Originally Posted by danzigs
I have an 83 Camaro with a 350, 882 heads (I know, I know), long tubes, dual 2 1/2" exhaust with Flowmasters, Eddy 1406 600cfm carb with a performer intake, and 4.10 posi. Lopey, solid lift cam ( I didnt build this but got a hell of deal on a nice solid car project), and a TH700R4. Fun car, runs good locked out timing at 36 degrees, I guess I will get to the point now. Rebuilt a 750 Demon for a friend and tossed it on my car while I had it. HOLY S#@T!! did it run! Idled rich and ran a little rich which Im sure its too much carb for my 350 but what a difference in the way it responded! Now I'm looking for a good Holley style carb I guess. Does anyone have any expierence with the new Summit Racing street and strip 600cfm carbs? My choice, I would think, would be the Holley Street Avenger 650. I have to keep the intake manifold because of hood clearance. Any of these combos good or is another better? Just on a tight budget and the Demon would probably be out. I've played with my present carb with metering rods, springs, primary jets, but never got the response like I got out of the Demon in which I understand is a Holley. Maybe I'm missing some tricks with mine?
600 or 650 cfm is too small no matter whose. I know the formulas commonly found in hot rod magazines and soft cover books say to use a carb with CFM that is equal to half the displacement times the max RPM's you'll turn it divided by 1728 which for a 350 will be about 608 cfm at 6000 rpm. However, except for a moderate street engine this isn't enough to pull the best power out of the motor. David Vizard includes a graph with factors to increase the carb CFM from that basic calculation based on cam timing, but in my experience even that doesn't get the carb big enough. The underlying problem is dealing with reversion, this takes considerable space within the intake passages and back through the venturies, I suspect the problem is not only sizing the carb to accommodate the reversion but to also be large enough to attenuate the signal it presents to the metering system. That simply takes a larger capacity carb to soften this disturbance than the formulas for feeding the displacement indicate. This gets to be a bigger problem with larger cams and or using high ratio rockers where there is more exposure of the exhaust system to the intake during overlap and longer exposure to the effects of the rising piston with late closing intake valve timing. So I'd recommend a carb of 750 to 800 cfm for best performance. 750 for a Holley or clone thereof and 800cfm for a Q-Jet or Edlebrock/Carter.
The Demon is an updated clone of the Holley but is not made by Holley rather by a former Holley engineer. They are very good and like the Holley offer easy tuning and a wide availability of parts where ever you are. I'm less enthusiastic about either the Q-Jet or the Edlebrock/Carter for the average guy. These can be made to work very well but it takes a lot more personal knowledge on the part of the tuner and parts are available only from a small number of sources what adds a lot of time and cost to getting them tuned.
The Performer RPM intake is a world apart from the Performer, if you can fit one, I'd actually start here rather than with replacing the 600 cfm Edlebrock/Carter. That intake will make the carb you have seem bigger to the engine. If you can't get the taller RPM under the hood and don't want to carve up the hood for a scoop, then I'd start with a Holley or one of its many clones. For mostly street with some track I go with the vacuum secondary, for a competition only application then a mechanical secondary model. Match the carb mount of the manifold to the carb being used, I find the Holley or square Edlebrock/Carter really isnít as good as it can be when used on a multiple configuration mount that includes space for a Q-Jet, too much turbulence. If you must use one of those intakes with a universal carb mount then use a spacer with down tubes under the throttle plates to carry the mixture an inch or so into the plenum.
Bigger cams with the Performer intake and a smaller CFM carb like a spacer, but again getting into the bottom of the hood may be a problem. Using a modern remote air cleaner such as found on contemporary fuel injection cars can be used to lower the profile above the carb letting a taller intake or spacer squeeze in without having to modify the hood.
Shop EBay for parts like the intake and carb, you can score some pretty good deals.