vintage chromoly

My 512 has an 800CFM carb on it.
It pulled past 6k and made 675 HP.

MILD THUNDER

Looks like an 870 CFM carb for the ZZ502 crate?

http://www.chevrolet.com/performance...02-deluxe.html

cabin fever

No he isnt offering to sell me a carb but he did say i could borrow his 850 to see if it makes a difference. When he was running it on the stand he used his quick fuel 850. He told me it would be better with a 850. Said as long as i wasnt running wide open for long stretches it would probably be ok but he didnt reccomend it. I trust him as he has treated me good.

Before i could barely get the motor to 4900 rpms. But the motor had serious issues too.

Im not sure now cause i didnt push it last time out. Only had it out once to verify everthing was ok and then it got to cold. I still have some bugs to work out and just wanna have as much as i can done before summer

Last summer sucked bumming rides.

MILD THUNDER

When it comes to EFI, everyone wants a big CFM throttle body. You never hear the EFI guys, saying "A 750cfm throttle body is plenty for a 650hp engine" .

You bring up a carburetor, and you'll hear guys suggest a 650cfm on a stout big block.

Its kinda like a cylinder head. A smaller head, doesn't always net more power , if the head's design sucks. A larger head doesn't always make more power, if the heads design sucks.

A well setup dominator, can be extremely responsive, idle well, and do everything well. Stick a 1991 dominator from a swap meet on your engine, and it could very well will prob run like chit.

It comes down to booster style, idle circuitry, venturi size, and so on. An old 800cfm holley double pumper, will probably easily get outperformed, by a modern Ultra XP 850/950, everywhere from idle to wot. There is power in fuel atomization, not just the cfm rating.

MILD THUNDER

From prosystems

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.

Thanks for the E's.

Have a great season everyone...see you at the races.

MILD THUNDER

Using the above formula

502 at 5200rpm=833 cfm
502 at 5500rpm =881cfm
502 at 6000rpm =961cfm

Each engine is different, so , the vacuum test is a good idea.

vintage chromoly

I always thought a guy could get away with a larger throttle body on an EFI because the fuel was introduced in the intake runner and the TBody didn't have to draw fuel through a booster and atomize the mixture.

GETTINBYE

That really makes the 1050 a great choice for the build of which I speak!

MILD THUNDER

Yes, that is true.

My point though, was that carbs have come a long way at how well they atomize the fuel. i woudn't go sticking a 1150 dominator on a 454 that turns 5200rpm and makes 450hp, but, if we are talking a 600+hp big block, they usually do well with a dominator , or really good flowing 4150. At least in a boat. Not many boats are running around with the throttle blades open 8% at 3000rpm, like a car cruising down the freeway. Usually, in a boat, at 3000rpm, there is plenty of velocity , as the engine is pretty heavily loaded at that point, and consuming some air to maintain 3000rpm due to the heavier load.

getrdunn

Anybody have any experience with


http://blp.com/cart/index.php?main_page=index&cPath=72

I like the 2 cicuit doms.