2 x 8-71 GM blowers

[AEDCarburetors]

hi guys ... just in the process of building wet carb flow bench .... i have 2 x 8-71 Gm blowers to move the air that are in good condition . They displace 436ci per rev each . Worked out that have to spin the blowers to around 3000RPM to be able to flow 1500CFM for the larger 2.1inch bladed dominators i do.

Question though... would the blowers still be efficient if they where mounded on top of eachother...... or would it be best to arrange them with there own plenum then transitioning into the liquid/air seperator drum.



Ray

RayJE Carburetors
Brisbane Australia
www.rayje.com.au

[ThomasVaught]

Probably the best deal would be to mount the two blowers in parallel so that the inlets are tied together with a common plenum. The blowers can be mounted so they sit on edge inside a metal frame.

Make the metal framework so that the "manifold side" of each blowers would mount to a side wall with the discharge from the blowers exiting 180 degrees from each other.

On the bottom of your framework mount your LARGE electric motor OR a small V-6 or V-8 engine with the flywheel end point the same way as the drive pulleys on the blowers. Use a heavy duty cog belt system to drive the two blowers at the same time.

A friend did a similar installation and left the engine/blowers outside his shop in a locked "lean-to" with ventilation for the engine/blowers. He routed the inlet air piping into his shop to his flow bench table. You could hear the engine running but with the exhaust muffled in was pretty quite.

He did not throttle his system but used "by-pass air" to control the suction pressure on the carbs. Less by-pass air to the blowers more suction to the carbs.

He used an orifice plate set-up in a reinforced cabinet made from a 55 gallon drum to measure the airflow in the system. The drum had an access plate to change how many orifices were open at one time. The plate had three orifices that were the same as a Super Flow 1200 bench (200 cfm, 400 cfm, 600 cfm) and a second 400 cfm orifice for a total of 1600 cfm. The bench used rubber stoppers to determine orifice flow.

The bench used a normal (Dwyer 246) 6" inclined manometer for the different flow ranges. It worked very well.

General Motors had a similar bench set-up using 6-71 GM blowers in their flow lab years ago.

Tom Vaught

[AEDCarburetors]

Thomas... or anyone ... do u have the formula to calulate what horspower motor would be required to spin the blowers .....

[ThomasVaught]

I have an excel spread sheet at Ford Research that would easily give you the number which I could e-mail to you after the new year. Other members may have a better rule of thumb but I use it takes 1 hp to move 1 lb of air/minute (at 15 psi) PLUS the mechanical losses from the boosting device
(assume minimum of 15%).

Assume 13.3 Cubic Feet of air per Lb of air (@ .075)

You wanted a 1600 cfm system so 1600 divided by 13.3 = 120 lbs/min

120 times 1 hp = 120 hp times 1.15 = 138 hp. Each supercharger would draw about 70 hp to move 800 cfm @ 15 psi.

Tom Vaught

[Tony]

Yes, stolen from Corky Bell's book "Supercharged" page 203

Thermodynamic horsepower to compress air = boost (psi) x CFM divided by 229. Say 1,500 CFM x 1 psi / 229 = 6.55 Hp

*note, 1psi is almost exactly 28" of water

Divide that by expected blower adiabatic efficiency, maybe 60% ??
Possibly 6.55/.6 = 10.9 Hp

Divide that by drive losses, belts, bearings, gears and so on.Maybe 85% ??
10.9/0.85 = 12.8Hp

I guess for wet flowing four barrel carbs you would be looking at 1.5 inches of mercury test pressure which is roughly 0.735 psi

A 15Hp motor would probably be up to it, but I would rather go larger than that if I was doing this myself.

[ThomasVaught]

I agree with your calculations but typically I never try and run a Boosted engine at 1 psi boost pressure. My back of envelope assumes much higher pressure/ depression, motor hp, lol! According to Corky's equation that 15 psi boost and 1500 cfm would be closer to 192 hp so his is conservation on the hp reqd. (More HP required).

Tom Vaught

[Tony]

At only 1psi or less, the blower may be a good deal better than 60% efficient, but I always prefer to be a bit pessimistic in these sorts of exercises, especially for other people.

Corky also suggests 90% for drive efficiency, whereas I used only 85%.

But 15Hp should get the job done o/k with a little bit to spare.

Ha-ha, yeah, 1psi boost is not a lot, but it is still surprising how much grunt it takes to move a lot of air against some back pressure. The first timer multi vacuum motor guys are often very surprised at the high number mains amps required to run a flow bench.

[AEDCarburetors]

thanks guys .... it was around where i was thinking.....im going a bit bigger .... 15KW motor with VFD that bit extra might come in handy..... good thing about GM blowers is the availability of pulleys i i want to spin them abit harder ....