So I've given up on my mechanical secondary Holley 4150, decided it's a little too radical for a stock engine that's street driven. My question is, being at a high altitude (7,500 ft.) should I get a carb that flows less CFM, or should I stick with something like a 650 and jet it down a few sizes to compensate? I wanted to try a vacuum advance this time around, but want to stick with Holley. Any suggestions on which carb to get? I was looking at the Holley 0-80783C. Has anyone had any experience with this carb?

 

I haven't done any side-by-side testing, but I do know that my car picked up after going from 650 DP to 750 DP - but not as much as it picked up going from vacuum secondaries (I assume that's what you meant, not vacuum advance) to mechanical secondaries.

Carb CFM ratings are "standardized" at a certain pressure drop. Varying the atmospheric pressure from sea level doesn't have much effect on the carb's flow needs - the change in density affects the booster venturi signal and the amount of fuel needed for the mass of air actually getting to the engine, but not the flow area (which is what the CFM rating is really measuring) of the carb.

As to vacuum vs. mechanical secondaries, the "issue" with mechanical secondaries is proper sizing of the carb for the engine CID and minimum RPMs going to full throttle. Too big, and it'll bog on you when you nail it. Too small, and you'll give up max RPM power.

But, for the street, it doesn't matter whether it's vacuum or mechanical secondaries if you're not getting into the secondaries. Too big, and throttle response and fuel economy will suffer, and there isn't much you can do about that.

 

Thanks for the reply, it was exactly what I was looking for. Problem was, I know I had a DP 650 CFM mechanical secondary (and I did mean vacuum secondary, thanks for pointing that out) but was told by a Holley fanatic that it had some *work* done to it at some point to increase CFM. He believed it was flowing more than 650, which by itself is PLENTY for my cars needs, and it had a mean bog from low RPM launches. I just figured that when the mechanical secondaries were opening the carb was pulling too much air, and when I tried to tune it for more fuel to match the A/F ratio at WOT, the engine hated it because it was just flat out too much of both. The mismatched mechanical secondary explains all of my problems, the car ran GREAT when I unhooked the secondaries (minus the power of the 2 extra barrels.) I ordered a new Holley 0-80783C so I know exactly what I'm starting with. Turned out I actually found a 4150 style with vacuum secondaries, that has jets in it instead of metering plates (which I think is the standard for vacuum secondary carbs?) It surprised me, I thought all 4150's were mechanical secondary. Sorry for the novel of a post, I've just been fighting with this carb forever and I'm glad it wasn't me being an idiot when it came to tuning it.