Thread: Low compression and good leakdown... what does it mean?

I have another thread about my current build but it seems to have lost focus and interest so I thought I would start this one with my current status.

Compression 1-4 150, 30, 150, 50
Leakdown % 1-4 5, 17, 5, 15

Are those leakdown numbers enough to affect compression that much? FAA regulation for internal combustion engines under 1000cc passes anything with less that 25% leakdown at 80 psi input. I built my own leakdown tester to FAA specs so that I would have somthing to compare to.

I'm leaning to think there is an issue somewhere in the valve train but I'm not sure where to look next...

I could really use some help on this. It's been about a year long project on and off and I'm really losing my patience.

Additional details:

Brand new bottom end with wiseco pistons. I personally file fit the rings within spec and made sure the gaps were spaced out in the correct locations.

Head and block milled before install and sealed with a cosworth gasket and mazworx head studs.

Valves were vacuum tested ok when the head was milled.

Lifters were initially installed not bled but have since been re bled and reinstalled.

Jwt s4 cams

All valves shimmed within spec using the Nissan valve shim tool. I only referenced the 2 valves to each other for each rocker arm. I didn't check to see if any of the rocker arms ended up being higher or lower than eachother. (maybe I should check that?)

I just bought a block tester dye kit to check and see if any exhaust is getting in the coolant but it's dark and I'll do it in the morning.

Help please! I'm buying drinks for who ever can help get this fixed if you're local. He'll I'll prpbably.even paypal you a bar tab if you're not lol!




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Damn thats a tough one for sure. Deffinitely wait for daylight to come up. I know when you do leak down tests you can hear the compressed air coming from certain areas like from the intake if the intake valve is bad and exhaust if exhaust valve is bad and also from the oil dipstick if you have excess blow by on the Rings and you can hear it where the radiator cap goes if it's leaking through the head gasket.

Skipped a tooth?

17% is not good and 15% is boarder line. Check the mechanical timing. Redo the leak down test and check the throttle body,exhaust, dip stick and radiator cap.

Do you think that mechanical timing would affect just 2 cylinders? I'm pretty sure the mechanical timing is good, but I will double check. I'll do another leakdown test too. The last one was done with the engine cold because i was doing it with the cams out of the engine. I'll repot back in a few days after I get some time to do some more investigating.

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if the timing is off then you could get low compression. Make sure the timing is right and do another leak down. How many miles are on the motor since it was built?

0 miles on the bottom end and the head has maybe 60k total. 40k or so since it was p&p and had over sized valves put in it. A couple valves have been replaced along the way too but I can't remember the exact mileage off hand

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have you started this motor yet.

Yes it starts and idles. The idle is a little rough but it keeps running if it's set above 1200 rpm or so. No bubbles in the coolant or smoke out the exhaust.

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If mechanical timing is.good... and tune is healthy... and your not.mixing fluids id say put a few miles in it.and.get things seated a bit and.re.check every because essentially.nothing is wrong especially if u know for a fact you assembled everything corrextly