stu.p

fitting up my boost pressure gauge on my zetroinix afr/boost gauge and was wondering where you guys think the best place is to take the accurate readings from???..
what bout pipe going between the inlet manifold and the ez69 unit?

DonE

For my boost guage in my dash, I take pressure from the brake booster. For the ECU on my Link system, I take it from the intake manifold.

spuggy

Should be accurate pretty much anywhere, so long as the diameter of the pipe is adequate.

Below the throttle plate will register vacuum but no boost spikes, before the throttle plate won't show vacuum but boost spikes will register.

JFairman

what do you mean by boost "spikes"?
Taking a vacuum reading below the throttle plate would give the same reading as anywhere on the intake manifold, accept that there could be some venturi effect skewing the vacuum reading if it's taken from a vacuum port right next to the butterfly while it was in a part throttle positon making extreme turbulance next to that vacuum port.

The brake booster vacuum hose is from the drivers side of the intake manifold.
T into into it down under the dash where it comes out of the center tunnel and crosses over above the pedals. Or right at the brake booster hose fitting.
You will see high vacuum on deceleration, moderate vacuum at idle or steady speed, and pressure when on boost from that hose.
The inline check valve for keeping some vacuum in the brake booster is right at the brake booster port.

DonE

I have to ask - what is an "ez69 unit"? I have my own definition, but its not automotive.

911rudy

You know Don, its the thing with the crack in it.

spuggy

Well, this is nothing to do with overboost caused by a wastegate or controller not regulating correctly.

This seems to be well-understood in the import tuning market, from a random VW forum:

http://forums.vwvortex.com/zerothrea...ostid=40377025

(the actual usage of "volume" in that paragraph is suspect, but it conveys the sense of it well - far better than I managed to write)...

Even if the recirculation valve has vacuum, it's a mechanical assembly with mass and inertia and spring pressure to overcome - it cannot react instantaneously.

And all the while, the turbo hasn't stopped spinning. What is that, 100,000 RPM?

So boost will temporarily go higher on the turbo side of the closed throttle plate than it is on the motor side (where pressure will drop into vacuum).

Boost spike.

The factory puts both the overboost switch and the boost sensor before the throttle plate. Coincidence?

If your recirculation valve is sluggish to react or stuck, you'll have a bigger/longer spike than if it's working correctly.

Also, find Jim2's video of his EFI 930 on YouTube; he has a dual absolute pressure gauge from an twin-engine WWII bomber, one gauge plumbed before, the other after, the throttle plate. Watch them do completely different things when he shifts, it's rather neat.

The motor itself never sees the spike, except possibly if you can get straight back on power after slamming the throttle shut, but before it's been bled off.

It can be significant because it can indicate issues with your recirculation valve (although leaking boost is probably a different failure mode), and also because some people posit that any overpressure getting forced back through the compressor blades (some racers don't run a recirc or BOV, saying response after the shift is better without) can damage them (that twittering sound is apparently the blade tips stalling in the charge air stream).

JFairman

Thanks for the explanation, that makes sense.

The "twittering" sound... do you mean the chirping like sound that blow off valves that vent to atmosphere make when closing the throttle(s) at high revs?
Like that sound a 962 used to make while off throttle and braking into a fast corner?

Are you saying it's the sound of the turbo impeller tips stalling or cavitating to some degree when closing the throttle(s) and you are hearing it in the air thats escaping from the pop off valve?

I always thought it was a high frequency reed like sound the pop off valve made as it opened releasing the air pressure build up when closing the throttle butterfly(s) at high revs and boost.

spuggy

You're welcome. Heh. I cheated anyway... I did start by typing up my own explanation, but the one I quoted was so much more coherent and concise that I used it instead

Uh, I'm not an expert on the noises that BOVs make. Out of curiousity, I listened to a WAV I found on a website one of those bolt-on units that make it sound like you have a BOV (for wannabe-ricers), and it just went Whooooooosh kind of like a CO2 fire extinguisher, as far as I recall..

I understand that that is what happens when the backpressure isn't relieved, and that it's audible, yes.

As to whether 962's make the noise - the links I found on YouTube sound more like a BOV venting when they shift to me, but they don't just go "whoosh", there's a on/off modulated thing going on there - I dunno what that's about...

Here's the thread on PP on the topic of running wthout one:

http://forums.pelicanparts.com/showthread.php?t=327438: