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Old 03-04-2011, 10:35 AM
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Crankcase Ventilation for Blown Engine

I have a 350 chevy with the smaller 142 Weiand blower. The engine has been rebuilt with forged pistons, crank, etc. appropriate for a blown engine. Before I rebuilt the engine, I used to go through rear main seals more often than usual and told that the blower may be creating too much crankcase pressure. The rebuilt engine will be show quality, with many billet parts. My question is what is the best and most attractive way of relieving this pressure? Will adding additional valve cover breathers do the job, or should I consider something else? Any advice would be appreciated.

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Old 03-04-2011, 01:53 PM
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I used a filtered breather in one valve cover and a single PCV valve in the other valve cover routed to the back of the carburetor. If your rings aren't worn out you shouldn't have any issues at all.
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Old 03-04-2011, 07:53 PM
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I used two filtered breathers, one on each VC, it wasn't enough, even on a fresh motor, I finally went to a PCV system, on an 8-71 383
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Old 03-04-2011, 08:06 PM
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Kind of odd that this post was came up. I spent about 2 hrs this morning researching this subject before I saw this post. Still as undecided as I was when I started. I finally went to Harbor Freight, Lowes and Home Deport looking for a water separator to make a catch can. No luck.

68NovaSS-Are you runing a standard PCV system? Any extra add on's?

There is a lot of info on the net about this subject, with many different opinions and advice, but no definite solution.
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Old 03-04-2011, 08:40 PM
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You got a leaky spark tube...
 
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For some reason the forum isn't letting me edit my original post to this thread.

I noticed I forgot to mention my engine was also a 350 with a 142.

trifive: I'm just curious but did you build your engine or did you get it from somebody else? I sold my K5 Blazer with that supercharger to a guy in the Navy that moved to San Diego a few years ago. I know it's a long shot but I have to ask.

Last edited by Blazin72; 03-04-2011 at 08:45 PM.
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Old 03-04-2011, 08:56 PM
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Quote:
Originally Posted by Blazin72
I used a filtered breather in one valve cover and a single PCV valve in the other valve cover routed to the back of the carburetor. If your rings aren't worn out you shouldn't have any issues at all.
Blazin72-Standard PCV? I have read that the crank case can still get pressurized, I don't see how as the carb will always see vacuum. Some talk about running a supra PCV or a Mazda turbo PCV.

I have ran two breathers with no issues, but noticed that my oil gets black real quick.

Not trying to hi-jack this thread I just have the same dilema.
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Old 03-04-2011, 10:06 PM
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Quote:
Originally Posted by NOlowrider
Blazin72-Standard PCV? I have read that the crank case can still get pressurized, I don't see how as the carb will always see vacuum. Some talk about running a supra PCV or a Mazda turbo PCV.

I have ran two breathers with no issues, but noticed that my oil gets black real quick.

Not trying to hi-jack this thread I just have the same dilema.
On a roots blower, the carb on top will always see vacuum around -12. It never goes to zero. On the manifold (under the blower), it will go to 0 and then into boost. Watch where you hook your various vacuum hoses! Power brakes never worked so well off the carburetor, but if you run the vacuum line to your th350 from the carb, you will be in for an unpleasant experience.

Last edited by Leoman; 03-04-2011 at 10:12 PM.
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Old 03-04-2011, 10:18 PM
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You got a leaky spark tube...
 
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Quote:
Originally Posted by NOlowrider
Blazin72-Standard PCV? I have read that the crank case can still get pressurized, I don't see how as the carb will always see vacuum.
I just walked up to the rack in Auto Zone and grabbed the one that looked like it would fit my application best and stuck it in. Nothing fancy about it at all. Worked great once I installed baffles in the valve covers. Before the baffles I was losing a LOT of oil through the PCV.
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Old 03-05-2011, 06:12 AM
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Believe it or not, I have my PCV hooked to the rear carburetor, on my blown Hemi, (engine in my Avatar) with no breather(s), and it works great. Doesn't suck oil, oil is clean, and I have no oil leaks. I figure that without breathers, the system is only pulling crankcase fumes when ever it needs to, like WOT, idleing, etc. There really isn't any overpowering vacuum while the engine is running, so sucking gaskets in hasn't been a problem...or hasn't appeared to be. Installing the PCV this way has taken care of any little seeps I ever had, and now the engine stays clean. Time will tell, but so far, it works great...
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Old 03-05-2011, 12:53 PM
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Get a crankcase evac kit...breather to header setup...it works like a charm. and you gain about 10 hp across the whole power curve. And yer not recirculatin garbage thru yer engine like a PCV system.
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Old 03-05-2011, 09:40 PM
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Thanks for all the advice. I will look into the crankcase evac system. Does this require drilling holes in both headers? Are the holes drilled below the collector?
P.S. to Blazing '72--no I rebuilt the engine--did not buy it already rebuilt.
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Old 03-05-2011, 09:56 PM
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You got a leaky spark tube...
 
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trifive: I replied to your PM before I read your response in this thread so disregard any redundant questions.
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Old 03-05-2011, 10:09 PM
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I have ran two breathers with no issues, but noticed that my oil gets black real quick.

Not trying to hi-jack this thread I just have the same dilema.[/QUOTE]
With two breathers and no PCV valve, all of the carbon from the blowby gasses stay in the crankcase and turn your oil black and is corosive also . Not a good idea. Install a PCV system to make your engine last longer and run cleaner .
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Old 03-05-2011, 10:13 PM
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Quote:
Originally Posted by NOlowrider
Blazin72-Standard PCV? I have read that the crank case can still get pressurized, I don't see how as the carb will always see vacuum. Not trying to hi-jack this thread I just have the same dilema.
The crankcase can still get pressurized at wide open throttle because combustion pressures are higher then.
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Old 03-06-2011, 12:27 PM
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This is the kit. .http://www.summitracing.com/search/?...se%20kit&dds=1.

It is pretty easy to do. You can drill the holes or use a plasma cutter. A good muffler shop could do it for around 30-40 bucks. There is no threat of it pulling oil out of the engine...probably pulls no more than 3-4 lbs of vac.
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