? Carb Cfm

[ROADGLIDE06]

Can someone tell me what the proper cfm is that I need for this set up?

318 bored .030 over stk. heads Speed-Pro Cam with .280 lift on both exhaust & intake, duration 278.

[23T-Wedge]

A rule of thumb ( and I don't have a clue where I first heard this but it should put you in the ball park..) is to multiply total cubes by max RPM and divide by 3456. So if you're at 323 cubes at you plan to turn 6000rpm it would be:

323 X 6000 = 1,938,000 divided by 3456 = 560.76 cfm. Again, that is a ball park figure, but should get you close.

[BJSracing]

I would multiply the end figure you came up with times .85 for volumetric effecincy. Unless it is built like a high dollar race motor it will have about 85% eff.

[ROADGLIDE06]

You're talking red line RPM right. If so does this look right 368x5500=2,024,000 divided by 3456=585.65 cfm

[23T-Wedge]

Roadglide,

That's correct, and like I said this is a ballpark figure, depending on how the engine is built it could go 100 cfm either way. I really wish I could remember where that formula came from but I've seen it many times through the years, enough that it was etched in my memory. As for BJS's recommendation, the 85% is a realistic figure for efficiency, I just don't know if that was somehow built into the original formula or not. If it was and you multiplied by .85, you would wind up with a smaller carb than you need, might hurt you in total HP slightly, but may also be a more drivable combination for everyday driving. It all depends on what your objectives are, if you're leaning toward more performance, go slightly larger, otherwise a slightly smaller carb will give better low end response around town.

[ROADGLIDE06]

Originally Posted by 23T-Wedge

Roadglide,

That's correct, and like I said this is a ballpark figure, depending on how the engine is built it could go 100 cfm either way. I really wish I could remember where that formula came from but I've seen it many times through the years, enough that it was etched in my memory. As for BJS's recommendation, the 85% is a realistic figure for efficiency, I just don't know if that was somehow built into the original formula or not. If it was and you multiplied by .85, you would wind up with a smaller carb than you need, might hurt you in total HP slightly, but may also be a more drivable combination for everyday driving. It all depends on what your objectives are, if you're leaning toward more performance, go slightly larger, otherwise a slightly smaller carb will give better low end response around town.

Thanks Wedge, the reason I am asking this is because I was told that this particular cam is a heavy duty two barrel cam, and the carter two barrel that I have is suposedly 330cfm. With that in mind the motor idle's fine but while driving and under load it feels like it is on the edge of starving for fuel.

[rumblefish360]

A 600 cfm will do fine. The problem here with formulas, even if correct, is the middle of the road engines in, size, performance, effeicenehnjk,,er, what he said above. At what percentage.

Best bet for a 318 with mild mods to some real nice ones is a 600 cfm carb. I have not noticed a 550 cfm carb yet.
A 500 would do fine for a stock engine to a few bolt ons. Like headers and a 4bbl. Thats it.

[rumblefish360]

Oh, 2 bbl carbs are rated sifferently than a 4bbl carb. You can not compare the 2 so easy.
A good write up apeared in Popular Hot Rodding in the Nov/06 issue. It's a good carb read and helps explain everything your going to need to know about these confounded things.

[dgc333]

The 85% efficiency number is not factored into the equation so from a pure are flow stand point less than 500cfm is the way to go.

I would disagree with Rumblefish's estimation that a 600 would be a good choice. I would recomend a 500 for such a mild engine. 600 will work but you will be much more satisfied with the improved throttle response in the idle to 5500 rpm range than the slight increase you will see in power over 4500.

Carbs like cams are one of those items that you are very tempted to fall into the bigger is better mentality.

I have a very stout 360 in my Barracuda and I have both a 600 and a 725 carb for. I definately feel some extra power over 5500 rpm but I keep going back to the 600 because it works so much better on the street. And, the 600 is no slouch on the car it still runs 13.7 at 101mph with the 600 and if I can improve my 60' times from 2.2 seconds I should be able to run under 13.5 with the 600.

FWIW, a 500 cfm 2bbl is rated at 3" water pressure drop where a 500cfm 4bbl is rated a 1.5" water pressure drop. 2bbl if measured at 1.5" water will likely be down at or uner 400cfm.

[rumblefish360]

Oh, forgot to ask, .280 lift? At the cam? As in .420 at the valve? Whats the duration @ .050. Headers? Raised compresion?

[it's all dodge]

that equation 23 T wedge brought up I think is in the Holley book, and maybe even the Carter book. Yeah, the 500 cfm 2 barrel is really not 500, when actually compared to a 4 barrel, we flowed a BONE STOCK 4412 500 holley, 389 cfm, which sounds right, since it is half the size of a Holley 780. Lots of people over carb, there is a 525 Eddlebrock, or Carter, but I think the others are right, the 600 will work good.

[rumblefish360]

RoadGlied06, iaDodge. I went to the Edelbrock site and found they make 500 cfm carbs in the AFB & AVS versions, then 600 in the AFB, 650 in the AVS, 750 AFB and 800 AVS. Next is the Holley site for there carbs under 600/over 500.
(Wish I had some spare bucks for spare carbs though. Just to see and play with. dgc333 has a point on a smaller carb, but where to get one over 500 is the question I'm having. I do not agree with a 500 being good. I think more cfm would be better. Not so much 600 worth, but more than 500. I could be wrong. I don't know for sure.)

[rumblefish360]

Interesting. I wonder is it a true 570.
http://www.holley.com/applications/C...lector/0-80570

[TK]

What CFM carb you chose, has a lot to do with what brand you chose.
For instance, a 750 DP holley would flat kill a 318, where a 750 EDL would run just fine, once tuned.

I run 600cfm Edelbrocks on all my 318's, and love them all, if that means anything.

[rumblefish360]

I disagree. Any carb can be tuned to an engine. However, depending on how you like your throttle response and that is the deal with real world driving.
(OK, just jabing at TK, but theres truth to it for a dyno.)

I seriously disagree with the Holley to Edel. comparo there.

I just came back from the Holley site. It says all carbs wet flow and tested. Like TK said, if that means anything to you.

[TK]

Grrrrrrrrrr............... Try to make myself more clear................

A mechanical secondary carb, too big, on a smallish, stockish, (his is smallish, and stockish) engine, will be a real bitch to get to run the way we, as a whole, like our engines to run.
Edelbrocks, thermoterds, and quadropukes, with they're velocity opened secondary door, makes our mistakes, and iffyoffs more, uh, dummy proof. (Thats me and you Rumble, go ahead and dispute that!)

[rumblefish360]

LOL, Nope. Key fetrures listed here;

with they're velocity opened secondary door

But not mentioned by you. Otherwise it almost seemed the comparo was the carbs were 2 different ones. 1 good, one crap.

[ROADGLIDE06]

Originally Posted by rumblefish360

Oh, forgot to ask, .280 lift? At the cam? As in .420 at the valve? Whats the duration @ .050. Headers? Raised compresion?

Yes, .280 at the cam and .420 at the valve. Duration @50 is 204. Stock manifolds, dual exhaust 2 1/4" with crossover flowmaster mufflers dumped at the differential. Other than going with the larger piston and decking the block and resurfacing the heads, made no attempt to raise compression.

[ROADGLIDE06]

However, depending on how you like your throttle response and that is the deal with real world driving.




Ok guys, let me throw one more wrench into the mix. In regards to this statement, for better response I took my distributor and had it reworked to elimanate the vacum advance diaphram and replaced the points with a Pertronix gismo, and set the timing @ 10btc factory states tdc. Could this be just compounding my problem?

[dgc333]

The vacuum advance only works at part throttle cruising when there is relatively high manifold vaccum. At idle or WOT it has not impact. The purpose was to improve fuel economy of the engine while cruising.

The initial timing should be set to produce the highest manifold vacuum at idle while not be so far advanced the car is hard to start. Was the mechanical advance also worked on? When you start increasing the initial timing you need to decrease the total mecahnical advance. A small block mopar engine with open chamber heads likes aabout 35 degrees total mechanical advance. A stock distributor with a spec of tdc for initial timing will have somewhere on the order of 45 degrees of total timing if the initial is bumped to 10 degrees. You need to limit the mechanical advance to keep it at 35 or you will have detonation problems and will not be getting max power from the engine.

Also, increasing the initial timing usually helps smooth out the idle and improve throttle response as you go more agressive on the cam.

[rumblefish360]

Good post dgc