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Old 02-02-2008, 11:36 AM   #8
greg64
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Join Date: Dec 2007
Location: Kimberley, BC, Canada
Posts: 799
Re: Tricked-out Chevy six cylinder engines

Hey slow66,

What power brake booster are you using? Was it a bolt-up? I certainly recognize the master cylinder.

For any others that are interested in hearing why MPI doesn't work on a Chevy 6, follow along. These 6 cylinders all have an intake port design called "siamese", where cyls 1&2 share a runner, as do 3&4 and 5&6. This goes right into the head, so the two intake valves are effectively side by side, drawing on the same runner. So what's the big deal? The valves are open an equal amount of time, so no problem, right? Well, no, because of the firing order which is 1-5-3-6-2-4. Obviously, any fuel injected (even at the port) can only really enter the cylinder when the valve is open. And any fuel injection system will have the injectors open up to 80% of the available time (when wide open). But, the valves are really only about 25% of the time, so the fuel just puddles on the backside of the valve. In SB chevys, this is no problem, because when the valve does eventually open, the fuel enters the cylinder, just as intended.

Okay, lets look at that firing order again (1-5-3-6-2-4). Notice how the space (and thus the time) is not equal between 1 and 2 (and also 5 and 6)? 1 fires, then 3 other cylinders fire, then 2 fires, then 1 other cylinder fires. What this means in terms of fuel is that #2 gets 2/3 of the fuel, and #1 gets 1/3 of the fuel. Not exactly an ideal mixture.

This doesn't happen with a carb, because all air contains fuel, so the cylinder breathes when it wants, and gets the same air/fuel mixture.

In my case, I was stumped by why #1 and #6 were always lean, and #2 and #5 were always rich (leading to bad stumbling at times).

Should have thought about this earlier, but then one day I discovered this link: http://www.starchak.ca/efi/siamese.htm
which explains the whole thing with far more mathematics than I want to get into here. They even propose a few solutions, but I would chalk them up as impractical.

Anyway, fed up with the MPI (and not yet knowing the explanation of why it didn't work), I switched the throttle body over to a GM TBI (real cheap and reliable, these things). Well, that cleaned the problem up right away, and had some side benefits: easier installation, less wiring ratsnest, cheaper, looks more like stock (if that matters to you).

I'm kind of thinking (but never tried) that a dual TBI on a dual 4bbl manifold would be the ultimate because it would cut down the runner length for the fuel and it would make the manifold more symmetrical for more even air/fuel distribution.
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64 GMC Suburban - 283, NV3500, 14 bolt
77 C10 swb - 292, SM465, 12 bolt
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