Supercharger Porting? Science or Voodoo?

[Majik]

My new playtoy is a M62 SC going on a little 2.4L engine. I was looking into porting the supercharger case since I already have to rebuild it. In my research on this subject I'm finding more hokey BS than anything else ever. I found a site that shows where someone did a CFD on the m62 to analyze it and noted that ported would provide minimal (5hp or less) benifits.

http://www.aerospaceboy.com/384/index.htm

Heat was the main issue. Inlets and outlets were not. Others in the industry that work constantly with these units also have dyno graphs showing that their "magic $600 Stage 40 porting service" will net you 60+ HP. While I'm sure there are gains to be had there is no information on the subject. I guess the question is how does one measure gains? (utilizing a flowbench? and a specified rotor RPM?) second. Is there any studies or reference material that goes into porting superchargers? So far I've found nill. I think playing with the contour of the intake could generate lower temps and a slight shaping of the exhaust outlet would help but neither would seem to be reaching any monumental gains. Most seem to hint that the angle of the outlet seems to generate a large gain but on the factory unit it is fully radiused as cast. Any insight and guidance?

 

[patprimmer]

Is this a Roots, Screw or Centrifugal blower

Regards

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[Majik]

Sorry the M62 is a Eaton Roots Style with the twisted rotors (not a full screw type) and teflon edge strips. A picture of it is located on the website linked.

 

[patprimmer]

That is a positive displacement blower, so an increase in outlet size will not significantly increase output air mass. The only increase is that by decreasing back pressure, you might reduce blow by at the rotor seals and you might reduce output air temperature. If you reduce back pressure you will reduce parasitic power losses.

As the Eaton has a good reputation, you must suspect that there is little to be gained here.

On the inlet side, the less the resistance to flow, the higher the input charge density, therefore a proportional increase in output air mass will result. That is where I would look to for improvement.

Blower speed is by far the most responsive tuning tool.

If you want maximum power, turn it as fast as you can, but keep in mind, as the rotor tips exceed the speed of sound, efficiency drops off rapidly.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Majik]

I know I can run a smaller pulley on it to gain more power. It would be a good thing to calculate the "maximum" rpm before it reaches a sonic state but knowing the localized pressure and temp in the tips would make it difficult to calculate the speed of sound in that region. A good guesstimate would probably be close enough. The way the inlet is on these where the air comes in across the top of the rotors and then down into the engine makes enlarging the inlet difficult without creating excess turbulence. In a probing for "real world" statistics. Most gained about 20-30HP dyno proven power on a 400-500HP engine with a ported blower no matter what "stage" porting it recieved and who ported it. The one strange stastic that came up in this was that the porting became less effective as the humidity and temp increased. Several racers reporting that in the south such as florida (where I live) once it was really humid(ie not jan or feb) the ported blower maybe made a little more HP but lost some torque. A $800 super race ported blower from stiegemeier (supposedly good for 60+HP and 16ftlbs) trapped slower than a stock blower. Off a good 2-3mph on the same 14lbs of boost. Also producing almost the same HP readings. I'm leaning more towards voodoo than improvement over stock casting.

 

[patprimmer]

Roots blowers used on V8s in drag racing typically start to drop off efficiency at about 9000 rpm blower speed.

You might be able to scale from there by comparing circumference.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BradJohnson]

Wouldn't the money be better spent removing downstream restrictions to airflow?

It seems, at least to me, that increasing rotor speed (and thus, air volume) beyond what the remainder of the intake system can accomodate only serves to increase parasitic power losses, intake charge temps, and accesory drive loads.

Not being very familiar with forced induction, is there some constant that can be observed to maximize efficiency in blower/turbo applications? Maybe something tied to overall port volume vs pressure to determine a "sweet spot" for the best overall flow efficiency vs power consumption vs charge heating? Or is this something that is so application specific there is no set-in-stone criteria for some universally calculable result?

A) Thanks in advance
B) It's great to find a forum where the discussions have a solid engineering base
C) Nothing to put here, I just like having three things on my forum lists.

Brad

 

[Warpspeed]

Brad definitely has the right idea here.

With a roots supercharger you will definitely be limited by the maximum boost pressure and maximum Rpm the supercharger can be turned. There will be far more to be gained by reducing the total back pressure after the supercharger, and that includes the cylinder head, induction system, intercooler, camshaft and especially the entire exhaust system.

Anything that causes chronic supercharger constipation is really bad news for airflow, and trying to improve the blower itself is going about it the wrong way.

Build yourself a traditional high performance engine, but with suitable cam and compression ratio. You will make more Hp on less boost, and much lower thermal sress on everything. Let the whole thing breathe.

The greatest initial gains are usually made with the exhaust system. There is roughly a 1:1 relationship between boost pressure and total exhaust back pressure. Reduce exhaust back pressure by 4psi, and boost falls by roughly 4psi.

You then turn the supercharger faster to restore the original boost pressure. Flow increases, power increases, but induction temperatures and the detonation threshold stay about the same.