Let's Talk Carb Sizes

[blykins]

Just wondering what formula everyone is using to spec carb sizes.

The two popular ones that I've found are:

1. (CID*RPM*VE)/3456

2. (CID*RPM*VE)/2820

Using my specs, I get two sizes.....

1. (420*6800*.9)/3456 = 743cfm

2. (420*6800*.9)/2820 = 911cfm

Now, I'm staring down two carb sizes that are pretty far apart....

Is a 750 too small? Is a 950 too big?

I need to pull the trigger on a carb for my new FE that's going in my Cobra.

I ordered my Victor FE intake with a Dominator flange....that way I could use a Dominator carb or use an adapter and a regular 4150 carb.....

Let's hear the ideas.

[DavidNJ]

It depends on the state of tune. Assuming a fairly wild motor, 743 sounds small for a 420 at 6800. It sounds like a prepped 850 or a 1050.

If it isn't a wild motor, why are we talking about 2.19 valves turning 6800?

[mrmustang]

What you need is a Holley 04778C 750DP. You will also need to wait until the motor is in the car and set up on a chassis dyno to properly tune it. Bigger is not always better when it comes to a carb for your engine.


Enough said.

Bill S.

[Power Surge]

Carb CFM formluas work okay for out of the box basic carbs, but nowadays there are so much better "prepped" out of the box carbs, than the standard Holley ones.

My advice, would be to decide on a brand of carb, and then go over your engien combo with them. For example, DeMon carbs discourages people from buying their carbs based on cfm rating, and prefers you call them with your engine and vehicle info so they can suggest the proper size and model.

But for the "general" rule of thumb people look for, my experiance is that a 750 is good for a mild street big block. Something hotter or a stroker will like something more like an 850. Full boogie big blocks will want something bigger, like a 900 or 950. I have no idea why you seem to be favoring a Dominator, that's a drag race application carb and really not that great for street use. As a matter of fact, Holley makes (or used to make) a 750 "street" Dominator for those who wanted the "look" of a Dominator without the drawbacks of using a real race version on the street.

But again, carb technology has come a LONG way in the past 5 years. Most of the carb specialty shops can set you up with a smaller "cfm" carb that will vastly outperform a larger out of the box carb.

My ultimate suggestion, would be to get a carb from Pro Carbs (I think they are now a vendor here as well?). I have been most impressed with their work and knowledge over any other carb company.

[DavidNJ]

Originally Posted by Power Surge

Carb CFM formluas work okay for out of the box basic carbs, but nowadays there are so much better "prepped" out of the box carbs, than the standard Holley ones.

My advice, would be to decide on a brand of carb, and then go over your engien combo with them. For example, DeMon carbs discourages people from buying their carbs based on cfm rating, and prefers you call them with your engine and vehicle info so they can suggest the proper size and model.

Good advice.

[trularin]

CFM only really comes into play at WOT.

Think about it. A "butterfly" or air regulation device controls the air fuel mixture delivered right up to the point it is perpendicular to the plane of the intake. The air/fuel mixture is relative to the throttle position, desired speed and flow of the engine. You are not flowing 750 CFM all the time. You flow what you want the engine to spin at by your foot.

However, if you WOT, you want the engine to jump to the fastest it will turn in the least amount of time. Hence the ability to deliver a Max CFM.

Just my opinion, I am completely nuts by several peoples standards, but I have a good time with it.

[blykins]

One reason why I went with the Dominator Victor manifold is that everyone that I talk to is out of stock of the regular ones....and won't have them for another 2 months or so. I can't wait that long on an intake. For one, I need to get this engine assembled....and two, I have to have the intake to see where the engine's gonna sit in the engine bay.....so if I have to "tweak" the angle, shorten the engine mounts, or use a drop base air cleaner, I'll be able to tell that right off.

Keith Craft had some Dominator style manifolds in stock....so I figured I had nothing to lose.

I have a Holley 4150 850 DP at home....I can slap an adapter on the intake and run this one if I choose....or I can find a good Dominator style carb and slap it on as well.

I wasn't looking at the 1050 or 1150 carbs of course....but Demon and Holley both have Dominator style carbs in the 750-950 range....and if I need to buy another carb to suit my engine's needs, then I can be looking for one.

[DavidNJ]

Any of the carbs will work. The difference is where it runs out of breath at WOT.

[blykins]

It just seems like there's an opinion for every person you talk to. I guess the only way to do it right would be to buy a handful of carbs and try them all on a dyno. Unforunately I'm a poor son of a gun....so that's not a viable option.

I guess I'm like everybody else....I want to maximize horsepower but I don't want the signal to be so low that it just loads up and falls on its face until it gets enough breath to blow the unmixed fuel out of the intake....

[blykins]

Shoot I don't even know if the Demon guys know for sure.

I just got off the phone with their tech line.

The guy asks me every technical question he can think of.....short of what color underwear I'm wearing.....and then says, "Uh.....I think a Mighty Demon 750 might work best."

[Hotfingrs]

Here's a website you should look at. Then give Don Gould a call. BG gets their tech info from Don. Most at BG tech line don't know what the heck they're talking about.

http://www.4secondsflat.com/

[mrmustang]

Originally Posted by blykins

It just seems like there's an opinion for every person you talk to. I guess the only way to do it right would be to buy a handful of carbs and try them all on a dyno.

I've already done that for you (4-6 carbs on each engine over a 2-4 day period for each one), both on a Ford Crate engine (351W/385HP) and a 427 Center Oiler (435/rwhp).......Again, unless your planning to drive at WOT 90% of the time (doubtful), you really do not need more than the 750cfm double pumper. The trick again is to find someone with the chassis (not engine) dyno to tune it to your engine/drivetrain. Once again, the choice is entirely up to you and your personal driving habits...........

Bill S.

PS: holley or the new Barry Grant only........If Barry Grant, make sure your vendor of choice is buying it directly from them, as their older stock had some issues with the metering blocks that has only recently (within the past month) been addressed by the production line..............

[Cobrabill]

And to throw all the formulas out the window,the carb on my .17 over Sideoiler is a 850 worked to be just shy of 1000 cfm.

[Bruce Robles]

I've got a Kirkham with a 484 cu. in. Sideoiler by Southern Automotive. It has Edlebrock Keith Craft CNC stage II heads, Blue Thunder Intake, SA flat tappet cam, 9.8:1 Compression etc, etc, etc. All the right stuff.
The motor came with a 850 double pumper and I switched to a 750 with vacuum secondaries. It now runs like a bat out of hell. Original 427 comp. Cobra's ran 750 cfm with the LeMans bowls which are no longer available.
I think if you are going to run flat out a large % of time, like on an Oval or drags, an 850 would be the way to go. But If you intend to drive on the street with occasional trips to hyper speed, the 750 is just fine.
You'll really only know if you try them both, on your engine.
It's just like racing, you don't what's the fastest unless you try.
We had a GT-350R back in the '60's, and they came with a 715 cfm Holley with the LeMans style float bowls and vacuum secondaries. In a test at Riverside we tried a 750 cfm with mechanical secondaries. The car was faster with the 715. So some times bigger isn't better.

Good Luck

[JayBar]

Ok, everyone says I am over carbed, but if the car responds has a crisp throttle, and the proper air/fuel mixture on the dyno, I think is is ok.
I have a 408 dart block with 185 AFRheads Trickflow phaseIII cam, Victor Jr manifold a a 950 HP carb. The motor pulls from 1500 rpm's up. Much better throttle response than the 750 HP the 950 replaced. Only bad thing is about 2mpg less.

[Cobrabill]

Jay-like i posted above-i'm just shy of 1000 cfm and i KNOW i'm not over carbed.Let people think what they want.

[NROTOXIN]

Just an FYI,
http://www.thecarburetorshop.net/bigger2.html
curtis

[jdean]

Jaybar, the Holley 950 HP has better throttle response than the 850 as well, and of course goes farther on the top end too. This is because the 950 has 750 venturis, but with radiused entry.

Proform makes a nice 950 HP type carb for around $550, with billet metering blocks, base plate, adustable bleeds and vacuum ports.

[blykins]

I'd like to stay with a Dominator carb I think since if I go with a regular base carb, I'd have to use a carb adapter....which would add at least .400" of height to the intake manifold.

So what would happen if I put say a 1050 on the engine. Would I be able to get the A/F ratio right? Would it just have crappy throttle response on the bottom end?

There's a Dominator 750, but I'm kinda scared that it would be too small.....I've been reading a lot of stories that say the carb manufacturers over inflate their cfm ratings.....i.e. a Holley HP950 only truly flows around 800 cfm....

[lovehamr]

My "Boss 372" (stroked clevor) is running a Demon RS set up for 1050 cfm at the moment and is in my 3200# Boss 302 body and runs fine. Now I haven't had it on a dyno but when that thing opens up there is no delay in throttle response. If dumped in 1st or 2nd it just lights the tires. These carbs are set up to have their flow rates changed for varying applications. I probably will try other sleeve sets for it to try it at other CFM ratings but for now it's fine. BTW, I picked mine up on ebay for less than $500. I second the advise to make a call to: http://www.4secondsflat.com/ I bought their tuner manual and there is a huge amount of info there.
Steve