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Question--2 Superchargers 1 motor
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clay
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Joined: 24 Nov 2002
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Location: South Carolina
318129.23 points


1972 Chevrolet Nova

PostPosted: Thu Oct 02, 2008 12:17 am    Post subject: Reply with quote

Actually I'll have to admit I'm just starting trouble - in a fun way though. The problem I'm describing is usually one of two things, #1 being the boost reference is being taken from the wrong place - being sourced from below the throttle plates instead of from the bonnet / enclosure. Supercharger is at a high rpm and has the bowls pressurized but the throttle plates are still closed enough that a vacuum still exists in the intake. Fuel pressure doesn't rise and the bowls run dry. Mine didn't have the lean pop when this happened, it just sort of faded out. #2 is power valve operation. Same problem with the bowls being pressurized on one side of the power valve but there still being vacuum keeping it closed. The vacuum level is below the opening point of the valve, however the pressure on the bowl side of the diaphram keeps it closed. Causes a lean condition and did cause an occasional lean pop in mine and Tiremines. We came up with a different method of power valve actuation that absolutely solves this. I still do have an occasional lean stumble under a very specific situation. If I let it coast down, then squeeze the throttle, it sort of hesitates a little - but not everytime. It never does it at the track, from a stop in the highway, just in this one situation. I have tried several cams on the 30cc pumps, now I have 50cc primary and secondary and I have tried squirter sizes all the way to something like a 0.050 or so. It has one other quirk that I know the cause of (difficult hot start), I just haven't devoted the time to actually fix yet. Clay
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big_G
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Location: Hutto, Tx.
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1968 Chevrolet Corvette

PostPosted: Thu Oct 02, 2008 10:29 am    Post subject: Reply with quote

Interesting...I show no boost until about 2,500, so I don't think the bowls are running dry. My theory is that when the throttle plates are not open very far, and no boost exists, there is turbulence in the bonnet/carb. above the throttle plates. The surge valve can't eliminate all pressure, just some. Feel how much air moves out of the s/c at the carb. bonnet at idle. This turbulence may be strong enough to disrupt the accelerator pump shot. I went with the extended nozzle tipped squirter, maybe it will help.
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d2180sx
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PostPosted: Thu Oct 02, 2008 8:30 pm    Post subject: Reply with quote

Clay, have you thought of running an electric fuel pump with a boost referenced fuel pressure regulator (one rated to handle higher flow) and set it to maintain at a minimum of 4 psi above the desired boost pressure level? This would most likely solve problem number one.
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87IrocTim
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PostPosted: Thu Oct 02, 2008 10:01 pm    Post subject: Reply with quote

if i am repeating what someone else said sorry, i got to lazy and didnt want to read every one lol.

my opinion is there is absolutely no need for two superchargers. one right sized vortech will be more then capable of what you want out of the car. if you are doing it to be quick, but more for appearence, then you would have to get two tiny vortechs, and no, they would not run efficiently at all. it would not be much different then one supercharger pushing an equal total boost of the two, except now the superchargers are going to create more backpressure on themselves, basically fight each other without the motor properly consuming it, causing more strain on the motor to turn them over. two 5psi vortechs will not make as much as one 10psi.

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clay
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1972 Chevrolet Nova

PostPosted: Sat Oct 04, 2008 9:55 pm    Post subject: Reply with quote

big_G - where is your blow off located? Sometimes if they are located to close to the carb they can cause strange things to happen with turbulence like you mention. I had a scary incident with a homemade bonnet. I had the inlet coming in the front of the bonnet and the blow off in the rear - out of the way. What happened when I started up is there was so much airflow across the carburetor it actually pulled fuel out of the bowls through the vent tubes and blew a gas fog out of the blow off. Scared to actually shut it off when I figured out what it was - didn't want a spark.
d2180sx - I actually have a blow though setup that works really well, I just like to stir the pot about them not working sometimes. Sorry. Clay

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big_G
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1968 Chevrolet Corvette

PostPosted: Sat Oct 04, 2008 11:09 pm    Post subject: Reply with quote

The surge valve is about 10 inches upstream of the carb. No air should be blowing across the bowl, but never can be sure. Wish I could find a good used EFI for this beast.
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d2180sx
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PostPosted: Sat Oct 11, 2008 9:17 am    Post subject: Reply with quote

Very Happy I figured! Yeah, that BOV venting atomized air/fuel is no joke. Could have been fried, dyed and laid to the side with that Very Happy
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240Z8
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1972 Datsun 240Z

PostPosted: Mon Mar 02, 2009 5:58 pm    Post subject: Reply with quote

clay wrote:
Actually I'll have to admit I'm just starting trouble - in a fun way though. The problem I'm describing is usually one of two things, #1 being the boost reference is being taken from the wrong place - being sourced from below the throttle plates instead of from the bonnet / enclosure. ....Clay


I know off topic, but running with some previous statments..
Clay, I know you have plenty of experience running these, your thoughts on my old system. I always ran my (mallory) regulator reference, and blow off from the manifold. When the butterflies shut, the regulator would instantly see no boost, shut the pressure down on the pump. Seems residual pressure in the bonnet would cause a lag in the regulation of the fuel pump pressure, even with a blow off valve. Maybe even to the point of blowing fuel out the carb? I also used a one way valve from a power brake booster, reversed, in line with the regulator to the manifold reference line. That way the regulator would never see any vacuum, just boost, and thus never let the pump run below the preset 7 psi value. Its been years since I've run my set up, but it worked really well.
I'm toying with the idea again, so maybe a few more questions coming.

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clay
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1972 Chevrolet Nova

PostPosted: Tue Mar 03, 2009 5:12 pm    Post subject: Reply with quote

I actually started with my reference from the manifold. I ran it that way for a while until I started getting other problems worked out and started driving it more. If I would run it through the gears and just use about 1/4 throttle, which wasn't enough to shut the blow off valve but still enough to be fun, it would run out of fuel in the bowls and just fade out and shut off. Problem was the carb is under whatever pressure is in the bonnet which is where the reference needs to come from to keep fuel flowing. However if you have a decently sized carburetor that doesn't allow much pressure drop across it and use the setup in mostly a drag race style, referencing from the intake will work - I only found problems with part throttle operation that I mentioned earlier. I did put a check valve in my PCV system similar to you as I had problems in the beginning with excessive crankcase pressure. Clay
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240Z8
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1972 Datsun 240Z

PostPosted: Tue Mar 03, 2009 5:35 pm    Post subject: Reply with quote

I suppose different combos will have their unique set of issues. Part of hot rodding fun.
My check valve was on my regulator reference line. My Mallory regulator responded to vacuum as well as boost. Took me a while to realize my pressure was dropping to about zip. Using the valve never let the regulator see vacuum, only 0- positive pressure. I think I started getting my ref. from the mainifold because I didn't initially have the relief valve. In my situation it worked well enough to leave for 4-5 years of street fun. Boost mongering 16+psi eventually killed my fun. Laughing

I'm sure I'll have a new can of worms with my V8.

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clay
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Location: South Carolina
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1972 Chevrolet Nova

PostPosted: Tue Mar 03, 2009 9:17 pm    Post subject: Reply with quote

Vacuum on my bypass regulator would drop fuel pressure some too. Not to the extent you were seeing though. Mine would drop from around 7 p.s.i. to the 3 - 4 p.s.i. range as well as I can remember. The high vacuum situations usually mean light load so that shouldn't be a problem as long as there is some pressure. Regardless you came up with a solution to the same problem - different than most, but that doesn't really matter if it worked. I like different - it spawns other new and interesting ways to do stuff. Clay
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