I just got a deal on a Street Demon 750 carb, part # 1402010. I was thinking about putting it on the 383 stroker I'm building, but I think it's going to be too much carb. The motor on build is a 383 stroker, 10.5:1 compression, Trick Flow 23 degree heads, Weiand single plenum intake, and a Comp Cams XR294 Hydraulic Roller setup with 1.6 roller rockers. You think this carb'll bog my motor down at WOT?

 

Put an 850 on it.

 

Why would I do that?

 

I have an 850 DP on my '79 Z28 and it works fine. The only real difference is I run a small solid Comp Cam (245/255dur, .504/.512 lift), and I run the old Gen1 Trick Flow Twisted wedge heads.

I didn't think I'd ever need an 850, but that's all we had laying around at the time, so we used it. It worked great, so we never changed it.

 

Yeah, I was looking at the specs of that 750 per Demon's website yesterday and it'll probably be fine. They suggest a 390-something ci engine with a max duration of 240 on the cam. I've got a 383 with a 242/246 cam with .575"/.599" lift on 1.6 rockers, so it'll probably do just fine. I scored it almost brand new, in the box, for $150, so I can't complain.

 

You'll be fine. I've run Holley vacuum secondary 750s on 350s w/much milder specs than your 383, no problem.

 

I used to run a Holley 750 on my 70 SS w/a 350 and similar specs to your 383. It did well.

 

Cool. It's settled then, the 750's going on the 383

 

Hard to go wrong for $150.

 

If you happen to get the opportunity to borrow a larger carb from a friend, anything between 850 and 1050 cfm; bolt it on, play with the jetting and see what it does for you.

That 750 will work great and have good velocity to boot, but you MAY benefit from more CFM. Because it's "may", borrowing one before buying one is the way to go.

And no...
I can't offer you a logical explanation of why you might want a bigger carb. Lets just chalk it up to observation over time and leave it at that.

 

Ignore any carb formulas you see. They're always underrating the carb. The formulas give a minimum carb required for an engine.

I've seen SBC engines with 2 1050 dominators. My 469 BBC has 2 850's on it but then the cam makes power to 7800 rpm also.

If metered properly, carbs will always give the proper air/fuel ratio no matter what size they are. The key to making power is getting more fuel and air into the engine. Superchargers, blowers and NOS all fill the engine with more air. With that air you can put in more fuel to make more power. NA engines need the engine's sucking power to pull in as much air/fuel as possible.

Not all carbs are the same. Buy 2 identical carbs and they'll probably behave differently. People have badmouthed a used carb they bought saying it gave terrible gas mileage or poor performance when it was probably just a really old worn out carb or a carb that just didn't work well with their engine. Holley produces a bunch of different 750 carbs and they'll all perform differently even though they all flow the same CFM.

Just from looking at what you've said is in the engine, I still say go with an 850.

 

My question kind of pertains to this thread so I figured I'd just reply instead of starting a new thread. I just bought a 650 CFM Ebelbrock carb for my 350 crate engine. We just bought the car with the engine installed and it has the Performer RPM intake on it but i'm pretty sure the internals are pretty much as ordered. Am I going to be killing power with such a low CFM carb???

 

I run a 650 performer on a stock '72 350 with a performer intake manifold. It runs just fine.

 

You can't leave out the rpm you are turning and some info about the heads. Using rpm, displacement, and VE (which you need to estimate and is why you need to know about the cam and heads) you can calculate how much air you need. A carb rated to flow that much air will do so at a stated pressure drop, typically 1.5" of mercury. This number is "conservative" in that it provides, on a typical setup, a strong enough signal to the boosters so that the car will run decently and allow the engine to make good power. Careful tuning allows you to use a larger carb and still preserve drivability.

The relevant point is that the amount of hp to be gained is relatively small. I am too tired to give you a sample calculation, but it is small. On a typical street 350 making peak power at 6,000rpm or so, going from a 750cfm to an 850cfm will net maybe 5hp. Now, turn that motor at 7,500 with a bigger cam and race heads and it will need a 1,000-1,100cfm carb and will make another 30-40hp compared to a 750. I think the 750 is a fine choice for a street motor that is not going to turn high rpm. In fact, a 650 would be adequate for a 6,000rpm motor.

Rich

 

FWIW we're running a Holley 700 DP with the RPM Air gap on our 350 crate motor. Good power and great driveability. (It's a daily driver. )