I have a hard time buying the stock WG holding 25 psi (even with the boost line closed).

If I remember correctly, the WG valve opens into the exhaust pipe (going toward the cat). Therefore, the exhaust going into the WG is exerting pressure on the valve in the same direction in which it opens. I can't believe that the stock spring is strong enough to hold the valve closed.

The functional advanage to the Tial is that it opens in the opposite direction of the stock wastegate making it impossible for exhaust pressure to open it.

 

i didn't believe this myself, until i tried clamping the cv line (on five different cars) to test it.

 

Isn't this how the stock WG functions?

 

Nize, I don't know if what Iam about to say makes any sense. But when I was running with the stock wastegate my vdo boost gauge was showing 19-20 consistenly all the way to 6000rpm's. When I installed the tial 38mm. I didn't make any changes to my boost settings and when I first drove the car the boost went all the way to 25psi+, I had to stop 3 times and turn down my boost level back to 19psi. What I think it was happening is, that the boost gauge was showing 19psi but in reality my turbo was working super hard @ 25+psi in order for me to achieve 19psi. Bottom line the stock wastegate was bleeding boost badly.

 

Big difference in making 25 psi and holding 25 psi. Assuming the turbo won't run out of steem far before that anyway in the higher revs.

 

I can't imagine a K26 anything sustaining 25psi for more than a moment.

lart,
I saw the same thing. I knew my WG was bad as I had to turn the boost up just to get 13-14psi. As soon as I put in the Tial it spiked to 22psi and I blew he HG......

 

Daniel, how long did you run @22psi?

 

Momentarily. Maybe a couple of seconds....

Lart,
Is that diagram I posted correct for the function of the stock WG?

 

Of course,
In the stock WG, the exhaust pressure "helps" the "control pressure" coming from the intake manifold through the CV or boost controller. These two forces add up, one pushing on the back of the WG valve, the other pushing on the WG membrane. In the Tial, the valve is "backwards" so the exhaust pressure will have no effect on it and the "control pressure" will have to open the WG on its own. Thats why you have to turn down the boost control when switching to Tial....
Tommy

 

Yes, that's the correct flow. Tial is opposite

see revised doc.

 

the diagram is missing the exhaust out on the opposite side of the wastegate, which leads to the crossover and turbocharger up-pipe.

and yes, i know i'm not lart.

 

here's a question regarding the tial, what happens if you develop a vacuum leak and the 'control' line fails? since the tial wastegate will not open via any amount of exhaust pressure, will you spike to +60psi of boost?

 

of course once the stock wastegate actually opens, enough exhaust will be dumped to bring the boost psi down again, so this is a given.

my point was that the stock wastegate (and spring) is NOT as weak as most people believe. not even after 20 years.

 

What exhaust post is missing the diagram?

check the next link from one of my auctions there is only 2 ports intake and exhaust. The top is the vacuum port.

http://cgi.ebay.com/ebaymotors/ws/eB...category=33742

 

You mean a boost leak instead of vacuum since the control line is under pressure. Easy answer; it can't over boost because one of two conditions will exist. If the leak is minor enough to allow boost to develop, the wastegate will still function normally when the pressure defined by the boost controller is met. Otherwise the leak is substantial enough that you are loosing pressure to fast to build boost and the leak acts as a relief valve.

There is no way overboost like you describe.