Boost spiking on throttle off

[Gowf]

OFF TOPIC: Try not to have overshoot when initially step on the gas. I have used EvoScan to log the knock, and I saw there was as much as 3x knock sum counts when overshooting. In this case, the ECU will retard the ignition timing, and I guess you know what will happen if ignition retards.

As far as i am aware, the ecu will only retard after a knock sum of 5 or greater has been logged. I really wouldnt worry too much about a sum of 3. If you use external knock detection, you'l find that the ecu does see knock when its not there, far too sensitive.

As for the boost spike, surely the bov will only open when it sees a pressure differential between pre and post butterfly? It will take a very small bit of time for the pressure in the plenum to stabilise and become vaccuous, so as others have said, it may increase slightly for a very small period of time.

If you've got a stock BOV, they are not the loudest of things, but sound totaly different to a stalling turbo. I personally wouldnt be overly worried about it, but if you are then i suppose the only true way is to have a BOV mounted as close to the throttle butterfly as possible.

[Turbo_Steve]

Quick question: Where is your EBC Measuring boost from: inlet manifold or inlet pipe?

[eyeballprawn]

It's measuring from the manifold, see here.

I agree that Tee'ing in a normal gauge will give a better idea of what's going on, now to find someone I can borrow one from!

So perhaps the wording of the manual is ambiguous or the EBC is working in an unexpected way. I understood LAST BOOST to show the reading of immediately before I took my foot off the throttle, and PEAK to show the highest boost since last spool.

More testing is required!

[wintertidenz]

I have found that when planting it and letting off the accelerator at high revs, I got a spike up to 10psi from 7 momentarily. This was measuring from the intake pipe before the throttle body - where the stock boost solenoid plumbs into.

However, if measuring boost pressure from the other pipe on the back of the manifold (the one beside the brake booster) it stayed at 7 and when I released the foot it went back into vacuum.

Note, this is without a boost controller... and I have a VTA BOV. BOV makes the normal sounds, so it sounds like it could just be the quick surge between the butterfly closing and the BOV venting the extra pressure.

Not an expert on this though but that's what it sounds like to me

[Davezj]

yes i would agree with you about the butterfly valve closing and causing a boost spike if your measuring the pressure before the throttle body.

but he is measuring the correct manifold pressure. so i think it is all to do with EBC, and either the set up or it just giving an odd reading.

[kc427]

oh, maybe I typed it wrongly, actually, I mean knock sum as much as 30+ I had logged, and the ignition was logged retarded a lot when there was overshooting.

As far as i am aware, the ecu will only retard after a knock sum of 5 or greater has been logged. I really wouldnt worry too much about a sum of 3. If you use external knock detection, you'l find that the ecu does see knock when its not there, far too sensitive.

As for the boost spike, surely the bov will only open when it sees a pressure differential between pre and post butterfly? It will take a very small bit of time for the pressure in the plenum to stabilise and become vaccuous, so as others have said, it may increase slightly for a very small period of time.

If you've got a stock BOV, they are not the loudest of things, but sound totaly different to a stalling turbo. I personally wouldnt be overly worried about it, but if you are then i suppose the only true way is to have a BOV mounted as close to the throttle butterfly as possible.

[VR4WGN]

then maby you guys may know my problem too then,sorry if you feel im hijacking guys but my legnum is spiking hard out,just jumps to 20,is it ok for me to restart the process and remove MBC from engine,get boost reading then start again adding boost?,now iv bridged the boostguage off a t form the MBC Exit hose(to intake hose before plenium),and now the guage doesnt go into vaccum out of boost like it usedto when tee'd fro mthe rear of manifold s normal tee positions come from,is there anyhting i may have missed?
~Q~

[Davezj]

i don't believe the MBC should be plumbed in where you have it connected to. it should be plumbed in, in the line that goes to the stock boost solenoid (has a red band on it). can't remember if it is on the wastegate side or the inlet pipe side. but are loads of threads about where to put it so i would advise you to check up on it.
But the boost gauge can be tee'd in to the smaller of the two line that come from the back of the plenum.

hope this makes sense

[wintertidenz]

I think the one from the big plastic intake pipe before the TB is what the car is meant to be boosting, and the one from the rear of the manifold is the actual boost. I found that when my boost gauge was tee'd into the one before the TB it wouldn't go into vacuum, but now it's tee'd off the back one it does go into vacuum.

[VR4WGN]

thnx dave and winter... um ill re plumb them accordingly,as mine is also where you did yours last,so not going into vaccum,,thanx guys
~Q~

[eyeballprawn]

Does the BOV activate on the gear changes under a WOT run in an auto transmission?

[AlanDITD]

Does the BOV activate on the gear changes under a WOT run in an auto transmission?

No mate, only when you lift off the throttle

[eyeballprawn]

I now have confirmation of what is really happening, seems the suggestions that the boost was spiking on the automatic's up changes were correct.. and not spiking on throttle off as I thought. These spikes were so quick that the digital readout couldn't display them in real time and I wasn't seeing them until the LAST BOOST flashed up after throttle off.

So, the Profec's manual is ambiguous and should instead read:

LAST BOOST
This mode is used to display the last peak boost that was recorded since coming into boost every time the accelerator was released for 3 sec.

[Davezj]

glad you got it sorted.

it is alway good to find out for definate what is going on. glad the suggestions that you got from this site were correct. Some of use do know what we are talking about after all.

[eyeballprawn]

Yes indeed I really appreciated everyone's assistance and couldn't understand why the controller was telling me those readings.

The downside is that I still have the spikes. But I can't really see how to get rid of them with the automatic tranny . Should I even want to get rid of them in an auto or will performance suffer under WOT up changes? Theoretically, if the BOV *did* actually activate, which it can't, would that be beneficial?

[Kenneth]

It is likely that when the change in RPM alters the engines air consumption rate, the spike occurs.

That being the case, it will always happen.

I wouldn't worry about it.

[eyeballprawn]

It is likely that when the change in RPM alters the engines air consumption rate, the spike occurs.

That being the case, it will always happen.

I wouldn't worry about it.

Nice concise explanation, that makes sense. All I can do then is to wind the boost down so that it never spikes too high to cause concern.

[Kenneth]

Log your detonation count and compare to your boost spike.
If it doesn't cause the detonation count to go up, don't worry about it.

[eyeballprawn]

Shall do, I have just ordered EvoScan.

Out of interest, how fast can the wastegates vent boost compared to a BOV? I'd assume they'd be less efficient?

What I'm getting at is, with a more intelligent EBC could it be possible to reduce this spiking by venting via the wastegates? Or would the lag between the turbos and the EBC reading the pressure at the manifold make this difficult?

[Kenneth]

Wastegates do not vent pressure, they let the exhaust gas bypass the turbine which results in the turbine slowing down. As a result, the compressor pumps less air and boost drops

I think controlling that spike is going to be difficult no matter how you approach it, mainly because you don't know exactly when the gear is going to change.

Again, if it isn't causing detonation, don't worry about it. All auto VR-4s probably behave this way. (At least ones with the same level of mods)