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Max RPM of roots superchargers when bypassed?

W

wmorrison65

New member
This isn't specific to L67's with M90, I'm just hoping somebody here knows in general.

But I'll use an M90 as an example. From specs I've read online, the maximum RPM for the M90 is 12000. Let's assume that's correct. My question is, is this an absolute, or only when the supercharger is under load? (Moving air, not bypassed.)

I'm asking relative to twin-charged applications, where a supercharger provides some low-end "grunt" until the turbo spools up, and then the supercharger can be bypassed (clear path between inlet and outlet, so even though it's still spinning, there's minimal parasitic loss.)

Normally, in a supercharger-only system, the supercharger wouldn't be bypassed at higher engine RPM. But in a twin-charged application, it would. That's why I'm wondering if the pulley ratio could be safely set to where the supercharger reaches its maximum RPM just after the engine RPM where it would be bypassed because the turbo is fully spooled.

Just planning for the future (i.e. "dreaming," and not for my Grand Prix.)
 


The speed rating is dictated by the bearings and nothing else. 14K was the number I remember for the M90.

Jeff
 
I was curious so I looked it up.

The rear bearing in snout if using SKF 6204, max rpm is 20,000.
Front snout bearing if using SKF 6203, max rpm is 24,000
The rear case needle bearings are INA fc 65477. Couldn't find specs on them. Found one a little smaller, and it's max rpm is 16,000. Larger diamater will likely drop the max rpm so my guess, Like Jeff already said, is around 14k for the 23mm INA's.
No idea what the front bearings are in the rotor pack, didn't go that far because I think they would be similar to the snout bearings.

So looks like the weakest link is the needle bearings, and max is most likely less than 16k.
 
bandook said:
I was curious so I looked it up.

The rear bearing in snout if using SKF 6204, max rpm is 20,000.
Front snout bearing if using SKF 6203, max rpm is 24,000
The rear case needle bearings are INA fc 65477. Couldn't find specs on them. Found one a little smaller, and it's max rpm is 16,000. Larger diamater will likely drop the max rpm so my guess, Like Jeff already said, is around 14k for the 23mm INA's.
No idea what the front bearings are in the rotor pack, didn't go that far because I think they would be similar to the snout bearings.

So looks like the weakest link is the needle bearings, and max is most likely less than 16k.
Click to expand...

Where did you find the information about what bearings are used?
 


Crazyguy03 said:
what would also help is a chart showing pulley size vs engine rpm vs sc rpm for gen5 and gen3
Click to expand...

That would be nice. You made me curious so I decided to see how horrible my math skills really are.

This was just a quick search so I'm not positive the diameter i'm using for the HB is right. But on summit the balancer is said to have a 7.156" OD. So I guess you take that and divide by pulley size. Using my 3.4", that gives me 2.1. Multiply that by say, 6000 crank rpm and the blower is spinning at 12,600rpm. Still in tolerance, so it looks like the smallest pulley you should theoretically go and stay in specs is a 3.1 on the M90, which would hit 13,850 rpm at 6000 crank rpm. But I don't know anyone who is hanging out above 6k rpm long, so I imagine you can safely go smaller.

Again, I probably butchered something, but it was interesting for me anyways :)
 
wmorrison65 said:
Where did you find the information about what bearings are used?
Click to expand...

I linked the bearing specs. Click on the rpm's. These are the bearings that are typically used in rebuild kits. And same part numbers I pulled out of mine when I rebuilt, just different manufacture for the snout bearings.
 
The smaller the bearing the higher the RPM. To be perfectly honest they will handle quite a bit higher we used to spin them to the moon. It's common for the Tbird SC to run a "10%" pulley which put it at a 3.03:1 this puts the blower speed at 18,180 at 6K engine RPM.

I I realize this is a different gen blower but here are some calculations I used, very accurate for that blower.
http://www.sccoa.com/forums/showthr...driven-blowers-!&highlight=Blower+drive+ratio


Jeff
 
bandook said:
I was curious so I looked it up.

The rear bearing in snout if using SKF 6204, max rpm is 20,000.
Front snout bearing if using SKF 6203, max rpm is 24,000
The rear case needle bearings are INA fc 65477. Couldn't find specs on them. Found one a little smaller, and it's max rpm is 16,000. Larger diamater will likely drop the max rpm so my guess, Like Jeff already said, is around 14k for the 23mm INA's.
No idea what the front bearings are in the rotor pack, didn't go that far because I think they would be similar to the snout bearings.

So looks like the weakest link is the needle bearings, and max is most likely less than 16k.
Click to expand...

You're right about the rear needle bearings being the weak link. I spoke with a guy from Eaton back in like 99'-00' (Ian or Ed, don't really remember) and was told that the reason for the 14K limit was the needle bearings and the fact that they use grease and not oil for lubrication. I was also told that the dynamic load on the blowers was also the limiting factor. We had one guy that ran I don't remember if it was the Texas Mile or the one out east that is a top speed run. He was in the 120 mph class he ran in 4th gear at 5.5K engine speed and I think his blower speed was around 16K he didn't have a bearing issue he actually had the rotors swell from the heat and when he stopped and shut the car off the rotors seized to the case. Most of the bearing failures I used to see were the Snout bearings, most of the time we replaced the needle bearings at the same time, kinda while your at it thing. The cartridge bearings have a lip of aluminum rolled over the edges to hold them in place. You never want to replace them because re-timing the rotors and gears is a PITA. I did it once to coat them in an epoxy polymide paint that was a really good thick paint and had a hell of a time getting it back together. A company called ESM used to rebuild the cartridges but I am not even sure he's around anymore. Mangnuson made the Magnaport series of blowers and ended up becoming a Eaton certified repair station if I remember correctly.

Jeff
 


GTP guys will never get close to max speed. You are only at 15.5K at max RPM so you're just barely over and just for s short time. Like I said we used to spin them over 18k and never had issues.

Jeff
 
So at idle with a 3.8 is "x" blower rpm

so take the number say x is 1,000 blower rpm and each .1 smaller pulley increases rpm by 200
That would make the blower rpm on say a 2.8 be 3,000 rpm blower speed at idle.
now if we had a chart we could see that the idle speed of the blower on 2.8 equals what the blower would be at on 3.8 while maybe at 4,000 engine rpm
it helps put into perspective the blower speed and heat etc with different pulleys at different engine rpm


I had always wanted to see this information put into a chart
 
I'll do it up. Why are everyone's screen names changing?

Basically just take the crank drive pulley and divide by the blower pulley then multiply engine RPM.

So 7.156/3.8 X engine rpm

Then just multiply the ratio 1.88 X 5000 = 9400 blower RPM
or 1.88 X 750 = 1410

1.88:1 with a 3.8" pulley
1.99:1 with a 3.6" pulley
2.10:1 with a 3.4" pulley
2.24:1 with a 3.2" pulley
2.39:1 with a 3.0" pulley
2.56:1 with a 2.8" pulley
2.75:1 with a 2.6" pulley

Jeff
 
Nice. So using a 2.8 at idle is turning the blower 510 RPMs faster then the same blower using a 3.8
and a 3,400 rpm increase at 5,000 rpm comparing the same pulleys - that's incredible


so using a 3.8 you would have to rev the car to 6,800 to achieve what the 2.8 can at only 5,000rpm
 


Not quite, as engine RPM increases the amount of air use increases as well so you would not make the same boost at that point. I need to take a look at my formula I think it may be incorrect in that thread I linked. Something isn't adding up....and I haven't had to do a lot of calculations in many moons. After I get back from picking up some bolts I will see if I can see where I went wrong.

Jeff
 
No worries. Just fooling around with the numbers - It's very interesting if you can see what I'm trying to get at.
 
Of course, if you look at that link I think that one was from 05'. That board only goes back to 2002 and I was doing all that stuff in the late 90's when all that was available was dial up lol. So I have been messing with superchargers for a few years. I have tons of tech articles that are in the members only section that used to be the TBSCEC which we only allowed tech info more to get around some of the BS that floated around the SCCOA and then they merged, but since I haven't paid me dues I can't see them. I need to pay up so I can get all my old tech articles. I used to be the guy that would help out all the people that came in with different cars with M90's on them like Mustangs and a Taurus and several other cars. I really like that kind of stuff. I'm not very good with Excel but I will give it a try when I get a chance. Super busy today.

Jeff
 
Ok I figured out what I did wrong the formula is supposed to be (25.58 x Supercharger CID x Drive ratio / Engine CID) - 14.7, of course that is for early 89-93 Ford blowers the GM one works better with about 29.98 instead of 25.58

So for a stock car the drive ratio gives you a boost pressure of about 7.17 psi for one with a 2.8" would give you about 15.07

This does not take into account cam, headers, or anything that effects airflow through the engine. Usually when you open up the exhaust you lower the boost. You are not losing power you are only pumping the air more efficiently, after all boost is only a response to a restriction. The supercharger doesn't really compress the air it only moves air from one side of the blower to the outlet side there is no compression. A Twin screw is a true compressor and so is a turbocharger, but a roots supercharger is nothing more than a pump.

Jeff
 
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