My Comp Test Result

[SargeRX8]

Just wanted to share my comp test result. First time I done one and I kind of shit my pants as nothing would read past 60psi. Turns out you gotta crank for about 15 seconds as the motor seems to push the pressure up about 30psi each time it goes around.

Anyway these are my results

160, 155, 160, 155, 153, 153.

I don't know if 5 and 6 are of a concern but I do have to keep my eye on it all. The spark plugs looked fine. I was abit worried as the car has seen a couple of instances of +100 knock and a couple around the 90 mark.

For the noobs who are doing their first comp test, be careful. The engine is at operating temp when you start so your hands are prone to being seared. Also have another car with jumper leads hooked up to your car as the battery will go fair quickly. Crank it for about 15 seconds or position the gauge so you can see it in the cabin when it stops rising. Its not always easy to position it so you can see it so when you aren't sure just crank it a few more times. Remember to disable fuel.

[Moodles2]

more than 10% variance between ajoining cylinder's is generally bad for RBs, I would be perfectly happy with comp results like that.

[markdem]

oh. are you meant to do a comp test on a warm engine? does it really make a difference? ive done it cold all my life

[SargeRX8]

From everything Ive read, you do it warm. I am guessing this is because you cycle oil around the engine and everything is warm and as tight as it should be. Cold motor, I assume, will always give lower results.

[tridentt150v]

Pistons and rings won't be expanded on a cold motor, valves and guides as well. You'll generally get lower readings and a lot more variation between cylinders. {lus as SargeRX8 says oil as well. Motor needs to be at operating temp - or at least very warm.

[DVS JEZ]

Also ensure the throttle is fully open, and disconnect the crank angle sensor before starting. So the injectors dont spray fuel

[PewPew]

Because of the intake plenum design on the RB engines, lower compression in the last cylinder is normal as it's usually the "lean cylinder".

[jjman]

Because of the intake plenum design on the RB engines, lower compression in the last cylinder is normal as it's usually the "lean cylinder".

are you referring to Rb26's here with the front facing plenum or all RB's? (25 included- such as in the example of this thread?)

I can see this being the case with the FFP of the 26's but not 25's. I thought the symettrical plenum on the RB25's would avoid this?

[SargeRX8]

I am fairly sure its a flaw in the RB26 intake manifold due to the cylinder being the furthest away. What I don't understand though is why it would cause a lean condition and not a rich one...

[GTRsean]

More air into rear cylinders due to manifold design, and all cylinders get same amount of fuel= leaner air/fuel ratio at the back, more heat, more prone to det.

Nismo plenum fixes it. We just run a bit more fuel in the rear cylinders.

[SargeRX8]

I would have thought that since the runners are further back, the air would be forced into the cylinders closer first but after thinking of it logically, the air would be gushing to the path of least resistance so it would be building up at the back of the plenum.

[GTRsean]

Right you are. If it was naturally aspirated there'd be no drama's, but runners are too short and plenum too small for NA anyways.