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Compression ratio issues...Daewoo VS Corsa

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Compression ratio issues...Daewoo VS Corsa

Postby Nubaroo » Thu May 23, 2013 6:40 am

Hi there guys.

Just some input/advice needed please.

My Daewoo sporting a freshly built setup has an calculated CR of 8.5:1, my Corsa with the old Daewoo motor (also freshly rebuilt) has a CR of 8.6:1. Both engines 2L Ecotec setup

This is obviously fantastic for a turbo car.

However something that I dont understand - When I do a compression test on my Daewoo with the new motor I get a reading of 1050KPA after swinging the motor till needle on the compression tester stops/stabilises. All cylinders read more or less the same. Now if I do the same test on my Corsa with the older Daewoo motor I get a reading of 850KPA, also more or less the same on all 4 cylinders.

What can cause these similar motors to have such different readings although they both have almost same CR??

My tuner suggested it could be different camshaft setup - the Daewoo now running 2L cams from an Opel motor en the Corsa running my old Daewoo cams (they have been slightly reprofiled)??

Any input??
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Re: Compression ratio issues...Daewoo VS Corsa

Postby exist3nce » Thu May 23, 2013 2:37 pm

Interesting question... I know you can not always take the compression tester reading as a direct relation to compression ratio. I believe as your tuner said various cam profiles can alter the tester reading, but I think there are also more variables. Also, are you 100% sure your calculations are correct?
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Re: Compression ratio issues...Daewoo VS Corsa

Postby gse_turbo » Sat May 25, 2013 4:42 pm

How did you figure the calculations on CR? The 2.0 Daewoo version has an advertised CR of 9.6:1.

As for the variations in test results, it may just be a matter of age on the motor. Although the cams may (or may not) be different they shouldnt have a significant affect. A compression test basically just test how well the motor is sealed and how capable it is in compressing air. (ie, the more the cylinders leak, the less capable of compressing air).
The compression testing is a basic way of testing piston rings, head gasket, valve seats and valve seals.

Now to narrow down the potential problem...
1. Although the compression is low, if it's even across all four then it's unlikely to be the head gasket.
2. A HOT test and COLD test comparison can point to sticky rings if the reading goes up after a heat cycle.
3. Putting a small amount of oil into the cylinder through the spark plug will temperarily help seal the rings. If the reading goes up you're looking at low compression from blow-by from the rings. Also, if blow-by is present in the motor you should be able to run the motor with the oil filler cap off and see excessive air pumping out (crank case pressure).
4. The last this is least likely, valve seals. You would be getting smoke out the tailpipe from oil seaping in if this were the case.
The valve seats are kind of impossible to test unless perhaps you have a leak-down tester. Even then I don't think it would conclusively point to valve seats.
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