CFM's of air on a 350

jnperin · May 14, 2005

I got into a discussion with my neighbor about electric superchargers. He has one installed on his Celica and thinks it will work on a 350. His charger pushes 1200 cfm's. I think when I am at 3k rpm the 350 must be sucking around 1000cfm and the supercharger will be ineffective. I tow 9500 lbs about 3 times per week. I am always complaining that when I go up a hill on the highway I can only maintain 45-50 mph.

Does anyone know how many cfm's of air the 350 vortec takes in at around 2500-3000 rpm.

Jim

Haulin' 8.1 · May 14, 2005

The math to determine an engines CFM goes like this:

CID x RPM / 3456 x Volumetric Efficiency (V.E.) = CFM

3456 is just short cut for a lot of math that takes into account engine volume drawn in at a certain speed with time in minutes. The fact that it actually goes in numerical order 3-4-5-6 is just a coincidence.

V.E. is the rating of how well your engine can fill it's cylinders compaired to ambient pressure that it's drawing from. This formula assumes sea level pressures. Without forced induction most engines never top 85%. An engine produces the most torque when it's filling it's cylinders as well as it can. You're asking about the CFM in the RPMs in which that engine makes the most torque, so let's be generous and say it's hitting 85% V.E. in that range. The formula would work out like this:

350 x 3000 / 3456 x 0.85 = 258.2 CFM

That being said I'd be surprised if that electric supercharger on his Celica would be able to build more than 5 PSI agaist the vacuum load of 5.7 liter engine. And then at 5 PSI your engine might hit 1.4 V.E.

Vernon

jnperin · May 15, 2005

The math to determine an engines CFM goes like this:
CID x RPM / 3456 x Volumetric Efficiency (V.E.) = CFM

3456 is just short cut for a lot of math that takes into account engine volume drawn in at a certain speed with time in minutes. The fact that it actually goes in numerical order 3-4-5-6 is just a coincidence.

V.E. is the rating of how well your engine can fill it's cylinders compaired to ambient pressure that it's drawing from. This formula assumes sea level pressures. Without forced induction most engines never top 85%. An engine produces the most torque when it's filling it's cylinders as well as it can. You're asking about the CFM in the RPMs in which that engine makes the most torque, so let's be generous and say it's hitting 85% V.E. in that range. The formula would work out like this:

350 x 3000 / 3456 x 0.85 = 258.2 CFM

That being said I'd be surprised if that electric supercharger on his Celica would be able to build more than 5 PSI agaist the vacuum load of 5.7 liter engine. And then at 5 PSI your engine might hit 1.4 V.E.

Vernon

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Thanks for that lesson!!!(and the answer) How much HP would the superschager make in a 350. Just an estimate would be great.

Jim

mcreyaufmiller · May 15, 2005

would probably cause a loss of HP. Jap Crap.

mudbug660 · May 15, 2005

wouldn't the cid have to be converted to cubic feet diameter for the cubic feet per minute to be correct or is there something missing?

Haulin' 8.1 · May 15, 2005

wouldn't the cid have to be converted to cubic feet diameter for the cubic feet per minute to be correct or is there something missing?

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Yes but that's all done in the 3456 divisor. Since those missing figures are constants it's simply X x Y / Z = 3456, so you can plug 3456 into the formula to cut down on the math. It's been a long time since I read the long version and there's no way I could remember it now. I've been using the short one since '86.

A couple of observations, due to the way things work if an engine was able to completely fill itself it's CID would equal CFM at 3,456 RPM and it's CFM would be double the CID at 6,912 RPM (3,456 x 2 = 6,912). Or at 100% V.E. a 350 draws 350 CFM at 3,456 RPM and 700 CFM at 6,912 RPM. It's helpfull for getting rough quesstimates of needed carberator sizes and stuff. Like a 454 would need a 900 CFM carb to run at 6,900.

Here's the formula for CFM of a boosted engine.

(CID x Max RPM / 3456) x ((Max Boost in PSI / 14.7) +1)

Hmmm, not trying to be a braggert but I've got a Whipple coming for my truck right now. I've not thought about it but I already feel the throttle is smallish for a 496. I wonder what kind of CFM that it will need at 5,000 RPM running the blower.

(496 CID x 5,000 RPM / 3456) x ((8 PSI / 14.7 Sea Level PSI) +1),

(717.6) x (1.544) = 1,108 CFM Yikes,

I don't think that 4.3 sized throttle will do the job. All it means is by redline the engine won't have as much V.E., therefore Horse Power, as it could due to flow restraints. Without a nice blower cam, large valve ported heads, and huge headers the engine won't be able to pull that much through the throttle anyways, to much flow resistance at all points. The quest for HP never ends until you're broke, you wanna play, you gotta pay.

Vernon

BigWheelinBubba · May 15, 2005

Don't put that crap on you're truck...those are marketed to make someone $, not make you horsepower. Spend the $ and get a real supercharger.

jnperin · May 15, 2005

Don't put that crap on you're truck...those are marketed to make someone $, not make you horsepower. Spend the $ and get a real supercharger.

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I don't want to put one in. Just wanted to know the feasibilty of if it will work. Thanks for the info everyone,

dogdays · May 25, 2005

It won't work because it is the wrong kind of fan. A supercharger is either positive displacement or else centrifugal to produce the type of pressure you need to get a noticeable result out of an engine. Sure it may blow 1000 cfm into 0.5" of water pressure but 1 Atmosphere is 34' of water and that's 14.7 psi.

So doing the math, 0.5 inch water equals a little less than 0.02 psi. So what you need isn't ever going to happen with some electric axial fan! You need 5 psi, not 0.02 psi! That's a ratio of 276:1. Save your money for some of those bead seatcovers, they'll have more of an impression on your backside than some "electric supercharger"!

R.

willieboy · May 25, 2005

It all depends on the electric super charger. Most of the boat blower type electric super chargers are total crap. However, there is a guy out there building real electric super chargers.

Check out http://www.boosthead.com/home.php