GT40s.com - View Single Post

Ron Earp · 12-17-05, 01:15 PM

Bob, is that with 44mm chokes, the largest that fit in the IDAs?

I'm interested in this too and have bought it up on a couple of threads. One fellow sent calculations for a particular motor/cam I was doing and indicated I'd be out of luck with Weber 48s above around 6700 RPM or so. And, I've seen a couple of articles with Webers and 4bbls with some interesting conclusions (I know about 289s comparison on this site, but some more detail is needed there and it is only one data point).

Some questions and issues - do you want the head flow so close to the max flow of the venturi? Or, would it be much better to have the carb flow a max of say 500 cfm per barrel while the head flows a percentage of that, maybe 70% or so?

Does a 4bbl have an advantage - a 4bbl carb might flow 850 cfm on a built motor, but, on a single plane intake the cylinder that is opening "sees" a large percentage of that flow and it is flow more than 850 cfm divided by 8. Reason is that only one cylinder will be drawing an intake charge from the manifold and carb at a particular instant, compared to a weber setup in which one cylinder can only "see" a maximum flow of what one weber barrel will produce, in this example 340 cfm.

There have been numerous people on the board that have experienced "choked" motors with various EFI induction setups. Hershal, Jerry, Mark, and some others all experiened it with some systems at bore sizes ranging from 40-44mm. Some went to carb systems and whammo - car ran like a bat out of hell claiming the flow did it for them. I think Paul Thompson has 58mm bore on their EFI setup on their stroker motor for flow reasons. They don't have flow problems on theirs, but 58mm is 42% larger in area than a 44mm bore and would flow accordingly.

Mike G, what sorts of rwhp numbers on a Dynojet are you seeing on that 347? Would also be useful to have cam specs. Flowing 305cfm at 0.600" lift is fine, but if you aren't at that lift but for a short time it doesn't mean much. I'm interested since I want to run webers for sure, but I don't want issues with choking my high RPM motor.

Ron

12-17-05, 01:15 PM	#4 (permalink)
Ron Earp Ron Earp Site Administrator Join Date: Sep 2001 Posts: 3,870 Rep Power: 54	Re: TWM/Weber manifold flow Bob, is that with 44mm chokes, the largest that fit in the IDAs? I'm interested in this too and have bought it up on a couple of threads. One fellow sent calculations for a particular motor/cam I was doing and indicated I'd be out of luck with Weber 48s above around 6700 RPM or so. And, I've seen a couple of articles with Webers and 4bbls with some interesting conclusions (I know about 289s comparison on this site, but some more detail is needed there and it is only one data point). Some questions and issues - do you want the head flow so close to the max flow of the venturi? Or, would it be much better to have the carb flow a max of say 500 cfm per barrel while the head flows a percentage of that, maybe 70% or so? Does a 4bbl have an advantage - a 4bbl carb might flow 850 cfm on a built motor, but, on a single plane intake the cylinder that is opening "sees" a large percentage of that flow and it is flow more than 850 cfm divided by 8. Reason is that only one cylinder will be drawing an intake charge from the manifold and carb at a particular instant, compared to a weber setup in which one cylinder can only "see" a maximum flow of what one weber barrel will produce, in this example 340 cfm. There have been numerous people on the board that have experienced "choked" motors with various EFI induction setups. Hershal, Jerry, Mark, and some others all experiened it with some systems at bore sizes ranging from 40-44mm. Some went to carb systems and whammo - car ran like a bat out of hell claiming the flow did it for them. I think Paul Thompson has 58mm bore on their EFI setup on their stroker motor for flow reasons. They don't have flow problems on theirs, but 58mm is 42% larger in area than a 44mm bore and would flow accordingly. Mike G, what sorts of rwhp numbers on a Dynojet are you seeing on that 347? Would also be useful to have cam specs. Flowing 305cfm at 0.600" lift is fine, but if you aren't at that lift but for a short time it doesn't mean much. I'm interested since I want to run webers for sure, but I don't want issues with choking my high RPM motor. Ron