problems with EFI car idle and pressure sensors

[Craig Dotson]

> This raises an interesting point. I've data logged at about 50Hz,
> and on a sharp throttle opening MAP changes almost instantly, i.e.
> in about 1/25 of a second. You can't move the throttle much faster
> than that.
>
> Why would I need TPS? (Except for overrun cutoff, etc)
>

Our FormulaSAE car runs a 600cc motorcycle engine with DOHC of an aggressive
profile.  As with any agressive, long-duration camshaft, the engine has very
little manifold vacuum at idle.  If you're running purely MAP control, your
idle will be over-rich and may even choke out the engine due to the
poor/erratic vacuum signal your MAP will see.

If you have such a camshaft, or have idle troubles, see if your ECU will
allow you to change the load parameter based on TPS setpoints.  On our
engine, we have a separate fuel map for throttle <~5% and another one for
throttle >~90%.  The percentages may vary, but somewhere in-between there
will be a useable MAP signal.

In my very limited experience, systems with both a TPS and a MAP are easier
to tune and run more reliably than those that eliminate either TPS or MAP.
But your results may vary!

Craig Dotson
crdotson at vt.edu
2002 VT FormulaSAE

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