I am running a stock 400 setup but thinking of making a few changes. Anyone running dual Q-Jets? If so, what intake manifold you using? How is the performance?
I don't know about the performance gains. The stock motor will only pull the fuel from the Q-jets it needs. You may have to jet down the primaries so it doesn't run rich at idle.
Other than using a bit more fuel than you would like and the WOW factor of 2 big carbs on top of the engine, I don't really see any need for it on a stock motor.
You'll probably have to use a side inlet Q-jet on the rear. Probably best to use a side inlet model for both. Says you can't use a big cap HEI dist.
Assume you'll have to rig your own linkage. Will probably need to be pretty accurate, for smooth street operation, since it appears as tho the front carb feeds the front 4 cyls, & the rear carb feeds the rear 4 cyls. But hey, Pontiacs worked with Tri-Power & 2 AFB's, so why not with 2 Q-jets.
Not sure if you could use a dual square bore carb intake, and use Q-jets on it, with adapters, or not. Probably could, but might take a little massaging. The reason I mention this is because I figure you're more likely to find a good used one, for a reasonable price. Assume there have been lots more of those sold, thru the years. Saw a '68 GTO with one of these, with AFB type carbs, a few years ago @ our local car show. Looked good.
There were also some reproductions of the Edelbrock P-65 intake. Have read on a forum that this one makes more power than the others. Don't have any idea which of these intakes you might be able to find.
Agreed, if you're going to run multiple carbs you need extensive tuning, a 400 only needs technically 650cfm, Quads ran 700-850 max claimed cfm, actual was probably less , you might be able to go with 2 400cfm's or similar but just putting larger carbs on may do nothing but puke your motor unless it's built for it, extensive head and intake work, chamber work, valves, ignition, etc all to flow match what you want to pull in, you're going to punch it and your motor is going to cough.
I've seen so many guys put on duals and a tunnel ram only to that set up is only good for 2500-7000rpm, rest of the time no hp no torque. Bigger carbs or multiple carbs delivers more air & fuel to the engine. All components must be properly matched in order for performance gains to be achieved. If the motor can't suck in the cfm delivered it will lose power, that was the main issue with the RAV, the heads and intake were so huge the air/fuel mixture didn't know which way to go, and zero Vac".
It's really just a way for some companies to sell you 2 carbs, now if you have a roots blower, maybe, still, extensive tuning.
