Holley ultra double pumper???'s

[Rt Jam]

If you don't go deep in the throttle, there is absolutely no difference in mileage. If you are concerned with mileage and it's your daily, then you will not be going WOT everywhere.

As for vacuum vs mechanical. You can get a vacuum close but from idle to wide open, the response will go to a mechanical. This will be seen in the 60' and especially manuals that are closing the throttle 3 or more times in a run.

What heads and camshaft in your 406?

 

[Ernest]

When it comes to that 406, just to play it safe, you're better off with a 750VS, especially if its mainly for the street when it comes to drivability concerns and you want/expect your gas to last a little longer, think about this, GM was putting holley 750VS on their 396BBC back in the late 60s, actually 780cfm, basically same carb.

The engine is only going to use what it needs when it comes to the vacuum secondary carb, so it makes absolutely no sence to use a double pumper, some people just cant keep their foot out of the secondaries, and thats where you run into running rich when the engine cant use the extra fuel being dumped in and therefore, gas milage is much worse.

Running that 670 may get you by, but it will severely, somewhat, limit your rpm, so theres a lot more to it when it comes to carburetor selection for any given engine...cid size, camshaft specs, compression, intake, exhause, ignition and more importantly, purpose, street or strip use, figure one or the other, or both, then theres tuning said carburetor to make the engine either roar like a lion or run like a sick dog begging to be put out of his misery, or yours due to frustration.

The concensis is... VS for an auto trans, heavy car on the street along with decent gas mileage, and DP for stick shift and light cars for race when you give zero fracks about the gas being blown out the tail pipe.

 

[dickeymoore]

Promaxx heads Comp cams hyd roller cam 510/520' lift,700r4 transmission 373 gears