Compression test #'s??

[Jason_in_KC]

Who has done a compression test on a 426 and what kind of numbers did you get? I have an 01' 426 that was bought new in Oct 00'..right around 100hrs on it. KCHusky and I used a tester and I did the regular starting drill 4-5 times and netted right around 125psi on the dial..other than a leak down test does this sound about right? W/o pulling the top end apart are there any other tricks to checking the piston/rings for wear? I have done searches here and on the WR side w/no luck on comp numbers from other bikes. Thanks, Jason

[Shawn_Mc]

it takes 90 psi min. to produce combustion. 125 seems light on a 12.5-1 engine. I would have thought 180 to 190 would have been the order.

question: did you pull the compression release while attempting this test.

If so your read is gonna be wrong.

Your probably going to have to pop it in 3rd and push it to turn the motor with out the compression release. Its what I had to do with my CR500 to get an accurate read. good luck

[Rusty]

I agree with Shawn Mc. 125lbs seems pretty low. I haven't checked my bike yet but the 460 Ford in my boat has 125lbs with 8:1 compression and bad rings (I know, apples and oranges but, still a frame of reference). He may also be right about the method used to test it. You obviously couldn't use the compression release at all. Anybody with a shop manual out there?

[Thumpologist]

If your compression is 12.5 to one and assuming a perfect engine with standard atmospheric pressure, temperature, and humidity at sea level being 14.7 psi then 12.5 x 14.7 equals 183.75 psi. At Kansas City with an altitude of approximately 3,000 feet your standard pressure drops from 29.92 inches of mercury to 26.82 inches of mercury. That would cut your compression by 10.37% which would give you a reading of 164.71 psi. Which explains why living in Colorado at 7,000 feet makes me wonder how much fun it must be to ride a 426 at sea level on a cold day with low humidity!

[ February 08, 2002: Message edited by: Thumpologist ]

[ February 08, 2002: Message edited by: Thumpologist ]

[kc_husky]

I agree with all of you but, if you do a test on a car or boat motor. You will be cranking the motor over for 4 to 5 comp. strokes at 600 to 800 rpm's. This is not so on the YZ. You have to set up for the regular starting drill each time. Which will not give you a hole lot of piston speed and the way I see it, your air volume will inturn be lower, giving a lower reading . I do like the third gear tire off the ground idea. The only problem with that is these bikes don't bump start to easy rolling down a hill or trail without pulling the decomp. lever a little to allow the motor to turn over. Jason I can bring home the cyclinder leak down tester and we can compair between the two bikes. ?

[DOZER]

I'd have to agree with Thupologist you need over 125psi to run minimally and should dlook for 145 - 180psi depending on sea level.

If you are testing properly and coming up with 125 then you should first check/adjust your valves!

Good luck

Craig

[Jason_in_KC]

Someone else mentioned compression testing when the bike was warm..thus allowing all internals to be tightened up to operating specs? Surely there are many here who have changed top ends and done this test. The valves were in spec..I was just wondering about my top end so we decided to do the comp test...the manual has no info on these numbers either.

[Shawn_Mc]

Like I said earlier in the thread, your going to have to push the thing in 3rd or 4th, throttle wide open. You could get the motor turning using the compression release. But your definetly going to have to use the old flat track style of bumpin the thing off to get a real read. Then you can dive into your altitude correction. Doing it with a warm engine is a good idea. Get the oil around and also closes some tolerances. Only thing is your going to be working with a hot engine. Dont fry a finger, been there done that!

[Dave_S]

As someone else mentioned, a compression test is done at WOT. I am sure it is impossible to kick one of these bikes through a whole cycle at WOT, I wonder if it is even possible to bump it over in 3rd, 4th even 5th gear without a tow on pavement or a good long downhill...

Engine should definately be warm so the rings are oiled. I think it would be OK to use the decompressor to get everything moving as long as it is released for several rev's for the test.

How did you perform the test???

Valve overlap can play a major factor in a high compression/high rpm engine. A compression test on a low compression engine with a cam profile optimized for low rpm power may indeed produce a similar reading to a high compression engine with a cam profile designed to be most efficient at 7-8000 rpm.

Dave S

[Shawn_Mc]

Here's a little story for ya. I had the cylinder on my CR500 ported and polished by CT, they milled the head, the whole deal. I rode it once, felt wrong. did a compression check, came back at 110#. This was low according to Alan at CT. So I took it apart, found nothing wrong. Put it back together rode again still felt wrong. Tested it again 110#. My test was kickin the bajeessus out of it WOT. 110#. I decided to push it. 210#. I found my problem in the carb, the point is the numbers went from 110# to 210#. Yes it needs race gas. "It's not easy, if it was everybody would do it. "