Losing Vacuum downtrack

June 04, 2018, 02:32 PM

391T

We've been having trouble with vacuum this year. It has always been around 12 at the finish line but this year it is only around 4 or so. Dragster with 438 sbf moroso 4 vane pump on methanol. Bottom end was new last year put total seal rings in it this year is really the only thing different. I had a few random aluminum pieces anodized black and had part of the regulator done. It is the kind that is on the pump. I put new vanes in it and that didn't help any. No blowby with the pump unhooked.

Is it a leak or the rings? We put a little shorter tire on it and the car is a little quicker this year. It has always pulled quite a bit of oil out in the puke tank but seems to be more this year.

June 04, 2018, 03:25 PM

John Heard

I'm gonna say it's either ring seal or you're got a leak, key thing is you're seeing more oil too in the puke tank. So it's moving more air volume, question is what is the source?

Better do a leak down/comp test and see where you're at.

June 04, 2018, 04:08 PM

Curly1

Yes, pulling oil is a sign you are moving a bunch of air through the motor. Like John said it could be a leak or rings etc. It could also be a bad sensor. On mine I found my sensor was off it had over 17 inches of vacuum when it was showing 3 or 4 inches.

Also they can leak around distributor, intake, valve covers, crank seals, timing cover or oil pan.

Smoke test may show up some issues but to put a vacuum pump on it and pull a vacuum on it and listen looks for leaks is better way to find them.

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June 04, 2018, 06:37 PM

SCDIV1

Real easy to connect an air line with a pressure regulator right to the fitting on the valve cover where the pump suction line attaches. 5-10 lbs of air and you will hear any leak. I just did my engine a few weeks ago because I knew something was wrong. Pump was part of the issue with stuck vanes but as fast as I hooked up the air line I found a major leak at the intake front bottom rail. Silicone failed. Sealed it back up and cleaned out pump and breather tank and back to normal. The air line with low air pressure in is a trick I have used on small engines to find oil leaks. You don’t need or want more than 5-10lbs...

June 04, 2018, 09:00 PM

davidsracing

My engine went from 11 inches to 3 inches between races and it turned out to be an intake gasket leak in the lifter valley area.

June 05, 2018, 10:27 PM

Strange Magic

What is the actual ring package you installed?

If there was a change in radial wall and the tension changed, you could possibly have an issue. If your oil rail and expander changed whereas it's lighter and allowing oil to be burned in the chamber, then it will run significantly richer and charge the crankcase accordingly.

Due to the stroke and deck height, you really need to pay real close attention to controlling the oil, keeping it away from the ring package, and making sure it goes down faster then it goes up.

P.S. At 158 degrees of water temp on alky, you will have excess un-burned fuel that will go to only one area, and that's past the rings. At 158, the chamber is not hot enough to allow for a complete burn. I would suggest to get it up another 10 degrees.

June 06, 2018, 08:55 AM

391T

Leaking in the front corner of the oil pan the low pressure air deal worked good.

Yeah we need to get it warmer.

Thanks for all the replys.

June 06, 2018, 09:40 AM

TOP38

I'll add this, your vac curve is opposite of rpm and your leak is constant (relatively speaking) and therefore your blow by is increasing with rpm of which your vac pump can't over come and therefore your losing vac. So you could have two issues here.

June 06, 2018, 09:41 AM

TOP38

quote:
Originally posted by Strange Magic:
What is the actual ring package you installed?

If there was a change in radial wall and the tension changed, you could possibly have an issue. If your oil rail and expander changed whereas it's lighter and allowing oil to be burned in the chamber, then it will run significantly richer and charge the crankcase accordingly.

Due to the stroke and deck height, you really need to pay real close attention to controlling the oil, keeping it away from the ring package, and making sure it goes down faster then it goes up.

P.S. At 158 degrees of water temp on alky, you will have excess un-burned fuel that will go to only one area, and that's past the rings. At 158, the chamber is not hot enough to allow for a complete burn. I would suggest to get it up another 10 degrees.

Strange

Unburnt fuel can go out the pipes too.

June 06, 2018, 10:50 AM

Strange Magic

quote:
Leaking in the front corner of the oil pan the low pressure air deal worked good.

Yeah we need to get it warmer.

Thanks for all the replys.

Can you repost two graphs here when you correct your leak?

The first at or around the same temp a shown in the current graph, and the second with 10 degrees higher in temp.

June 06, 2018, 10:59 AM

SCDIV1

One real common place for a bad air leak is the distributer at the split collar on adjustable height MSD units.

End rails on intakes and corners at oil pans.

A couple years ago I had a one piece pan gasket, blue silicone type with steel inserts fail at the front. It shifted and was hanging out forward and leaking. I was at a National event and cut out the damaged part and siliconed the heck out of it.

I remember it real well because I got gored up by wire tie cutoffs doing the repair....blood and silicone......a nice mix....

I was dead late in the first round .....Not a good event....