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Old 08-06-2009, 11:04 PM
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Dual plane manifolds and spread carb. jetting?

Was reading up on this...

In Team Chevelle, a member ran his engine on the dyno while checking his individual cylinder a/f ratios. When it was said and done after many controlled pulled he ended with a spread difference between the front dvr. and pass of 6 sizes. On the rear dvr. and pass. he had a difference of 5 sizes. His jetting gave him consistant mid 12's on his a/f ratios.This was a conventional BBC with a dual plane intake and holley 850 model 4781.If my memory is correct he netted around 30 more hp. also.

Also found an article that explained that Chevrolet intentionally staggered their jetting on the factory holley 850 carb. The model number was a 4296. My understanding of this was they figured out the a/f ratio was lean on some cylinders and rich on others due to this dual plane intake.

As a starting point they suggest to add 2 jet sizes to the pass side front and driver side rear corners to even up the distribution.

Anyone ever bother doing this? I'm running a dual plane manifold myself on a mild (300-330hp) SBC with a 600 dp Holley.

Using a summit vortec intake...


Anyone run one of these? Have any information on spread carb jetting?

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Old 08-07-2009, 01:16 AM
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Every engine tends to be different, you're comparing BBC's to SBC's which don't seem to respond to stagger jetting quite as much, and you would need a dyno or several days of drag strip testing to ever find out if it will help or hurt the power curve of your particular engine. It's not a "just do it cause it ALWAYS works" type of thing. You'll never figure it out just on the street.
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Old 08-07-2009, 09:05 AM
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Thanks for the reply,I kinda figured. That's why I was asking if anyone else was running this manifold and had experience with spread jetting. Had it on dynos etc... Hoping someone already did the hard work. I know it won't be an exact science, just hoping for some free hp....
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Old 08-07-2009, 09:45 AM
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Holley Jetting

Hey Friends,
This is an interesting topic. I too have wondered about mixture distribution on different intakes. ForceFed86, you are correct in that the holley r4296 was intentionally stagger jetted. My info says the primary jets were 78 choke side & 82 throttle side, the secondaries were 82 choke side & 80 throttle side. I think, but I'm not sure, that this was due to mixture distribution issues with the factory rectangle port dual plane intake. I own a 502 big block Mercruiser(boat) engine. It has the big rectangle port heads, with a matching dual plane chevrolet intake #6269318. Interesting part is the special built holley carb for the engine which is a 715cfm(carb sized to exact volumetric requirements @5000 rpm max outdrive speed) is stagger jetted, but only on the secondaries, 80 choke side & 90 on the throttle side, the primary jets are matched w/73's. It is well known in some circles that Mercruiser built(some call them the black engines) engines are dyno engineered, then built with bullet proof blocks and internals, with precision tolerance. I'm guessing that the Merc engineers did some AFR readings or exhaust temperature readings in each cylinder when they decided on the jetting. So, it seems a given that this factory GM aluminum dual plane(rectangle port) had some mixture distribution issues. I don't know if the big block aftermarket dual planes corrected this or not. I also don't know if the same issue exists on small block dual plane intakes, but it's an interesting topic. I've been wondering if you could use one of the laser thermometers and check header tube temperature differences to determine if a given cylinder or more is needing additional fuel. What do you think?olnolan
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Old 08-07-2009, 10:30 AM
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Without cutting the threads off the plugs they all look pretty much the same. Maybe whom ever designed this manifold for the vortec heads did a little better on the distribution?

I don't see any obvious excessively rich or lean plugs. Runners on the intake look to be the same diameter, externally at least. S'pose I could throw in new plugs at the track and remove them after a pass.

I'll look over on the super chevy forums and see if anyone has done specific tests on this manifold. Keep hoping I can find someone that's done the hard work for me.

thanks all...
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Old 08-07-2009, 01:54 PM
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There has been various things used to help w/distribution besides staggered jetting- like turning the carb flange, plenum floor "turtles", various porting schemes, under carb spacers, etc.

But to gain an advantage, there needs to be a way to read the A/F ratios of the individual cylinders. This can be done by reading the plugs- IF the one reading the plugs has enough experience w/it, but most will use some sort of instrumentation, like an O2 meter, an exhaust gas analyzer, and/or exhaust gas temperature in order to tell what's going on in each cylinder.

I read of one guy who adapted a VOM (IIRC) to either an O2 or gas analyzer of some sort- I need to research this deal...
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