This is more of a question for sofa, really.

He had, or still owns a 3:73 T56 400sbc with a Comp 288 hyd roller AND a #6211 800 CFM Holley mech sec spread bore carb.



Just wanted to know, from an idle 1st gear stomp, did Sofa’s own Holley tuning procedure sticky help prevent a bog when all 800 CFM flopped open?



Never have heard any specifics on just how well that combination ran.



(I have a 400 sbc mild cam waiting in the garage for my T5 ‘88 IROC with either a 6210 650 sprd bore or a 6211 800 sprd bore.)

A major sticking point on my end might be

the 3:08 rear end.



I brought up this question because I rode in a friend’s ‘86 m, with a Blueprint Engines 355 with a 700r4 , 3:42, 650 DP Holley, on a dual plane & long tubes that’s not quite tuned & won’t take more than 5/8 throttle yet.

 

I never really drove it like that, because (like any other setup) it wasn't the fastest way.

But no, it didn't "bog". It got enough fuel to match the air flowing through it, at least, above 2000 RPM or so. That's One Of Those Things about carburetors in general... it takes a certain minimum amount of air to lift the fuel up from the bowl to the venturis, which means, no matter what, the RPMs have to reach some minimum threshold before the main system begins to feed. Not like EFI nowadays, where there's no such "threshold".

It ran like an involuntarily fornicated ape. On the dyno, I pulled in right behind a 454 with a Comp 308 Magnum cam and lots of other goodies, and my car did 373 ft-lbs to the wheels, where the big block did 33x. Some jaws dropped. (including the BBC owner's) It was a muggy overcast hot day in the deep South.

You're right, the 3.08 rear will hurt, because it will prevent the engine from reaching that critical RPM before the pump shot is consumed, if you just stomp on the throttle at a dead stop like it's a cockroach skittering away.

 

Sofa,
Thank you for the reply. Without too much smoke blowing up your etc, I really appreciate your expertise here on this question.
I’d buy you your favorite adult beverage if I could! Kind of beneficial for a lot of people here that a person of your scientific & technical background still enjoys 3rd gens at our age.
I’ll have to see about getting a 3:73.

 

Nice reply. A lot of people have a hard time dealing with Sofa's answers because they really don't want to hear the correct answer in the 1st place. They are looking for an easy fix (work-around) to their problem. Sofa gives the info straight-up right-in-your-face and many can't handle that. But make no mistake, the info is correct and straight to the point, with no extra BS.
I, for one, appreciate his answers.

 

As Sofa said, there just has to be enough pump shot to cover the time it takes to get the RPM up high enough for the main circuit to take over. This is an area that give DP carbs a bad rep. That and not enough stall converter for the cam, or said another way, too much cam for the stall converter.

This is as good a link as you can find on "tuning" the pump shot (includes the chart below).
https://www.crankshaftcoalition.com/...celerator_pump


Most DP applications can be dialed with the two stock 30cc pumps. For those that can't (and for aggressive applications for the vacuum secondary carbs), there are 50cc accelerator pump conversion kits. On a semi-related note (unrelated to this post), the annular discharge DPs are a lot easier to tune when the camshaft and stall speeds aren't properly matched.

 

Thank you motorq, I like to pass along the hard-learned lessons I suffered through in some rather financially adverse pathways. I am, unfortunately, rather ... experienced.

What is it They say: my good judgment is the result of my experience, but my experience is the result of all my bad judgment...

That Holley chart will help you understand what you need to do to your pump shot to tune it into the requirements of your car. Ideally though, you (a) tune the carb to where you don't need too much of that, and (b) learn to drive the car to where you know how far you can go with the pump shot you've got, without exposing that transition interval that creates the "bog". Just like pro racing, the tuner is either constrained, or liberated, by the skill and sensitivity of the driver.

 

Thank you Sofa & BadSS.
Wish I’d seen that chart in the mid 70’s.
I will be following Sofa’s Holley tuning sticky, of course INCLUDING tuning the pump shot.

Then,
(a) tune the carb to where you don't need too much of that, and (b) REMEMBER TO learn to drive the car to where you know how far you can go with the pump shot you've got, without exposing that transition interval that creates the "bog". Just like pro racing, the tuner is either constrained, or liberated, by the skill and sensitivity of the driver.
You gentleman are truly a huge help here.