Roots Blower efficiency data 2

[patprimmer]

I have a dragrace car with a GM, Roots type blower.

It is a 671 at 14% overdrive at the moment

In trying to figure the efficiencies of extra O/D, I had the following thoughts.

The power to drive the blower faster will increase this is because it is spinning harder. The friction on the blower parts will be based on the original static friction plus dynamic friction. The dynamic friction will be load and surface speed dependant to some degree. The increased RPM obviously increases the surface speed. The increased boost obviously increases the load on the rotors. The extra frictional heat, compression heat and shear heat as air blows by between the rotors and the houseing, all add to the parasytic load.

The speed at which these parasytic losses are lowest in percentage terms is the speed of maximum blower efficiency.

For a GM blower, I guess, based on the RPM and O/D ratios successfull people seem to run, this must be about the 5,000 to 10,000 RPM speed range at the blower.

I would really like to see some air flow figures at various RPM, for GM blowers, and some figures for temperature rise and boost pressures with an unrestricted outlet, and against a few fixed size orifices (orifii?).

I would also like to see figures for the power consumed to while running each test

With these figures, we could then remove a lot of the guesswork.

Does anyone know where I might get some real data, instead of the anecdotal bull that seems to abound at drag race meetings Regards
pat

 

[Metalguy]

First, how well is your huffer sealed-teflon strips?

Don't know about the math part, but drag racers don't use Roots blowers for their efficiency. They use them for the throttle response, and experiment to find out what works best.

 

[patprimmer]

It's not teflon stripped, but that was a consideration

I know that a Roots blower is positive displacement and therefore works efficently at idle, and therefore give virtually instantainious throttle response.

I know they, and we, experiment at the track, but it all seems to be comments like, well that's what so and so did and it worked for him, or do it this way because it just makes more grunt, with five variations of which way, from five different people.

Basically, I am looking for data to calculate starting points for tune, and to calculate which changes might yeild the optimum results Regards
pat

 

[evelrod]

Pat, you need to get in touch with the folks over at B&M . Their units are positive displacement and I would guess they have a tech line.

Rod

 

[Metalguy]

I remember (barely) an old article in Hot Rod Mag., probably 20+ years ago, where they and B&M (IIRC) did a bunch of instrumented tests on different Roots blowers. I don't remember if the engines were running or not, as they measured airflow, pressure and temp. of discharge using different manifolds and blower RPM.

I think the main point of the article was that one shouldn't spin any blower too fast, because the discharge temp. skyrockets-they got temps. around 400 deg. F even when the pressure and flow didn't seem to correlate well. When they used a 6-71 at high airflow they got much lower discharge temps. at the same pressure than when using their little street blower, so I think they concluded that small roots blowers belonged on the street and not on the strip.

 

[SteveHuck]

There is a book by Pat Ganahl called Street Supercharging.
Covers things like Blower Efficiency, Adiabatic Efficiency, Mechanical Efficiency, Ect. Very good book, lots of math. Tons of info for the roots type blower. I have a 540 with a 8-71 myself.

If your car is street drivin or you keep the boost under 15 lbs., forget the teflon striping. They come apart and need service. Teflon is for high boost applications and people with street cars that like to brag about there "Racing Blower"

 

[patprimmer]

Mine is a so called street car.

Our class rules call for the car to be street registered, but there is no way we could actually drive it on the street.

It is currently sending our 20psi gauge off scale.

It is a 671 with 14% OD

I have pullys for 35% OD, but if efficiency does not fall of signifficantly, our Enderley 80a will not supply enough fuel.

I am currently trying to figure, do I buy 25% OD pullys, or do I buy a bigger fuel pump.

I think I have a book on supercharging by Pat Ganahl, by HP or SA. I obviously need to dust it off again and see if it is the one you refer to. Regards
pat

 

[SteveHuck]

Must be a small CI engine to get 20+Lbs. I drive mine all the time on the street. At 18% OD i made 7Lbs and at 25%OD i made 10.5 Lbs boost.

Sounds like you may need teflon if you plan to increase drive ratio. If you increase boost too high you may need to go to a hard anodized or even a billet rotor to keep the rotors from flexing. If you go with billet, you may reduce wear and tear on the teflon and get more life out of them.

What are you running for fuel? Don't want to pack it in to tight and start knocking!

 

[patprimmer]

It's injected (Enderley Bird), running methanol.

Its a 350 CI SBC, ported Brodix track 1's, 2" primaries into 4" collectors, 0.610" net lift roller about 310 degrees at 0.020, supermag 4.

Everyone here says we should only get 15 lbs boost, maximum, but gauge spikes out at well over 20 lbs. We also seem to need more fuel than advised, but we need to flow the pump to confirm this.

I think we just got lucky with a very good, tight blower. Regards
pat

 

[kimbo1]

In my limited knowledge of roots blowers i thought they were far inferior to turbocharging at boost above 10 psi or so? (not stirring anyone this is genuine question)does your class prohibit turbocharging?

 

[patprimmer]

Roots blowers have greater parasitic loss than a turbo, but they are positive displacement and crank driven, so lag time is zero. This helps launch a drag race car.

We are currently getting about 27 psi peak boost.

I need to know what boost and inlet charge temp rise I will get with the extra O/D, so I know how much extra fuel I will need, and if my current system will supply it.

We just ran 8.8 seconds in a 2400 lb car, at 148 mph.

Speed is down as it got loose and we backed off after we hit top gear.

I recently bought Desk Top Dyno 2000, but it is next to useless in this regard Regards
pat

 

[patdaly]

Pat, just a thought, would you be better served looking to an air compressor manufacturer for data? I believe at least Ingersoll-Rand still makes Lobe compressors, and I am sure there are others. I do know this, your IATs are very, very high, perhaps in the mid to high 300's, and possibly a better scheme than overdriving it more would be to fix an aftercooler to it. Just for giggles, have you given any thought to a centrifugal compressor such as one made by ATI? I know 27 PSI is pretty high for blow thru S/C, but I know of many who are running 24-25 PSI, and with the cost of aftermarket EFI dropping by the day, it really isnt a terrible financial hardship these days.
I assume you are running roughly a 350 inch engine, is this a bracket car or a class racer?
Just to give you a couple of examples, ATI has a carburated, 427 inch BBC with their D3 blower and 2 450 CFM carbs that ran 8.81 at 155 on 91 octane pump gas with 24 PSI boost. The car is pig heavy, perhaps as much as 3700 line weight.
My street driven Nova carries a 489 BBC, D1 blower, and EFI Line wt is 3980 ( Im a big boy )and though I havent made it all the way down the track, I did run several 121 MPH 1/8's, which should put me low 9's/high 8's in the 1/4.
I am going to step up to a F2 this year, as I am pushing the D1 past its Impeller speed and running in the transonic region, which does nothing but spike IATs and kills belts.

 

[patdaly]

Pat, forget the stupid question RE class rules, I just re-read the thread.....I need more coffee if I am going to keep working Saturdays.

 

[patprimmer]

I just got the answer to my question from another source. Seeing as many contributed, but could not supply the answers I was looking for, I decided to "publish my findings".

The drop off in efficiency is fairly sudden at a specific speed. This speed is as the rotor tips reach the speed of sound, which for the large dia 6:71, and 8 throughto 14:71 is about 8500 rpm blower speed.

My conclusion then is that the blower pretty well retains it's VE up till that point, so you must provide extra fuel in direct proportion to the increase in blower speed, then use a lean out device to kick in at about 8,700 or 8800 rpm blower speed.

Some of this info re blower sizes and rpm was aquired from the site

http://www.kuhlsuperchargers.com

The info re the rotors going supersonic was from a contributor in another forum, then as they say, "the penny dropped" and the path forward was clear.

We bought a fuel pump with about 90% more fuel flow, and we are currently running at correct mixture with 110% fuel flow to the engine compared to the previous pumps capacity.
We also have a 30 deg and .050 bigger cam, and OD up from 14 to 20% and a better inlet manifold. We are still pumping about 27# boost and looking to very low 8's on our first serious pass. (We ran 8.9 sec on a half pass first time out)

As the last set up used about 88% of the previous pumps capacity, we must have picked up about 200 HP from about 900 to 1100.

Regards
pat

 

[Warpspeed]

Pat, you will lose Ve long before the rotor tips go sonic.

The problem is turbulence at the rotor inlet, the rising lobes at the centre of the inlet port stir up the air there, and prevent the descending lobes around the casing from being properly filled.

So as you increase rotor RPM the filling gets worse (Ve falls), and you quickly get into diminishing returns. If you want big airflow numbers get a bigger blower and use less blower speed. If you want fast boost rise in a street car, use a small tight blower and turn it faster.

Another thing you can do is use either a turbo or centrifugal supercharger to fill your roots blower, and run it less hard. You get the response and low end grunt, and also the top end airflow of the centrifugal.

I have staged a turbo and a roots blower on a small 4WD car, and it is absolutely the best and most efficient setup I have ever had. Rules may not allow you to do it for competition though, too bad.