Q: Beyond 9lb pulley size above 5000 feet; math?

Comptech 9 lb kit does around 5.3 pounds of boost at 5,200 feet, according to dynos of two different '91 NSXs in the Denver area. This is discussed, in part, here
http://www.nsxprime.com/forums/showthread.php?t=4248
and here http://www.nsxprime.com/forums/showthread.php?t=74826
So. Theoretically, the whipple CTSC runs out of efficiency somewhere around 8.x pounds of boost.
Is this true at 5,200 feet? I am sure someone on this forum knows the forumulas that can tell me whether I can increase boost safely with a smaller pulley (beyond the Comptech 9 lb kit) at this altitude so that I can get more than 5.3 pounds. Not a lot more. Just a bit... and... what size pulley?
(with some apologies from posting to an old thread, not quite knowing when to do that and being off-topic, vs. starting a new thread... but I know how to search).
Thanks and apologies in advance,
--bruce

That's a great question and one I'd like to know as well. I'm at 5300 feet down here in Albuquerque, and I've been thinking about getting a CTSC one day in the near future.

You'd think that (because of the less dense air up at altitude) that it would take more rotations of the SC to pump the same amount of air molecules into the cylinders as opposed to at sea level. Like you, I wonder if a 9lb pulley at altitude would be very similar (and as safe) as a 6 lb pulley down closer to sea level, and there might be a way to go smaller than the 9lb pulley size for even more boost. The only thing I can think of is that the speed the SC is turning and the small size of the pulley may be limiting factors as well.

Anyone?

Viper Driver said:

...only thing I can think of is that the speed the SC is turning and the small size of the pulley may be limiting factors as well.

Anyone?

Click to expand...

That is correct. Any blower (centrifugal, screw, or turbo) has theoretical rpm limits. Once you're past that threshold, spinning the blower faster won't add additional boost. If you are familiar with fluid dynamics, cavitation occurs, and the blower looses efficiency. It can even be damaged from being over-driven.

Get details from whoever your blower mfg is, to find out what is the peak rpm spec. Size your pulley (or turbo) accordingly. If they tell you to "spin it as fast as you can", you need to talk to someone else...

If you are at altitude, and your source air is lower density, it is a natural side effect to have lower peak boost output.

Everything I am going to refer to is for the 3.2 motor not the 3.0 motor.
I have spun the1.6 liter whipple charger to 16,500 rpm using a 2.9 inch diameter pulley and this resulted in 9.4 lbs of boost after the intercooler so add a pound or so before the intercooler. I was told buy Comptech they had spun a whipple to 14 lbs on the 3.0 motor during some testing. IAT's were in the mid to high 200's.

More boost is possible but it increases the intake temps so much you have to use a cooler and after market engine management. Any thing past the "high boost" option from Comptech and you are dealing with a real heat and timing problem.

The new Autorotor is about 25% more efficient so you could get more boost with less heat. The problem is if you set it up like this for altitude you would run lean at sea level.

Joe

I want to start by requesting that everyone who reads this thread cease using the incorrect "9-lb kit" nomenclature...and that they correct others when they see this misnomer used. "High-boost kit" is the name given by Comptech and SOS for their respective kits and is the correct term. When applied to the whipple-based CTSC (what they were originally designed for) they do not produce 9psi, even at sea level. I think I've read numbers around 4.5-5psi for the regular kit (often incorrectly called the "6psi kit") and 7.5psi for the high-boost kit...at sea level.

BTW, I understand that the 6-lb and 9-lb terms are finally becoming somewhat accurate when used to describe the autorotor kits...but even there I think there are better ways to describe things since boost will vary given conditions, etc. My main gripe here is that since the CTSC came out I've seen many people (lots of them at sea level) upset that they're not getting 6psi and 9psi from these kits (respectively)...all because people call these kits by these terms.

-----

Anyway, to relate that back to this discussion: I think it's more appropriate to shoot for ~7.5psi (not 9psi) if one is trying to match the sea-level output of the high-boost kit.

I do think Bruce's theory is correct (smaller pully to make high-boost kit output at altitude what it does at sea level), that jdnsx's warning is also correct (with such a setup driving to sea level would not be advised without switching back to the regular high-boost pully), and that "the nsxnut" is giving good advise to seek out the manufacturer RPM specs for the blower and then stay within those.

Where I'd question things is RE temperatures and the risk there.

There will be a little more heat heat generated in spinnig the faster at higher altitude (than spinning it slower at lower altitude to reach the same PSI) because of bearing & gear friction. However, I'm tempted to think the amount of heat over the high-boost kit at sea-level will be minimal since I'm under the impression that a lot of the heat issues in spinning an SC faster (i.e. to generate 14psi as in example from jdnsx) are a result of friction between the SC and the air (and within the air itself). In this what-if we're talking about generating the same density of air on the output side...so I think the air-on-sc and air-on-air friction will not be significantly higher (and thus the IATs would be similar to the high-boost kit at sea level).

Someone with a degree in this area would really be useful (to confirm or refute my hypothesis on this). Having the luxury to try this out on a dyno and measure the IAT's would also be a good way to figure it out.