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Thursday, March 19, 2020

Compression Check and Leak Down Check on RB26DETT


~180 psi is one of the highest I have seen while compression testing an RB26. However they were nearly all similar

I really don't like compression tests. There are 100 ways to do it wrong, and 1 way to do it right.  The only thing I really care about is the spread between holes.  If you have 150,150,150,150, 150,150  then its good.  I am not that worried that it isn't 165, 180, or whatever magical number that someone told you it should be.   To find out the true condition of an engine, do a leak down test. Compression tests are a good easy test to get a general idea of engine condition.


If you want to have "proper" results follow the procedure.  There are no cold numbers listed in the RB26 English service manual. Just warmed up, or hot compression figures.

Part 1 of RB26 compression test procedure

Compression test procedure for an RB26dett part 1
Part 2 of compression test procedure

Compression test procedure for an RB26dett part 2

  • Warm up engine
  • Release fuel pressure(I unplug CAS, if you unplug the cas the fuel injectors stop)
  • Remove ALL spark plugs
  • Start at #6. If you are going to find anything wrong, its normally here. 0 is bad, anything much above that is ok. Higher is better, but similar to the other cylinders is much more critical
  • WOT with the accelerator.  People miss this all the time
  • Crank the engine until the compression needle stops moving. That isn't 2 times, or 3 times, that is until the needle stops moving
  • I normally leave battery connected to a jumper box, so it spins over fast enough. 
  • Note numbers. Take a picture or video, for the people that think you are faking the numbers. 
  • If one cylinder is low, you can drop a little oil in it, to see if its the rings or the valves. If the compression number comes up with oil, then the engine may be getting tired. Its leak down time. 


Now for the grain of salt discussion.  When was the last time your gage was calibrated? If its a 0-300 gage then its normally most accurate the middle of its range. However 10% off is not unheard of for some mass manufactured gage. That is 30 psi on a 0-300.  Spinning the engine too slow, will probably see lower numbers.  Not holding WOT will usually see lower figures. Not pulling all the spark plugs will make the engine spin over a little slower. Not letting the gage settle/stop will show lower figures.  All in all, again, what you want to see is balance among the holes. If you live and die by the exact numbers, you are the type of person that I hate.

Compression test standard and low limit for RB26dett  171 psi standard, and 128 psi low. 14 psi between cylinders




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