how much difference does higher compression make?

[jake27081991]

hi all,

my 948 is currently running at about 90psi in all 4 cylinders..

i have heard around the grape vine that the standard/new pressure is around 160psi..

how much difference does this make to the engine, and is it worth either reseating the valves or replacing the rings??

what sort of work is involved in replacing the piston rings? i imagine that once you get to that level that you might as well replace /renew a bunch of other stuff however.?

[billlobban]

I'm afraid that I cant quote what the cylinder pressure in your 948 should be but several others will be along soon with the info. Is your engine emitting clouds of blue smoke? Using lots of oil? if yes the answer is probably a rebore with new pistons rather than a set of rings. If no then leave well alone.

[MarkyB]

Did you have the throttle open when you did the compression test ?
You need to in order to get the correct reading.
Also take all the spark plugs out when doing the test to give the battery and starter motor an easier life.

[mike.perry]

The workshop manual gives the 1098 pressure as 130psi LC, 150psi HC which will give you a rough idea. 90psi is rather low, you will notice the difference on hill climbs

[Ratbag]

What's the 1098 CR, and what scope is there for a bit of head skimming to bump it up a bit?

Phil.

[bmcecosse]

90 psi is pretty desperate, are you sure you measured it correctly ? It should be nearer 130/140 psi. Not much point raising compression ratio on a knackered engine. When it's hot/idling - take the oil filler cap off and rev it up. Is there a plume of smoke/fume ? Is it leaving a trail of blue smoke behind - and is it burning much oil ?

[jake27081991]

not much smoke, and it looses very little oil ( maybe 200mls in 5 weeks)

how ever when it does loose a little oil is when it been driving hard...ie hy waydriving
then sitting at traffic lights after this long drive..

i dont think i had the throttle open when i did the compresion test, i guess i will have to do another one,

the question is still weather it is worth while fixing the problem, what ever it may be, to get some more power from it....

does any body have a figure in mind or a calculation that i can do to figure it out??

they are rated at 37hp i believe?

[mike.perry]

C/r on a HC 1098 is 8.5:1 and on a HC 948 is 8.3:1, so I would expect similar pressures of around 140-150psi on a good engine.

[autolycus]

not much smoke, and it looses very little oil ( maybe 200mls in 5 weeks)

Volume of oil lost per mile is more meaningful.

how ever when it does loose a little oil is when it been driving hard...ie hy waydriving
then sitting at traffic lights after this long drive.

Is that based on seeing smoke when it's idling or when you start off again?

.i dont think i had the throttle open when i did the compresion test, i guess i will have to do another one,

Yup. The whole point of a throttle is to reduce the quantity of air (and thus fuel) getting into the cylinder. When the throttle is almost closed, less air gets into the same volume of cylinder, so its pressure at the start of the compression stroke is lower, so its pressure at the end of the stroke (which is what you measure) is lower, too - in both cases "lower" meaning "lower than it would be with the throttle wide open". To stand any chance of getting a test result you can compare with anyone else's, the throttle must be wide open.

the question is still weather it is worth while fixing the problem, what ever it may be, to get some more power from it....

If there's a problem at all...

does any body have a figure in mind or a calculation that i can do to figure it out??

Calculation needs physics to somewhere around Year 11 level. I explained it on here a while back.

Don't forget that your gauge will not be properly calibrated, so don't worry about small differences between your results and anyone else's. Post your "full throttle" test results, and see what people make of them.

Kevin

[kittyfell]

Engine hot after a run with battery fully charged, all plugs out and yes, throttle propped open for which a brick is good. 140 would be good, 120 would be poor and 160 would be excellent. If it goes well enough and you can live with the oil consumption I would leave well alone.