Stock oil bath limitations

Bret Young · Posted: Mon Feb 27, 2012 10:30 am Post subject: Stock oil bath limitations

Okay guys. Have a question for you guys. I know people are always saying that the stock carb is a bottleneck, or a serious limiting factor on an engine that is larger than stock. But this question does not pertain to this. My question is this:

Does the stock oil bath air filter have a limiting effect on the amount of air the engine gets? I know that the engine runs better with one due to the velocity stack that is built into one.

The reason I ask is I am building a torquey stroker engine that will be built for my split window bus, but will be running the mexican EFI from the Mexican bugs and controlled by Megasquirt. Even though it's a single throttle body, it's 43mm and has been proven to flow well on decent sized strokers under a certain RPM. Since it's going in a daily driver bus, high RPM's won't be seen hardly ever (except stock freeway RPM).

I'm just curious to see if I run a stock oil bath oil filter on top of the TB, will it choke the motor? If the TB will not be an air restriction maybe the air filter won't make a difference?

Please share your thoughts. Would be appreciated.

krusher · Posted: Mon Feb 27, 2012 10:48 am Post subject:

depends what oil bath you us as to how restrictive it is, why dont you just use what the FI bug had for a filter?
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torsionbar · Posted: Mon Feb 27, 2012 10:50 am Post subject:

you can easily test this yourself. once the motor is complete, do some back to back dyno runs and see if the power differs appreciably between the oil bath and whatever other filter you are contemplating. it sounds like your engine and requirements are fairly unique, so i doubt anyone would have numbers that would be useful to you. low rpm operating band is working in your favor if you intend to keep the oil bath.
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Eaallred · Posted: Mon Feb 27, 2012 11:16 am Post subject:

Since you are fuel injected instead of carbureted, you can get away with a restrictive air filter (if that happens), you will just have your target a/f map take care of that. As the rpms go up, manifold pressure goes down if there is a restriction. It will just act like you are giving it less throttle.

You could also datalog between different air filter housings with WOT pulls to your desired redline. If MAP goes down from one air filter vs. two with increased rpm, you will know it is restricing power, and you'll know where it is doing it at so you can decide if it is a big deal to you or not.
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[email protected] · Posted: Mon Feb 27, 2012 11:24 am Post subject:

get a vacuum gauge and measure various points in the system, look for places with a large pressure drop, and address that area.
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Bret Young · Posted: Mon Feb 27, 2012 12:06 pm Post subject:

borninabus · Posted: Mon Feb 27, 2012 12:30 pm Post subject:

you may find this thread intersting:
http://www.thesamba.com/vw/forum/viewtopic.php?t=424278&highlight=flow
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Bret Young · Posted: Mon Feb 27, 2012 9:16 pm Post subject:

Thats not a terrible thread you got there. I don't know if that filter would filter the air as well as the stock oil bath because it sort of eliminates the oil bath part correct?

But I have considered possibly running a bay window side-mounted oil bath in the split with a removable pedestal to increase the amount of usable air. If I went with that setup, the above mentioned modification would be a possibility.

craigman · Posted: Mon Feb 27, 2012 9:25 pm Post subject:

Yes, the stock oil bath is restrictive.
Just run one of those fancy chrome aircleaners. Could get real fancy with it and put a K&N filters in it for maximum flow and filteration.

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[email protected] · Posted: Mon Feb 27, 2012 11:05 pm Post subject:

I think putting the cone filter in the stock housing was a fantastic and innovative idea.
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skabonner · Posted: Tue Feb 28, 2012 1:09 am Post subject:

I'm actually wondering the same thing, so if you do do the test, please post the results. I'm gonna be building a mexican efi set up too, and recently noticed that a stock oil bath fits perfectly. Only difference in my motor is that I will probably be building an 1835. As far what the mexican efi ran stock: paper filter? No thanks...
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borninabus · Posted: Tue Feb 28, 2012 7:24 am Post subject:

[email protected] · Posted: Tue Feb 28, 2012 8:54 am Post subject:

that shows how much that stock oil-bath was choking the engine. Measuring the WOT vacuum between the carb and filter in both situations would be pretty dramatic IMO.

Furthermore, your #s are with the lesser airflow at 5k' elevation, the guys at sea level have it even worse!

borninabus · Posted: Wed Feb 29, 2012 8:36 am Post subject:

part of the mod in that thread is cutting the extra hole in the bottom of the intake snout...
which also helped quite a bit Wink

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raygreenwood · Posted: Wed Feb 29, 2012 6:07 pm Post subject:

I posted some numbers on the STF a few years back on teh difference between teh stock oil bath air cleaner and the pleated filter...both of which came on different years of the same engine for the 1.7L type 4.

The oil bath filter most probably filters better because of how it works....which is a combination of both oil wetted fibers to attract smaller particles....and the wicked high velocity 180* turn the air makes right above the oil pool which drops out heavier particles which cannot make the turn.

But....the oil bath cleaner was definately more restrictive. At idle and just above....there was no real difference...if memory serves...to about 2k rpm. But above that it was moderately restrictive.

Common logoc would say that its just a volume problem. The internal volume of the oil bath cleaner on a 412 is about 1/2 or less than the pleated filter.
The problem is that if the volume of the oil bath cleaner got too large...it may not have the velocity needed internally at high rpms at that 180* turn point to drop out he larger more detrimental particals.

I will have to look a around for my flow numbers. I used a digital anemometer to check flow velocity, volume and temperaure at the inlet at various rpms. Ray


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