Leak down numbers - What should a healthy engine read?
07-31-2017, 07:47 AM
#1
Rennlist Member
Thread Starter
Leak down numbers - What should a healthy engine read?
Back in the old days when my 997 actually ran a bit, I did a leak down test on it. I seem to remember something like 9% in 5 out of 6 holes, and 22% in #6. Unfortunately I didn't note that anywhere.
If you were buying a used engine, what would you be looking for in leakdown numbers?
If you were buying a used engine, what would you be looking for in leakdown numbers?
07-31-2017, 07:59 AM
#2
leak down is around 10% from cylinder to cylinder.. a cold engine is gonna leak more than a warm engine where the metal has expanded..so ur looking for consistant numbers from cylinder to cylinder...
07-31-2017, 02:58 PM
#3
Race Director
Back in the old days when my 997 actually ran a bit, I did a leak down test on it. I seem to remember something like 9% in 5 out of 6 holes, and 22% in #6. Unfortunately I didn't note that anywhere.
If you were buying a used engine, what would you be looking for in leakdown numbers?
If you were buying a used engine, what would you be looking for in leakdown numbers?
'course, if the engine can't be started and run and the car tested on the road then the 10% sober_owl provided is one I recall from back in the day.
You have to be careful your compression testing doesn't give you erroneous numbers. These engines have piston oil squirters and at cranking speed the oil may not flow right away so some cylinders can be "dry" and others tested later after the pump has had time to prime and pump some oil can be less "dry", wet with oil. This can have the numbers different.
You can mitigate this so some extent by with the spark plugs out adding a bit of engine oil -- tablespoon or two -- into each spark plug hole so each cylinder has some oil in it and this just before you test the the cylinder. That is add the oil. Crank the engine a few seconds to spread the oil around a bit.
(Get an "eyedropper" or a plastic medicine syringe from the drug store to use to put the same amount of oil in each cylinder.)
There can still be some differences so after you check the last cylinder if any of the other cylinders were reading low recheck them to see if they still read low.
Be sure you have a real healthy battery. Ideally you should use "shop" 12V power so the starter receives the same power when cranking the engine for the last cylinder tested as it received when cranking the engine for the first cylinder tested.
And keep the starter cool. An electric fan coupled with giving the starter a reasonable cool down period after each cylinder is tested helps to ensure consistency. And an undamaged starter.