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boost drop off

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Hello i have a ford zetec engine with a t3 turbo on it and vipec v44.

With no duty it peaks boost about 12/13 psi at just under 4000 rpm and by 6500 rpm its down to around 7 psi the drop off is linear.

Thats not a problem as i could hold a flat boost line by playing with duty.

The problem is when you start adding duty i can make it peak 30 psi just under 4000rpm but by 6000 rpm it will be around 18/19 psi.

I have tried 3 diffrent actuators on it and its the same on all of them i tested the system for boost leaks but couldnt find any.

Has anyone got any idea what could cause this thanks max

The issue you're having is due to the turbine side becoming excessively restrictive at high rpm. This isn't uncommon, particularly with smaller sized turbos. The result is the back pressure in the exhaust manifold becomes so great that it begins to over ride the wastegate spring and force the actuator open.

The solution would be a larger turbine wheel, larger exhaust housing or both. Keep in mind though that this will result in slower boost response.

Think boost drop does also happen if you get out of the efficiency range of the turbo and consequently the turbo can't flow enough to hold the boost pressure at higher revs. That's quite common on std. or small turbos.

I can't make sense of your comment, "Anonymous User". How can you have MORE boost with MORE duty-cycle??

Marek what shoud ve wrong with that? If you have plumbed the wastegate right, higher numbers in the wastegate duty cycle table makes usually more boost and not less.

Am i missing something here?!... more duty-cycle = more open waste gate = more exhaust gas bypassing the turbine = less boost.

You don't give any duty cycle direct to the wastgate valve. Let's take a turbo with internal wastgate for the explenations.

The internal turbo wastgate actuator is actually pneumatic controlled via the boost control solenoid (bcs). Of corse, as more duty cycle you give to the bcs, as more it opens the athmospharic port. That means less pressure is applied to the wg actuator and the spring holds the turbine bypass valve closed.

Summary:

If you disconnect the hose to the wg actuator the turbo will produce as much boost as it can. (bad idea to test)

- If you connect a hose betwenn turbo compressor outlet pipe direct to the actuator, you will get only base wg pressure, which depends on the actuator spring and preload setting. (no problem to test)

Marek, it's all going to depend on the type of valve you're using and the way the wastegate is plumbed. With the very common MAC 3 port solenoid that I use for boost control wore duty cycle will either bleed more boost away from an internal wastegate or supply more boost to the top of an external wastegate which has the effect of increasing boost.

It's also safest if the system is set up this way as the system will revert to minimum boost if the wastegate solenoid gets unplugged for some reason.

Hello ive retested the intake side for leaks and with more pressure found 4 leaks on the inlet manifold so resealed that and just have to test it

A leak on the compressor outlet side can obviously effect your boost, however you usually need quite a large leak to have a significant effect on the boost curve. The turbo can move so much air that a few pin hole leaks or instance have a negligible effect. That being said though, obviously no leaks is the preferable situation.

having looked more closely at the old inlet manifold gasket it looks to have completely failed.

as you say the small leaks shouldn't make such a difference will confirm tomorrow when the new gasket arrives