Compression test?

[Graham Honnor]

Hello All

I have recently carried out a compression test on my vehicle for no other reason than curiosity and wondered
if somebody would be kind enough to analyse the results for me please.
The engine is a '37 Ruby M2443** 2 bearing and I did the test on a cold engine.
Cylinders 1 - 4    80, 83, 85, 80  respectively.
As I said, I am only curious as to whether these results are in the right field because the car is running fine.

Many thanks for your time.

Graham.

[Martin Prior]

Hello All

I have recently carried out a compression test on my vehicle for no other reason than curiosity and wondered
if somebody would be kind enough to analyse the results for me please.
The engine is a '37 Ruby M2443** 2 bearing and I did the test on a cold engine.
Cylinders 1 - 4    80, 83, 85, 80  respectively.
As I said, I am only curious as to whether these results are in the right field because the car is running fine.

Many thanks for your time.

Graham.

My RP, with over 80,000 miles since its last rebore, gives readings of around 82 - 86 psi and the PD, with perhaps 20,000 miles, 88 - 92 psi.

[Graham Honnor]

Many thanks indeed for that Martin.
Not got too much to worry about then.
Have you seen our mutual friend Glyn lately?
I keep meaning to pop over the garage and see him and Adrian.
Thanks again
Graham.

[Bob Culver]

As mentioned before, compression testing only became common when crank handles disappeared. A problem with Sevens is that now many permutations of oversize, head and c.r. An  h.c engine with wide ring gaps or broken top rings may give the same readings as a perfect l.c engine. Testing is usually done hot, throttle open. Stock heads seem to have been nominally 1.5 inch deep.

A lot can be discerned with the crankhandle. All compressions should be similar with a a degree of bounce. Sevens hard driven used to be plagued by burned valves every 6,000 or so ; I dunno what the experience is with modern valves, presumably of modern (post 1960) steels. Curiously, whilst burned valves seriously sap power, and may cause a miss when idling, wide ring gaps often seem to have little effect on running.

Until the 1950s and better oils and thermostats many smaller car engines were rebored at 40,000 or so. My car was rebored at this and 60,000 later .010 wear, very rarely encountered now.

[Chris KC]

That result looks fine to me; I think it's supposed to be checked on a warm engine though. Look for 80-90 psi and no big difference between cylinders.

[Derek Simmons]

Hi, it's helpful to hear what has been said about cylinder compressions as my 1937 Ruby has pressures of between 85 & 95psi and I was wondering if it was a bit on the low side because I recall testing an old 1960's Mini and getting 140psi plus!! If you squirt engine oil into the cylinder before taking the pressure and you get a higher reading it could indicate bore wear. Would that be a fair assumption?

[Chris KC]

Hi, it's helpful to hear what has been said about cylinder compressions as my 1937 Ruby has pressures of between 85 & 95psi and I was wondering if it was a bit on the low side because I recall testing an old 1960's Mini and getting 140psi plus!! If you squirt engine oil into the cylinder before taking the pressure and you get a higher reading it could indicate bore wear. Would that be a fair assumption?

(1) Get the engine thoroughly warm - at least 10 mins running (shroud the radiator)
(2) Remove all four spark plugs
(3) Disconnect coil from battery
(4) Prop accelerator pedal down with a broom handle (open throttle valve)
(5) Screw gauge into each cylinder in turn and measure highest pressure reached on starter (may take 5 - 10 turns)
(6) n.b. a steady cranking speed is needed, if battery is low results will tail off
(7) If weak rings suspected introduce a teaspoon of engine oil into cylinder and measure again (a marked change confirms an issue)
(8) If adding oil doesn't make a difference suspect a burned valve

Best level for a 'sports' A7 in peak condition: 115 PSI
Typical level for a road-going car & perfectly OK: 80-90 PSI (variation is not only wear but head type, skimming, valve overlap etc.)
( ref. ~100 psi minimum for a modern OHV engine)

A "fault" is indicated by 1 bar / 15 psi variation between two cylinders
n.b. It's well worth checking your tappet clearances from time to time!

[Graham Honnor]

That is very helpful Chris.
I shall endeavour to re-do the test as you have explained and come back with the results.
Many thanks again.
Graham.

[stevenm]

Hi Chris, 

Hope you dont mind im having some trouble getting a 1928 austin 750 engine to start. 

What would be the reading for a cold compression test ? 

Im showing 50 50 48 50 1 through 4. Im failing to get more than a puff puff and some exhaust gasses so somethings happening but i'm failing to get her running.

Ive checked spark ( good converted to 12 v) so strong spark all plugs.

I have a brass updraft carb ( zenith 22 ) and i'm getting suck at the air inlet. No idea if the carb is good or how the mix is controlled but theres some fume smell in the chambers but not a lot. i worked my way from the chamber through manifolds and sealed up all leaks.

Ive adjusted timing right round and appears quite retarded is best setting currently but again nothing more than puffs smoke and as i cant run her 10 minutes to check hot compression wondered what would be expected, seems very very low to myself.

[bob46320]

Tappets OK (caught me out first time after a re-build)?
Handbook says start with ignition advanced.
Try a bit of propane gas up the carb whilst cranking.