what carb for this setup?

I would (and did) go with the 650 double pumper...you'll give up a *tad* throttle response on the street but make up for it on the top end. You definitely don't need any more than a 650. As for the turbo, i've even seen 650s on blowthrough setups...it's all about fuel delivery and not so much flow potential of the carb since it's forced induction.

I have heard that Barry Grant Speed Demons are better for roots superchargers but i'm not sure why or if they would be for turbos either. Most guys i know who do blow throughs call up The Carb Shop or some other carb builder and have one built specifically for their setup.

Modular2v said:

thanks for the response......i really would just like to have a nice little 12 second street car. I know exactly what you mean as far as the throttle response vs. top end issue goes. My 331 with a 750 is exactly that way....throttle response is a little hesitant but it screams at the top end. So you think i should definetly go with a mechanical secondary carb?

Click to expand...

Personally i've never had any luck with vacuum secondaries but some people swear by them on a street car. I've tried all the hop up springs Holley sells to make the secondaries open sooner/quicker, but they still don't do it for me. With mechanical, you can adjust the "bar" for the secondaries simply by bending it and that adjusts how soon they open. Plus, i like that i can actually feel through the gas pedel when i'm getting into the secondaries which actually helps save some gas since on vacuum, every time you step into it they open up and dump fuel. I just feel like i have better control over the gas that way.


If you're running a big head/cam combo'ed stroker, you probably want a 700-750 for a drag car, but even then if it were a street/strip car i'd probably opt for a 650HP and jet it as needed. Those carbs are BADASS but they're really expensive.

http://store.summitracing.com/partd...063+400178+306299+4294791224+115&autoview=sku

That is one badass 650 right there....too bad it's twice what i paid for my 650. I'm thinking about milling off my choke horn since it's chokeless anyways.

Heh...or if you're just plain crazy and wanna run a 300 shot of NAAAWS then you could opt for the 1000cfm version

http://store.summitracing.com/partd...063+400178+306299+4294784716+115&autoview=sku

I agree a 600-650 would be perfect... Don't use the blowthrough on a NA motor...

I have always found this article from Pro System helpful..

One of the most asked questions we receive at PRO-SYSTEMS is, how big will my carburetor be, how much will it flow? Most customers are expecting to hear big numbers. They want to hear that their 4150 will flow 1150 cfm or that their 2 inch throttle blade Dominator will flow 1400. Unfortunately, that's not going to happen. In all the designs I've flowed, designed, calibrated and tested, it's just plain physically impossible. Builders will give out dry numbers (meaning cfm ratings without the disturbance of the fuel cone) but these numbers are not what the engine actually sees and are for reference only. Some builders still use the inflated cfm creating 28 inch rating used by an aftermarket manufacturer. Enough has been said on that subject.

But really cfm is not the main level of importance. Fuel shear, atomization properties and fuel curve are your main areas of concern.

To size a carb for the application, you're looking to achieve minimal restriction at the finish-line yet have enough signal at launch as to be sure that the booster is atomizing the fuel and supplying the proper air to fuel ratio.

Horsepower equals air flow (of course). Launch rpm/trap rpm equals a reference of the range of the air flow.

If the carb is too big or signal/curve is too poor at the launch rpms created airflow, the fuel does not properly atomize and plates out (turns back to raw fuel) on the intake. Losses of 10-12 percent of available torque at launch can easily be recognized without a lean cutout or backfire. Then as rpms increase, the plated fuel is picked up and alters the air to fuel ratio down-track as it is cleaned out of the intake. More loss of power. So you jet it down to compensate for the plated fuel being picked up and the launch gets even worse. See the dilemma.

The wider the range of rpm you're going to subject the design to, the more you need to look at the range of airflow and available options.

I'm sure you remember this old formula:

CID x RPM x V.E. / 3456 = CFM

Well that formula is still being quoted by magazines and companies etc...but times have changed and carburetors are operating on almost immeasurable amounts of vacuum. 10 years ago a carburetor would require 10 inches of water to pull signal and shear fuel. Now they can can pull and shear fuel at only 3. Remember 20.4 inches of water (wet) is the cfm rating guide with reputable designers so we aren't looking to match cfm requirements with cfm ratings.

20.4 = 1.5 hg.

You can see that going from 10 inches of water as a requirement at launch to only 3 inches as a requirement really allows a serious increase in cfm size. This removal of restriction really pays off in cylinder head flow numbers and hp of course. Imagine altering this upstream restrictor when flowing your heads.

Because, most of you have specific application designs, a custom shop/unit is typically the plan.

In the future, use this calculation as a general rule on a modified carburetor:

CID x RPM x V.E. / 2820 = CFM
350 x 6600 x .9 / 2820 = 737 CFM

Now you'll be a little closer.

A .9 Volumetric Efficiency (V.E.) number represents a pretty good combination and a 1.1 V.E. number represents an all out assault on the engine blocks stress handling capabilities.

Remember, if we have a heavy vehicle and a two speed we will require a slightly smaller carburetor, than a light vehicle and a stick. Also, if we have a booster/emulsion/air bleed configuration designed to operate and shear fuel at lower rpms we can increase the cfm. An increase in cfm is usually a guaranteed increase in power, but it takes a design that'll still pull and shear fuel at launch to pull that off. That's when the builder starts altering the entry and exit angles of the booster, the emulsion layout, air bleed configuration/well diameter, etc. All in an effort to fan the fuel cone to increase impact, supply the proper air to fuel ratio throughout the rpm band and emulsify the mixture prior to decrease plating for the air speed being encountered. All those mods cost money and they're not easy to do.

But return on investment is the deal when purchasing a carburetor. Oftentimes a customer is thinking of purchasing a this or a that, when the same money spent customizing his current model will yield more performance.

Remember, as we talked about earlier, the loss of torque we record at launch and the subsequent rate of acceleration you lose at the start of the race will be carried throughout the rest of the shifts. So a good leave (excellent fuel shear and proper air to fuel ratio at launch) is getting the reciprocating mass to carry this rate of acceleration to reduce E.T.'s. But if you have too much restriction at the finish-line, the mass will be slowed as a result and E.T.s will increase and none of us want that.

What i've always heard that really makes it all make sense to me is, about airflow, you know that the bigger the opening the slower the air has to move to get through it...and the smaller the opening the faster it has to move to get the same amount through...

The straw example works great for this...think of drinking through a straw from a 1/4 inch diameter straw vs. a 2 inch diameter straw. You plain old don't have the strength to draw fluid up the 2 inch straw, and if you oversize your carb, the engine is going to have a harder time filling the cylinders with air, which is basically what an engine does, it sucks and blows air. The faster you can fill the cylinders (example, forced induction) the more power you can make.


Just like porting, people think hogging out their heads/intake is the fast track to more power, but it's all about air velocity and the turn radius that the air has to flow through. On the right size carb you'll be the perfect blend of speed and quantity, and that's going to make the most power.

if your staying N/A then a 650 is fine like it was said, but if you PLAN to go with a blow through,do it now, my carb ran me about 800 bucks for my blow through set up, and my other carb is a door stop now.