Hello, I have studied this site and others, and have compiled an EVAC system design for a GM style 671 supercharger that is carbureted (no EFI/MAF). I do not want to vent to the exhaust or to the atmosphere. I have reviewed Moroso and Trick Flow’s sites, and reviewed other’s builds. I know that there are a number of us running this setup, and there seems to be no basic standard for a system that prevents internal sludge build-up, detonation, a mess in the engine compartment, under the car, or in the exhaust. Will this concept work? Please review and comment. Thanks! Jim
EVAC? Do you mean PCV? If so, you are over thinking this. If so, put in a regular PCV valve, and run it to either the big port on one of the carburetors, or to a port on the carburetor adapter, whichever you happen to have. Make sure that there is a baffle under the PCV valve. If there is not one, use a baffled grommet. Have a filtered vent somewhere in the system. Unless your rings are in terrible shape, you should not have so much blowby that you need catch cans
FTR: This is blower engine #71, and it is for my own build: (Yes, it is installed backwards) It will have a screw in PCV valve in the valley pan (Corvette, I think), and a pair of Offy breathers are headed for the valve covers. The hose from the PCV valve will go directly to a manifold vacuum port. I put a baffle under the PCV valve. I have never done anything more that this.
Thank you for the detailed response. I do understand how a simple PCV system works. This is a new build, so no bad piston rings (that I know of!). This is my first engine build specific to a GM style supercharger that will be used for street/strip. I used a local machine shop experienced with many blower builds. An EVAC (evacuation) system has been suggested. I have been following a supercharger forum here: https://www.superchargerforums.com/...ilation-how-to-breathe-when-youre-blown.2472/ The block is a bored and stroked 1966 283. The factory valley pan breather has been removed and the hole plugged. My understanding is that when under boost pressure, contaminated oil mist will be a problem. Your suggestion is simple, and works for you. I will take that into consideration! Much appreciated.
I ran a blower for years on a 454 that just had a simple PCV valve on one valve cover, and open breather on the other side. It worked great, lots of miles, lots of drag strip p***es, never spit oil out the open breather even when the rings got tired, before I overhauled it. But I put the blower on a fresh 427, same valve covers, and it would spit oil out the breather. I eventually put a normal drag racing type header evac system on that side of the engine, with the old AIR type check valve. The PCV remains on the other side. Interesting thing....this engine will draw a vacuum in the crankcase at low throttle opening, and it doesn't leak, and it uses very little oil, far less than any other big block I've built. But if you have some specific reason not to vent to the exhaust, I guess you'll need a complicated system like you designed. Unfortunately, my brain has deteriorated enough that I can't follow what it does, so I can't offer any useful critique.
Nicely drawn! Not sure I understand your whole system, so here are a couple questions that I have. If you have breathers open to the atmosphere in your catch cans, won't they provide essentially a "vacuum leak" at lower speeds, negating maintaining vacuum ? Why would there be pressure to push air/oil from one valve cover while the other valve cover has a vacuum? Would they not both be the same pressure or vacuum ? Where is the oil going that gets separated in the oil separator. When under boost, won't the air flowing out/backward on the fresh air side contain oil and contaminates? Maybe I'm looking at it wrong, and don't mean to be negative, but I thought I would ask.
I’ve had 3 blower motors (2 yblocks) and 5 factory blowers off topic. None had anything as complicated as yours, mine were all down as described above. I had 1 problem with a baffled breather grommet, but that was probably peculiar to the layout on the Yblock. I installed a valley pan baffle and problem solved.
Thanks for the reply, and the questions! As others have indicated the possible vacuum leak issue as you have, I will need to re-think this. Not sure where and how a vacuum leak can, or will occur. My thoughts were that the breathers on the catch cans are no different than the typical breathers on valve covers. No vacuum leaks on NA engines with valve cover breathers? Maybe I have the check valves in the wrong location. My design criteria is as follows (although it may be flawed) Under a low speed condition, crankcase vacuum would pull the PCV valve open and allow flow out of the PCV side, through catch can #1, filter through the air/oil separator, and back into the engine though a fitting in the carb adapter plate. Under vacuum, the fresh are side would pull fresh air in through the breather on catch can #2. Durning a boost condition, the PCV valve would be pushed shut under pressure, restricting flow out of the PVC side. Under boost on the fresh air side, flow would be pushed out, through catch can #2, filter through the air/oil separator, and back into the engine though a fitting in the carb adapter plate. No comments/advice taken as negative! I highly respect the the depth of knowledge and experience of HAMB members! I fully realize this appears to be complex (sorry, I am retired from an engineering career and in recovery!). As many of you have, I have a substantial investment in time, money and resources in my project, and I just want to build the best solution I can.
No part of the outlet side of the PCV system should be attached to the boost side of the system. It gets attached above the blower.
Everything under the carbs and above the rotors is vacuum all of the time. The only place you have boost is under the rotors. I think you are over doing this. I would connect a pcv to a port of the carb adapter plate and run it as any N.A. engine. I believe you will be better off with a pcv system.
What type of super charging set up you using 6:71 as , Positive roots or blow threw set up , with a positive roots just pcv in valve cover , or just breathers , or evacuation, covers to collectors . Or Vacuation pump, but this will pull constant vac in oil pan / valve cover ****ing oil out to catch can/puke tank, You should be Ok with just Pcv unless your doing something Extreme. Whats is you combo , ring type , Standard **** Gapless Gas ported What the cid Now & rpm's you plain on turning, How much boost, Quite a few of us here run superchargers Street & full race , My last full race was 50psi. The link you provided is really for blow threw / turbo , boost before throttle body , carb blades
Good, as I ***ure you my intent is to be helpful. I think my (mis)understanding is basically how you will have vacuum existing and at the same time have pressure existing within the engines valve covers. Another way of saying it: If you have pressure in the crankcase, why will it go out the pcv valve instead of simply traveling back thru side 2s open air filter ? I do like to see people try to think things thru to find something that suits their purpose rather than just go along with existing tech........but many times existing tech turns out to work just fine.
Thanks to all who have commented. For those of you who thought that I was overthinking this and made it more complicated than needed to be, you were right! I have had detailed conversations with Moroso, Trick Flow, and Motion Raceworks, all of which manufacture catch cans and air/oil separators. I now have enough information to re-design a simple, functional system. I will post when complete.
Ok, here is what the experts say will work for my setup: Use positive locking, baffled valve cover breathers on each valve cover (not push-in grommet type) Use dual inlet, baffled and filtered open element air/oil separator (could use 2 – 1 per side). Run #8 or #10 braided hose from AN fittings at breathers to AN fittings at catch can inlets. Functions properly under positive (boost) & negative (vacuum) pressures. No oil mist will occur at breathers or at catch can filter. PCV valve unnecessary. Check valves for directional flow unnecessary. Return line to crankcase unnecessary. - Filtered oil is contained in catch can (can be easily drained) - Filtered vapors vent to atmosphere Plug vacuum ports at carburetors. Boom!
Here's the pcv valve setup I previously used on my street driven 2-stage nitrous sbc... Worked great and didn't pull oil, but I further refined it to this... Even pulls crankcase vacuum, as inside the breathers there are check balls... The screw in the top of the pcv valve elbow allows adjustment of minimum flow volume of the valve without disabling the valve's reverse flow check-valve function. In the end I get crankcase vacuum down the highway, so no oil leaks or blowby smells in the p***enger compartment. If/when the nitrous pushes the engine into positive crankcase pressure, that pressure gets vented thru the breathers. This graph shows crankcase vacuum during a 6.25sec wot 3rd gear street pull... ...Red line- engine rpm ...Blue line- WOT switch ...Yellow line- crankcase vacuum Started out at 12.6"Hg in the crankcase, 6.25sec later it still had 3.9'Hg left. Grant
Still completely baffled as to why you are not considering just taking the simple route. If you think about it, you did get different advice from different sources. You must weight that advice against the knowledge that some of it was given to you by those who have an interest in selling you something.
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