Leakdown Testing A SBC

[BogBeast]

I have my SB Chevy out of the car at the moment and I thought I might leakdown test it with a tester I bought from Summit whilst in the US.

The instructions are telling me to put the pistons at bottom dead centre, loosen off the rockers and then test there, which doesn't make a lot of sense to me as I would have thought that TDC would be where ring and valve seal would really matter.

As I have no particular desire to mess around with my rockers so I tested at compression TDC on each cylinder. I am getting leak downs of between 10 and 15% across all cylinders.

I am testing cold, as the engine is, well. out of the car, I assume these % would 'be better' when everything is closed up a bit

Should I really be testing a BDC? Am I wasting my time testing whilst cold? What is a reasonable leak down % for a SBC?

Thanks for any advice..

{edited for seriously poor spelling & typos..}

Edited by BogBeast on Thursday 25th February 11:20

[stevieturbo]

Only reason I can think of....if you try and test at TDC, there's a good chance the air pressure in the cylinder will actually try and turn the engine over unless you can lock the crank from rotating.

If you can lock the crank, I'd say even try it at a few piston positions, ensuring valves are closed and no risk of similar damage anywhere else.

That said, a good compression test is as handy as any

[BogBeast]

stevieturbo said:

Only reason I can think of....if you try and test at TDC, there's a good chance the air pressure in the cylinder will actually try and turn the engine over unless you can lock the crank from rotating.

If you can lock the crank, I'd say even try it at a few piston positions, ensuring valves are closed and no risk of similar damage anywhere else.

That said, a good compression test is as handy as any

Thanks for the response

It certainly does turn the engine over if you are not right a TDC, I usually use a breaker bar to 'wobble' each piston around TDC to seat the rings and then hold it there.

Whilst I had the engine out I just wanted to see if there was any need to do any more work, I was under the impression that a leakdown test was 'better' then a compression test.

Looking around the web it seems that the leakdown %'s I am getting are not outlandish, I think I will assume that its ok....

[Steve_D]

My experience is that the test itself is not very consistent but the beauty of the test is that if it is leaking you can go around and listen for the leak. Noise at....oil filler = rings...Carb = inlet valve..Exhaust = exhaust valve..top hose = head gasket etc.

Steve

[BogBeast]

Steve_D said:

My experience is that the test itself is not very consistent but the beauty of the test is that if it is leaking you can go around and listen for the leak. Noise at....oil filler = rings...Carb = inlet valve..Exhaust = exhaust valve..top hose = head gasket etc.

Steve

Thanks Steve, I am also finding that the readings I am getting are not all that consistent. I am going to put a squirt of oil into the cylinder and then around the valves (manifolds are off) and try one more time.

Conscious that I am not wanting to waste time messing around just for the hell of it

[LarJammer]

i find the testers are a bit random, you will be able to adjust your percentage leakdown by fiddling with the air pressure. this isnt much use unless you have something to compare your test to. however, a leakdown is very useful as already mentioned.

[stevieturbo]

I wouldnt say it is better than a compression test, just different.

One may highlight things others wont, as as others have said you can listen for leaks too.

But a compression test can reveal a lot.

[BogBeast]

So dripped some oil into the inlet and exhaust valves (and put some oil into the cylinder and tuned the engine over a couple of times)

And pretty much all my valves are blowing bubbles: video

The exhaust seems to be slightly more bubbly, plus listening down the distributor hole I can hear leaking by the rings.

The leakdown %'s are still 8>13% which for the age of and build of the engine (they are not zero gapped rings for instance) I am assuming is not unexpected? Unless any experienced SBC out there care to differ?