962 948cc MM - Compression Test
Posted: Mon Mar 11, 2024 5:52 pm
by svenedin
I have a 1098 which is completely standard. I have always had a habit of having at run at a hill if the conditions allow. The power band (such as it is) is quite narrow so getting the revs right is quite important. Hills that the car finds quite easy with just me in the car are quite a different proposition with a passenger and a large dog. On really steep hills I have to drop to 2nd gear and on rare occasions I have had to stop to go into 1st. On even rarer occasions I have had to ask passengers to get out and walk! Those are very steep hills though. One was the road to Hartland Quay
Stephen
Re: 1962 948cc MM - Compression Test
Posted: Mon Mar 11, 2024 6:28 pm
by oliver90owner
Your compression pressures appear fine - 120psi + or - 5psi is well within the expected normal range (spread). Pressure gauges are notoriously inaccurate, so the readings, while consistent, may not be totally accurate - I would be first checking with another good quality gauge, as the values do seem lowish.
There may be other reasons for low compression, such as changes by previous owners. Pistons with different crowns, or a non-standard cylinder head, could have been installed.
Assuming correct timing and carburation, does the engine breathe heavily from the crankcase? If not, the engine is likely in fair order - and if so, I would not be interfering with it!
There may be yet another alteration to the car which might lead to lower performance on inclines - if a higher ratio differential had been fitted.
The only vehicle I have had to alight from, for a steep incline, was in late ‘66 or early ‘67. That was from a Fiat 500 to allow it to negotiate a steep Mendip hill!
Just a few practical checks, before condemning the engine as the culprit.
Re: 1962 948cc MM - Compression Test
Posted: Mon Mar 11, 2024 6:59 pm
by svenedin
oliver90owner wrote: ↑Mon Mar 11, 2024 6:28 pm
Your compression pressures appear fine - 120psi + or - 5psi is well within the expected normal range (spread). Pressure gauges are notoriously inaccurate, so the readings, while consistent, may not be totally accurate - I would be first checking with another good quality gauge, as the values do seem lowish.
There may be other reasons for low compression, such as changes by previous owners. Pistons with different crowns, or a non-standard cylinder head, could have been installed.
Assuming correct timing and carburation, does the engine breathe heavily from the crankcase? If not, the engine is likely in fair order - and if so, I would not be interfering with it!
There may be yet another alteration to the car which might lead to lower performance on inclines - if a higher ratio differential had been fitted.
The only vehicle I have had to alight from, for a steep incline, was in late ‘66 or early ‘67. That was from a Fiat 500 to allow it to negotiate a steep Mendip hill!
Just a few practical checks, before condemning the engine as the culprit.
There were 5 rugby playing lads in my car when I had to ask 2 to get out to get up a very steep hill. I think the clutch was on the way out as well. It was over 30 years ago.
Stephen
Re: 1962 948cc MM - Compression Test
Posted: Tue Mar 12, 2024 7:27 am
by oliver90owner
svenedin wrote: ↑Mon Mar 11, 2024 6:59 pm
There were 5 rugby playing lads in my car when I had to ask 2 to get out to get up a very steep hill. I think the clutch was on the way out as well. It was over 30 years ago.
Stephen
Ahh, so the engine was not the reason for any inability to climb the hill? My memory must be better than yours, I think.
We need facts, not claims that are later, in effect, withdrawn as above. It does not help the OP one iota.
Re: 1962 948cc MM - Compression Test
Posted: Tue Mar 12, 2024 9:28 am
by philthehill
When the 948cc 'A' series engine is in good condition the compression reading should read around 140 - 150lb.
When the engine starts to wear the compression reading will get lower.
Even if the gauge is not perfectly accurate the relationship between the compression readings indicate that there is some wear to the engine.
The indicated 110lb is low but as I said above I personally would not at this moment overhaul the engine.
Re: 1962 948cc MM - Compression Test
Posted: Tue Mar 12, 2024 6:58 pm
by Klooid
Thanks everyone for your comments, which is much appreciated, and has also given me some confidence to persevere a bit longer with the engine as is.
Hopefully , the weather will now improve to allow me to concentrate on some bodywork repair instead.
Thanks again.
We need facts, not claims that are later, in effect, withdrawn as above. It does not help the OP one iota.