I see dozens of cfm calculators on the internet. All of which give me very similar results. Why don't they ask about cylinder head flow when typing the info in? Must not be necessary.

If you are talking about carb CFM it is because they assume a set voumetric efficiency.

So the better flowing the heads, the better the VE%? If so, I wonder what % would represent a good flowing set up.

With this one you can account for the Volumetric Efficiency:

http://www.virtualengine2000.com

http://www.caliwampus.com/Dyno/dyno1.18.jpg

Yea, there are a couple calculators on the internet that lets you adjust the VE%. But even then a 350 (ZZ4) that has a fairytale 100% VE, that shifts at 5800 (heck, even 6000) still only need a sub 600cfm carb. And thats with 100%VE:mad:

Why does this not coincide with GM using a 750 on a stock one? Or, why everyone seems to DEFINATELY recommend something larger than a 600. These are accurate calculators right?

Heck, look at the example in the photo above... a 355CI ran up to 6500 RPM only needs 567cfm... Geesh!

Shall I keep my 600cfm on there and forget about it?

I have seen a car run just as good with a 600 but almost always better with 750 and I have never seen a 350 run slower with a 750 apposed to a 600.

A bigger carb may give you a few MPH more on the top end, but your bottom end and low end throttle response will suffer. I have an 825 mighty demon carb on my new 468. The dyno showed us at 102 % VE on the motor, and the cfm showed me at 830 cfm on some pulls. I called BG and they told me not to worry this 825 cfm carb will flow every bit of 940 CFM and that i dont need anything bigger. The dyno showed them to be correct.The motor was not suffering on the top of the rpm range. now this motor is very crisp on the bottom end, great throttle response Dont get sucked into the bigger is better deal get the carb that will serve your pupose well. If this is a street car you will want good bottom and mid range response. call your local engine builder and see what he has to tell you?

OK a couple of observations from the dyno and clarification.

VE is a calculation based on what is actually happening with airflow, not something you change in a simulation program to make the numbers come out where you want them. In other words it is what it is, based on the combination. Dyno simulation programs should calculate out the VE and unless some component is changed it doesn't vary.

Any carb will flow "more" than it's "rated at" if the engine "needs" the air. The problem with that though is IF you run say an 825 CFM carb on an engine that is pulling 830 CFM AND it is a restriction it will/can/often causes a loss in power. The check on this is recording manifold vacuum on the dyno. If you are pulling a significant depression,say 3" of manifold vacuum in the upper RPM band there is almost certainly power to be had by upgrading carburator sizes. 1" or less and typically not an issue.

When I first built my motor and installed it on the dyno I pulled over 850 CFM through a HP 750 Holley. On that run it cost me over 50 horse as it struggled to make a tad over 610. Manifold vacuum readings showed almost 6" of vacuum. Voumetric efficiencies were high at slightly over 105 to 106 percent so that data alone does not give the total picture. A restriction is a restriction.

Holley appears pretty close on their flow ratings versus what we see on the dyno for restrictions. BG might fudge things a bit giving a bit more headroom prior to the carb being a bottleneck. I do agree it's MUCH easier to get better overall driveabilty with a modest size carb and a 650 CFM carb is usually much closer to what's needed on a typical street 350 than a 750. Even in the 440 HP range an engine with 100 to 102 VE will consume only around 550 CFM.

There are many many factors which make one carb work better than the other besides CFM rating, although that is an important one. Swap a 750 CFM 3310 Holley for an HP 750 and I'd be you'll pick up power. We dynoed back to back 6 various 4412 2bbl carbs on a limited late model restricted motor and ALL of them produced different power levels with a spread of more than 10 HP between them. Some were tuner carbs and some were customer carbs.

"Even in the 440 HP range an engine with 100 to 102 VE will consume only around 550 CFM."

So you'd think a 600cfm would be plenty for a 420HP engine with 95%VE? my deal is, I don't want to spend a few hundred bucks for a 650 just to find out that I can't tell one darn bit of difference in power. But I'd definately spend the money if it were guaranteed.

Bolt it on, jet it and see how it runs. If you want to see if the airflow is restricted hook a vac gauge to manifold vacuum, tape the gauge to the windshield and make a wide open throttle run. Note any vacuum readings on the gauge.

Other than that the only other test is to run the engine on the dyno and also analyze the brake specific fuel consumption numbers in conjunction with the ACTUAL VE's it's recording, EGT's and the vacuum.

It may be a bit small, remember that CFM ratings on carbs ARE based on a depression of mercury like a flow bench is, although I don't know what the depression rating is.

And what would I see in the readings that would indicate a restriction?

To make it worth considering changing carbs, around 2" of vacuum.

Cool. I'll post what i get and tell me what you think if you want...

And just because I'm debating with you about MotorKote, doesn't mean you can't still help me....:crazy:

MotorKote smotercoat.......just don't drink the stuff.

I was given a 750 dominator, so when I built my 427 I planned to run it cause well it was free. My engine has a dominator flange intake cause it was the easiest way to use the carb. I wheel dynoed it and other than finding my exhaust sucks big time I found that buy upgrading to a "way too big" 1050 my hp gained 300 rpm and about 25 more horse power. The jetting was way out of range (104s on all corners) when done to gain the proper a/f. My guess as to the requirement of the 104s is that at low booster signal (below the 1.5 Hg most carbs are rated at) the engine is not able to pull the same amount of fuel it would use if the signal was higher. Plus the carb doesn't use power valves. In the real word this carb is way to big but I plan to only use it with out the exhaust and at the track and run the rest of the time I will keep the 750 and my cheap sucky exhaust.