Carb size

[Philip47]

I have heard allot of talk in car and boat circles about carb size, here is the equation
I have relied on
Cubic inches times max RPM divided by 3456

example a 383 cube motor running 5,500 RPM
383 x 5,500 equales 2106500 divided by 3456 equals 609.51967

So a 650 dp should be plenty of carb for this engine.

I checked with Holley and they use the same formula

[Forrest]

That's assuming 100% volumetric efficiency or VE (percent of cylinder filling) which is almost imposable to achieve with a naturally aspirated engine. You can figure on something between 80% to 90% VE on most NA performance engines that you will see in a boat. Depending on the amount of boost above atmospheric pressure, supercharged and turbocharged engines will have a VE well over 100%.

[Philip47]

how would you then allow for that?

[Forrest]

(CID)*(RPM)
----------- * VE = CFM
3456

. . . in the case of the 383 small block turining 5500 RPM we will estimate VE to be 85% since there is no practical way for us to measure it.


383 * 5500
----------- * 0.85 = 518.1
3456


. . . however, a smaller carb and other intake restrictions all contribute to reduce VE. In this case if we hit the VE right on the money with a 518 CFM carb, it would be ideally sized for highest intake velocity vs intake restriction. Real life has it where a readily available 600 CFM carbruator will slightly increase VE at the expense of a slight decrease in intake velocity and will provide satisfactory results. Now, if you were to install a 750 CFM carbruator on this same engine, you will slightly increase VE over the 600 CFM carb, but there will be a decrease in intake velocity to a point where fuel atomization will not be complete. Some slight top-end HP gains may be found by going to the oversized carb, but the trade off will be less low- and mid-RPM performance as well as poor fuel efficency at all RPM ranges.

[Philip47]

thanks Forrest,
When speaking of fuel atomization thats where intake plenium depth/carb heighth comes into play where a low rise manifold running realitively high RPM will cause the air flow/fuel mixture to "slam" the bottom of the plenium causing fuel dropletes to come out of suspension, thats where carb spacers come into play. I think anyway

[GeneD]

Uh, just for giggles, tell me...how many of you guys out there are running 5500+ RPM's?

[CDMA]

I do I do I do....well more like I did...once by accident....that cost $1500....

Dem the brakes....

Chris :rolleyes: :rolleyes: :rolleyes:

[Philip47]

Gene
I just picked 5,500 as a random number, BTW rode in a big block boat the other day that turns 5,800 at WOT so I would think 5,500 is not out of the realm of reality for a pro built small block

[BigGrizzly]

I do Gene. The Corsican uses a 600 cfm holley 4010, ( faster than the 750 cfm) at 5600, you've seen it. The Criterion pulls 5700 with a 825 cfm Demond. However I don't recomend anything over 5200, lower units won't take it. I have had engine failures at these Rpms several times and I use the best parts avilable. So unless you can really afford it don't. The last time, the heads cost more then most peoples entire engine.

Randy

[TORYSMINX]

I use a 750 edelbrock marine carb and I have been fooling with it for a while. I changed the oil the other day and it is giving it too much fuel and fooling plugs. I am going with a different setup next year. Any suggestions for carb. The cam is 220 duration with .510 lift. I hear these demons are pretty good. Maybe BigGrizzly or somebody else could give me a suggestion.

Thanks
Tory

[BigGrizzly]

I tried the Edelbrok You can lean it out but it tales forever. I have found that in rough water the needles drop even with the offroad setup. I prefer the Holley 4010(600) or 4011(650) they are a two piece carb -> no float bowl gaskwts and jets change without spilling fuel. I also found the 600 CFM Holley to be faster and better on fuel on the Ford 351 Cleveland big port mortor in my Corsican,then the 750 I have. I twist it to 5600 frequently, and it has over 400 hp. On the semi stock 454 that came in the Criterion the 750 is a tadd too large, but i left it on anyway didn't want to screw with it since I was building a 502 blower motor. that motor uses a 825 Demond and pulls 5700 at 698.5hp ande 700ftlbs of torque. The Demond is just as good as any carb I have used and is easily tunable, like a Holley and they are just 35 miles from me. Thoes guys do know their stuff and will make a carb for your specs. My 825 has only one vaccum port just for tuning purposes.

Randy