Impeller RPM or RPM+Boost, death of the Blower?

[Buck-Strickland]

Is impeller speed of 42,000 rpm at 8psi, the same as 42,000rpm at 4psi? Does the slipping or friction of the ball drive on the output depend more on the boost (measure of resistance) or is the impeller speed the culprit of potential damage? I realize that the impeller has a maximum efficiency point rpm wise, but I was wondering what really is the cause of damage for SN units. Is it the rpm? Or the RPM + boost? I have read a lot of threads that say don’t over spin the blower. I was just curious if anybody has an opinion on the matter. Thanks

[Michael]

The way I understand it, there are two reason not to overspin an SN:

1. Above 40,000 RPM (approx.) is beyond the impeller/scroll design's efficiency, so very little boost gets generated beyond that point.

2. Significantly increased levels of heat is developed beyond 40,000 RPM.

Heat is what breaks down the ATF fluid within the SN which is what ultimately leads to the bearing races/balls being damaged. And from everything I've read, heat breaks down ATF very quickly, so the more heat that's generated, the faster the fluid breaks down. That's the reason so many people run an external cooler on their SNs.

I don't think it's both RPM and boost that causes the damage, since the SN's pretty much max' out at about 5-6psi. I think it has more to do with over revving (in search of more boost) and not replacing the fluid on a more frequent basis.

[Buck-Strickland]

I concede that any RPM over 40k is useless with the standard SN or SN HO impeller. I was just wondering, as I have seen receipts of rebuild that state “Customer wants 10psi, torque to 25in/lbs”. It seems to me that the higher the boost the harder it is for the impeller to spin, thus more resistance on the output-shaft is required to prevent the slipping or skipping. Yes, a cooler is a perfect way to control fluid temperatures, and reduce the breakdown of the lubricant. As an aside, the VR-4 if I’m not mistaken requires far more load on the ball drive. Is that due to an increase in the rotational weight of the impeller, or is it due to the fact that they are intended to run 12-15psi? I am ignorant, and freshman when it comes to knowledge on these superchargers, so I’m just curious.
On a side note, does anybody know what material the supercharger brackets are made from and what thickness they are? I am building a custom bracket and I was thinking ¼” steel should be strong enough.
Thanks again.
P.S. This forum is a treasure.

[pavement pounde]

Michael nailed it pretty good.
I have rebuild mine 4 times as an SN93 and 2 times as an VR4, and due to some stupid mistakes on my side it overheated several times.
Twice to much ball pressure, ones with a to small blower pulley and overreving the unit, and ones with the VR4 on the highway for several hours while the oilcoolerpump wires conectors fell of, so no external cooling wich is a MUST on a VR4.
Everytime the impellorshaft start falling apart then the balls go out of round and turn blue then the races get knicked.
The SN never showed more the 6 psi and the VR4 never more then 10 psi, but that is on a 454 well breathing engine.

[Buck-Strickland]

Yes, it appears that the heat is the major issue, but with a good cooling system for the blowers they should last. Would not running a by-pass valve contribute to the heat, as it would back up presure to the impeller.

Are the standard cooling kits helpful, it seems like they come with a small cooler.

Marco,

You got 6psi on a 454 with a SN93. Did it have the improved impeller? What pulley combination were you using, crank and blower? What RPM was that at?

Thanks,

[Blown V8 Capri]

The way I understand it, there are two reason not to overspin an SN:

1. Above 40,000 RPM (approx.) is beyond the impeller/scroll design's efficiency, so very little boost gets generated beyond that point.

2. Significantly increased levels of heat is developed beyond 40,000 RPM.

Heat is what breaks down the ATF fluid within the SN which is what ultimately leads to the bearing races/balls being damaged. And from everything I've read, heat breaks down ATF very quickly, so the more heat that's generated, the faster the fluid breaks down. That's the reason so many people run an external cooler on their SNs.

I don't think it's both RPM and boost that causes the damage, since the SN's pretty much max' out at about 5-6psi. I think it has more to do with over revving (in search of more boost) and not replacing the fluid on a more frequent basis.

sorry to highjack the thread,
has anyone any more info on the cooling on the SN series please?
I have a SN92 that we are fitting onto a Rover (buick) 4.6 ltr v8 later this year and the car will be used for shows and drag racing etc,
and i may go down this route now before every thing is fitted and thinking to myself, i wish i had done that first :)

[pavement pounde]

Marco,

You got 6psi on a 454 with a SN93. Did it have the improved impeller? What pulley combination were you using, crank and blower? What RPM was that at?

Thanks,

I used a big tranny cooler with the dipstick pump.
It did have the improved impeller and showed 1 extra psi compared to the stock impeller.
I don't remember the pulley combination but the max impeller speed was 49.000 rpm.
I made 60.000 rpm ones and that lasted 1 day of driving and some dragrace runs however it did not showed more then 6 psi. just about everything was shot inside.
I kept my VR4 also at the limit of 49.000 rpm

[Michael]

sorry to highjack the thread,
has anyone any more info on the cooling on the SN series please?
I have a SN92 that we are fitting onto a Rover (buick) 4.6 ltr v8 later this year and the car will be used for shows and drag racing etc,
and i may go down this route now before every thing is fitted and thinking to myself, i wish i had done that first :)

The cooling 'system' used on SNs is a relatively simple affair. It's largely just a couple of tubes and a small heat exchanger (cooler) which you'd mount in an area that gets adequate airflow. It's very similar to an oil cooler setup you'd install on an engine.

The two tubes I believe are connected to the supercharger itself via the dipstick tube, but I don't know if there's an adaptor/valve which allows the supercharger's internal oil pump to pump fluid through to the cooler or if there is an external pump. From what I've seen, the size of the cooler itself isn't very large, but installing a larger cooler would not be difficult so long as the external pump has adequate flow.

I believe Paradise Wheels sells a cooler kit for the SNs. I think it comes with everything needed to install the cooler - fittings, tubing, cooler, pump, etc... Don't know how much it costs though, but I can't imagine it would be too expensive.

Hope that helps.

[pavement pounde]

This is the cooler: http://www.jcperformance.com/oil_cooler.htm
Paradize wheel is way cheaper and has a much bigger cooler btw.
On another sn92 I drilled 2 holes in the case and concted 2 hoses and an inline pump that pumps 5 psi trough an old oilcooler, 1 hole in the bottom and 1 hole above level.