Bingo. Carb numbers, or model numbers for that fact are sometimes misleading. Talk to a carb professional before you jump into the fuel pool heads first.

Seriously though, I do understand that for general purposes, the VE to CFM sizing does work. My point is that for today's motor with so much really really good performance stuff out there, general terms in many cases just doesn't fit. Example: Look around. Several 496 marine motors builds making max. HP at 6000 RPM are using a 1050 dominator.
496 x 6000RPM / 3456 = 861.11 and that is at 100%. The days of using poor flowing peanut port heads are few. And that is a good thing. We all benefit from the aftermarket desiging and building great performance products.

Why you crappin' on Rotax ? he makes many valid points.

Here's my quicky opinions:

Holley's cfm recomendation chart is very conservative. This is is so the general public doesn't get themselves into trouble. Yes, it is easier to hurt power/driveability with 'too big' of a carb vs 'too small' but what is too big today with modern cams, heads, and 500cid's ?

Many engines with modern cams, intakes, and heads will have good driveability and very good top end with a carb sized somewhere around 120% with a single plane and around 150% with a dual plane.

Yes, carb #'s are all monkeyed around. The Hp950 is closer to 830cfm. But they work well because it's metering is very good. Not because of big airflow. Why does the HP500's 800cfm carb work well on bigger motors ? It meters fuel well.

If you had a more airflow capable carb and kept the good fuel metering, then bang, the engine would make more power. The trick is to find an off the shelf holley that meters fuel as well or to find a carb tuner to build one for you.

Sometimes, the airflow demand makes a carb not meter so well, but more often than not, a carb that does not meter so well is because the carb metering system suks.

Don't always blame a poor performing carb by saying 'it's too big.'

BTW: as long as there is vacuum measured under the carburetor, it is restricting airflow to the engine. It's almost impossible to make a carb big enough to cause absolute 0" vacuum. A 1250 won't even do that on a 502.

Got out my dyno sheets from my session with 502 ci with custom cam and 305cc AFR heads.

Stock (old) 500HP blue carb with choke and stock dart intake-563 hp at 5400 rpm, peak tq at 590lbft at 4400 rpm.

Switched to Holley 950 cfm Racing with 81 primary jets and 82 secondary jets up from the 79 square-586 hp @ 5400 rpm, peak tq at 605lbft at 4500 rpm.

Switched to Edelbrock Vic Jr. intake with 1" open spacer-595 hp @ 5500 rpm, peak tq 613lbft @ 4600 rpm.

These are corrected numbers, CF was .99 that day.

I think that Wags382 massaged a stock 500HP carb and did well with it by removing the choke along with some additional work.

Edelbrock intakes are also much less expensive than the Dart intakes.

On my 548, on a dyno, using a massaged Victor manifold, my motor made 12 more ft/lbs of torque at 3000 rpm with a 1050 annular discharge carb, than it did with a 950HP. The 950 laid over and died by 5000 rpm. The 1050 was up over the 950 by 31/hp at 5000.

The motor made 685/tq at 4600, and 682/hp at 5750.

Thankyou Bob Madera aka RMBuilder.

Chris

we kinda got off the post here... how about with stock rect heads and cam ??

For all out top end on a 502, I would pick a Victor Jr. It will probably beat the RPM air-gap by 10-12 hp at 5200-5400 rpm. The RPM air-gap will beat it untill about 4500 rpm.

I would use a Holley HP1000 carb, which is just a 850 DP with the choke horn removed and better metering circuts.

Been there... tried all that... on a dyno... with that motor.

Chris

P.S. I would have a cam done by Bob Madera aka RMBuilder. The guy really knows his stuff !

thanks....some good info from this thread....Rob