So I've been debating with myself about how to go about reaching my performance goals with my car. Basically, I'm looking for ~450rwhp while maintaining good manners, and I'd like to keep fuel economy in mind, too. I also want power on demand -- no filling nitrous bottles. I'm willing to lay down a lot of money to build it right if necessary. Those goals pretty much scream forced induction.

So anyway, I've been trying to decide if I want to go turbo or supercharger. The "free" power of a turbo appeals to me, as does the potential for more boost at lower RPMs.

STS had my attention due to easy install and low cost. And Kraest, I've seen your posts before about the benefits of a P1SC over the STS kit. No need to repost them here.

Anyway, I was talking to a friend of mine that has a WRX. He's a mechanical engineer and knows his stuff well. He told me that on Subarus, instead of blowoff valves, they use bypass valves. In this setup, rather than venting excess boost to the atmosphere (and wasting a bunch of work done by the turbo), it vents back to the intake pipe, an inch or so before the compressor inlet.

This makes a lot of sense to me. Your compressed air isn't lost, and you can maintain much more of the pressure between shifts and such.

So anyway, to use a BPV on an STS kit, you'd have to plumb yet another pipe to from the engine to the back of the car, which just seems stupid. So, no STS for me.

Now I just have to decide between a Procharger (probably P1SC... not looking for a lot of boost here), a homemade turbo (scary, I don't have much experience), or a more expensive underhood turbo kit.

 

If you go centrifugal, get the D1,D1SC, or Vortech T trim...they will allow you to boost more later and have a higher step up ratio.

 

I'd go with the D1SC over the P1SC for the reasons mentioned above

Also, check out www.exoticperformanceplus.com. They have the cheapest pricing around on Procharger stuff.

 

Not to mention, to meet NHRA rules for 2007 the turbo(s) must be in the engine compartment.

 

No offense, but your post doesnt make much sense to me .

I believe a bypass valve works the exact opposite of a BOV. A centrifugal/roots charger uses a bypass valve to vent boost back to the intake side of the compressor to prevent part throttle boosting/surging. Once you go WOT then the bypass valve shuts and lets all the boost go to the engine. A BOV vents boost when there is a dramatic change in pressure (i.e. throttle body is open under WOT and then you lift off he gas). The boost in the charge tube has nowhere to go. The BOV pops open and vents preventing damage to your turbo or blowing silicone couplers all the time. These are two different things. Now, a bypass valve could be placed in the charge tube right near the sts turbo and vent back in to the suck side pretty easy. No reason that the bypass valve would have to be up front, just on the charge side. However, I dont see a need for bypass valve with a turbo application.

 

Yea, there are alot of reasons to not get an STS, but this isn't one of them.

A bypass valve and a blowoff valve do the same thing, the only difference being where the boost if vented to. Whether it's vented to the inlet of the compressor or atmosphere is of no consequence, depending on where the MAF is.

Venting boost to the suction of the compressor doesn't retain the boost. After it goes through the bypass/BOV, the energy is already lost. The suction of the compressor IS the atmosphere.

Mike

 

I've been looking around and it appears that roots/screw blowers have bypass valves that work the way i stated and that a centrifugal compressor (whether its a blower or turbo) uses a bypass valve like a BOV.

Purpose of a boost bypass kit according to http://www.kennebell.net/faq/faq-answers10(5).htm

What about bypass valves? During non boost driving, bypass valves reduce NOX emissions and allow the supercharger to operate at a cooler temperature by circulating compressed air (boost) back to the inlet. If the supercharger is located in front of the throttle body (blow through) as in the case of centrifugals, and the throttle body closes at high rpm, the compressed air has no place to go. The resulting back pressure and surge can destroy the supercharger.

 

So for clarification, location of compressed air determines what a bypass valve does. If the compressed air is after the TB (roots/screw charger), the it works different then a BOV. If the compressed air source is before the TB (centrif. or turbo), then it is the same as a BOV.

 

Also, another term for bypass valve is diverter valve.

 

I will accept that I might have misunderstood him, but I guarantee you that he knows what he's talking about. He is not your typical WRX owner. Please just trust me on this one.

It hadn't occurred to me that the high-pressure side of a BPV could be right next to the turbo as well (as shown in rob's drawing). I suppose that eliminates this as a reason not to want an STS kit.

Nonetheless, I still maintain that for turbo applications, a BPV that dumps RIGHT before the compressor inlet is superior to a BOV that dumps to atmosphere. Assuming that the turbo is spooled (which it should be, if the BOV/BPV is dumping air), there should be a low pressure zone between the air filter and the compressor inlet, and dumping the air there should cause it to be sucked directly back into the turbo, thus maintaining MAP at whatever your defined maximum is, rather than letting it all bleed off to the atmosphere.

Am I missing something here?

 

Well, my brother just sold his Sti, and he is a mechanical engineer, and his Sti had a blow off valve that was routed back into the intake, the sole reason being that there is almost no noise when the valve opens.

 

The stock setup on all turbo Imprezas (and possibly all turbo Subarus, but I'm not that well informed) is a bypass valve, which is what you described. They are indeed quieter than blowoff valves, but that is not the only reason.

No. Don't you remember my thread from a couple months ago where I said I was learning to powershift?

Some vacuum is better than none. By dumping all of that air in the intake, you momentarily increase the air pressure at the compressor inlet, which should in turn momentarily increase the efficiency of the turbocharger. It may not make much of a difference, but my only argument is that it's better than dumping to atmosphere.

 

Now that is a point I can get behind.

The 2007 NHRA rules are the real kicker anyway. No STS for me.

To those of you who think I should go with a D1SC over a P1SC, tell me more. What's the price difference, and what are the limitations of each? Are there any downsides at all to a D1SC? I'm quite sure that I'd never want more than 7-8psi of boost.

 

You'll see

The price difference is minimal - a few hundred bucks.

 

Well, OK, I know I'd "want" it, but I don't want to put a roll bar in the car, and I like to drag race once in a while, so it can't run any faster than 11.500 in the quarter mile.