Imo000

I'm looking to locate a vacum line for the BOV on my '85 928. I need a location past the throttle plate. The BOV needs a vacum line that is different when the throttle is closed. They operates on the principle of difference in pressure. All the ones I found under the hood so far are under constant vacum. This keeps the BOV open all the time.

shaaark89

lines to the fuel dampers and fuel pressure regulator come from past the throttle plate.

T_MaX

Brake booster line!

Imo000

Tom,
Thanks for the info. Sent you a PM regarding the FMU.


Curtis,
I think the brake booster line is under constant vacum, regardless of the throttle plate position. Or am I mistaken?

BrianG

Typically the brake booster is straight off the intake manifold. On the '86.5 it's off the left intake plenum.

The feed to the "spider" is "port timed" and is found on the throttle body.

T_MaX

Yes and No. Under boost you go into positive numbers and when you get off the gas or at idle, you will go into vaccum.

What side of the TB do you want to be on for your BOV?

Here's how I ran my last turbo BOV

Imo000

Ok, let me get this straight. Since I’ve never used a BOV in any of my previous SC applications its operation is not completely clear to me. I thought it was but it’s apparent that what I think I know is not accurate.
A BOV will vent during idle and off boost, right? It will also close as soon as vacuum reaches 0 and goes into boost, right? Is it necessary to run the vacuum line for the BOV past the throttle plate?

As for the BOV location, it's located at the middle of the radiator so eithe side of the TB will do. The easier to get to the better it will be for me.

T_MaX

I'm sure some of the 928 SC guys should be able to answer this better than me. I have only played with turbos on non-928 cars

FlyingDog

BOV is for venting excess pressure. If you have the throttle wide open then close it the turbo/sc is trying to cram air into an engine that doesn't want it. The BOV, prevents over boost. That's why you here the pop and squeel when boosted cars shift. The revs drop, more air is provided than needed, pressure goes up, excess air is 'blown off'.

I guess you could also use it to get boost earlier without over boosting at higher revs with an SC. That's one of the purposes of a wastegate on a turbo.

Edited to add: I would think you'd want your vacuum/pressure line to be on the intake side of the throttle, not the engine side. If it was on the engine side, that line could have vacuum while the other side is still building (too much) pressure.

shaaark89

imre
yes, yes, and yes.
easiest access point given your location is the front fuel damper line.

Imo000

AAhhhm never mind, I asked before i read the whole page Curtis has. At the bottom, explains the operation of a BOV (factory and aftermarket). For now I'm using a '93 ZX300TT BOV and it operates different than the aftermarket type. I will "T" into the fuel pressure regulator as a sorce.

Thanks for all the help!

heinrich

Here ....

heinrich

What, nobody got the bovine thing?

FlyingDog

I got it, just couldn't think of a good response... still can't.

Imo000

That was good! A picture of a whale's blow hole would be even better, after all they both sound alike.