Boost spiking on throttle off

[eyeballprawn]

Thanks for clearing up my misconception. I was just thinking ahead to if I ever wanted to do further mods and up the boost and the possibility of hitting fuel cut during these spikes.

[Kenneth]

If the X axis is seconds, then the spike does not last long enough for the ECU to fuel cut on that alone.

You might find with increased boost that the spike isn't as big.

[eyeballprawn]

I have taken the previous comments on board, but please just humour me for the sake of discussion & further knowledge.

With a manual car, does the boost drop right off at the gear change as the BOV vents and if so then by how much? Would we expect to see a dip at that point instead of the spike?

What is actually the best possible scenario? Hold boost from where it tapered off to in the last gear, then smoothly ramp up to the maximum desired boost level again without spiking?

[Turbo_Steve]

The "best" scenario would be for the boost to be held at the target ready for the next gear. This neccesitates keeping the turbo spinning, which requires exhaust gas flow.

So ALG allows air to bleed into the exhaust system (usually via EGR-type pipes) and injects and ignites at the exhaust part of the cycle, resulting in no power, but a hot gas mixtures exploding inside the exhaust manifold and driving the turbo round. A good (read "bloody expensive" ALG system actually maintains a boost target based on the RPM select for the next gear. Most systems are a big cheap and cheerful and just keep the turbo spinning.

ALG will pretty much make your car illegal for road use and emissions as well as noise, puts more torque load on yoru drivetrain (in a manual) and tends to gradually destroy exhaust manifolds and turbos as the temperatures are very difficult to manage, and the forces involved considerable.

It's also virtually pointless on a car with teeny tiny turbos like ours!

[eyeballprawn]

Sorry Steve, what's ALG? Anti-lag something?

[Kenneth]

A BOV releases the pressure when the accelerator is lifted. As such it works the same for auto or manual transmissions.

As boost is usually measured from the plenum, the fact that the plenum goes into vacuum is due to the throttle being released (and closing the throttle plate). This would happen with or without a BOV.

The BOV when activated, just lets air continue to flow through the intake system without having to go into the engine.

That being the case, you would expect a pressure dip which looks similar to your log where you lift the throttle.

Neither is good or bad, just different.

[AlanDITD]

Have i missed the point here, but dont all internal wastegates have a boost spike to some degree. External wastegtes have always been the alternative, and dont produce spikes as the gasses are completely clear from the turbo when they are vented.

[Turbo_Steve]

ALG=AntiLag, indeed.

[eyeballprawn]

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.

I just did a logging run, and in general there is some knock under hard acceleration at high boost, the highest recorded was a knock sum of 14. However, the gear change spikes had only 2 or so. Is that ok?

[Davezj]

what fuel did you have in the tanks when logging it, as the lower the octane below 100 the more knock you will get under hard acceleration.
i am talking about uk fuel rateings here, don't know if they differ in AU.
if i use 95 RON and try logging knock it can be massive. but if i log with 98 RON or 99 RON in the tank the knock is minimal under hard acceleration. Knock of 2 tops.

[eyeballprawn]

I'm running 98 RON, perhaps I should try with a tank full from a different service station to eliminate that possibility.

So when people talk about knock count, is it the same thing as knock sum?

[Davezj]

thats a good point i was monitoring knock sum. which i think is a added count hence the name.

but if i remember rightly it was not a gradual build up and drop off of knock but big spikes.

i think the one you shold be monitoring is knock count, which will give an indication of knock at that particular momenrt in time.

[eyeballprawn]

During the auto's gear changes Evoscan logs confirm the RPM is dropping while at the same time the boost is spiking.

Apart from this, is there ever a situation where you would actually want or expect the boost to increase while revs went down?

[Turbo_Steve]

You mean apart from Anti-lag?

[eyeballprawn]

Yes apart from in an anti-lag system. On a normal road car, in everyday driving.