Blow-by testing

[Dave Harmison]

I’m trying to determine the condition of my 5.9L Cummins in my ‘93 Dodge. Turns out Cummins doesn’t believe in leak-down or compression tests but blow-by testing.
Plug all connections to crankcase ( oil dipstick) and put known diameter orifice in vent (15/64 dia). Run engine at various rpm and measure delta pressure on orifice to get flow rate of blow-by to get idea of engine condition at given rpm’s.
I know many of us use leak-down, but this is dynamic, engine running data. Anyone use it for their race engines?
BTW: spell check / auto correct was a bitch typing this. The B didn’t like anything.

[Charlie Schmidt]

FWIW...Ford had a similar procedure for the early 7.3s

[DanW]

Blow-by testing will tell you compression ring condition. It may tell you if you have a bad cylinder if you detect an increased pressure pulse, but really won't give you info on the valves.

[scott fairchild]

when I was working on street cars (been 6+ years ago), I'd use a pressure transducer and lab-scope to record running compression testing. I'm sure there are videos on YouTube that can explain what to look for, but it was useful for many mechanical checks without tearing into a job too deeply.

Another useful trick can be checking relative compression with an Amp clamp (ignition disabled).

[Art Smith]

I’m trying to determine the condition of my 5.9L Cummins in my ‘93 Dodge. Turns out Cummins doesn’t believe in leak-down or compression tests but blow-by testing.
Plug all connections to crankcase ( oil dipstick) and put known diameter orifice in vent (15/64 dia). Run engine at various rpm and measure delta pressure on orifice to get flow rate of blow-by to get idea of engine condition at given rpm’s.
I know many of us use leak-down, but this is dynamic, engine running data. Anyone use it for their race engines?
BTW: spell check / auto correct was a bitch typing this. The B didn’t like anything.

yes. I use crankcase vacuum for both dynamic (ie: engine running at temperature under load) ring pack state of health assessment and ring pack/bore finish development. assume belt driven oil pump with significant scavenge capability, crankshaft seals good to 30 inches of Hg at 7500rpm, teflon valve guide seals, engine assembly using NO silicon sealant, cast dry sump pan & valve cover, .........

Art
artesmith@earthlink.net

[Steve Demeter]

Correct me if I am wrong, but it seems that the blowby test merely gives an overall condition of the engine. It would not appear to give condition of individual cylinders, right?

[Dave Harmison]

Yes, the blow-by would confound all the cylinders together. I think trying to measure it fast enough to separate cylinders would get into all sorts of fluid dynamics and resonances that would be pretty hairy.
Seems to me leak-down as many of us do is good for identifying each cylinder and spotting valve sealing issues but blow-by has the advantage of actual running engine sealing of the rings against the cylinder walls.
Like I said, it’s Cummins preferred metric and I’m working on my 5.9 diesel but I thought it would be an interesting thing to discuss as it may apply to our cars.
Yeah, I just can’t stop engineering things. It’s a lifestyle/obsession.

[DanW]

Yeah, I just can’t stop engineering things. It’s a lifestyle/obsession.

There's no 12 step program and all your friends are either co-dependent or enablers.