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Oil Blow-by a Big Issue Under Boost?

Joined
15 November 2001
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High in the Rockies
I can't find much of anything on the forums on this topic...I've heard of a number of people who have had this problem and as I recall, it's more prevalent on later model NSXs, but ICBW. I've never experienced it on my '92. Since I'm getting close to installing a SC, how concerned should I be about it and is it recommended to install a catch tank?

I should mention that I may well be going to a high boost arrangement like Marc Weinberg's with all the appropriate short block and head mods so the engine can handle it reasonably.

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You should probably be more specific about your definition of blow-by because it is not always the same.

For me it is when the crankcase is pressurized by leakage past the piston rings. If you do a leak-down test on your engine and all is well, (very low % loss) then boost should not be a problem either. On the other hand, if you have significant leakage then the pressurization of the crankcase will be that much higher under boost and start pushing the oil out. But of course, that's just a symptom of a problem that needs correcting anyway.

FYI, running too rich even under just certain RPM and conditions is a top reason for boosted (or any) cars to develop blow-by in a hurry.
 
Originally posted by sjs:
if you have significant leakage then the pressurization of the crankcase will be that much higher under boost and start pushing the oil out.

Where does it go?

Originally posted by sjs:
FYI, running too rich even under just certain RPM and conditions is a top reason for boosted (or any) cars to develop blow-by in a hurry.

Why? Does the extra fuel create more combustion pressure?

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[This message has been edited by Track Junkie (edited 06 June 2002).]
 
Originally posted by Track Junkie:
Where does it go?

Back into your intake (between the filter element and the throttle body) to be burned off by the engine.

I have a 1991 model, so I'll use that as an example. Notice that there is a hose coming off the throttle body going up to the positive crankcase ventilation (PCV) valve that sits in the top of the crankcase on "side" of the engine closest to the passenger compartment? That let's air "in".

Now look at the other "side" of the engine... nearest the trunk. On the edge that faces the intake and throttle body, there will be a hose coming out of the crankcase which turns into a hardline metal tube that connects to the intake between the airbox/filter element and the throttle body. This is where the "blow by" oil goes... to be returned into the throttle body and burned off by the engine.

A catch tank just connects to this point and collects all the oil instead of letting it go back into the engine. Useful really only to know how much oil is going through this system and getting burned off.

EDR
 
Actually, I believe the bigggest problem is unwanted detonation. For a while you can cover the increased temps associated with boosting a hi-comp engine by adding extra fuel. To do it well, RPM and boost need to be mapped. Also to a smaller degree a higher octance will help. If you are after more power and want to maintain driveability you can then cool the charge in some designs with an intercooler. Next is to lower the compression ratio below stock like Marc did. Then its all a matter of how much power vs how reliable you'd like to be. I'm aiming for 400 RWHP for the street with a bit more tunable for my track buddies :), hint its in the cooling Luke, may the Boost be with you.
 
SJS, don't you mean running lean? I think running rich will cause a slower death of the engine from cylinder wash resulting in scored cylinder walls.

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Originally posted by ScienceofSpeed:
SJS, don't you mean running lean? I think running rich will cause a slower death of the engine from cylinder wash resulting in scored cylinder walls.

That's exactly what I meant. Perhaps I was not clear in my haste. I was saying that running rich is a common reason for developing blow-by (due to cylinder wash).

"FYI, running too rich even under just certain RPM and conditions is a top reason for boosted (or any) cars to develop blow-by in a hurry." Still sounds right...

Anyway, apparently FactorX and Mark B have seen the oil go find another way out when not vented. Too bad, but all part of the game.

But it is not necessarily a slow death if you run way rich while tinkering with aux injectors under boost.


As for causing unwanted detonation, only if you leave it vented back to the intake, and even then it takes a lot, which again may mean you have other problems.
 
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