PaulW

I think Randy and skiracer were recently discussing this.....

air flow (in CFM) through a motor is equal to:

<b>[</b> displacement (in CID) x RPM x 0.5 x E_V <b>]</b>/1,728

Where:

RPM is redline RPM
0.5 is for a four stroke motor
E_V is Volumetric efficiency of the engine
1,728 converts cubic inches to cubic feet

In my supercharger book, it states that "Engine volumetric efficiencies vary, but for general purposes it is reasonable to use 80% for two-valve engines and 88% for four-valve engines."

So am I safe to use 88%?? Randy? skiracer? Bueller? Anyone?

And if I did, I would get:

<b>[</b> 254.5 x 6200 x 0.5 x 0.88 <b>]</b>/1,728

= 401.8 CFM

Does this sound right?

pw

skiracer

variable length intake runners. I would guess that the efficiency would be higher than 88% at lower RPMs and at least that at the higher RPMs. If you are using this to calculate the required flowrate for the centrifugal fan you are investigating, I would use 88% at 6200 RPM as a starting point, but I would also run the calcs at 5% intervals from 80% to 95% just to see what affect varying this number makes to the final design solution. You may find that the results are not very sensitive to this number.

PaulW

I will do different calcs.....and I think you're right.

And to answer your previous question on how the intake runner works.....I scanned my page on it....it changes over at 4200 RPM increasing and drops out at 3800 RPM decreasing....I bet with a centrifugal blower it's going to cause something to surge! I hope it's the car moving forward!!! ;-)

<img src="http://www.audipages.com/upgrades/supercharger4.JPG">

jrryan

Any where between 75%-85% (Common range for motors).Also keep in mind w/a centrifugal s/c you get parasidic loss in torque at low rpm. Gear box drive s/c's eat some power on the low end.Don't overcomplicate you're math...Do it correctly and Ve will be a whole new # at 105 plus...have fun.

ryoung

Volumetric efficiency varies,

... 90% @ 4000 rpm
... 98% @ 5400 rpm
... 94% @ 6200 rpm

The volumetric efficiency (and hp) at any rpm is hyper sensitive to cam specs and to the length and diameter of the intake manifold runner, intake ports, exhaust ports, and exhaust pipes.

This is based on an early model developed with <a href="http://www.performancetrends.com/Engine_Analyzer_Pro_v3.3.htm">Engine Analyzer Pro</a> from Performance Trends. This is very sophisticated software (it even allows modeling a 5-valve head) and is the same software I used to develop our race motor. I had to make quite a few assumptions with Audi's 4.2L motor, but they couldn't have been too bad ... the model predicts 311 hp. Does anyone want to loan me an intake manifold and head so I can get precise measurements :-)

For the ABZ motor, I just need to change the cam specs, which I'll do tommorrow, but the results will be in the ballpark of the AKB motor. And btw, it's very easy to add a supercharger to the model. Since the software requires specs for 12 different centrifugal supercharger parameters, it doesn't seem to miss much.

PaulW

Those seem like high numbers.....is that one of the reasons why it has a high hp/ci ratio? I would think so.

PaulW

The output of the ABZ engine is 300 HP @ 6200 RPM.....is that flywheel HP or HP to the wheels?

tozoM8

I think since 72' HP is always wheel HP.

ryoung

Spin it fast, and you've got serious naturally aspirated hp from not that many cubes. The only "catch" is that the hp is peaky, and you need to use the gearbox to keep revs near the sweet spot. It's amazing what you can do with proper tuning. Volumetric efficiency ~80% is more typical of old American iron.

BTW, the bone stock naturally aspirated 2L motor in the Honda S2000 gets 240 hp. If our 4.2L motors were that good, they'd put out 504 hp!

PaulW

And that's why muscle cars had such high HP ratings.