CARB CFM FORMULARS

jwojtasi · Post by **jwojtasi** » Mon Jul 04, 2005 6:40 am

HI ALL HAVE BEEN PLAYING AROUND WITH THE VIRTUAL ENGINE CALCULATOR AND PUTTING NUMBERS INTO IT . MAX RPM/CUBIC INCHES/ VE OK NUMBERS ARE 6500 RPM 308CI VE 85 AND CFM SAYS 490 ODD CFM.? CARB. NOW THIS DOES NOT ADD UP TO ME ? TO PRODUCE 380HP AT REAR WHEELS . THIS IS DOES WITH A Q-JET WITH RE NEEDLING AND AIR VALVE ADJUSTMENT. THEN TRIED AN EDELBROCK AFB 850 CFM MADE MORE POWER AND OWNER REPORTED LESS FUEL USAGE? IF THE CALCULATOR SAYS 490CFM WHY AM I RUNNING A LARGER CARB FOR ? DOES THIS HAVE TO DO WITH PRESSURE DROP ACROSS THE CARB. THIS MAY SIMPLE BUT THANKS TO THOSE THAT CAN HELP.

ozrace · Post by **ozrace** » Mon Jul 04, 2005 6:31 pm

Aussie V8 - Both the Q-Jet and the AFB have secondaries that respond to engine demand, so they tend to size themselves to the engine.
The Q-Jet flows over 700 cfm, but Holden were able to put them on stock 253's because of this.
490 is a bit small I would say, but something around 650-700 cfm would be ideal.

BTW - 380 RWHP from a 308 at 6500 rpm - Wow.

bill jones · Post by **bill jones** » Mon Jul 04, 2005 11:18 pm

-you have to remember that carburetor airflow numbers are measured with 1.5" of mercury (vacuum) showing meaning that if you installed a vacuum gauge and could read it at speed, when it comes up from zero (at WOT) to 1.5" while the engine rpm is way up there and the throttle is wide open, that's when the carburetor is getting close to what it was rated at.

jwojtasi · Post by **jwojtasi** » Tue Jul 05, 2005 3:57 am

DAM FORGOT ABUOT THAT? THANKS BILL , ACUALLY THOUGHT THAT ONLY HOLLEYS WERE RATED AT THAT, HAVE THE HOLLEY PORTIG FROM DON . DO YOU KNOW IF THESE PORTING METHODS WILL WORK ON OTHER CARBS, Q-JETS , CARTERS . JAP ONES TOO. THANKS AGAIN .