I searched previous threads, and couldn't find any talk about dual/tri intake manifolds. What are peoples opinions on them? I was looking at an Offenhauser dual quad for my stock 455. I know you set the carbs up so that you only engage the second one if you need it at WOT, but how do they generally rate on the Gas-O-Meter? Are there any hidden issues one would need to be aware of if purchasing one? Besides looking bad ***, would there be any real benefit to having this setup on a stock 455? Any info would be great!

 

I have a 425 with the Offy dual quad intake. The engine is built up, cammed, and I run a steep gear out back. I have twin 500 cfm carters on it. It look s cool but I really dont know if its any better than a single 4bbl carb. Hood clearance too would be tight on a non hood scouped car.....the carbs are linked together so they both open together at the same rate. It works for me but I dont think it would be the ticket on a daily driver. Back in the 60's Hot Rod or Car Craft Magazine did a dyno test with the 425 and bigger cams and the rare Eldelbrock dual quad intake. The numbers HP wise were not much or at all greater except at RPM's you probably wouldnt see on a street car. Its your choice but I would say to stick with a 4bbl.

 

If you have any issues with the Gas-O-Meter then put fuel injection on your engine, not carburetion.

 

A single 4bbl that's properly set up and matched to the engine will almost always provide more HP and less hassle than a multi-carb setup. It just isn't as cool looking.

The Offy dual quad manifolds are not the best designs for the street. They are flat, single plane intakes that have very poor flow characteristics. I've run the long out of production Edelbrock C396, which is a dual plane, dual quad intake for the BBO. I also used twin 500 CFM Carter AFBs (that's all that would fit on that intake) with a progressive linkage and in a 71 Cutlass with built 455 that setup would regularly get 15 MPG on the freeway. Unfortunately that car got away in the first divorce. I've been looking for another one of those intakes ever since.

 

Wow! I would never think that a dual quad intake could provide 15 mpg! What kind of transmission / gears did it have in it?

 

Yeah, I thought it should be higher also...

Seriously, the car was a 71 Cutlass with a built 455, dual pattern cam (the specs are unfortunately lost in the corners of my failing memory) and switch pitch TH400 with Type O 3:42 posi axle that I personally removed from a 70 W-31 in a wrecking yard (hey, it was the mid 1970s and muscle cars were just considered gas hogs after oil crisis #1). Rear tires were fairly tall BFG 275/60-15s. Again, this was a dual plane intake, so the mixture velocity was kept up. The carb linkage was set up so that the primaries on the back carb opened first, followed by the primaries on the front carb, then the back secondaries, and finally the front secondaries. All eight barrels reached fully open at the same time. By the way, it did NOT get 15 MPG when you did that.

Oh, and there was a custom-made nitrous plate between the carbs and the intake also.

 

One exception to the single 4 bbl vs. multiple carb generalization in favor of the single: In 1975 I tested a Torker/800 spread bore Holley vs. Tri-carbs on a mid-12 second 442 on street and strip. The Tri Carbs were 1-2 tenths better on the strip, both setups had good drivability. 850 Holley straight-bore carb was worse on the Torker, which had been designed for Qjets and similar carbs by Edelbrock.