I'm racking my brain over my engine and need some help. Here's the deal. . .

The engine is a forged 383 LT1 with a home-build PT-76 GTS turbo. The crankcase vent hose is attached to the inlet of the compressor.

I ran it for a while at 12 - 14 psi with no real issue, but it dyno'd low (520 rwhp versus 540 rwhp with the old supercharged setup). I traced the problem to a restrictive IC - TB pipe, so I built a new larger one and removed the BOV.

With no other changes, the turbo now produced 18 psi. I ran it like that for a while even though I heard hints of detonation now and then.

When I removed some intake piping to change a gasket, I found alot of oil in the IC - TB pipe. As a diagnostic step, I ran the crankcase vent hose into a plastic bottle.

Last Friday night, I brought it to the track and was underwhelmed by the performance. I ran some 11.0's at 125 mph at 15 psi, and then raised the boost to 18 psi for a 10.7 at 134 mph. This is disappointing because it ran 10.40 at 132 at 13 psi with the supercharger.

During the 3 runs, it accrued about 1 cup of oil in the bottle, which sounds excessive to me, but more noteworthy was the fact that it blew the dipstick out and sprayed oil over the top of the motor.

Today, I did a compression check and a leak-down test with the following results:

#1 124 psi 4%
#2 125 psi 3%
#3 134 psi 2%
#4 125 psi 3%
#5 124 psi 3%
#6 132 psi 3%
#7 134 psi 4%
#8 134 psi 4%

This is me now: :confusd:

In the past, it was always easy to tell which one was bad because it would be down 30 psi or more.

Also, I've read that anything below 5% leakdown is very good, especially if you consider my ring gaps are pretty wide (.025").

So what the hell is wrong with my motor???

Mike

 

Did you ever fix the detonation problem?

I think that is the most likely explanation given your compression test and leakdown numbers.

(losing ring seal under detonation)

 

Yes, I turned the timing down a bit. Also, I mixed in some C12 at the drag strip.

 

i'm inclined to think its more an issue of your setup than bottom end?

cam? exhuast setup?

 

I hope you're right. . .

Cam is 224/236 - 114, 0 adv from Comp, exhaust is single 3" downpipe, over the axle, to a Hooker Aerochamber, thought I don't understand why either of these would cause oil to spew out of the valve cover.

 

When "As a diagnostic step, (you) ran the crankcase vent hose into a plastic bottle" did you dead-end it in the bottle, or did you use the bottle as a knock-out pot and run another hose to the inlet of the compressor?

For high boost S/C systems, I thought either an oil separator in the crankcase vent line, or a stand-alone vacuum pump was often used.

 

Leakdown looks good but compression pressure is a little low. Any good engine should be around 150 psi and a very good engine around 180 psi. 134 max to me is a little low. Chances are the boost pressure is easily pushing past the rings.

 

Injuneer, the vent hose went into a plastic bottle with no lid. That way, it could collect the oil, but not block the hose.

Stephen, the engine is only 8.3/1 compression and the cam is installed on a 114 ICL, which bleeds down compression more than normal.

 

ohh, didn't know it was doing that! somehow i missed that!

 

Do you still have the metal pan on the bottom of the intake? That should knock out some of the oil. I would assume you've removed the vent line from the TB to the valve cover. Not clear if the "crancase vent hose" you are referring to is the original PCV hose, or the TB vent hose. Apparently you were not running any oil separator in the line previously?

My only thinking on your bottle test was that you lost the eduction caused by the rapidly moving air in the compressor inlet pipe, and that lost you a bit of crankcase vacuum, maybe enough to lose a bit of seal on the rings, increasing blowby and oil entrainment. Just thinking out loud.

 

The metal pan is still on the intake.

The "crankcase vent hose" is the one off the passenger's side valve cover. The PCV is still like original.

The only oil separator is the baffle in the valve cover.

Mike

 

Since the PCV is connected to the intake manifold, isn't it possible the valve may be letting some pressure through to the crankcase? I know its a spring loaded check valve, but possibly it wasn't designed to seal against 18psi. The high boost S/C guys I know delete the vent line from the valve cover, add a breather to the cover, then run what was the PCV vacuum line to the compressor suction side, usually with an oil separator. The baffle in the valve cover isn't adequate to keep oil out of the vent line in a stock engine.

 

This sounds right to me, sounds like your forcing pressure through the PCV valve back down into the crankcase, and it has nowhere to go, a good reason the dipstick would blow off. PCV valves are open when they are in the middle posistion, and closed when towards the intake, but the spring is only designed to force against up to 14.7psi (atmosperic pressure), so you are forcing all 18psi back down into the closed off crankcase. I think you need to run just a breather setup on the valve covers, easy enough to try it and see if it works.

 

I use a one way positive check valve along with the PCV valve in all blower cars. This solves the problem of pressurizing the base.

 

PCV valves and race cars don't mix. I don't want to be sucking oil and water vapors out of the crankcase and feed it back into the intake. It doesn't burn too well.

That still doesn't explain why you blew the dipstick out unless the boost pressure prevented the PCV from sucking out the crankcase pressure until it just built up too high.

Take out the PCV and add a second filter. Having a filter breather in both valve covers will just vent out excess crankcase pressure. If lots of oil gets past the breathers and covers the rocker covers then you may need some sort of evac system.