Compression test results - Questions
05-09-2011, 08:29 PM
#1
Pro
Thread Starter
Compression test results - Questions
Ok, so I'm at the point with a potential purchase that I had the car sent to a shop to do a compression test. I get the call today and the shop manager says "Theyre all consistent" but then tells me four of the cylinders are 105 and two of the are 100. The test was done cold.
I say that seems pretty low and he suggests it's the consistency that is important, not the numbers.
I call back and tell him that I'm concerned with the test results, as I was expecting higher numbers even if doing a cold test.
Am I right to expect 140-150 cold and 160-180ish hot?
What could they be doing wrong with the test to get 100-105? Would a gt3 with 100psi compression even run?
So confused and the potential purchase hangs in the results.
Thoughts?
I say that seems pretty low and he suggests it's the consistency that is important, not the numbers.
I call back and tell him that I'm concerned with the test results, as I was expecting higher numbers even if doing a cold test.
Am I right to expect 140-150 cold and 160-180ish hot?
What could they be doing wrong with the test to get 100-105? Would a gt3 with 100psi compression even run?
So confused and the potential purchase hangs in the results.
Thoughts?
05-09-2011, 09:15 PM
#3
Lifetime Rennlist
Member
This may simply be the calibration of his compression gauge Vs another. Perhaps that is why he stated that consistency is important over the actual raw number.
05-10-2011, 12:41 AM
#4
Nordschleife Master
Join Date: Oct 2004
Location: Vacuuming Cal Speedway
Posts: 7,306
Likes: 0
Received 7 Likes
on
5 Posts
What you really want to see the results of a leak down test. I'm guessing his guage is off but regardless, you really want to see is leakdown results for all 6 cylinders warm, not cold.....
05-10-2011, 08:36 AM
#5
Nordschleife Master
Exactly what JR said. The compression test is way too variable from shop to shop and very difficult to get an accurate reading unless they really know what they are doing and have accurate equipment.
Leakdown is quick and easy and more telling to WHERE the problem exists.
Leakdown is quick and easy and more telling to WHERE the problem exists.
05-10-2011, 09:31 AM
#7
Nordschleife Master
For starters, it's almost idiot-proof to to screw up the leakdown. With compression there are too many variables that if not all the stars are aligned then you could get very spurious results from cylinder to cylinder.
Trending Topics
05-10-2011, 09:46 AM
#8
Pro
Thread Starter
You never know.... these might be a special breed of idiot that you're not considering.
They were also very reluctant to do anything with a warm engine as they might burn their hands.
They were also very reluctant to do anything with a warm engine as they might burn their hands.
Good point.
.....
05-10-2011, 12:26 PM
#11
Race Director
Ok, so I'm at the point with a potential purchase that I had the car sent to a shop to do a compression test. I get the call today and the shop manager says "Theyre all consistent" but then tells me four of the cylinders are 105 and two of the are 100. The test was done cold.
I say that seems pretty low and he suggests it's the consistency that is important, not the numbers.
I call back and tell him that I'm concerned with the test results, as I was expecting higher numbers even if doing a cold test.
Am I right to expect 140-150 cold and 160-180ish hot?
What could they be doing wrong with the test to get 100-105? Would a gt3 with 100psi compression even run?
So confused and the potential purchase hangs in the results.
Thoughts?
I say that seems pretty low and he suggests it's the consistency that is important, not the numbers.
I call back and tell him that I'm concerned with the test results, as I was expecting higher numbers even if doing a cold test.
Am I right to expect 140-150 cold and 160-180ish hot?
What could they be doing wrong with the test to get 100-105? Would a gt3 with 100psi compression even run?
So confused and the potential purchase hangs in the results.
Thoughts?
A compression test is in some cases unnecessary.
The power output from each cylinder is constantly monitored by the engine controller at every power stroke. Each cylinder's contribution to the flywheel speed is constantly checked and if one is found to be under (or over) spec the check engine light is turned on and one or more misfire error codes are logged.
If you can drive the car the best checkout is with the engine dead cold and with the A/C off get in and turn on the key and verify all warning lights come on, including the check engine light.
Start the engine. It should crank smartly and fire right up. Cold idle will be a bit rough and at a higher speed than hot idle.
The engine should be quiet (as quiet as these engines can be) with no ticks, knocks or other noises.
As the engine warms up it should smooth out and the idle speed drop.
Let the engine idle a while then go for a test ride. Have the driver follow a route that gives you a chance as rider to pay attention to the car's behavior/performance under a variety of driving conditions. This wants to include city driving with stop/go, cruising at low, medium and high speed with some hard acceleration runs starting at idle to redline.
The engine should pull strong and smooth from idle to redline and do this more than once.
The route should be around 15 miles long.
Back at the starting point give the now hot engine another listen.
Now you take the car and folllow the same route driving the car the same way.
This gives the engine controller time to id a weak cylinder or any one of the other possible problems with the engine's mechanicals or electronics.
Sincerely,
Macster.
05-11-2011, 11:33 AM
#13
Race Director
Regardless, try to give the car a thorough test by first a test ride then a test drive. You want to hear/experience the car the engine in a variety of scenarios and you want to give any existing issues (if any) a chance to make themselves known.
Sincerely,
Macster.
