i've done a lot of searches on tc and sc. i'll be staying N/A for a while, but eventually i want to go FI. i've never been in an sc'ed car, but i've driven an evo 8 before, so i know how turbos feel.

basically i have 2 questions about sc:

1) i've seen that in a lot of threads, people upgrade to a smaller pulley for more boost. this makes sense because you increase your crank/pulley ratio to allow for high max pulley speed. however, wouldn't it bog you in the low RPM range? i guess it's like starting in 3rd gear instead of 1st? to me it seems that a smaller pulley would hurt low (and perhaps mid) range throttle response, as more engine torque is required to accelerate the smaller pulley. i guess my real question is...is there an "optimum" pulley size for the street, where mid-range and trottle response is more important (to me at least) than max boost?

2) It's pretty clear that sc builds power linearly, as boost is a function of rpm. however, can we use a "wastegate" to limit boost like turbos do? i know sc isn't exhaust driven, and a limitation on the actual charger unit is propellor speed.
assume I want 8 lb of boost max, but I want it before redline. now assume a sc is running a 14 lb pulley such that the propellor speed at redline still doesn't exceed its limit. it seems that if the extra boost can be bypassed, then the sc can achieve max boost and hold it til redline. has anyone tried this idea with sc's? when i go FI, i want to be able to run low boost, but for a greater range of rpm.

thanks

 

I would like to know this as well

 

Turbo...

Centrifugal SC does not provide power the way you are looking for.

You are talking about a ROOTS type SC.

 

Sounds like a lot of wasted potential to me.

 

Thanks for the input.

I like the efficiency and light weight of the centrifugal SC, but you're right, a roots type SC will definitely make boost early...maybe too early. i don't mind building boost, in fact, i would prefer that if i consider fuel economy. a setup that reaches the boost i want at 5-6k rpm and hold it to redline would be ideal for me.

from what i understand, the wastegate in turbo setups prevent the turbines from spinning too fast, and since a SC is crank-driven, that speed is only controlled by your right foot. but still, if it's possible to leak some pressure past what i want, that would be ideal for me.

 

plus...roots style sc's are too...rotary

 

The answer to number one is no. It only takes a small amount of hp to spin the superchrger at lower speeds. It does not bog the engine like larger tires, higher gears, etc. The way the centrifical superchargers pull with this engine is like having booster rockets. It just goes faster and faster.

 

i like faster and faster

and i've always wanted to strap some rockets to my car!

 

anyone else with SC want to comment on the 2nd question in the OP?

 

You would need to use a variable speed drive to do this with a centrifugal. Using some type of blow-off or pressure relief valve may work - but it would be very difficult to keep repeatable.

If you want an SC that build boost quick and holds it over the RPM range you may want to consider a roots type SC. I have one on my car and it hits 3-4 psi as soon as you put your foot in it, reaches 7 PSI by 6k and holds it till redline. This is my dyno, max boost was 6.8 PSI.



You can read more on my install here: https://www.s2ki.com/forums/index.php?showtopic=467853