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Blow-by curve

General engine tech -- Drag Racing to Circle Track

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Warp Speed
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Re: Blow-by curve

Post by Warp Speed » Wed Jan 07, 2015 4:45 am

MadBill wrote:I can't quite make out the blow by CFM on your screen, but assuming it's ~ 4 CFM, another 9 CFM of air leakage is a ton. If you can track down and eliminate the source(s), the pan vac. should go up big time and significantly help power and sealing, even without anything too trick in the ring department.
He said in his original post that it starts at 2cfm and went to 11cfm at the end of the pull.
Measuring total loss at the pump exit he is now seeing 15+CFM at the start of the pull!
Huge leak somewhere!

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Re: Blow-by curve

Post by jdperform » Fri Jan 09, 2015 7:46 am

Put 1 psi of air in crankcase, check entire engine seams with soap bubbles. If no leaks found quick close off your air hose attached. It should hold some pressure for at least 20 to 30 seconds. With this check you can be 100% sure you have no external leaks. Also a good way of checking for potential oil leaks.

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MadBill
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Re: Blow-by curve

Post by MadBill » Fri Jan 09, 2015 8:20 am

A vacuum gauge permanently plumbed into the crankcase is useful. On shutdown the engine should hold good vacuum for a minute or more. If it falls like a rock, something's not sealing, be it rings, main seals, the hole in the pan where a rod escaped...
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Re: Blow-by curve

Post by Warp Speed » Fri Jan 09, 2015 9:58 am

jdperform wrote:Put 1 psi of air in crankcase, check entire engine seams with soap bubbles. If no leaks found quick close off your air hose attached. It should hold some pressure for at least 20 to 30 seconds. With this check you can be 100% sure you have no external leaks. Also a good way of checking for potential oil leaks.
The problem with this is most crank seals are designed to hold pressure. They can very well seal while doing a pressure check, but leak terrible under depression!

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Re: Blow-by curve

Post by RW TECH » Fri Jan 09, 2015 12:37 pm

Warp Speed wrote:The problem with this is most crank seals are designed to hold pressure. They can very well seal while doing a pressure check, but leak terrible under depression!

Right on.

I have an EVAP smoke test machine I use on projects at home & right now I wish this worked the opposite way.....A BBC with smoke pouring out of the rear main seal with near zero pressure in the crankcase & no way of really knowing whether it plugs up with vacuum in the system when the engine runs.

So what could've been a partial teardown to change out some higher-mileage aluminum rods has become a full teardown to do some register measurements & replace a seal that may be OK as-is.

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Re: Blow-by curve

Post by MadBill » Fri Jan 09, 2015 1:42 pm

I've often use a running engine as a vacuum source for such purposes. Teed into a vac. line (lately it's usually been the MAP sensor hose) and used 20 ft. or so of aquarium tubing to reach the engine on the stand, teed a vac. gauge into the line at that end and sealed it to the crankcase via a vent hose. (or in the last build, a dedicated fitting for a full-time monitor gauge) Fire up the 'vacuum donor', wait for the gauge to max out, then pinch off the line downstream of the gauge and see how fast the needle drops. I figure if there's any vac. left after 60 sec., it's all good...
Felix, qui potuit rerum cognscere causas.

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Re: Blow-by curve

Post by MadBill » Thu Oct 03, 2019 6:14 pm

Exhuming this five year old thread, if only for general knowledge:
1. Was the cause of the high blowby/low vacuum ever resolved?
2. Once we learned the top ring gap was actually 0.022-0.024", the second gap looks way tight; could have brought on inter-ring pressure, unseating the top ring and inducing flutter.
Felix, qui potuit rerum cognscere causas.

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Re: Blow-by curve

Post by williamsmotowerx » Fri Oct 04, 2019 2:24 pm

Run it with 2 stroke oil in the fuel. 32:1 or 24:1. See if that helps. Would be a fun quick test

I didn't even notice this was a super old post... my bad

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