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Re: Split Second looks pretty good..... (archive)

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Posted by Shawn Fogg on June 10, 1999 at 10:06:16:

In Reply to: Split Second looks pretty good..... posted by Cheuk in Seoul on June 10, 1999 at 08:46:12:

Cheuk,

"She thinks its fuel problems at partial throttle along with pinging."

That doesn't suprise me on a turbo. IMO, the problem is that your engines flow rates (at any RPM/thottle positon combo) aren't even remotely fixed due to turbo lag and such. Because of the adaption of the DME it keeps chasing it's own tail trying to get the air/fuel ratio right.

"I will look for other solutions to include a generic system to control fuel with boost."

That is what the boost sensitive fuel pressure regulator does. As boost comes up it increase the fuel pressure in your fuel rail. The DME opens your injectors for X amount of time. Since the pressure is higher in the fuel rail more fuel is squirted into your engine, hopefully to match the added air that is being forced into the cylinders by the boost.

Where I think you run into problems with the DME is that under identical conditons (as far as the DME is concerned) RPM/throttle position the engine may or may not be running at the same air/fuel ratio as measure by the O2 sensors. So the computer adjusts X amount of time slightly but the next time you are at the same conditions (again to the DME) your boost level may be different (lag) and the correction the DME applies to X actually makes the air/fuel ratio worse... if it leaned you out under boost you will ping. This cycle keeps going and going and going.....

Now there may be some electronic gizmos that MIGHT be able to help with this. Split Second has a device that alters your O2 sensors output under boost to force the DME into open loop mode where it doesn't use O2 sensor feedback. This will probably trigger a check engine light though (the DME may think a O2 sensor just died) and even then I don't think it will totally solve the problem. HKS might make similiar electronics but their stuff is mostly for cars that came from the factory with turbos. In those cases the DMEs will be monitoring boost themselves and can deal with the changing airflow conditions.

Contrast this with a supercharger... since it is directly tied to engine speed it's boost is fixed at a certain throttle position and RPM. The conditions don't change (at the same RPM/Thottle position) so the DME can properly adapt to get the air/fuel ratio where it wants it and the next time the engine is at the same conditons the air/fuel ratio is pretty much the same.

Shawn


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