The early ones cause an annoying "yo-yo" under part throttle. The valve is fluttering, causing manifold pressure to fluctuate.

If you run a DT BPV, it builds boost faster, builds more boost sooner, and eliminates the "yo-yo".


I would say its one of the top ten important mods you should do.

 

I agree. Mines a June 04 build but I noticed a substantial difference with the BPV. I was told early to look into it. Maybe mine wasn't great? I dunno but it was worth the $125

 

Any car will have some benefit. You didn't have any "yo-yo" to get rid of in the first place, but you still get the other benefits.

 

Tell us more about the DT BPV. I have an '06 JCW, loaded with RMW stuff and a 15% pulley, at ~245whp it's no dog, but it seems like I could always use more on the bottom end.
Will a DT BPV be of any value to me?
Would this improve driveability?
Would this kill gas mileage? It's already not that great.

 

there are tons of pages on the modified by-pass valve, but they are lost to history because they are so many threads ago... but that's another subject.

The bypass valve does a two things:

1) It adds a stiffer spring so that the open/close point and transition charecteristics are modified.

2) It has well set stop points so it's not as leaky as some valve as they come from the factory.

So, what's this all do? First you have to know how the stock valve works..... The stock valve is there so that you remove most of the SC load at light throttle and idle. It does this by having a vacuum diaphram that opens an air return valve so that air can go from the high pressure side of the SC to the low pressure side. When you install the shorter, stiffer spring, what you find is that it takes more vacuum to open the valve, and this means that you're "on boost" earlier, and "off boost" later. You also get a more abrupt transition. Is this good? That depends on what you want. Many here have posted that they like the effect, myself, I didn't so I did my own mods to the valve... While the earlier boost is good for some, it comes with the price of a difficulty at light throttle power modulation, and I found this a PITA in some steady sweepers. It's a personal preference thing for sure.

As for make the dreaded "yo-yo" go away, there are mixed reviews on this. On some cars, it induces it! This is because it's not the valve per se that causes or supresses the effect, it's the interaction of the valve with the TB and the ECU programming that results in the yo-yo (one of the best burried threads here is called "The Yo-Yo Chronicles" and should be a must read for all SC owning NAMers....).

One other thing, is that the DT BPV (aka the Ryphix II, one of the long time NAMers is the original source of the idea) doesn't open the valve all the way. This isn't really a big deal, the TB will just shut a bit more such that the manifold vacuum is what it should be. But overall, you get a slight hit in MPG. Not much, but real and repeatable.

The carefull adjustment of the stop on the valve used to be a big benefit for peak boost, but I understand that they are better set from the factory now.

Anyway, I"m not in the "must have" camp on this one. Some people like it a lot, some don't, and for some it causes more trouble than it's worth and it comes off the car.

Matt

 

The Yo-Yo Chronicles

Ryephix2 Bypass Valve Spring discussion

The DT BPV

FWIW, the Yo Yo thread was number 20,115 here on NAM, the RP2 thread #32,276 and the DT BPV thread is #89,593 and this thread is #159620! Some things never go away!

Matt

 

Thanks Dr O, plenty to read and plenty of debate and info.

Thanks Again................