948cc compression

[andrew.searston]

hi everyone
i have a 948 moggy engine with a 1098 head on it (skimmed by 60 thou), just done a compression test to find 3 cylinders around 95psi and number 4 about 90
what should the compression be as this seems very low?? does seem to be running ok if a little underpowered. havnt done a wet test yet.

[liammonty]

That's pretty rubbish. Was that with a warm engine and with the throttle open?

I can't remember the exact figure, but I think 140-150 psi or thereabouts would be fair.

[bmcecosse]

Yes - your readings are hopeless.....check the gauge on another car if you can? And are you taking the reading on a hot engine, with all plugs out, and a brick on the throttle pedal? Are you sure the head was skimmed by 60 thou.....of an inch? Next time it's off - measure the thickness and report back.

[ampwhu]

forgive me for being stupid, but I don't get the bit about having the plugs out and a brick on the throttle? it wouldn't start without plugs, so why touch the throttle?

[IslipMinor]

Compression tests are always done with full throttle, plugs removed, and a warm engine to allow it to spin over quickly and get the maximum amount of air through the engine whilst cranking to check the pressure.

[ampwhu]

Sorry, I dont buy that. My engine spins over freely when cold with the plugs out and I get good readings of around 160psi on all 4. I dont need to touch the throttle. I just cra nk the engine on the starter.

[bmcecosse]

Oh well - we're all doing it wrong then, I guess.......

[liammonty]

Ampwhu - never mind whether you buy it or not, that's how you are supposed to do it. If somebody asks for advice, it's good to give the right advice. Any engine will spin over fast when cold - that's not the reason for warming it up first. Also, of course you'll get a reading with the throttle closed, it just won't be the same reading as if you do it right, with the throttle open!

[ampwhu]

but what has the throttle got to do with it?

you are spinning the starter motor on the key or switch under the bonnet. This is in contact with the flywheel that is attached to the crankshaft which has the pistons attached to it. these move up and down in each cylinder. you get your compression reading from screwing a compression tester into each spark plug hole in turn and take your reading from that.

it has nothing to do with the throttle.

[liammonty]

It's a shame that this argument has rather detracted from the OP's question. I'm a little confused though Ampwhu; this started out with you asking a question, but it seems that you knew the answer all along, so why did you bother asking? It's good that you're so self-assured ;-)

[bmcecosse]

Oh dear----yes it has.. If you can't see that a closed throttle seriously limits the amount of air entering the cylinders (that is precisely what a 'throttle' is designed to do, after all) - then I'm afraid we can't help you..... Good luck. Oh and I've added you to my 'list'.

[kennatt]

it also will draw more petrol in to give a denser charge to compress thus copying the readings as if the engine was running.but maybe the gauge makers got it wrong eh,same as you BM on my list as well

[andrew.searston]

i did the test with the engine cold and throttle fully open, also removed plugs one by one, does anyone know what the compression ratio should be??
think it just a bit tired and due a rebuild or that it has a 1098 head even though i skimmed it by 60 thou
thanks

[liammonty]

Standard 948 CR is 8.3:1. Try it with the engine warm, that could give better figures and put your mind at rest! Also try some oil down the bores as a quick way of telling whether you've got issues with rings and/or bores. That 60 thou skim will have maintained the std CR I think - it's only about a 2 cc difference between the 948 and 1098 heads, so that won't be the cause.

[bmcecosse]

Ideally the engine should be warm (partly to thin the oil, and partly to make sure the battery is well charged..) - and you must take ALL the plugs out........ What pressures did you get Andrew?

[ampwhu]

I think you are still missing the point. a compression reading is a mechanical reading. moving parts separated by liquid (lubrication).

when a vehicle doesn't work as it should, it is down to three things and three things only. if it stops and doesn't go, one of those three things will be the problem. it will be either fuel, air or mechanical. does not matter which way you 'experts' dress it up, it will be that.

when doing a compression test, it isn't going to be 2 out of the 3 isn't it?

if you think otherwise, buy yourself a large cornetto when it's scorching and see how long it lasts.

and as for being on a 'list'. that's cool with me. I passed you in my C63 a while back.

[bmcecosse]

I think you live in a different world to the rest of us ampwhu.... Good bye.....

[kennatt]

do the test hot,you will def get a better reading,unless the bores are realy badly worn,if so when putting oil in if the readings improve dramatically then you will know for sure that they are.

[Boomlander]

I think you are still missing the point. a compression reading is a mechanical reading. moving parts separated by liquid (lubrication).

when a vehicle doesn't work as it should, it is down to three things and three things only. if it stops and doesn't go, one of those three things will be the problem. it will be either fuel, air or mechanical. does not matter which way you 'experts' dress it up, it will be that.

when doing a compression test, it isn't going to be 2 out of the 3 isn't it?

if you think otherwise, buy yourself a large cornetto when it's scorching and see how long it lasts.

and as for being on a 'list'. that's cool with me. I passed you in my C63 a while back.

You forgot electrical.......That's four things and I'm no "Expert"

[chuffit]

I think you are still missing the point. a compression reading is a mechanical reading. moving parts separated by liquid (lubrication).

when a vehicle doesn't work as it should, it is down to three things and three things only. if it stops and doesn't go, one of those three things will be the problem. it will be either fuel, air or mechanical. does not matter which way you 'experts' dress it up, it will be that.

when doing a compression test, it isn't going to be 2 out of the 3 isn't it?

if you think otherwise, buy yourself a large cornetto when it's scorching and see how long it lasts.

and as for being on a 'list'. that's cool with me. I passed you in my C63 a while back.

Talk about missing the point Found this post highly amusing , amazed at the patience of you all trying to help