I think I made a mistake when I put a decent 4MV back on the 327 when I sold the 68 and kept the 650 CFM Holley. BUt the Holley was new, and I like the electric choke, so here I am...

The carb is a Holley Model 4175 spreadbore carb. The particulars are:

- Holley Part No. 80555C
- 650 CFM
- vacuum secondary
- 62 primary jets, 54 secondary metering plate, 0.040 primary nozzle, 30 cc accelerator pump

My big block is very mild. It is a 72 truck 402, .030 over, with big open chamber heads (stock) and a Crane restoration grind that is supposed to be the 396/350 Camaro/Chevelle cam (396/360 in the Vette). I am running stock exhaust manifolds. The motor is in the car, but I still have the underside from the doors back to scrape and paint/undercoat, so there's no fuel tank and I can't start the motor now. Maybe in a month or two...

I do have a good Holley book, and I think I have all the equations if I actually wanted to sit down and figure out if this will work, but I wanted to see if any of you all had an opinion on what it's going to take to make this work. I'm thinking that the secondary metering plate may be on the small side, and if I have to start changing plates costs are going up for a carb that may not have enough CFM anyway.

I'm going to try to sell the Holley (see the classifieds below...) and get a new or rebuilt 750 Q-Jet, but I was just curious what you guys thought it'd take to make the Holley work.

I'd sell it and save up for a Proform HP750 or HP950, you can piece them together pretty easy. Main body is $99 for the 750, and $190 for the 950, Base plates are around $130.

I say try the Holley first. If the motor really is that mild the carb will work fine probably through about 5500 RPM. Much over that and it will probably give up some power to the bigger carb.

I tested a Holley 650 double pumper on my 388" SBC (only 14 cubes smaller than your BBC) and the 650 seemed to pull hard through 6000 RPM. The carb had excellent throttle response. The carb did really hold the engine back over 6000 though . . .

I've also run a 650 (DP, on a 406) it ran strong and had great response. Probably did give up a little on the top end compared to a larger carb, but ran great on a daily basis.

I think it will run fine but just run out of air earlier than it would with a
larger carb.Like Eric said, up to 5000 rpm or so it should be fine, just jet
it properly.

mine ran to 5800... 108 in a traction linited 13.23, with an RV cam!

The formula for figuring minimum carb CFM is (Highest RPM * Cubic Inches)/3456.

So with your car
(650 CFM*3456)/402 cubic inches= ~5588 RPM.

Thanks for al the input guys! I will probably give it a try, as I haven't even gotten one offer for it and I don't really want a $300 paperweight.

If you've got one, try it. A square bore 650 would be ok, but a spread bore would be pretty small on the primary side. I'm sure it will work fine, but the BB motor probably would like at leat a 650 square bore or larger.

Heck, it should fit and work FINE on your MILD 408 BB street motor!!

454 P/U truck motors came stock with 750 Q-Jets on them...

pdq67