RB26 Leakdown test

[Ricky_Bobby]

Hey guys I have a quick question for you rb26 guys out there. . . Im looking to buy aRB26 and i have one lined up. . . he says with the leakdown test it runs 185psi across the board. . . . is this good??? i know a leakdown test is different from a Compression test, so i was wondering if 185 was good??? what kind of numbers should i be looking at for with a leakdown test. . .

[MarusGTR]

Hey guys I have a quick question for you rb26 guys out there. . . Im looking to buy aRB26 and i have one lined up. . . he says with the leakdown test it runs 185psi across the board. . . . is this good??? i know a leakdown test is different from a Compression test, so i was wondering if 185 was good??? what kind of numbers should i be looking at for with a leakdown test. . .

Wet test right?...Because 171psi on air is brand new and you should be looking for less than 20% pressure loss on a leakdown test.

[Ricky_Bobby]

what do you mean by Wet test?? im not familiar with leak down tests at all, also as far as i know, the motor is removed from the car. . .

[TW]

leakdown tests basically consist of pressurizing each cylinder at Top Dead Centre to see how much pressure it retains. It can help identify where compression is being lost. It can be done with the motor in the car.

[Ricky_Bobby]

ok so im pretty sure the guy said 185 across. . . is this good?? he was hard to understand, but im 99% sure he said 185 across. . . i will verify tomorrow. . . i just dont want to buy a bunk engine, so im just trying to educate myself on this method of testing, before i go throwin this guy $100 bills.

[TW]

Just ask to do a compression test yourself.

[Ricky_Bobby]

ok he says he just wires up a battery directly to the starter, and thats how he turns the car over with out it in the car . . so he uses the conventional style of compression testing, by removing the spark plugs and getting compresion readings through the spark plug holes with a compresion tool. . . . but still 180 across for an engine with 80-100k on it . . . . you would think it would have lower compression than 180psi across right?? ill prolly end up bringing my own compression tester down there and doing my own analysis. . . hes all down for me bringing a mechanic down there with me as well to look it over. still though, isnt 180psi across a little too good to be true? is there any RB26 mechanics out there that have seen numbers like this with a motor that has 80-100km on it? just want to know what ppl think before i spend 12 hours of driving to go look at this thing...

[GTR-Dad]

The only way an RB26 will have those compression numbers is if it's got a higher than stock compression ratio and is in great condition.
I doubt the the values are accurate, but the engine could be just fine. A video of the test might shed some light. Spec rpm for this test is 300.

[caliber676]

What he is doing is a Wet compression test. This is usually done to determine the condition of the piston rings, not an actual compression reading. Ask him to do a dry compression test and then compare them to the wet compression test numbers he gave you. If the numbers for the dry compression test are significantly lower, that means your piston rings are most likely not sealing properly. Its a good idea to do a cylinder leakage test (not a leakdown test). This will give you an indication to how the cylinders are functions and if/where they are leaking. I would only trust doing these tests myself or if you can't, get a trusted mechanic to. $100 is a smaller loss than the thousands you may spend on a worn engine.

[MarusGTR]

I've googled it and a guy got 200psi all cylinders on wet compression, variable 150ish on air. Not sure if the air test was a leakdown.

Compression test: You crank the engine with a manometer screwed in the sparkplug hole. After a couple cranks at 300RPM you should get the maximum compression your engine exerts on the piston head. Leaks would indicate degraded piston rings or headgasket.

Leakdown: The reason why he probably did this is that you dont need to crank the engine. You only bring the cylinder to TDC, lock the flywheel, and the tester is placed in the plug hole. The tester injects air at the factory compression in the cylinder. The manometer pressure indicates pressure in the compression chamber. If the pressure injected in the engine doesn't match the pressure read inside the compression chamber, you have a leak. Normal seepage is expected to be less than 20% difference.

Basically Ricky_Bobby, go to crappy tire and buy yourself a universal compression test kit (40$) and crank up that engine with the flywheel.

Edit: Hope this helped

[Ricky_Bobby]

thank you for the prompt replies, i have another question, what is the difference between a " dry " and a " wet " test. i think i have an idea, but i just want to hear it from someone who knows exactly, and what kinda of parameters i should be looking at for decent compression PSI wise. .

[jdms13rhd]

wet test: Adding oil or like fluid into the cyl to seal it up.

Dry test: No fluid added to the cyl before testing.

If he really did a wet test its going to be hard to indicate if its a truly good engine without it running to find out. Once you get oil in there without letting the combustion burn it all off your not going to have the same reading you would get when running a dry test first.

I've never met anyone that does compression tests on a engine and starting it off with a wet test. dry is always done first. This is for many reasons.

To give you an example. If I had a engine with lowish compression, "Ie worn rings". I know its low, I get blow by or whatever. Numbers are approx 100-135 on a few holes and got another few down in the 80-90. engine still runs good. You wouldnt know that its low by driving it necessarily without knowing what your looking for. Thinks like oil in your catch can, lack of power, smelly exhaust ect. With that said. I pull it out, throw some oil in there and suddenly the numbers climb up. The three holes are still a bit low but I keep cranking on the starter/battery cable and you see a high number that can fool the buyer.

Almost every engine I sell/build I always run a dry comp test and a leak down and then if im not happy with the numbers and want to know the problem I use a wet test to 100% diagnose whether its rings, HG, valves or whatever else. Normally 6 crak rotations does it. keep the test consistent.

Most leak down testers dont have a psi on it. Most ive seen have a poor/fair and excellent rating. PSI all depends on what you have your hose regulated at or how much psi you have in your air tank.

a 15 yr old engine could be in around the 0-8% from oem for compression and should all be in the excellent mark on your leak down tester or very close pressure to whats regulated, think crappy tire gauge telly you to set at 165psi?