Carb CFM?

[Krooser]

Okay... in a couple weeks I'll be assembling the carb I will be using next season on my DLM. 388ci, iron block and heads, flat tappet, alky fuel, heads flow 294intake @ .600, T&D rockers, MSD ign and dizzy.

This is an ex-WISSOTA mod engine that had 575HP it it's original livery but I'm hoping the extra cubes, lighter valve train and improved carb should get me to 650HP. I plan on doing a bit of bowl work on the heads just to help flow around the guides... and maybe open up the intake ports a smidge as suggested by a reputable local shop.

The engine originally had a 750 Braswell carb... an alky conversion from gas (mid 90's deal). Last fall I bought another used Braswell but an 850cfm deal. Sold the 750.

Just looking around online Holley sez I need a 950XP for my 388 to run right at 72-7500rpm. I don't really question their suggestion but most of my buddies are still telling me they run a 750 on their 360-390 cube engines. Funny how they are always selling an 850 to "improve" their program but still insist they run 750's!

I've talked to a few aftermarket builders and they are all over the place from 750 to 830 to a 750 with an 850 baseplate to a full bore 950 that they claim on produces 850.

I do my own carb work... have been since the 60's. No offence to the boutique builders. If I buy a $900.00 carb I'd like it to be spot on out of the box. Dyno results will tell me quite a bit, too.

Ideas?

[perfconn]

Carburetor minimum requirements:
Cubic inches devided by two(you only have 4 cylinders sucking for each rpm)
Multiply by maximum rpm intended to run
Devide by 1728(cubic inches in a cubic foot)

[Krooser]

Based on that I need 808 cfm. My 850 should do OK then.

[fastford]

on a dirt track , we found a 750 was sufficient to 400 cid , it pulls better of the corner , now im not talking about a box stock holley.....

[over4T]

Perfconn's formula is a good general one but as fastford says the 750 works well. The touring group we often run with has no engine size limit but an 800 cfm carb limit and there's a lot of 434c.i. and 440c.i. engines in that bunch that run very strong under those constraints.

Of course, they've also got the "no exotic carbs, Predators, etc." clause. I'd love to take that big old empty box Predator off my shelf and show them how "exotic" it really isn't. 'Bout the simplest and most adaptable carb I've ever run, both circle track and off road rock crawlers.

[perfconn]

The formula tells you what size carburetor your engine needs but it doesn't tell you if your cam is way too big that a smaller carburetor will work better.I had one customer down in Georgia that had a 1.560 venturi (about 1150) that I swore would not come off the corner,but it worked great for him on a 360ci steelhead.

[Jking24]

From my personal experience I would say you don't need anything bigger than a 850 and I would agree that the "750" may race better. But this is one of those things that can only be found by running it on your car because it can be more than just engine combo sensitive. The racetrack and driving style has a great effect on witch carb will yield the most success

[overachiever]

I have not done the math for this in a couple of decades as I have out of auto racing for about that long, but a good explanation of CFM requirements follows: "The formula for calculating how much CFM (cubic feet per minute) your engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456. Any ordinary stock engine will have a volumetric efficiency of about 80%" Hope this helps.