Making a spreadbore out of a squarebore manifold

[road chicken]

Anyone ever try this? I have a Eddy Performer RPM, ( which can only be had in squarebore for the B blocks) and 800 cfm thunder series carb. I'm not happy, ( read that as anal retentive) with the low end response. I know the carb is a bit overkill and the big primarys make the off idle a bit dicey. Even with the 750 it's the same thing. SO my cockamamie idea is to have material added to the secondary area and have the manifold re bored for a Thermoquad. That should take care of the off idle issue and bump the mileage 2-3 mpg on the highway, ( it's getting 15 now) IS there anything I should be careful / aware of ?

[74roadbird]

"Back in the day", I ran a Holley spreadbore on my 318. My 74 Runner now has a 383 running a Thermoquad. In both cases, I had a square bore manifold and used the 1" thick, spreadbore phenolic adapter. The adapter also helps keep the fuel in the bowls a little cooler and both engines ran fine.

[road chicken]

I should have stated I don't have room for the adapter plate, and have heard they compromise the flow of the carb. I have a been comparing what Eddy did on their GM Small block Performer to the RPM I have and it looks very doable. I'll only need to get the material added and I can do the rest of the work on the mill.

[74roadbird]

What engine are you running? How much room do you have? It may be easier to machine the phenolic plate down. The material does help keep the fuel cooler.

I'm pulling the Thermoquad off the 383 and going with a 650 cfm Thunder. Besides the Thermo being too much carb for the engine, it kept backfiring through the carb when I really got into it. I've been running a "loaner" Performer carb and the backfiring is gone. Granted, I don't know the Thermo well enough to really tune it, but I'll put it aside until I find someone that does.

[road chicken]

No room to play with. I have a 383 with a performer RPM intake and the air grabber scoop. I have it pretty well figured now. It will take about 5-6 hours to build up the material and then machine it to the TQ pattern. They have a Ford RPM AIRGAP intake that has the same runner layout with a spreadbore pattern.

YES! TQs are super adjustable carbs, but those adjustments all interact so they can be a finicky proposition to dial in. Surprisingly You tube has the factory tech-training movies on line. However, in this case, the fuss is worth the time, to pick up the throttle response and mileage while keeping ( or bettering) the WOT performance.

[road chicken]

Well after goofing around with all this I settled on a Demon 1904. I forgot who made the suggestion to use a Demon but you were definitely right. It is certainly a modern version of the TQ with the short comings of the original engineered out. Of all the holleys and carters i've had on the car- this Street Demon just leaves them in the dust for street use.

[dangina]

Well after goofing around with all this I settled on a Demon 1904. I forgot who made the suggestion to use a Demon but you were definitely right. It is certainly a modern version of the TQ with the short comings of the original engineered out. Of all the holleys and carters i've had on the car- this Street Demon just leaves them in the dust for street use.

have a look at proform(QFT), seem to be the best, I haven't read a bad thing about them yet!

[road chicken]

Thanks, but I've had my fill of Holley based anything, ( Just personal experience with my engine combos) and I've always wanted the TQ back under the hood. They just work best for my applications, which are usually pretty feisty off idle. The triple primary boosters are the best mixers short of fuel injection for that and mileage-