Boost Creep

[cutsnake]

Gday All,

I am not sure if I am using the right term (boost creep?) but I have a problem with boost spiking more than it set for, before settling to its setting. For example with either 8 or 10 psi boost maps (custom edits), when nailing the throttle it goes over the boost setting by around 4 psi before it goes back to normal. This isn't happening all the time but it is definately noticeble on kick-down (auto) and early in the morning (cool air).

I am little concerned the afr's will be way out when this is happening, plus it pings which can't be too good for the motor. I have noticed this with the factory map, but its no where near as noticeable.

Has anyone else noticed this? Is it caused by a lazy wastegate?. Is there anything that can be done to correct this?

Thanks

Cutsnake

[cutsnake]

OK, I think I have boost spike not creep. Found this explanation on the net (Still searching for the cure..).

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Boost creep is rising boost due to not enough exhaust bypassing the housing. For example, you're set for 13 psi, and when you floor it, you get a quick rise to 13 or 14 psi. But then as rpm's climb, the boost creeps higher and higher: 14, 15, 16, 17, etc. Enlargening the wastegate hole is one cure for boost creep, as you did.

Spiking is when your wastegate hole is large enough, but your actuator can is taking too long to fill with compressed air, so you *temporarily* overshoot your boost level. Example: you're set for 13 psi, and when you floor it it quickly rises to 16 psi, but then gradually drops downward towards 13 psi again.

A common problem with modded engines is a combination of spiking AND boost creep. That's where you're set for 13 psi, and when you floor if it overshoots to 16 psi, *begins* to drop back down, but as the rpm's rise it goes up again!

Normal boost rise when set to 13 psi = shoots up to 14, then 13, 13, 13...

Spiking = shoots up to 16, then 15, 14, 13, 13, 13...

Boost creep = shoots up to 14, then 13, 13, 14, 15, 16, 17...

Spiking and boost creep = shoots up to 16, then 15, 14, 13, 14, 15, 16,17...

Set a manual boost control to 11 psi and floor it slowly in high gear. It should go to 11 psi and hold it pretty well. If it starts to slowly climb over 11 psi as the rpm's get high, that's "creeping". The boost creeps higher at high rpm's because there's too much exhaust flow for the wastegate hole to bypass.

Now go to 11 psi again, but tromp on it quickly! It will shoot to 13, or 14, or 15 psi, then drop down to 11 and stay there. That's "spiking".

Spiking is a quick flash of high boost that goes a little beyond your set boost point. If you are set for 11 psi, and you tromp on it quickly, this might happen: boost "spikes" to 14, quickly drops to 11, then slowly "creeps" back to 14 or more as the rpm's climb. Now you are seeing spiking followed by creeping!

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[<XR6T>]

Have you got a cold air intake?? If you have with a standard tune you will get a boost spike and pinging due to the turbo spooling up quicker (from having less restriction due to CAI) than the program has been programmed to. This happened to me, I blocked the CAI and it fixed the problem straight away. You will find the power will increase somewhat aswell.

[cutsnake]

Yeah, I have the DIY under the headlight job with a K&N filter.

Even though I have a custome tune, I might block it off and see if it makes a difference.

Thanks for the tip..

Cutsnake

[<XR6T>]

No probs mate, hope it helps. If it turns out to be the problem it might be as simple as getting your tuner to take a degree or two out of the timing.