Compression pressures

[Barry Townsend]

What should the compression pressure be on my 1931 RN. Last time I checked they were all in the 50/60 psi range.

[Tony Press]

I wrote in September 2018 and I quote-


The standard Compression Ratios seem to be

1923-1933 4.8 (or 4.9) to 1

1933-1936 5.2 ? to 1

1936- 1939 6.0 (or 6.2?) to 1

I understand that Compression Pressures are between 17 and 20 times the Compression Ratio in a good engine giving :

4.8 to 1 = 80 to 96 psi

5.2 to 1 = 88 to 104 psi

6.0 to 1 = 102 to 120 psi   

I get around 100 psi from the standard early head and 110 psi from the 1936 head with dry hand cranking and throttle open.

As suggested all cylinders should be within around 3-5 psi of each other .

[Chris KC]

If the engine is running well and compression is similar in each cylinder (say within 5 psi) I'd leave well alone. 50-60 is a bit on the low side, but before 'fixing' anything I'd suggest checking you have followed the instructions on the compression tester to the letter (sorry if teaching to suck eggs, but many people don't).

[Barry Townsend]

Hi
Thanks for that. 'Alfie' is running like a little sewing machine, starts readily and takes me wherever I want to go with good fuel consumption figures so I'm loathe to start ripping it apart.
I think I'll put it off for a time unless it starts to play up.

I did wonder if it had the wrong head gasket, i.e. thicker one. I've only owned the car for a few months so I wouldn't know

---

Thanks for the info.

I'm puzzled because they are all very close rather than one or two being out.

[David Stepney]

You may find that a good 'decoke' and valve grind may restore the compression. It's surprising how slightly leaking valves can reduce power and compression over a period of time.

[Chris KC]

I'd certainly try that before anything more drastic. And before even that check your valve tappet clearances.

[Bob Culver]

As Chris KC implied, I trust open throttle is being used.

Compression testers only became common when crank handles disappeared.  Reveal little that the handle cannot.

The indication of compression is variable. At one stage my Seven had quite good compression but .010 taper and burned oil in vast quantity. Failing valves had a marked effect on performance but were immediately apparent on the handle.

For 30 years and 150,000 miles my 1964 1600cc car had handle compression less than the Seven! Eventually replaced broken rings and very springy comp obtained. Blow by eliminated, idling much improved and slow speed pulling, but performance at normal touring revs, economy much the same. Oil consumption reduced but was always low.

Consistent much reduced compression could be associated a gross cam timing error. Very easily approximately verified as valves  "rock" about 8 deg after tdc.

[Stuart Giles]

You can do a useful test to get an idea of where the compression is being lost. After doing the compression test in the usual way with the throttle fully open, shoot a squirt of engine oil down each plug hole onto the bore walls; the cold oil provides a temporary seal. So, if the compressions are markedly better on the second try, it's highly likely that the problem is piston ring/bore related rather than valve seepage.

[dickie65]

Just a note on my tuned engine.
I have done a test on number one cylinder with the engine warm and running at 1000 rpm

I am getting a reading of 125 psi

This is with a ported block race spec camshaft big valves and a Ricardo Head.
1 1/2" pressure fed crankshaft so not worried about causing a tempory miss fire.
The engine had a freshen up with new pistons about two years ago.

[Zetomagneto]

Dickie, it would be interesting to see what pressure you get cranking on the starter.
Never heard of checking an engine while running, but likely to give a high reading.