One other thing I'm battling- I have always bought Vac. secondary carbs, but I thought mech. may be preferred when drag racing. Any thoughts here? I was going to go with a 750 Holley, just wonder if vac or mech. more appropriatte. car is heavy, 3800 lbs or so.

For drag racing as long as you leave with the secondary either fully open or fully closed a vacuum or mechanical secondary makes absolutely NO difference. If you use a vacuum carburetor and the secondary is partially open at the launch it's very difficult to be consistent.

I have a methanol 750 that's a vacuum secondary and it's been used by lots of my customers that want to try methanol or experiment with a vacuum secondary on their traction limited circle track car.

One customer who I can think of right offhand used my carburetor, liked the results and then had one done which was exactly the same except his core was a double pumper. His car ran identical ET and speeds when at the track trying both carburetors back to back which was a good fun test.

that the metering blocks were different. I was under the impression that the DP metering was more "performance oriented", whatever that means. Dave, I assume you are not talking about out-of-the-box VS vs. DP? If we are talking OOTB Holleys, and not custom carbs, I would choose a DP over a VS. Good luck, James P.

is it an auto? i too always used vacuum secondary carbs. and wondered what what it would be like if i switched to a mechanical carb

I would think the choice of carburator would better be addressed by where you want to drive the car. If it is more likely to be driven on the street and for better gas mileage I would stick with the vacuum secondary type. If you what the "seat of the pants" feel I would go Doublepump and thats not to say a vacuum sec. can't be reworked to that state of feel. If you like you could have two carb. for street and strip but I don't think I would like to be switching back and forth.

Thanks for the responses- I'm running a top loader 4-speed, and think I'll give the holley DP 750 Mech. secondary's a try. I do have a vac sec. 750 edelbrock; it could be interesting to swap between the two and see how they differ, or not...Thanks again for input!

It is true that out of the box Holley double pumper carburetors are calibrated differently than the vacuum secondary versions. The double pumpers do have a different metering block with a smaller power valve restriction and what this does is make you run a larger jet for the same full throttle power mixture which is the same with either carburetor. The vacuum secondary carburetor has larger power valve channel restrictions and this lets you use a smaller jet for the same full throttle power mixture but at part throttle the mixture is leaner than with the double pumper.

The reason for this is probably to make the double pumper carburetor have better drivability with an engine that has a performance camshaft. What I've found personally is that it tends to make the carburetor burn excessive fuel and run too rich at part throttle.

I think that the calibration of the idle circuit is close to identical on either carburetor.

In my case I was talking about some carburetors done by Bobby Oliver at Competition Carburetion. He does a really nice carburetor package in either a double pumper or a vacuum carburetor and he's taught me a few things about this stuff.

One trick on a Holley that I found works really well:

A customer had an 850 straight booster double pumper on a 454 Chevy in a jet boat. The engine makes 500 horsepower at 5000rpm and has a Comp 280 street roller cam(good combination). It was very rich in the transition circuit from off idle to nearly full throttle. Cutting down the idle feed restrictions in both blocks helped the fuel economy and overall drivability a TON! It was just way too rich in that area even with the wide open throttle mixture in good shape. Easy to fix and it saved a lot of fuel. I think he said that his cruise fuel consumption dropped by about 1/3! Not that jet boats are ever going to get great economy but why waste fuel?

wow, think of the damage all that fuel is doing to the rings.not good

I suspect the PVCRs on a dbl pumper can be unexpected in size depending on whether or not there is a secondary PV.


When the subject of tuning with a wideband is brought up, most folks seem to jump straight to the bleeds.....but I've often wondered what someone without a wideband could do with just removable PVCRs, jets, and a little bit of test driving.