Roots blowers- maximum speeds

[pontiacjack]

Hi. I'm new to this particular forum, and hoping for some guidance on a project that's in the early planning (dreaming?) stage.

I bought a proven "lakes" race car (belly-tank lakester) and will be building a blown-alky four cylinder hemi engine for it (182 c.i. for class-F, where the SCTA record is 262 MPH). In order to get anywhere near the record, I'll need about four HP/c.i. (~720 HP). Engine simulations (using actual measured port flow of my head) reveal the need for 33 PSIG manifold pressure (@ 14:1 static c.r.). This is do-able, but I'm concerned about rotational speed of the blower- an 8-71 would need to be driven 47% over crank, which would put the blower rotors at darn near 15K RPM at peak-power speed of the engine (~10K RPM)!

"Conventional wisdom" won't help me answer this question, since I've already run a BDS 6-71 much faster (and longer) than it "should" have survived- two full competition seasons at rotor speeds just shy of 10,000 RPM. [However, it's a small-bore 6-71]

I see two areas of concern: 1) rotor integrity (explosion?)
and 2) behavior of the fluid flow at such elevated rotor tip speeds (turbulent and inefficient?).

Any and all comments are welcome...

 

[Lou Scannon]

I suppose a turbo is out of the question for some reason.

 

[TheBlacksmith]

What are the top fuel dragsters running and at what speed? Theya re pretty much cutting edge on these blowers. Rotor seal retention also comes to mind as a limit; I think most of the high speed applications open up the lobe clearance a little to allow for thermal expansion and install pheriphal seals.

 

[patprimmer]

Top fuel has a maximum overdrive allowed so that limits blower speed.

I only have anecdotal evidence that 14,000 is about as hard as you should rev a GM X.71 type roots blower. I think this is more to do with efficiency. Once again, anecdotal evidence suggested that as the peripheral speed of the rotor approached mach 1, the airflow through the blower choked. Some measurements and calculations proved that to be false.

I would also expect that he longer blowers like 12 and 14:71s would have more issues with over speed due to the longer rotor flexing and binding on the housing more.

It is common practice to use Teflon strips as seals on the rotor apex as rotor to housing seals and Nylatron brand graphite and molly filled nylon strips on the rotor flanks as seals between the rotors.

Regards
Pat
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[MiketheEngineer]

Average life time of a dragster engine is about 2 or 3 runs... go figure.

They blow up all the time. Hope you are rich!!!

 

[turbomotor]

The cost (in terms of power required) to boost air from 14 psi to 33 psi is very high. (About 1 hp to drive the blower for every 6 hp at the engine flywheel, assuming my quick calcs are correct and also assuming a 70% shaft-power-in-to-isentropic-air-power-out efficiency.)

So it really makes sense to consider a turbocharger.

If not a turbocharger, than at least a shaft driven compressor. Roots blowers (no internal compression) typically don't compress air very efficiently.

Have you consider a shaft driven centrifugal compressor. (I'm assuming that you have considered and decided against a turbocharger due to exhaust system side complexity.)

Check out this link to Vortech Superchargers:

http://www.vortechsuperchargers.com/global_item.php?gik=328

Dick

 

[patprimmer]

According to the OP this is not a drag racer but a lakes racer where the intent is maximum speed with quite a long run up and shut down area. Quite a different thing to drag racing.

While various methods of boosting an engine all have their own advantages and disadvantages, many races have been won and records set with roots blowers. There exceptionally flat boost curves vs rpm and time are the obvious advantage. Exceptional exhaust gas scavenging is another advantage. Simplicity of selection, installation and tune with an MFI system is another.

Regards
Pat
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[pontiacjack]

Thanks for the input; it's all good...

I agree that a compressor (centrifugal blower, screw-type Roots, whatever) offers much more performance than a Roots "fan". Problem is, this project is all about image- the car is meant to look like a "tank" built fifty years ago (body is fiberglas copy of an actual P-38 belly tank, etc.). Hopefully, I'll create the same-vintage appearance for the engine- the hemi head is a M/T casting from about 1961. Jimmy blowers pretty much dominated back then, so that's what I'll use. I frankly don't expect to be in contention for the class record anyway, so making less than maximum-possible power is acceptable. But I've set a personal goal of running comfortably over 200 MPH, which brings me to these blower-speed questions. And, yes, it will be mechanical fuel injection.

I neglected to mention that I've developed a fondness for the zero-maintenance aspect of "street" versions of Jimmy blowers (no Teflon, simply clearanced), which is what I plugged into the engine simulations so far. I'll run additional simulations with a Teflon-stripped 8-71 to see how much rotor-speed reduction could be gained (can't do that until I recover files from another computer).

 

[Lou Scannon]

Any chance of running a bigger blower, e.g. 12V-71?

 

[patprimmer]

High helix rotors give you about 15% extra capacity and look identical from the outside.

Regards
Pat
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[pontiacjack]

Thanks for the reminder about high-helix rotors; I'll look into that.

 

[bentwings1]

I don't see any reason you can't use one of the new "6-71" blowers commonly used on the nostalgia funnycars. We have 2 cars at the shop with these and they can make close to 30 pounds of boost on a 500 cu in motor with just 19% OD. They run 8500 engine rpm run after run.

Some of cars use 417 cu in stock stroke motors and run even more boost with the same OD.

You could run probably 40% od and have all the boost you need.

If you plan on using a stock GMC case I don't think it will hold up to the pressure with good rotors. Actuslly a stock case may have too large of ID to start with. Finding a good case to start with is pretty hard today. We bought 6 cores before we had enough parts to make just a streetrod blower.

I don't really know what the max sustained speed can be. I ran 60% OD on a good 8-71 with strips. Engine rpm was 8500 and the blower out lasted the motor(s).

99 Dodge CTD dually.