New Flow Bench Build: Some Questions

[head_tech]

Hi all
I have wanted to build a flow bench for sometime.
I really wanted to take my time and build a bench that will ‘stand the test of time’ so to speak
A reliable, versatile bench that will flow in both directions, to cover a range of engine sizes and hopefully wont cost me an arm and a leg to build.
I first started reading this forum back in 2009 and then rejoined this new forum earlier this year. After looking through this forum and researching I think I have settled on a design to fit my requirements.
An orifice style based on the designs tried and tested right here on this forum.

One of the biggest costs for my build is the vacuum pumps and the amount needed for a high flowing bench, I have checked the link from this forum to vacuum pumps and even at a conservative -say 6 pumps at $99 US dollars, converted to New Zealand currency @1.303 as of today. Plus freight and the import tax I have to pay based of purchase value of product.
You get the idea the cost goes way up^^ so I thought about it for a while.

I had an idea several months ago about using a roots type blower driven by and electric motor. I knew I could adjust the frequency of a 3ph motor to control speed, direction, and therefore air flow, I knew it had to work and I figured I wasn’t the only one thinking of this type of set up, I now see after checking out this forum there is a post over in flow bench air movers PTS Flowbench Forum - View topic - 8V92 Roots Blower Test that is exactly what I had pictured- blower mounted below electric motor driven by belt, mounted into cabinet, much like the PTS design. Digital manometers connected to computer etc.
Sounds good I thought but what about cost?

To me the first logical step is to build a good versatile cabinet with suitable air mover for my requirements to cover air flow for 2.0L 4 cyl through big block. But before starting any cabinet I need to know what pump and motor I’m using to accommodate their sizes into the cabinet.
That means I need to purchase my blower and electrical equipment and adjust the sizes of any cabinet I build to suit? Sound like a good plan so far?

Ok e-bay has remanufactured GMC 6-71 superchargers for around $500 US+freight+tax etc
Locally I can find GMC 6-71 second hand in need of remanufacture for $450 NZ, not a problem I have rebuilt one of these before new bearings seals, clearances etc.

I made some enquires about a new 2 pole 3ph motor, but how much horsepower would I need to drive a GMC 6-71 supercharger. I have worked with/on a 3-71 and a 6-71. I realized that the amount of power needed would increase from moving large volumes of air, and from my limited experience I knew they didn’t take Hugh amounts of power, although I’d never measured how much my best guess-termet was between 3-5hp
Turns out a new motor quoted from SEW-Euro drive NZ. 2pole 3ph 5hp is about $1500+ GST, plus a new VFD @ $800+GST.
I start thinking maybe the USA vacuum pumps aren’t so expensive after all.

I have this weekend looked at second hand electrical motors, which are obviously much cheaper.
After reading Im really thinking the GMC 6-71 supercharger with good second hand electrical 3ph motor and VFD is the best option for me. I see in the 8V92 roots blower topic that the member is using a 10hp motor.
There is a post from a member mentioning a roots pump type flow bench achieving good flow, enough to cover the range I’m aiming for. This encourages me to go ahead with this type of setup.
I have a few questions I’m hoping the members might help me with.

Ok how much horsepower does a GMC 6-71 require to drive? Is 10 hp needed or will 5hp be ok?
Is there a cheaper way of doing the VFD? Or am I better to buy this new?

Is it worth buying the PTS cabinet plans? I would imagine the plans are easily tweaked to allow for the size of my pump and motor?
Have I missed anything? Is there any pitfall’s a novice might be falling into?
Those reading this who have experience and built your own benches please offer any constructive criticism/advice.

Hope I have covered everything for this first step in my new build as I construct my bench I hope to post pictures and get input from the forum/members as to the best designs/methods
Cheers.
Grant

[Tony]

Grant, I really like the way you are thinking here.

If you can score a low cost secondhand roots blower, it may be pretty worn out, but should still work fine in a flow bench without any reconditioning at all. You can always do that later on.
We are not trying to make 20 psi boost at 20,000 blower rpm.
More like 1.5psi at 2,000 blower rpm, or less.

The drive power will not vary much (within reason) no matter what type of air blower you use.
A very rough guide to start from might be 5Hp = 300cfm and 10Hp = 600cfm for a typical bench.
A three phase drive motor should not cost more than $50 to $100 secondhand, but the variable frequency drive to run it will be by far the most expensive item.
As always, if you are very patient, some great deals pop up from time to time.