Murdog BMF

Here's a stupid question I'm sure-I'm swapping my LG4 in my 83 Z28 with a 400 ci SB-what size carb should I use? I don't want to use the Quadrajet, I'm looking at either a Edlebrock or holley. Also what would be a good manifold/head/cam combo? Any help would be welcome! Thanks!

chevy350

Cant say what a good combo will be w/out hearing the performance specs of the engine (high reving, stroker?) cuz that effects the needs. High reving= a need for high powerband intake and different heads. The same is said for a stroker or a bored over engine. A good carb. would be the holley 670 or maybe the 700. If you want good gas mileage, might go with a 650.

IHI

iTrader: (1)

At minimum 750, possibly up to a 950HP-Holley carbs that is.

Air_Adam

yes, 750cfm sounds about right, but i dont reccomend a Holley.. yea, they can make good power, but they require alot more constant adjustment than an Edelbrock or Speed Demon carb... so i heard anyway

IHI

iTrader: (1)

I'd like to know what people have to "constintly adjust" on their Holleys. Both the ones I've had, once I got the floats adjusted, proper jetting, power valve, idle mixture setting I've NEVER had to mess with it again. I think people try to "tune" too much and don't know WTF they're doing, hence the constint adjustment. I don't know? Others, please enlightnen me by personal experiences so I can better understand what's the problem, as maybe I've been "lucky" the past 2 Holleys I've run. Not tryin to sound like a "know it all *****", but everybody I hang with has no problems either, once they're set-up-PROPERLY!!

AJ_92RS

I think they're referring to the combinations of

1) Holleys are used more in competition than any other brand
2) They need to be tuned at different tracks for different temps and altitudes
3) They are infamous with floats sticking open.
4) Holley's float bowls are set up in a way that the gaskets are always subjected to gasoline which in turn causes them to break down faster.
5)You have gas all over the place when ever you change the jets.
6) Then there's the "blow out" valve which is really only caused by backfiring.

They aren't bad carbs. Actually they're the easiest to tune which is why they're so popular. But I have to say for everyday driving, or a car that sees little track time, an Edelbrock is much easier to maintain. And they perform rather well. They’re just not as easy to tune as a Holley.

Don’t take that wrong either. Physically they’re easier. All you gotta do is pull a metering rod and or change a jet and you don’t ever have to drain the bowls or even re-adjust the floats. But because they’re “metering” rods, there are more variations and combos. This requires more time to tweak, and in the pits there isn’t a lot of time to do that.

AJ

AJ_92RS

Oh..... forgot to answer the question.

The formula for finding the minimum required CFM for an engine is

CID x RPM
3456

400 x 6000 = 694 CFM
3456

That's at 100% VE (Volumetric Efficiency)

If you want better streetability then you have to know the VE. Most stock motor are 8-85% VE and most HiPo engines are 90-95% VE. Multiply 694 or 700 for ease by VE%

700 X 95% = 665 cfm

You will loose top end HP but you'll increase throttle response and MPG.

I wouldn't ever put a 650 cfm on a 400 There's nothing wrong with oversizing a carb unless you get too crazy. Like putting a 1000cfm on and try to drive it to the grocery store.

A 750 cfm, IMO, would be perfect.

AJ

IHI

iTrader: (1)

Yeah, when I first tried that formula when I built my engine I bout **** myself laughing said I needed a 616cfm carb, I did start with a 600VS cause that's what I ran the 305 with, great low-end response, moved up to a 750 VS and it'd pull all the way through the rpm band and not fall on it's face at 4500 and higher, now I'm finally running the 830 DP and picked up .3 with that bolt on alone, but I can literally watch my gas gauge go down in a 1/2 drive.

AJ_92RS

Yea..... It cracked me up to. But I guess it is the MINIMUM.

An additional part of the formula to find CFM for an engine that has higher HP than CID that I made up which I like a lot is to take the cfm after the VE% is found and...... well I'll show ya But it only works if there is more HP than CID. Otherwise the previous formula is much better. Anyway......

700 CFM at 95% VE is 665 cfm right?

Now take max HP and divide it by CID.

450 HP = 1.125
400 CID

Then multiply that figure (1.125) by 665 cfm.

665 x 1.125 = 748.125 cfm or 750 cfm.

I made that up myself but after I did it about 50 times with 50 different engine combos it works pretty well. But like I said, it only works with engines that put out higher than 1 HP per CI.

AJ