6/carbs: ? about tall 2ndary venturi's, rich, I miss you. Options

[brant]

carbs: ? about tall 2ndary venturi's
Hi all... thanks for the help in advance.

I want to try a set of the tall 2ndary's on my IDS-3C carbs.
I'm running the stock ones and want to try the tall ones, that I've heard called "906-style"

its a race car and withoug re-hashing all of my jetting and dyno history, my questions are:

1) pierce manifold says they have 2 different types in stock a 3.5 size and a 4.5 size. I believe this measurement is in reference to bore and not height. Pierce said that both sizes are the "tall" type. Which do I want? Motor is a 1967 2.0/S motor at altitude.
(the pierce catalogue references that the IDS carbs came with 4.5 short, but I'd like confirmation before I order)

2) Does anyone know if these tall secondaries interchange on the IDA carb versus the IDS carb. I would assume so, but I once bought some F33 emulsion tubes from Pierce that do fit the IDA, and found that they would not fit the IDS. I didn't know there was that much difference between the two carbs, which surprised me. But again, before I order do people know if the 906-type tall 2ndaries will fit an IDS-3C body?

again, thanks in advance everyone!
brant

[Thorshammer]

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Reversion takes place normally at a given rpm and is "fuel stand off" meaning you can actually see a small cloud of fuel above the velocity stack. This is caused by many things, but the actual process is quite simple.
Due to one or a combination of things, a pulse is sent up the intake tract from the combustion chamber and actually makes a portion of the fuel reverse and it is contained (hopefully) by the velocity stack. Jake said he uses the shortest stack so that it just contains the reversion. This is right. All engines (non direct injection)even fuel injected engines can have reversion, although it is more rare for a fuel injected engine to have reversion. The key here is the pulse being tuned to draw the combustion chamber down at the right point, and at a given rpm, this is usually meant to enhance power at peak torque, although some engine builders do things differently. Below this rpm if the exhuast pulse is returned earlier (there is a delta for temp) then reversion can happen, from the returned exhaust pulse going up the inlet tract during valve overlap. Equally so, if the inlet valve opening is very early, then the cylinder pressure can actually push the mixture back out as well. This is why some "s" cams can have a mid range issue, because the inlet valve opening IS very early and there can be a pressure wave up the intake towards the carbs. I like to run that cam in a slightly advanced state to reduce this. But everyone reading this needs to understand that since these factory cams were produced we have learned so much more about pressure waves and valve action, its not even funny. What was not possible due to metalugy years ago, is now possible, cam lift rates that could not have been utilized, are now being so. This is very important when building a "cookbook" engine. And this is why People like Jake Raby get the money to build type four engines, No cook book shit here, just hard work and proper testing to find the RIGHT combination.

The best way to control reversion and to reduce "flat spots" is to build engines that are true packages, If you have 906 cams and are running headers that are 40 inches long, your wasting your time, probably better off putting in some E cams if you only have 40 inch headers, so remember it's a package. Brant is on the right track. BTW, the 3.5 or 4.5 refers to inches, not id or od. you want 4.5. This will increase the signal strength and make the carbs easier to tune. Trust me. 4.5. And remember these are very crude castings and must be massaged to make them work well, At least that is my opinion.


Erik Madsen