Sources of crank case presure...

[ssdurango]

I ran my Supercharged/propane 3.4 on the dyno, Got ~295 Bhps But now...

During the 2nd pull on the dyno, A plug popped out of the valve cover and I bunch oil mist and smoke went up. Now There seems to be alot more crank case presure. You can feel it building up if you plug the pvc hole with you finger. Soooo.... Should I go right for the rings or could there be any other possibilities to look for.

[geoffinbc]

Yea blow by is what causes it. But is yours really excessive? Every engine will have it. That is what the PCV system is designed for.

[merlot566jka]

how is your pcv system set up? you could be pushing boost into the crankcase some where! (ie blown gasket)

checked the compression yet?

[Joseph Upson]

Sounds like you've busted or burnt some rings. When a plug pops out of the valve cover under boost you're getting way to much pressure in the crank case. I've done it twice and each time elements of what you described is what happened; a hose blown off the valve cover from over boosting followed by continuous puffing of smoky air and oil mist from the valve cover to a severe case of oil being blown out of the valve cover, all detonation related.

In the first instance I had to disconnect the PCV system because the engine sucked the air and oil mix into the intake and caused a lot of smoke to stream from the exhaust. It also increased the static compression because it still ran fine and was driven for a while before I tore it down to find a thick layer of carbon covering the combustion chamber.

Hope I'm wrong but I doubt it given what you've described, blow-by is one of the things I monitor to determine how well the engine is holding up to boost. Some blow-by is to be expected but that's to dramatic a display.

[ssdurango]

Thanks guys, I try a compression test, and go from there.

[ssdurango]

#6 Piston had 90 psi cranking pressure, the rest had 190. I put some oil in the cylinder and the pressure went up so I pulled the engine, and stripped everything down.....#6 piston has cracks in both ring lands. Leaving in those speedpro 3.4 DOHC piston when I supercharged it didn't pay off. Obviously, 10.9 compression, 11 psi boost and full timing must of been just too much for the lands to take. .......sooooo my question for you boost guys is, should I drop the compression to stock or do I really need forged pistons at this point. Do GTPs, WRXs, ect have forged pistons?

[ForcedFirebird]

Are the pistons hypereutectic, or cast? Cast pistons are actually stronger when detonating from boost

[ssdurango]

They are hyer-eutectic which is a cast piston. How are they stronger? What pistons do you use?

I don't think they cracked from detonation. Propane is rated at about 109 octane and has a ignition temp of about 800 degrees F. (Gas is about 350F). It wasn't pinging.

When I ran this engine a few years ago on mid grade gas, it would ping. Maybe the cracks started years ago, now that it's making some good power, it finally gave out.

[Monzsta]

Detonation and pinging are two different things. You also need to consider that just because you're not detonating, you still can reach peak cylinder pressure before TDC, which is bad. Your ignition timing should follow a gas engine's untill your fueling is right. Then timing can be advanced in increments until no further power is seen, then backed off 1-2 degrees. Your max timing shouldn't have been more than 11 deg BTDC under boost.

[ssdurango]

Point taken. I was running alot of timing. I though I would be safe because propane is a slower burn and denser charges take longer to burn too. I'm running a malory unilite distributer so I'll have to block out the centrifugal advance mechanism. Here's a pic of the damage, BOTH lands busted off...