How to make a well performing BOV for under $40

[ZiG]

Basically a blowoff valve does two things; it holds boost while you are boosting, and it releases boost when you are off the throttle. The important things to look for in a blowoff valve are that the boost is held without any leaks, can vent all of the air quickly, and have a short response time.

This writeup shows how to take a Bosch ‘diverter valve’ (DV), (aka 'compressor bypass valve', [CBV]) and use it on your car. These valves are used on many stock cars, including turbo Audis, Volkswagens, Porsches and Saabs. They can hold about 15 psi of boost unmodified, and have a very quick reaction time.

The first thing to do is actually get your hands on a valve. You can go to a dealership and say you want a CBV for, oh, a 2001 Jetta 1.8t. Expect to pay about $80 that way. Or, you can go to a junkyard or ebay and get one for $10 or less.

It will look like this (Note that boost goes in the SIDE, and comes OUT the BOTTOM):

Both ends are for a 1” ID hose. Now, the ones made for the newer Porsches have slightly stiffer springs and thus can hold slightly higher boost. If you find that it does not hold enough boost here’s what you do:

1. With a hack saw, very carefully cut the top off the valve, you will have to turn it as you cut to avoid damaging the spring inside.

2. Sand the cut surfaces til they are smooth, then sand the body a bit more as needed. This will have the same effect as crushing a DSM valve. Basically it makes it shorter so there is more preload on the spring, which will make it hold more boost. However, doing this will also make it open less easily, just like tightening an adjustable bov.

3. Glue it back together with either epoxy, or, better yet, plastic weld.

If you intend to use it as a recirculating valve, stop here. If you want to use it open atmosphere, read on.

Depending on which actual valve you got, and whether or not you modified it as described above, it may or may not leak at idle/cruise. If you’re recirculating it, who cares, but if you’re running it open atmosphere like this, it will be sucking in dirty air, as well as throwing your MAF off if it’s placed between the maf and the engine (or if you even have a maf?). There’s a real easy solution to this problem: a simple one way valve. This will likely be the most expensive part, at ~$20. Go to the hardware store and look in the plumbing section. Look around the kitchen area or the hot tub parts area. Somewhere around there you will find one way (check) valves.

I can’t find the exact one online, but here’s a random picture of it I found. I guess it’s used in hot tub plumbing systems or something. Each end is threaded on the inside, and yes, I painted mine.

Notice there are arrows on this thing. They point in the direction of flow that it will allow. You want to attach this to your Bosch so that the arrows are pointing AWAY from the valve. Both ends of the valve are threaded on the inside. What I did was I went and got a pipe coupler that had threads on one end and was flat on the other, then I attached that to the Bosch and screwed on the check valve. You can also screw something into the other end. I used a reducer and drilled some holes in it, like on a GReddy RS bov. Different things screwed into this end will produce different sounds, so go ahead and experiment, A smaller opening will make a higher pitched sound, and then you can drill holes and stuff to make it whistle.

Here’s mine all ready to be painted and put on the car:



It looks pretty long there, but really it’s only like 6” maybe? You can make it shorter if you glue the 1 way valve right to the Bosch.

As far as attaching it to the car, what I did was I got a hose connection, the kind with a hose barb on one end and threads on the other. I simply cut a hole in the intercooler pipe, screwed the hose connector in a few turns, covered it with epoxy (didn’t have any JB weld handy) and that was that.

Here's a pic of it on the car. Please note that the plugged hose you see going into the intake beneath was for when I had it recirculated..



As far as sound goes, I'll get a clip soon. It makes a sort of wooshing noise. I like it because it doesn't make the 'HAI GUYZ LOOK AT MAH BOV!' type of noise..

Modified by ZiG at 9:18 AM 3/4/2008
Modified by ZiG at 9:19 AM 3/4/2008

[CMG]

Dang, I got ripped off when I paid $30 for an HKS SSQV.

[jbracy7]

i got a turbonetics bov and a manual boost controller for 45 bucks from a guy on here

[Spickler240]

wow thanks for the info

[ZiG]

Dang, I got ripped off when I paid $30 for an HKS SSQV.

What, one of the ebay knockoffs that hardly ever seal properly? I guess you just bought it for the noise, eh?

[krazydriver]

AHAHAHAAH!!!!ZIG, that's the greatest!thanks for sharing.

btw, my ka-t project will hopefully be back on the road in a month or so.