Underdrive my blower

Faster7

Registered User
What would be the easiest way to underdrive the blower on my '95? I know the 94/95 pulley should already be driving it slower than if it had a 89-93 pulley.
 
I think the easiest way is to buy a jackshaft pulley to overdrive the blower. You have the ability to make the supercharger spin faster on these cars three different ways. The supercharger pulley, jackshaft pulley or crank pulley. I think the jackshaft is easiest to change. I recommend no more than 5% overdrive on a fairly stock car.
 
I think the easiest way is to buy a jackshaft pulley to overdrive the blower. You have the ability to make the supercharger spin faster on these cars three different ways. The supercharger pulley, jackshaft pulley or crank pulley. I think the jackshaft is easiest to change. I recommend no more than 5% overdrive on a fairly stock car.


He wants less boost if I read this correctly. There are a few 1 to 1 jackshaft pulleys floating out there. Mike Tuck can machine one for you if you want him to. I use to have one but sold it off.

Ken
 
the Pontiac supercharger uses a huge pulley. not sure if they are inter-changeable though. please enlighten us as to why you would want to underdrive it? if your thinking is to make it last long or go easier on the supercharger, well it wont. the bigger pulley will help the snout go bad faster and in stock form these things last well beyond 200,000 miles. if your intent is to slow it down then just get a Honda or Toyota.
 
He wants less boost if I read this correctly. There are a few 1 to 1 jackshaft pulleys floating out there. Mike Tuck can machine one for you if you want him to. I use to have one but sold it off.

Ken

Ahh good point, technically he did! Didn't mean to mock you! I bet he meant overdriving the supercharger though.

Chris
 
the Pontiac supercharger uses a huge pulley. not sure if they are inter-changeable though. ... the bigger pulley will help the snout go bad faster and in stock form these things last well beyond 200,000 miles. if your intent is to slow it down then just get a Honda or Toyota.

The GM cars use a smaller number of ribs, I believe, so no, the pulleys would not interchange (at least in that direction).

Why would the snout go bad faster? More centripetal loads on the bearing based on the larger diameter pulley?
 
I do want less blower rpm. I guess a 1:1 jackshaft pulley wouldn't be terribly hard to make....

I very much enjoyed beating up on my friends with SC's with relatively tame turbo cars and would be "putting my money where my mouth is" if I quickly got the SC I picked up to be in the same league.

I'd like to try a bit more engine rpm.... Appropriate cam, tune for E85 full time, and a ~75-100hp wet shot when it needs to "go." I know the stock M90 can only move so much air and I'd like to skip past where most start to run out of steam.

Although if I wanted to try something silly like a rear mount turbo I would be asking the same question...

:D
 
we have alot of people here that run higher rpm, and i have never heard of anyone under driving to sc because of such. infact most all are running some level of od. the most common with engine mods(heads, headers, cam, lifter springs, tb, maf, and at the least a ported blower) running 15%. its not so much the rpm that makes the blower run out, its the amount of air its able to push. by under driving it your going to make that outcome even worse. you need to open the blower up or get a magnum powers blower if your wanting to do higher end rpm. i implore you, do some research before you do any mods. you can search for days learning almost everything about the cars in the search engine. also visit Eatons web site. they have a few articles that might help you learn more about roots blowers.
 
I do want less blower rpm. I guess a 1:1 jackshaft pulley wouldn't be terribly hard to make....

I very much enjoyed beating up on my friends with SC's with relatively tame turbo cars and would be "putting my money where my mouth is" if I quickly got the SC I picked up to be in the same league.

I'd like to try a bit more engine rpm.... Appropriate cam, tune for E85 full time, and a ~75-100hp wet shot when it needs to "go." I know the stock M90 can only move so much air and I'd like to skip past where most start to run out of steam.

Although if I wanted to try something silly like a rear mount turbo I would be asking the same question...

:D

The jackshaft wouldnt be hard to make if you are a good machinest. There are a lot of diffrent pulleys out there that look good but have a lot of belt slip do to points on the ribs or other inconsistancys.

If you build an engine capable of going higher rpm you will find underdriving the blower is going to starve the motor. Are you building a bottom end with good rods and pistons that will hold together at higher RPMs? What size injectors are you looking to go to? E85 and nitrous, remote turbos, pretty heavy stuff are you going to build an engine with supporting mods to handle these plans or are you just dreaming. Your talking a lot of money and problems that even some of the big dogs here havent worked out.

Ken
 
Are you looking to Underdrive, or reduce the overdrive?

Some things to keep in mind.
1. as engine RPM increases, CFM requirements will increase regardless of the blower shaft rpm.
2. blower efficiency is based on pressure ratio vs Rpm. simply changing rpm doesn't ensure an improvement in efficiency.
3. too much underdrive and you won't enjoy the car anymore. The extra load of turning the blower, with lower airflow at low rpm may turn a heavy car into a worse slug than a N/A 3.8.

That all said, give it a shot. If you are going to go through the expense of making a pulley, I'd make 3 in varying drive reduction amounts so you can experiment and find the best. Stock overdrive ratio is around 2.77 (2.77 turns of the blower shaft for each turn of the crank shaft) so you might try dropping down to 2.25, 2.0 and 1.75.

A stock m90 moves 90 cubic inches of air (at 100% efficiency) each shaft revolution. At 100% efficiency a stock 3.8 block moves 116 CI of air per crankshaft revolution. Thus at a 1:1 ratio, the stock M90 is providing 26ci less air than the motor can use. At a 1:1.75 ratio, the M90 would provide 41.5 CI more air. Neither a M90 or a stock 3.8 block are particularly efficient, and their efficiency change as RPM and Load change.
 
Are you looking to Underdrive, or reduce the overdrive?

You're right, "less overdrive" would be the proper description. Straight under-drive would be a waste. :) 2.25-2.4:1 instead of 2.77:1 might be in the ballpark.

If you build an engine capable of going higher rpm you will find underdriving the blower is going to starve the motor. Are you building a bottom end with good rods and pistons that will hold together at higher RPMs?

I'm not planning to spin it to the moon.... but I've never seen a stock heaton no matter what the overdrive do anything but fall flat at 5000-6000rpm VS say a turbo split port motor, still pulling strong at 6000. Long term I see going to a turbo split port motor or 302. This good running SC somewhat fell into my lap or I wouldn't be in this situation.

What size injectors are you looking to go to? E85 and nitrous, remote turbos, pretty heavy stuff are you going to build an engine with supporting mods to handle these plans or are you just dreaming. Your talking a lot of money and problems that even some of the big dogs here havent worked out.
Ken

I have 55, 72, 90, and 120lb injectors. The 55's are fresh and since I "only" have 6 of that set, I think a bit of blower underdrive and E85 full time could make nearly the same amount of power as stock since the E85 should let me run plenty of timing, without high intake temperatures at ~5500-6000rpm. 250 flywheel HP shouldn't be a problem on E85 and 6x55's. I've daily driven on and off a 92 NA V6 tbird that I've had since '97 so I know how slow it "could" be.

I don't see a problem spinning a stock SC shortblock 5500-6000 and making 350-375whp with good tuning on a wet nitrous shot, E85, and a mild cam.

If I had to buy everything from scratch I don't think I'd get quite to what an MPX blower upgrade/rebuild would cost which is the first sensible level of blower upgrade that makes sense to me, yet I already have fuel pumps, injectors, turbos, a moates J3 chip adapter and programmer, extra megasquirt, etc.

If I've built EFI harnesses from scratch and installed and tuned standalones and turbocharged non turbo engines in the past, does this "underdrive my blower and run E85 and wet nitrous" project still seem like "pretty heavy stuff" ?
 
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I'm not planning to spin it to the moon.... but I've never seen a stock heaton no matter what the overdrive do anything but fall flat at 5000-6000rpm VS say a turbo split port motor, still pulling strong at 6000. Long term I see going to a turbo split port motor or 302. This good running SC somewhat fell into my lap or I wouldn't be in this situation.



I have 55, 72, 90, and 120lb injectors. The 55's are fresh and since I "only" have 6 of that set, I think a bit of blower underdrive and E85 full time could make nearly the same amount of power as stock since the E85 should let me run plenty of timing, without high intake temperatures at ~5500-6000rpm. 250 flywheel HP shouldn't be a problem on E85 and 6x55's.
I don't see a problem spinning a stock SC shortblock 5500-6000 and making 350-375whp with good tuning on a wet nitrous shot, E85, and a mild cam.

yet I already have fuel pumps, injectors, turbos, a moates J3 chip adapter and programmer, extra megasquirt, etc.

If I've built EFI harnesses from scratch and installed and tuned standalones and turbocharged non turbo engines in the past, does this "underdrive my blower and run E85 and wet nitrous" project still seem like "pretty heavy stuff" ?


Well Im glad you got it all worked out. If you had provided all this information in your first post, I likely would posted a diffrent reply that would fit your question. But since you know what you are going to do anyway and got it figured out I will refrain from offering you any advice

Ken
 
Well Im glad you got it all worked out. If you had provided all this information in your first post, I likely would posted a diffrent reply that would fit your question. But since you know what you are going to do anyway and got it figured out I will refrain from offering you any advice

Ken

I very much appreciate all of the input.... and respect that the responses were written with sincerity and the desire to help out.

Still can't say that I have it all worked out, I just thought there would be some easy ways to drive the blower a bit slower than stock. Like in the case of belt slip, where if you were to get the jackshaft to blower speed up say 30% from stock and run a bigger blower pulley to get it back down to stock or a bit faster, the increased belt wrap I'd think would help with the slip. Or all of the different big blowers that folks upgrade to, I'd also think there would be some common need to drive some of them slower. I'd love a big screw to play with and if I came across a deal on one....
 
I dont think you are going to gain much. You will loose torque by lowering the boost. I highly doubt you will make it up in RPMs, since the engine will just demand more and more air. To combat heat most upgrade the IC or put a fan on it or even use water injection. If you are running nitrous then you really dont need to worry about heat. A stock early sc can still be overdriven to at least 5% before heat affects performance. Leaving it at the stock od will be the better choice even if you want rpm to the moon.

But like others said you will the first to try it out. You can get a custom OD pulley made just give bring your stock pulley to a machine shop that can do the job and tell them what you want to do.
 
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I very much appreciate all of the input.... and respect that the responses were written with sincerity and the desire to help out.

Still can't say that I have it all worked out, I just thought there would be some easy ways to drive the blower a bit slower than stock. Like in the case of belt slip, where if you were to get the jackshaft to blower speed up say 30% from stock and run a bigger blower pulley to get it back down to stock or a bit faster, the increased belt wrap I'd think would help with the slip. Or all of the different big blowers that folks upgrade to, I'd also think there would be some common need to drive some of them slower. I'd love a big screw to play with and if I came across a deal on one....

If you underdrive the M90 you will likely get less belt slip. Most people here will look at you descision to turn the blower slower as not a good thing. The M90 is 1.5 liter and on a stock SC will make 10 to 12 lbs of boost. If you install cam or free up the exhaust you remove restrictions and the boost drops, sometimes a lot, even though the car is making more power. If you underdrive the blower as well you will likely end up with 6 or 7 lbs of boost. People overdrive the heaton to make up that diffrence. They also spend a ton of time and money figuring how to cool the heat the roots blower makes.
Going E85 is a popular thing now and a bunch of guys are doing it. I havent heard anyone here on this site doing R85 and nitrous. But Im sure there are other sites that have that imput. I run a 2.3 blower and still am running a 5% jackshaft pulley and the car is setup to make power to 6300 rpm. I beleive the M90, whether you underdrive it or over drive it just cant make power at higher rpms, even the good MPX struggles at those levels. My 2 cents anyway

Ken
 
If you plan on making power on E85 55's wont be enough injector. Unless you plan on actually making just the 255 flywheel hp you mentioned. In that case why bother doing any of this. Underdriving the blower or reducing rpm will just make the engine fall on its face that much sooner. Reducing the M90's RPM isnt going to give you any extra room to rev if thats what your getting at. The engine will more than likely be running out of blower that much sooner.

As a whole we have a pretty good understanding of what the m90 can or cannot do here. Reducing blower RPM while increasing engine RPM is not one of them. On your car an S port of the blower would do you best for increasing efficiency of the blower. Also it can never hurt to reduce restrictions on the inlet side of the blower
 
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