Hey guys, just a quick question on Tial BOV springs. I am getting my car together and ready for some dyno time. My waste gate spring is 8.7psi, and the only 50mm Tial BOV I have, has an 11psi spring.


Will there be issues? I understand that it takes 11psi of charge pressure to overcome the BOV spring, in order for the BOV to vent. How important is it to be as close to the W/G spring pressure as possible. I know that vacuum plays a role in this as well.

Some input would be appreciated.


Just worried that it will not be responsive enough and cause compressor surge, since the spring is fairly strong.

 

It does not relate to boost pressure or wastegate like you are thinking, rather it relates to vacuum. You need to measure vacuum on the engine and then Tial has a chart to find the correct BOV spring.

 

The tail BOV springs are not rated in boost but vacuum. if you have an 11psi spring it means it takes -11psi of vacumm to open the BOV. Thats why the vauum line for the BOV is sourced from the manifold after the TB so it can see vauum when the throttle closes and the pipes stay charged.

If you have stock cams the 11psi spring might work but it was way too stiff for my car. I ended up having to run the -8psi spring (Tial Q BOV) to get it to open right.

Just measure your vacuum at idle fully warmed up and see which spring you need.

http://www.tialmedia.com/documents/w3_tial_bov50_sp.pdf

 

Thanks guys. I understand that vacuum opens the BOV, but the spring rate has to do with how fast the valve opens, how responsive it is I guess

I guess I have to put it all together, start the car and see what the vacuum looks like. I have a stock rev-up. IIRC the vacuum was -22hg, in which case the spring will work fine.

 

Isn't the BOV seeing pressure from both sides (thats why there is a line going into it), when the line going into it goes to vacuum (or less than charge pressure) it opens?

So like Hal said, that springs is there to overcome the engine's natural vacuum, I think, or something along those lines lol.

Tom

 

There is close to equal pressure on both sides of the valve during boost. There reason there is different spring pressures is to adjust the responsiveness of the valve for engines with different levels of vacuum. The spring that came with my Tial Q was way to strong and the valve would barely open and when it did it was slow. Fixed that by using a much lighter spring. Another reason for the different springs is so that you can turn the BOV into a bypass valve for superchargers so that the valve is open under all vacuum conditions.

 

While in boost, pressure on the plunger (in the charge piping) and pressure on the top port is equal. The spring is there to add just a bit of pressure to keep the valve closed. In theory, a 1psi spring should keep the valve closed. So if am seeing 10psi of boost, the top of the valve is getting the 10psi + 1psi of spring pressure, keeping it closed.

When the TB closes, the vacuum overcomes the spring pressure and opens the valve, venting the charge pipe. If it is too strong the vacuum can not over come the spring pressure causing problems. Compressor surge is my main concern, because if the valve does not open the air is forced back through the compressor making it spin backwards. This is very hard on the turbo bearings/shaft.

 

I'm running the STS kit, which I have found has some more compressor surge issues than a standard turbo. I ended up running a -9 psi (IIRC) spring with a stainless washer as a spacer. This allowed the BOV to open faster without (as much) compressor surge when letting off the throttle at lower boost pressure than the -11 psi spring did.

I would say just try out the -11 psi spring, see if it's closed at idle and opens quickly without any surge. If not, just order the -9 psi spring.

 

Looking at this Tial table: http://www.tialmedia.com/documents/w3_tial_bov50_sp.pdf

and now knowing my vacuum (-21) the 11psi spring should be ok. I will know more when I get to the dyno in a couple of days.