Air cfm's for two cylinders

A lot of guess and giggle numbers involved -- but assuming the 830 is about 470 cu in and the max RPM is a shade less than 1125 PLUS assuming it has a volumetric efficiency of about 65 per cent would give you somewhere close to 100 to 110 cfm flow requirement. As you move toward long duration, high lift cams and tunnel ram type intakes; you increase the VE. So for around 100% VE you'd get a flow number of around 200 cfm.

The displacement and max RPM numbers can, of course, be pegged exactly. The big guess is how efficiently the valve train and manifold can perform to suck in air. The formula to figure the VE is a bit complex and I'm more experienced working on these calculations with gas engines. I'd be a little surprised if anyone popped up and said they'd run the numbers on an 830 -- so until that expert shows up, the above 60% VE and 100 cfm number is probably a decent guesstimate.

Steve

Phil - I'm guessing at how you got that number, tell me if I'm wrong. And smoker, neither one of us gave you a formula to figure out the cfm requirement of an engine.

The quick and dirty way to do it is displacement times half engine speed times volumetric efficiency (or RPM x displacement /3456 for a 4 stroke or 1728 for a 2 stroke). So 470 cu in = .27199xxx cu ft. Half RPM of 1125 = 562.5. VE assumed at 1, so .27199 cu. ft x 562.5RPM = 152.99 cfm. If you change to a .65 guess you get 99.44 cfm. For performance engines, they use formulas and assumptions that give you a bit more headroom for flow demand, so you get results that approach the 200 cfm mark.

The electric blower thing is interesting, but you get down to trying to force 10 lbs into a 5 lb bag after a while, just like a bigger carb stuck on an engine without improving the lift and duration of the cam and the flow of the intake doesn't do much good. From another perspective, I think a 450 cfm fan is only going to add at most an inch or two of mercury to your intake system. Contrast that to a turbo charger turning 100,000 rpm and sticking another atmosphere, or 30 inches of mercury into the engine. The flow requirements of a 2 cyl are a lot jerkier than a turbo charged V-8 so at least you'd need the fan or the turbo blowing into a big log plenum. Even with that, I wouldn't hold out any great hope for the fan to really change things.

If you're still interested - there's a thing called an E-Ram which is the same idea using a 750 cfm fan which draws 50 amps and claims a 5% increase in HP in engines up to about 302 cu in. Bigger fan for bigger motors is available - but start thinking about a massive extra battery - and as always, it might be pure snake oil, so caveat emptor.

Steve

Phil

Thanks for the reply. I think the performance numbers we used for even flat tappet engines fudge under the general life theory that it's better to have too much than too little. You're right about the "correct" number. I'd remembered that street engines are about .65 so used that fudge factor - it might actually be less for a 2 cyl JD. There is a guy who sells some software to calculate that VE number now - but just measuring to get the input numbers is a job and a half.

smoker -
I forgot to leave you a link to the e-ram device. Also, their hot setup for BIG engines draws 100 amps (wow). I also just found a web page written by a guy who actually dyno'ed a pancake 6 Porsche with ~240 hp. He got 2- 3 more hp and 10 or so more ft/lb of torque with the 750 cfm fan. Not too encouraging. here's the link -
Link