Compression pressure

[Barry Townsend]

When I originally bought my RN I checked the cylinder pressures and they all came out 45/50 psi. The car ran pretty well and achieved the performance a standard car of this type expects.
I decided to do a thorough refresh this winter primarily with the top end.
Having done a thorough job including replacing all the valves with new, cutting the seats etc. I now have a compression pressure of 51/52psi across all four pots.
I'd Expected it to be in the 80/90 psi range. the engine runs like a little sewing machine and pulls as well as I would expect but I can't explain the pressures.
I'm a retired mechanic and experienced engine builder and have built loads over the years, particularly race engines, but new to the quirks of the Austin 7.
I've even checked the gauge against my compressor and they agree exactly.
The bores are perfect with +40 pistons that look like new and come level with the block face at TDC
What's throwing me is that all four pressures are almost exactly identical which is making me wonder if I have some mismatched parts from a previous owners build, i.e. wrong head or whatever.
As I said the engine starts instantly and runs sweetly and seems to have the requisite power but it's stumping me, any suggestions would be most welcome



Buy an Austin 7 they said, It's easy to work on they said !

[David Stepney]

My RP saloon also appears to have little compression, but runs well, climbs hill sturdily and idles evenly. There is no evidence of any crankcase pressure and it doesn’t burn any oil. Given that the standard compression ratio is 4.9 to 1 I don’t suppose that the cylinder pressure is very great anyway. If it goes well, I shouldn’t worry too much.

[John Cornforth]

I spent my university days in north Wales. A friend had an enthusiastically driven tuned mini, and was always fretting about the reading on the oil pressure gauge. On the narrow roads lined with stone walls, it was a bit nerve wracking for me as a passenger when his eyes were off the road. I found the solution one day by raiding the first aid kit and sticking an elastoplast over the b...y thing ! Moral - gauges aren't always a blessing.

[Bob Culver]

Supposed to be taken with wide throttle.

[John Mason]

Hi Barry, I am not like you a trained mechanic but have messed with cars all my adult life. My Ruby gives compressions of around 100 on three piston and 90 on the other. Something I am going to investigate when the weather warms up a bit. Not wishing to teach grandmas etc. With your engine having done all the valves there should be no leakage there. Although your pistons and bores look good have you done a compression test with a squirt of oil down each plug hole to check if there is any leakage there.
Having said all that if it starts and runs well what is there to worry about. If it ain’t broke then don’t fix

John Mason.

[Chris Garner]

I spent my university days in north Wales. A friend had an enthusiastically driven tuned mini, and was always fretting about the reading on the oil pressure gauge. On the narrow roads lined with stone walls, it was a bit nerve wracking for me as a passenger when his eyes were off the road.  I found the solution one day by raiding the first aid kit and sticking an elastoplast over the b...y thing !  Moral - gauges aren't always a blessing.

" Oil pressure at two thousand, hot, is forty pounds or so,
" And if it is not forty pounds, your motor will not go."
Thus spake the learned makers in their clover little book,
And so, at that small instrument, I seldom dare to look.

Sometimes on biting winter morn, when bitter frost abides,
The needle crawls to a shaky ten, then sleepily subsides;
Oh clueless clot though I may be, is not the car a hero,
Have we not gone ten thousand miles, oil pressure, hot, at zero.?

W.H.Charnock 1951

---

...alternatively Fougasse and McCullough in their 1935 " Complete Guide to the Road " state:

The Oil Gauge is a little clock with " Pounds per square inch " on its face. It has only one hand, which moons vaguely about. A sudden return to zero can mean a hundred different things to the expert, but never more than one to the beginner, who won't have noticed anything anyway.

[Colin Morgan]

I have been puzzling over the apparent lack of compression on my Ruby for a long time.  When I start the car on the handle, it is easy to turn over - there appears to be very little compression on any cylinder.  On-lookers in the know tend to laugh.  But it starts, runs as it should, and does not smoke.  And it has been like this for many years. 

Then one time, when the car was parked, an old guy I know who started his apprenticeship on cars back in 1949 and is capable of feats such as balancing triple carbs, turned the engine over on the handle quite a number of times, ignition off.  At the end of the day, when I came to start the engine to return home, I could hardly turn the handle, there was so much compression.  But the next day it was back to normal.

[Colin Wilks]

This topic seems to have become cross threaded (sorry!), so bumping this one to see if anyone has come up with an explanation for Colin Morgan's temporary high compression?

[Ivor Hawkins]

Barry, as you are lucky enough to have a compressor, have you tried a leakdown test?

[Ruairidh Dunford]

Leakdown testers are very useful and cheap to buy.