DR

>I’ve tried to use 2 similar bypass valves on my car and found them way too small for the SC setup.

Just for clarification, I assume you are meaning a Centrigal "blow thru" SC setup, correct?

Imo000

Yes, that's what I ment, cetrifugal SC. I thought if they are good for a turbo application they whould be ok for the SC too. They are, but only if they are at least 3 times the size. One was way too small and two was almost enough.

DR

>Yes, that's what I ment, cetrifugal SC.

Just wanted to verify because the Twin Screw Bypass Valve works a little difference, it is more about reducing pressure to eliminate drag and cavitaion when "crusing" instead of the concern of blowing open the throttle plate.

> I thought if they are good for a turbo application they whould be ok for the SC too.

It is funny how they all react differently, at first glance you would think boosted air is boosted air, but evidently not. The valve I spoke of was original equipment on all 944 Turbos and 90-94? 911 Turbos and obviously worked for them.

Imo000

I agree, each mode of boosting requires a slightly different way to setting things up. The bypass valve size is a perfect example. The main reason the centrifugal SC need such a large bypass valve when compared to a turbo is that the turbo will slow down a lot faster when the throttle closes, the centrifugal will just keep pushing the same amount of air because it’s mechanically connected to the engine by a belt. Therefore the venting hole (valve) needs to be pretty big. The HKS unit I have is not a blow of valve. It’s a bypass valve and works just like the 951/911 unit. It vents all the time except when the manifold is under boost. Blow off valves work like a safety valve on an air compressor. They will vent only when the air pressure reaches a pre set level. This is not a good way of venting, because the pressure in the intake will always bounce from really high to high, it will never go to low. This is the reason why a blow off valve makes the typical chirping sound. Bypass valves on the other hand, if are properly sized, are silent when open.

DR

Imo000,

Have you tried one of these (or is this the one you are using), they are 1.5" throat with a true throttle plate and we can sell them for around $100.

Sorry for the small photo..

Imo000

No, the HKS unit that I’m using is just like this one http://cgi.ebay.com/ebaymotors/HKS-S...spagenameZWD1V . Has a second vacum post bellow the diaphram to make it open even faster than a regular bypass valve. It's an excellent product for my setup.

porshhhh951

[QUOTE=Lance J
6psi w/o intercooler, est. power 290rwhp
8 psi w/intercooler, est power 335 rwhp
and 12 psi w/intercooler&propane injection. 400rwhp
numbers may be very optimistic numbers but hey. what ever i lack i'll make up with nos. lol[/QUOTE]

those are very realistic #'s.

Im trying to convince my father to go turbo on his s4 but, there just arent enough kits out there yet and the new 928 specalists roots blower kit is truely a work of art. Centrifical superchargers suck as far as powerband go's the roots however will make insane tq and boost at just off of idle rpms

now I am partial to turbos myself but, he is a old school kind of guy. People just werent using turbo's back in his day. It was all about s/c or nitrous. Which he says he wants to use both.

He has yet to beat me in a road race or street race in his car mostly because its stock..he hopes with the new motor s/c and everything he can take my new setup that im building.

we will see

Imo000

Why do you say that when the turbo works the same way? The only major difference between a centrifugal and a turbo is the way they are driven. They both make majority of the hp in the upper rpm range. You can’t say one sucks over the other. At the end, all 3 types of boosting have their advantages. They all add hp in a different way and that’s all that really matters.

porshhhh951

No a turbo does have lag just like a centrifical blower...but, centrifical blowers actually cost hp the more they run.

Not only that but, they dont build boost nearly as quickly as the turbo does because a centrifical blower is only has good as the pulley its hooked up too and how many rpms the motor is turning. So a centrifical blower wont really even make make boost pressure until way into the rpm range.

While the turbo driven by exhaust gas can actually make full boost much sooner in the rev range= a much much better powerband and more hp.

If you took a s4 928 with a nice centrifical supercharger kit on it running lets say 10psi vs the same car with a nice well done turbo kit running 10psi both comprable trim specs and wheel specs the turbo'd 928 will pull out much better with more tq and lower end grunt.

Simply put a centrifical supercharger is the step child of most street setups. The reason why most people run centrifical is because its easy to install and everyone and their mother is worried about price. The centrifical units are cheep......easy to put on and dont require alot of work

While the turbo requires ALOT of work especially on a car that was never ment for boost in the first place. Its alot more expensive but, in the end makes more hp and tq and gives better response.

The only supercharger I really look at and respect alot is the roots. Its wonderful sitting ontop of the instake manifold building boost almost instantly right off of idle. making great power and giving insane powerbands.

The only draw back to a roots blower I can see is you cant run as much boost through it as the other two.

But, turbo's have lag so they all have draw backs. The centrifical superchager being the step child of the group. and I have plenty of expeirence with all three so its not like im a rookie when it comes to FI.

porshhhh951

btw I wanted to add. Whoever makes that roots blower kit from 928 specalists.

Its a work of art and my hats off to you. When I buy my 928 and I go FI. Only two things will touch that car. The 928 specialists roots blower kit or a turbo kit. I have been doing tons of research on Fi in the 928 world and you guys are just now coming around to the whole world.

It seems like centrifical units have been out forever and just now are the really good kits showing light. The 928 specialists roots kit is prolly a first of a new breed of 928 FI cars. Were people arent worried so much about getting away the cheepest they can and want the best kit they can get.

Between a turbo 928 and the specialists roots one....god thats a hard choice I guess it would depend on what I used the car for and what amount of boost I wanted to run.

Anyways just wanted to say the kit looks like its the real thing and I bet you are going to be selling a ton of them when people see the results of what a real good FI kit can do.

Imo000

To keep this tread from getting out of hand I will not debate this.

Step child or not…..look at my avatar and imagine…….all that for under $1000.

porshhhh951

I agree with you the centrifical s/c is a performance bargin....but, dont be under the impression that its better than a roots blower or a turbo. Cause it isint.

**edit** I wanted to say. The 928 is a outstanding car as it comes factory stock. So one with any sort of FI on it is simply cool. PERIOD.

Imo000

I totally argree 100%. I never did and and never will say it's better.

heinrich

Better? In what way?

In that it allows the stock induction to remain in-place, or
In that it requires no real mods or
In that it costs little or
in that it develops less heat and encapsulates less heat due to its size and location or
In that it can deliver the same amount of horsepower as a twinscrew?

There are so many pros and cons. Each works and each makes sense. I doubt there's such a thing as "better". Better in what sense, is the question. Regarding sc vs turbo. you guys crack me up. So now the 928 list here has singlehandedly undone the many decades of technological engineering truths? namely that a supercharger is generally able to boost at far lower revs than a turbo, because the turbo can only work once the engine has revved (at least a little)? Or, the fact that a turbo needs to be sized for the entire rev range and most turbos I've driven have NO TORQUE at all below a goodly amount of revs, mostly 4k ..... or how about the fact that turbos usually require lower compression.

How is it that we're making these changes to what has been known for decades regarding the technology?

Turbo is obviously just as cool as supercharger. Better? suuuuuuuuuuuure.

m21sniper

It's better for one thing.

If you anticipate post-blown launch traction issues a centrifigul is the best way to go because all the extra power is up higher in the RPM range.