Okay, I am one of the many people on here interested in doing the do-it-yourself supercharger setup on a Na mine being a 86' GXl with 9k on a rebuild I plan to go pretty exteme with it(I probably won't be happy untill I crack 300rwhp and I'd like to go further) In the other threads there have been some comments made about Throttle body placement when using a roots style blower in a supercharged setup. I'd like this thread to be a specific point counter point arguement so we can "settle" this or at least come to an agreement on the risk/benefits of going one way or the other.

In the other threads the main arguement for placeing the TB before the blower was "because that's how the OEM guys do it. Here is my problem with that. In every stock aplication I've seen that uses a roots style or any possitive displacement supercharger the SC itself is placed in the same spot as the Intake manifold. The large size of the superchargers usually requires that it take a position like that, and when oriented on the engine like that it is much easier to directly run a belt of the crank since the IM will always run the same when the Engine does.

The technical arguement against running it prior to the TB is that even with a BOV it can create high pressue loads on the throttle body when it's closed, well the TB will only be completely closed in two situations. Idle, I don't car how big your SC is your not going to be generating enough pressure at idle to threaten the throttle butterflys. And, during engine braking. Well, that would pose a problem since the SC is belt driven if the engine is spinning 5k the SC is producing like it normally would(unlike a turbo which wouldn't have enough exhuast pressure to spool. The problem with this argument is that in a TB before the SC setup the SC is still trying to suck in enough air to make that pressure and the TB is closed. So the TB would be under alot of stress from the SC trying to suck it off the hinges. In this scenario it seems the advantage would go to the TB after setup because at least you have a bypass valve inbetween to make the TBs life easier.

So really, I conclude that the only reason the OEM manufacturers put the Throttle body before the SC is space and with a properly vented system an SC(centrifical, possitve displacement, or other) can be treated just like a turbo.

Besides, just look at Vortech's kit for most B-series Hondas it's a centrifical blower before the TB. And, yes a centrifical blower acts just like a roots they bolth create boost under load or not, so long as the engine is spinning fast enough.

 

I believe the reason most OEMs put the TB before the SC is so that under light throttle applications the SC is operating w/ very little inlet air and this decreases the pumping losses. Better gas mileage from less parasitic loss.

Put your hand over a positive displacement SC inlet and spin it by hand- easy. Put your hand on the SC outlet and spin it by hand- much harder.

The other way SC can cut the parasitic loss is to clucth it like an AC compressor.

Besides, just look at Vortech's kit for most B-series Hondas it's a centrifical blower before the TB. And, yes a centrifical blower acts just like a roots they bolth create boost under load or not, so long as the engine is spinning fast enough.

Centrifugal SC is different from positive displacement SC when it comes to pumping losses and TB placement.

 

Also, if the SC goes before the TB plates, you'll need a blow off valve.

If it goes after the TB plates, a blow off valve is not needed.

 

Why? What makes it different. The produce boost in the EXACT same way, unless a clutch is involved(which I haven't found any yet). I could see a difference in pumping losses, but the possitive pressure is still going to be there on the TB and a BOV has taken care of it in every instance.

Also, if the SC goes before the TB plates, you'll need a blow off valve.

If it goes after the TB plates, a blow off valve is not needed.

Well, yes but there is an upside here. With the vented system you have alot more control over boost. Suddenly low boost high boost settings are a push button away, while if someone with a 03 Cobra wants to run pump gas for a while, they are swapping SC pulleys.

 

No they don't. Search for 'superchargers' on google and do some research; centrifugals are completely different than positive displacement blowers in the mechanism of boost production . .

 

Your right I could have worded that better. But, they produce boost at the same times. Granted a possitive is more linear and a centrifugal will be more "peaky" The point I was going for is that at higher RPMs they are both producing boost throttle body open or not. As opposed to Turbos that wind down the second they aren't underload anymore.

 

But that's the real beauty of the positive displacement vs the centrifugal: They DON'T make boost the same way or in the same pattern. The centrifugal creates a boost pattern that rises with RPM, thus making the motor peaky. The positive displacement type produces the SAME boost from idle to redline, meaning that you have access to full boost right off idle. Also, when you put the TB before the blower the boost is throttle-dependant, so you only have boost when you're on the throttle.