4-cylinder GM - AC Delco MPFI

How to Overboost your Esprit

A Lotus Tech showed me a neat trick I had not heard before

If you rev the engine to over 5000 RPM momentarily, but the vehicle speed is zero (i.e. the car is in neutral and/or the clutch is in), then for the next 15 seconds or so,  the ECM will essentially skip over the lines of code that tell it to open the wastegate at its regular setting. This means that on an SE or S4, which normally allows about 0.84bar, you  will be allowed to go to a full 1.0bar. The S4s already goes to 1.0bar,  as does the gauge, so I don't know how far beyond that it will allow you  to go. Let's just say the needle was pegged!

Lotus manuals state that on the SE and S4 max boost is 0.65bar. At coolant temperatures between 75 and 115 degrees Celsius, and at throttle angles greater than 75%, the boost control solenoid valve is activated which allows up to 0.84 bar of boost. Under wide open throttle (100% - WOT) conditions, a short duration of additional boost up to 1.0bar *may* occur.

If boost in excess of 1.01bar is detected for more than 3 seconds, the fuel pump and ignition are shut off as a protective measure. This overboost trick will allow you to get 1.0bar for about 15 seconds, thus skipping the 0.65 and 0.84bar limits temporarily.

On the S4s, 1.0bar is normal max boost and doing the trick gives even higher boost although I have yet to find the specs as to what is the overboost maximum.

Also, note that the maximum boost readings of the boost gauge in the instrument panel will tend to rise with increasing altitude and decreasing atmospheric pressure. Actual boost is controlled by values determined by the MAP sensor which is independent of these variations. What this means is that you can't always believe the readings you get on the gauge as they can be higher that the actual boost.

I was told that the reason overboosting is done is so that Lotus can get lesser 0-60mph times for the Esprit's specs. By allowing overboost under these conditions only, they can get max performance for short bursts, while making sure that owners don't regularly blow their engines.

This would also help explain why magazines get such different 0-60  times. Maybe some magazine's test drivers know this trick and others don't. Add to this, driver skill, weather and track conditions,  heat-soaking of the chargecooler, etc. and you can see how the numbers might vary.

By the way, I feel compelled to put a big disclaimer on this: If you blow your engine trying this out, don't come crying to me! 

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