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Pulley size vs. SC vs. boost

porshuh

porshuh

New member
Do different superchargers (M90, GenV, etc.) make different amounts of boost at the same pulley size? Here is the reason I ask:

I am unsure of my pulley size - I measured 9.75" on the bottom of the grooves and 10.125" on the top of the grooves. I'm not sure which you're supposed to measure, but it gives me a pulley size of either 3.1" or 3.2". At WOT, I see about 8-9 lbs of boost. I would think for this small a pulley, it should be higher? I am running a GenV SC, and the previous owner seems to have done lots of upgrades (headers + full exhaust, tune, intake, etc.).

Is there some chart for determining proper boost levels for a given SC and pulley size?


Question 2:
Can someone explain how the "clutch" or "activation" system works for the SC? You can gently drive up the rev's to redline without seeing any boost at all (though the SC pulley is spinning every bit as fast, obvoiusly), so I know there is some sort of clutch of feathering system to control the actual supercharger independently of the pulley. If someone can explain how this system works, I'd really appreciate it.

edit: Here are some pics in case it helps

Oh yeah - does this look like a quick swap modular system like this one? - http://www.3800performance.com/Merc...e=QS-MPS&Category_Code=SCP_PS&Product_Count=0

DSC01983.jpg


DSC01984.jpg


DSC01986.jpg


And here's a picture of my dog, in case that helps lol he gets bored when I tinker with my car for too long

DSC01987.jpg
 
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I do not think you would be running a pulley that small without an intercooler and some serious work on a GenV.

The SC does not have a clutch. You do not get boost because of the "load" on the engine. You mash the throttle, you force more air into the engine, putting the super charger to work. Ease into the throttle, less air, nothing for the supercharger to do.
 
there has to be some valve train work to be running a 3.2 pulley. i was barely able to run one on my car.
 
JoshMcMadMac said:
The SC does not have a clutch. You do not get boost because of the "load" on the engine. You mash the throttle, you force more air into the engine, putting the super charger to work. Ease into the throttle, less air, nothing for the supercharger to do.
Click to expand...

See, I just don't get that.

Say you are driving at 5,500rpm, just cruising at this rpm so there is very low load on the engine. The SC is spinning just as fast as if you were WOT at 5,500rpm and making full boost. So with the SC spinning just as fast and making no boost, if there is no clutch of any sort, where is all that air from the full speed SC going?
 
porshuh said:
See, I just don't get that.

Say you are driving at 5,500rpm, just cruising at this rpm so there is very low load on the engine. The SC is spinning just as fast as if you were WOT at 5,500rpm and making full boost. So with the SC spinning just as fast and making no boost, if there is no clutch of any sort, where is all that air from the full speed SC going?
Click to expand...

All what air? Regardless of how much that supercharger is turning think about the throttle body. It regulates how much air makes it to the SC. At cruise or low throttle the throttle body is barely open thus not much air is making it into the supercharger. The output of the supercharger is very little because it isn't moving a lot of air...less than the vacuum created by the engine naturally. Because of this, there is a negative pressure (vacuum) in the Lower Intake Manifold and this keeps the Boost Bypass Valve open letting that little bit of air bypass the supercharger completely.

When you mash the gas, more air is coming through the Throttle Body than the engine can pull in on its own and the blower will now be able to build boost in the LIM with that extra air. When there is positive PSI in the LIM (Boost) the Boost Bypass Valve closes and 100% of the air going through the TB is getting into the SC.

Does that help? I think I explained that alright.
 
there is no load. no load = no boost, but if youre cruising at 5500 rpm youre more than likely going to be JUST under making boost because of the load the car will see just maintaining that.

also, what troy said is right about the BBV and TB being more of a contender than the blower is.
 


Robotech, yeah that helped a good bit. I didn't know there was a BBV or how it works. I now understand how the vacuum in the LIM keeps the BBV open.

This is my (incorrect?) thought process: At high rpm, the supercharger is spinning very fast, and whatever mechanical part of the sc (lobes, vanes, etc.) that spins to make boost is still spinning. I understand that the TB is almost completely closed and is not letting air get to those vanes...but how does that not create a huge problem? You have this thing spinning as fast as it would and working just as hard as it would be when you're at WOT and making full boost, but at high rpm/low throttle % it's starving for air...

I guess it has all to do with the BBV?
 
yes. the BBV does a lot of work to keep the motor from boosting when its not supposed to, including under torque management.
 
Ok, the second question is pretty much covered, for your first question, no, there's no chart or anything to tell you what boost level you should be at. These are positive displacement blowers, so for every revolution, they move a specific amount of air, in this case 90 cubic inches (hence M90). Your boost levels will depend on how efficiently your engine can use that air. Things like headers, high ratio rockers, etc help your engine breath better, so they would actually reduce your boost (and increase performance).
 


genV doesnt have the bolt hole on the top for the engine cover. that is a genV. it also has a longer "neck" than the genIII.
 
Just to make sure I am clear...is stock in an L67 a GenIII? Is the GenIII the same as an M90? or is an M90 a GenV? lol someone hip me to this. Thanks!
 
both are an m90. the genV is on the 04+ cars and is a little more efficient. a lot of people in the 97-03 crowd upgrade to a genV instead of getting a ported genIII and see pretty good gains (something like 20 whp with a full blown cammed/intercooled setup).
 


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