Ok, I have a little dilemma.
Right now I have a 330HP 350, mild cam. I need to get a new carb and intake. I am definitely going with speed demon, and edelbrock air gap rpm manifold. I have a 3000 stall and i manually shift so I am also going with mechanical secondaries. But the problem is that Im replacing this engine with a wild 383 stroker in about a year. I dont want to have to buy a new carb. I know the 383 will need the 750 carb, but im not too sure the 350 I havre now will run right with it. What is the worst thing that can happen If I over-carb the 350. Should I just deal with having to buy another carb in a year??? Thanks in advance... - Joe

 

Get a 750.

 

Yep, go with the 750. A mild 350 will run just fine with it and your wild 383 will run fine with bigger jets. BTW, there's a guy on the classifieds selling a holley 750 DP mech. sec. brand new in box for $360 shipped, that's $60 off retail. If you want his email let me know.

 

Thanks for the info guys.
Cant wait to order this stuff...

 

I have a question to ask along these lines. I'm begining to build what will be a mild 350 that will be non-CC carbed. I'll be going with a double pumper, but I've become confused as to what size to get. Here you two have said 750 cfm would run fine with a mild 350, but I've been reading the "Holly Carburetor Handbook" by Mike Urich and in his carb selection section, it looks like I would want to get a 700 cfm model, or maybe even a 650. If you follow these URLs, you'll see a few charts that I've been reading:

Double Pumper Carb Chart (2nd down)

Air Flow vs. Engine RPM

If it helps, I'm looking to build this 350 to make at the least 300 hp.

What do you all think is my best option, CFM-wise?

 

We probably all tend to over-carb. With vacuum secondaries, you can get away with that more easily than you can with DP, especially with an auto tranny.

The Holley formula on my 396, assuming 85% VE and 6500 RPMs, says a tad under 650. I shift now at 5700 RPMs, which says something like a 560 CFM.

I was talking to a guy last season with a '70 Nova, all steel (no front bumper, though), full interior, 454, slightly more cam than I've got, heavily ported heads, Stealth manifold w/1" spacer, who was running 12.0's (don't forget our 5800' elevation) - he had a 750 DP on top of it at the advice of his engine builder.

I'm borrowing a 650 DP this weekend to see what it will do on the 396. The guy I'm borrowing it from is a fellow racer who had it on a 350 in a '74 Camaro last season (it was quicker than my car). This year he's running a 468, and he said he chose the carb based on the Holley website formula. Size = 830 CFM DP. Runs mid-12's.

 

I think I'm going to go with either a 650 or 700. It all depends on which one I find a deal on first. I plan on buying a used one either locally or on eBay, and then rebuilding it using my book and a kit. Building this motor and all that comes with it will be a major learning experiance for me, and I can't wait to get started.

But like I said, I'm not lookin to build a hot rod, I just hope to get somewhere in the ballpark of 300 hp. So, I think I'll be just fine with a 650 or 700.

 

according to this book i have about holleys, the formula is:

(engine CID x max RPM)/3456=required CFM

so if you have a 350 that'll max out at 6000 you need 607cfm

 

I was reading an article i forget where it was, but barry grant, i think hes the owner of demon carbs or something, said that a healthy 305 should go with the 650 and 350 with the 750. Now my 350 engine supposably making about 330 HP should be able to handle the 750. At least I hope it will because that carb was expensive and I dont think I can return it once i put it on.

 

Does one or the other hamper gas mileage? (650/750)

 

You will get slightly lower mileage with the 750 since it is delivering more fuel.

 

Get the 750, you'll be very happy you did. If I can run a 750 Speed Demon on my 305 without any problems, you should be able to run one on a 350.

 

ordering speed demon 750 mech. secondary, air-gap polished intake, holley high volume fuel pump, msd billet distributor, and some nice braided fuel lines, gauges, fittings, ect. on other open web page right now. My credit card is in for a violent beating in a little while....

 

Well, I put a bid on a 650 cfm on eBay. If I get it, I get it. If not, I think I'll start to look around for a 750. Thanks for the help everyone.

 

The issue is that for the same engine speed, the larger venturi carb will flow the required air through a larger area, therefore lowering the velocity of the air moving through the venturi. That has the effect of reducing the booster venturi signal, and reducing the vaporization effect in the air stream. It also tends to reduce vacuum. Those add up to reduce fuel economy to some degree (and more importantly, throttle response), all other things being equal.

 

You might not appear to have any problems, but it most certainly isn't optimized. I'm "getting along fine" with a 750, but I suspect it is too large for my combo/usage.

 

Thanks for the explanation five7, I didn't know that... I'll have to file that for future reference upstairs...

 

I'll tell you this, it's much much better than the 670 that I was running. I had messed with that 670 a lot and it never ran or drove as good as it does now with the demon on.

 

That probably has more to do with the change in manufacturer (although Jester probably won't agree with that) than it does with the increase in flow capacity. Had you opted for a 650 Speed Demon, you would probably be even more impressed with the difference.

I've never been that impressed with what I've seen or heard about SA's, either (I think Jester would agree with that).