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Hi, I have a 1989 turbo MX5, but running at low PSI and have around 155HP. I had a compression test today and they came in at 140, 143, 140 and 143.

Although these are consistant, are they a bit low?

Thanks Nicky

How was the compression test conducted? (was it taken to operating temp first, wide open throttle, how many cranks did you do? etc)

Rule of thumb is 135psi is usually on the low side. At least across all your cylinders the pressure is fairly even.

Sorry Mr Revhead I cant be more specific, I just dropped the car off and left it with them for half and hour. The engine was warmed up well by the time I got it there. I dont really know much about this type of thing.

should be more around 170 - 190

did you have throttle 100% open?
crank over till gauge didnt climb any further?

emily_mx5 wrote:should be more around 170 - 190

I seem to recall 180 being the norm, so the range above sounds about right.



When the engine was turbo'd you didn't do anything like lower the compression etc? (thicker head gasket etc?)

The other option is, it's not an engine from an Automatic NA is it? I seem to recall the auto ones had less lift on the cams, and I think from memory also had lower compression ratios than the manuals.

emily_mx5 wrote:should be more around 170 - 190

did you have throttle 100% open?
crank over till gauge didnt climb any further?

I wasnt there when the test was being done so dont know. I would have thought the mechanic would have know what to do for best results.

hks_kansei wrote:

emily_mx5 wrote:should be more around 170 - 190

I seem to recall 180 being the norm, so the range above sounds about right.



When the engine was turbo'd you didn't do anything like lower the compression etc? (thicker head gasket etc?)

The other option is, it's not an engine from an Automatic NA is it? I seem to recall the auto ones had less lift on the cams, and I think from memory also had lower compression ratios than the manuals.

I put in a replacement engine which I had the mechanic put in new rings, rehone? the cylinders, APR studs, Cometic head gasket. (and the turbo from the previous engine) He said the head looked fine and the bottom end looked like new. When I bought the replacement engine it had a manual gearbox bolted to it so gather its always been manual? Im running it a bit rich to avoid any overheating problems on the track.

Arent you a mechanic?

Anyways, The test needs to be done with a very healthy battery or jumper cables hooked up and full throttle.

Dann

NitroDann wrote:Arent you a mechanic?

Anyways, The test needs to be done with a very healthy battery or jumper cables hooked up and full throttle.

Dann

Wish I was Dann! But your comment may have helped me. My battery is pretty stuffed and apparently the RAC said I need a new one, so maybe
I should get that and then try the compression test again.

Get another car hooked up to it and running just as if you were jump starting it.

Dann

NitroDann wrote:Get another car hooked up to it and running just as if you were jump starting it.

Dann

The things I learn here, presumably you want the starter all juiced to turn the engine over.

sticky nicky wrote:I had a compression test today and they came in at 140, 143, 140 and 143.

Bit of a contrast from a year ago - hope it's just the tired battery!

sticky nicky wrote:Just had my replacement engine (1.6, long nose crankshaft) tested for compression before it goes in. Results were 225, 225, 225, 175.

viewtopic.php?t=54462&f=29

Yes it is, can a bad battery make that much difference?

Of course, come on, do the math.

Dann

NitroDann wrote:Of course, come on, do the math.

Dann

Sorry Dann, I havent got a clue how the battery works except for starting the car, I think next week a new battery is on the cards and then another compression test. Ill let you know the results.