Help needed very low compression
Discussion
Hi guys bought a escort cosworth last week and it’s had a full top end build so I went to have it all checked over as you do to make sure it’s fuelling ok On dyno and have a health check etc etc before going car had no symptoms so had no reason to think anything is wrong just wanted to make sure fuelling was ok so I can run the engine in
Car drives lovely no miss fire no smoke runs sweet as a nut so they done a compression test and it came up 65-70-65-70 psi we were so baffled as it runs so great so they performed a leak down test and that came in 2%-3% so seems perfect checked all timing and everything is spot on
Surely an engine to have that low of psi across the bored wouldn’t run at all or if it did would be smoking it’s tits off and cutting out went on rolling road and fuelling was great it was under power but it was only boosting 0.5bar but that’s a different issue hopefully any help would be great cheers
Car drives lovely no miss fire no smoke runs sweet as a nut so they done a compression test and it came up 65-70-65-70 psi we were so baffled as it runs so great so they performed a leak down test and that came in 2%-3% so seems perfect checked all timing and everything is spot on
Surely an engine to have that low of psi across the bored wouldn’t run at all or if it did would be smoking it’s tits off and cutting out went on rolling road and fuelling was great it was under power but it was only boosting 0.5bar but that’s a different issue hopefully any help would be great cheers
Yep...most likely a case of a faulty tester ( ie the idiot doing the testing )
because no sane person who works at cars would accept those readings as accurate.
Not even sure a car would start with such low readings.
I would get a new tester, and perform the check again. But the fact you say it starts fine, runs fine, and there are no indications of a problem, and all 4 cylinders are well balanced.....suggests there is no problem
because no sane person who works at cars would accept those readings as accurate.
Not even sure a car would start with such low readings.
I would get a new tester, and perform the check again. But the fact you say it starts fine, runs fine, and there are no indications of a problem, and all 4 cylinders are well balanced.....suggests there is no problem
stevieturbo said:
because no sane person who works at cars would accept those readings as accurate.
Inaccurate testing seems the most likely explanation, but is it possible this is aggravated by a low CR designed for high boost, or very long cams so the test isn't seeing the actual static compression?A compression test is never a 'measurement', but a comparison, across the block.
The gauge is crude, and the test very dependent on technique and even the weather.
Your pressures are well within the usual 5% variation, and the performance of the car shows that there is nothing wrong with it.
Enjoy!
John
The gauge is crude, and the test very dependent on technique and even the weather.
Your pressures are well within the usual 5% variation, and the performance of the car shows that there is nothing wrong with it.
Enjoy!
John
Thanks for all the replies guys I can’t fault the mechanics as they are well known in South Wales for building 1000bhp race engines for evos etc I’m just hopping it’s a faulty gauge they told me not to worry about it because the leak down test was good and said it couldn’t be down to a weak starter motor or weak battery because it did sound slow if I remember
GreenV8S said:
That seems like a pretty fundamental mistake for somebody tuning engine professionally and doing this test as part of their work.
About 11 years ago I was pottering about at a rolling road place a friend ran doing some cylinder head porting as a kind of busman's holiday for a few weeks. One night they hired the rollers out to a tuning guy of 30 years experience who no longer had his own rollers. He was setting up a Ford Crossflow rally engine for a customer. I watched on and off for about 2 hours as he messed with fueling and ignition but never got anywhere close to the expected target output. It was about 30 bhp down. Twice I suggested do a comp test but was ignored. He just kept tweaking parameters which were clearly, to me at least, close enough for the engine to be working pretty well if it was in good health. "It's been rebuilt recently, it can't be wrong" I got told. Eventually after burning through many gallons of fuel and loads of the customers cash they listened to me and did a comp test. It was 180 psi on three pots and three fifths of FA on the fourth. Bleedin obvious really and the entire expensive session could have been avoided with a few basic checks to start with. Doing something for a living is no guarantee of doing it well IMO and many years of experience can equally just be many years of never learning things properly.GreenV8S said:
That seems like a pretty fundamental mistake for somebody tuning engine professionally and doing this test as part of their work.
About as big a mistake as accepting such readings as ok and as an engine problem.Too many people have rather inflated notions of the abilities of some of these "professionals"
T33omm said:
Hi guys done the test myself and I got 160-155-160-155 they clearly had a faulty gauge thanks for all your help guys
But you don't question why any garage would actually have given you the incorrect results when anyone with any experience would know they were not accurate and shouldn't be passed onto the customer...Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff