vacuum secondary or douple pumper
 

Hey guys, I'm sure you've answered this 100 times. I know this isn't a hot rod question so I appoligize. I have a mildly built 454, I'd guess around 400hp. It is in a heavy 1 ton truck with a 5 speed manual. I use this truck to tow a good bit and just cruise around. I have a 750 holley vacuum secondary on it now. It does pretty good for cruising but when loaded I have to get the rpms up enough for the secondaries to open. I'd rather have a douple pumper so if I push the throttle enough I can open the secondaries at a lower rpm. I tried a really light spring in the secondaries but without the pump shot it makes it bog when they open. Every place I read says you don't want a double pumper for towing but no one says why. Would it be a bad idea to switch to a dp? Also would it kill my fuel mileage? I'm getting about 11mpg empty with the vs... pretty good i think for a 6600lb truck.

double pumper uses a lot of fuel,mostly because they are fun to yank open.
for towing you need a Q-jet or even better F.I.

I had tbi and it ran terrible. I had the chip made for the cam and heads. I switched to holley vs and it made it feel like a whole new engine. Why do you recommend the q jet? I'm sure the dp will use more fuel if i don't keep my foot out of it, but yea, that will be hard to do.

Q-jet has very small primaries,is very good at getting mileage and it flows 700 CFM or more.The bb q-jet needs a little tlc to work best. intake manifolds make a difference

vacuum secondary or douple pumper
 

Most likely if you switch to a double pumper while towing if you don't have the rpms up it will probably bog. Why don't you just try going up a couple sizes on the jets on your 750 carb. Go from 72 to 74 on the front and on the rear go from 76 to 78. Keep an eye on the plugs to see if running too rich.

It is a 4160, no secondary metering block.

DP's are better off with manual trans so for your application with 5 speed that is one good thing, personally for a rig like that I would want more control over the secondaries and the DP will do that, add to that the right gear selection.

MPG > guess you could sqeeze the same out the same milage as a vacuum secondary if you keep you foot out of it.

Going out and dropping >400 bucks for a new one may not be worth the $$$ though ??? your call.

vacuum secondary or double pumper
 

You can always convert the 4160 secondary to a 4150. Here is the kit and instructions. This way you will have a way to adjust the fuel. http://holley.com/data/Products/Tech...110%20scan.pdf

I am going to suggest vacuum is a better way to go. With vacuum secondaries, the engine opens the secondaries when the engine need the extra air. If you have a double pumper, the engine may not need the air, it is at too low RPM. Then air velocity falls too slow through both the primaries and the secondaries, and the carburetor loses its ability to accurately control the fuel air ratio.

The idea with a double pumper carb is the secondary accelerator pump squirts a shot of gas to cover up the opening throttle bog, and by the time that gas is burned, the engine has increased RPM to be able to use the air coming through all those wide open holes, and then can control the mixture.
With a heavy truck, towing something, the rpm will not have had enough time to increase pulling the load, and the engine may just briefly run off the accel pump shot, and then bog down, being too lean.

no way in hell would I put a DP carb on that. Nor would I bother wasting money on a quadrajet. Your current carb will work perfectly- if it is cleaned up and tuned properly.

BTW, why do you want your secondaries to open sooner? That would be the first question I would want to answer. Next, if you need to change your pump cam, do it. You can also change your squirters if that is the problem. And as suggested I would convert to 4150 immediately.

You have an untuned carb, and any untuned carb is NOT going to perform well. Changing to a different untuned carb still will not fix your problem.

If your looking to buy a new carb I would sugest a Eldebrock 750 for this application due to you mostly are using the truck for towing. my dad had a 750 vacume secondary holley on his 454 in a 78 dually and you could not keep power valves in it. He switched to the Eldelbrock and truck ran well. In my opinion a Holley carb is intended for performance applications. I think you will be pleased if you decide to go with the Eldebrock.

Bowtie, For your application you really need a 650 vacuum secondary. You need torque not RPM to hall all that heavy cargo. 750 is to big to do the job.

lg1969,I looked at the video of the 406.Sounds super mild?what kind of power numbers do you have on that engine?

a 750 with mechanical secondaries is too big, with vacuum secondaries it really isn't an issue as they only open with enough air flow, and not a second sooner no matter how hard you stomp on the throttle.

500 HP @ 6500 RPM
530 FT lbs @ 4500 RPM
Cam I'm using is Comp Cam 280 HFT
There is lope to it when it's idling.