Thumper33

So my questions is specifically about a saleen supercharger, but I think they all work the same.

Decreasing the pulley diameter means that they supercharger is spinning faster so it gives you more boost right? Well if they say that you get full boost by 3000 rpm or whatever with the smaller pulley, then why is a bigger pulley necessary to get more boost? Isn't the waste gate on the supercharger going to control how much boost you're using? All I can figure that a smaller pulley will do is give you more boost at a lower rpm.

Am I right that you'll have to adjust more than just a pulley to change the max boost? If so, then how is this done?

luckythirteen13

sc dont have a waste gate. thats a turbo.

smaller pulley makes more boost, bigger pulley makes less......thats on the sc. a larger crank pulley will make more boost.

yes a pulley swap is all that is needed for more boost. the saleen sc makes full boost faster then a centrifical. all that means is that if your pullied for 10psi, youll hit 10psi at 3000rpms and hold 10psi all the way up through the rpms. a centrifical will start making boost at 3000rpms and hit 10psi at around 4500 and hold 10psi all the way up

Thumper33

I assumed that there was a waste gate on a supercharger as well. If not, then how does the supercharger hold the pressure constant after 3000 rpm, and if there isn't anything other than the pulley change, then what tells it to raise the pressure it's letting by? Maybe I'm just looking at the system wrong.

luckythirteen13

its just that type of design. they make boost early and as the rpms raise, the engine needs more air and that sc supplys just enough to keep a constant pressure. if it was a centrifical (completely different design) youd get the boost later and it climbs more slowly.

i think your looking at it wrong, the sc isnt letting air by, its sucking it in through the filter and jamming down the engines throat

Thumper33

I think I understand what you're saying now....

It's kinda like it doesn't start building boost until 2k-3k rpms, but after that point, it's pushing as much air per intake stroke as to be consistant per rpm... so the engine is using a certain amount to keep it from building higher pressure. So the pulley is actually just giving it more air per rpm perportionally since they're directly linked by a belt. Does that sound right?

luckythirteen13

you go it

Sap

The bypass valve provided in the supercharger kit is essentially the same thing as a wastegate. Once a certain boost level is achieved the valve opens venting the air back into your intake or to the atmosphere. Same thing as a wastegate I believe.

luckythirteen13

the bypass is for venting boost air out of the discharge tube when you let off the gas. it doesnt control boost. a bypass opens when under vacuum, a waste gate opens when its under a certain pressure.

newb2000GT

A centrifugal S/C never stops making boost until you exceed its max RPM and shreds itself. The faster a centri spins the more boost it makes.

My ProCharger for example starts making boost around 2800 RPM (depending on throttle position and load) and increases boost pressure up until I shift. If I add mods that allow me to rev higher, I will make more boost than I do now with all other things constant. I've seen as high as 9 psi on my 8lb pulley. This was due to me revving up higher than usual. I usually hit 7-8psi by the time I shift.

A by-pass valve simply releases the compressed air back into the system when the throttle body closes, preventing the compressed air from trying to go back into the head unit from where it came.

tt460

There are some cases where the bypass valve on a postive displacement S/C is used to control boost, but they're all OE type installations. The Cobalt SS does what is called "torque management" in which they close the bypass at lower RPM's, but they "actively bypass" the boost at higher RPM's to reduce manifold pressure (and lowering HP) to make components last longer (transmission, clutch, etc). This effectively provides more torque down low, but decreases or maintains torque it as revs climb.

I'm pretty sure GM Performance sells a kit that disables this feature since it's not desireable in a performance app.

In a performance application, the bypass would never be used to throttle the boost. It's only opened when the engine isn't under load and you don't need the boost. This way the supercharger has less parasitic drag and consumes less HP at cruise. This is a good thing for fuel economy.