[toohighpsi]

One important factor to remember is never spin the blower harder than you have to, if you're trying to push the limits, it's critical to make sure you have adequate fuel, octane, and component strength to handle the boost level you are looking to run. So many time I see setups running high boost levels and no timing, this is a bad situation for boosting device - you're consuming a bunch of power through the belt to make that boost that you're not efficiently converting into horsepower with only 16 or 18 degrees of timing.

As for altitude, a positive displacement device is a Pressure Ratio (PR) device - it provides a given pressure ratio for the given drive ratio that couples it to an engine. The PR will stay the same no matter the altitude which makes it easy to calculate the boost pressure you will see at different altitudes.

For example:
You have a 2650 pulled for 2.0 PR - at sea level ATM pressure is 14.7 psi x 2 (PR) gives us 28.8psi manifold absolute pressure, subtract ATM (14.7) leaves us with 14.7 PSI boost.

So we now move to 5800 ft.

2650 pulled for 2.0 PR at 5800 ft - ATM pressure is 11.87 psi x 2 (PR) gives us 23.74 psi manifold absolute pressure, subtract ATM (11.87) leaves us with 11.87 PSI boost.

Of course I picked a 2.0 pressure ratio because that's easy - atmospheric pressure always equals boost pressure under that condition

If we're at 1.8 PR it looks like this.
2650 pulled for 1.8 PR at 5800 ft - ATM pressure is 11.87 psi x 1.8 (PR) gives us 21.37 psi manifold absolute pressure, subtract ATM (11.87) leaves us with 9.5 PSI boost.

To get the same amount of air into the engine at altitude that you can at sea level, you just need to spin it faster (increase PR) and pay attention to minimize the inlet restrictions. The SC will multiply (PR) whatever density you can get into it, so as you get to higher altitudes these SC inlet restrictions become more critical (A bigger throttle body is better to minimize the inlet pressure drop).

There isn't a specific max RPM you can claim that will fail the 2650, or any other Eaton SC for that matter - it's more application specific. Bearings and shafts (without rotors) have been tested to over 40,000 without problems so there's no worries there. The majority of failures occurs based off temperature gradients. Once you're in the mid 20,000 RPM range with a TVS the inlet face of the rotors become the restriction, causing additional internal work to be performed in the supercharger, resulting in higher power consumption and increased heat. Its a fairly delicate balance to keep these temperatures in check as the rotors get hotter and the case continues to to be cooled by external airflows. Once the rotors grow too big to fit in the case - you guessed the result - no more spinny...

Now crank the boost up and get that thing ripping!

[ZL1Atlanta]

Always full of great info! Tested to 40k RPM, huh…? Sounds like we have some more room to play on this combo :-D. Still going through the traps at under 100 degrees in the blower now.

[JSH]

Originally Posted by ZL1Atlanta

As a frame of reference, from 20-23 psi on the dyno it gained 100 RWHP.

Yea, we're shooting for 22-23psi but I don't have the right upper pulley today

[laynlo15]

Originally Posted by ZL1Atlanta

Always full of great info! Tested to 40k RPM, huh…? Sounds like we have some more room to play on this combo :-D. Still going through the traps at under 100 degrees in the blower now.

Under 100 degrees is stellar and I know mine were about 110 IATs and my 2300 was 145 to 150. I remember when we first started the Jack Shaft 2650s and talking to a lot of to the Copo guys and they couldn't believe they were going thru the traps and IAT were 75 degrees, they'd been trapping with the 2.9 Whipples at 110. Now the new 3.0 Whipple might be different but back then they were smiling a lot with those very cool IATS of the 2650.

[JSH]

Originally Posted by toohighpsi

One important factor to remember is never spin the blower harder than you have to, if you're trying to push the limits, :

Not going to push the limits because I don't trust the short block above 1000hp. I'll run 104 octane with as much advance as possible and up to 23 psi, might go to a 112mm..

[JSH]

Originally Posted by toohighpsi

To get the same amount of air into the engine at altitude that you can at sea level, you just need to spin it faster (increase PR) and pay attention to minimize the inlet restrictions.

My post about air and fuel was this:
Say I'm limiting the blower to 22k rpm and making 23 psi in Houston at 0 DA. If I put it on a trailer and tow it up here to 8000 DA, I will not be able to makeup for the lost manifold pressure, because I'm either unwilling to or can't increase the blower rpm enough to recapture the drop in atmospheric pressure and lost boost.

[laynlo15]

You'll have to install a smaller upper or just suffer the loss of boost. At Magnuson we'd sell a blower for a Jeep that was supposed to put out 6 lbs of boost at sea level and they lived in Denver so the customer would have to install a 5mm smaller pulley to make bring the boost back to 6 psi. Usually 5mm is about 1.5 to 2 psi either way you go up or down.

[ZL1Atlanta]

Originally Posted by JSH

My post about air and fuel was this:
Say I'm limiting the blower to 22k rpm and making 23 psi in Houston at 0 DA. If I put it on a trailer and tow it up here to 8000 DA, I will not be able to makeup for the lost manifold pressure, because I'm either unwilling to or can't increase the blower rpm enough to recapture the drop in atmospheric pressure and lost boost.

If I was in your position I would find the pulley combo that gives you the desired manifold pressure at your typical elevation/DA, and if you intend on going elsewhere, one for lower DA to maintain the same. At 22k you have a lot of room to play.

[JSH]

Originally Posted by ZL1Atlanta

If I was in your position I would find the pulley combo that gives you the desired manifold pressure at your typical elevation/DA, and if you intend on going elsewhere, one for lower DA to maintain the same. At 22k you have a lot of room to play.

I agree. At what blower speed do I get into belt tensioners, etc.?

[ZL1Atlanta]

Originally Posted by JSH

I agree. At what blower speed do I get into belt tensioners, etc.?

Over 22K I’d start considering the 10 rib drive setup like TooHighPSI offers. It was around that point that the 8 rib started seeming overwhelmed.