Cosmo Kramer

Boostards:

I am putting together a group buy for a bunch of pressure switches to put the LH and/or EZF/K into open loop under boost. Under our current setup our cars have open loop with no O2 sensor intervention only above 80% throttle. Our cars are well into the boost at this point causing a lean tip in condition at part throttle.

I have sourced a switch that will flip the LH to open loop at .5 psi (it is actually adjustable from .5 - 2 psi). This allows the richer map to come into play and the O2 sensor will not be shortening the injector timing trying to lean the AFR back to stoich. It will also add a bit of additional ignition retard (if required) without having to wait until the pedal is mashed right to the floor.

The manufacturer has changed the materials inside the switch to make sure it will work in cold and warm environments (range is -46 - 205 F) and is tested to operate over 10 million cycles. The hookup is relatively simple and can be set up to flip both computers at once or just the LH.

I got the manufacturer to sell me one switch to put on my car and I have been running it for a few weeks. With no Sharktuning and just a FMU, at part throttle and low boost my AFR's are in the 12's, before they were in the high 13's and now the power is much smoother. If you are Sharktuning this will make your job much easier as you won't have the O2 sensor working against you when trying to get lower AFR's.

This will work on any turbocharged or supercharged 928 with a LH Jetronic fuel management.

If I have 10 or more switches to order, I can get them for $50 shipped within USA/Canada. I will make up some instructions to send out with them as well.

Please PM me if interested.

BrianG

When SharkTuning why not just shift the target A/F ratio to 12.6 for everything above the 50% TPS point?

Cosmo Kramer

From my understanding, the WOT map works in conjunction with the base map. The TPS only contacts at idle and WOT. The MAF % is part of the closed loop map.

With a sharktuner it is possible to tune to allow a richer tip-in, but it is much easier with the WOT map activated. It allows you to pick a more precise AFR. Without sharktuning on stage 1 systems it really works well.

Perhaps AO will chime in as he has a pressure switch to bring his car to open loop and is sharktuned. I am just running a basic system with a Begi FMU, no sharktuning and it is working really well.

jcorenman

That won't work if the car is configured for cats and has a NBO2 sensor. The LH then normally runs in "closed-loop" mode, monitoring the NBO2 sensor and adjusting fuel as required for a AFR of 14.7. When the WOT switch closes then it goes to open-loop mode, ignores the NBO2 sensor and follows the maps to determine fueling. The LH also goes into open-loop mode at higher loads or RPM, short of WOT (the 80% point that Ted mentioned.

The high-load areas of the fuel map can be adjusted to 12.6 or whatever you want, but until the LH goes open-loop it will happily adjust the fuel back to 14.7 as directed by the NBO2.

The pressure-switch is one way to force the LH into open loop before it normally would, alternately (without cat's) you could change the jumper or use ST's "force no-cat" mode and run open-loop all the time. Another possibility is to use the pressure-switch to disconnect the signal-lead from the NBO2 sensor, this will kick the LH into open-loop mode without also engaging the WOT maps.

Besides forcing open-loop, the WOT switch also tells the LH to use the WOT map, which adds additional fuel to whatever the base-map says, and also switches the EZK to a different WOT-map for ignition advance, instead of the normal cruise-map. With an ST, both of these can be ignored (zero the LH WOT map and disable the EZK WOT map), and do all of your tuning in the main maps.

John Speake

Bear in mind that the LH goes open loop in many situations before the WOT switch is activated. e.g when you hit the throttle to accelerate, the LH has to go open loop to allow the instantaneous enrichment necessary. If you still have a lean spot, there are edditable accelaration enrichment parameters in the SahrkTuner to modify the required fuel enrichment.

dprantl

Is this for a PSF102-7651-714 from Farnell UK?

Dan
'91 928GT S/C 475hp/460lb.ft

Cosmo Kramer

That is true, kind of like the "accelerator pump" on the older cars with carburetors. On my wideband this only lasts a second or so and only with a quick throttle opening to compensate for the additional air coming in.

When I had the Vortech FMU, it wouldn't increase the fuel pressure until about 2 psi of boost and if I was accelerating moderately at the transition point into boost I was running crazy lean (like high 15-1). The car would actually stumble a bit until I put my foot into it a bit more. For a while I just learned to drive around it kind of "all or nothing". I switched to a Begi FMU which starts to raise the fuel pressure at 2 hg which is great. Now with the open loop switch the AFR is right where it should be and the engine pulls nice and smooth everywhere.

When cruising under light throttle/vacuum my wideband cycles right at stoich exactly where it should be.

Cosmo Kramer

Very similar, has a different diaphragm for cold weather and can handle a bit more current that that one. I am getting them from a distributor right here in Canada.

killav

Ted,

Did you ever get enough interest for a group buy? I would still like one.

Thanks,

AO

Sorry I missed this thread.

As Ted explained, the pressure switch if connected to the WOT lead on the LH will kick the LH into WOT as soon as it sees boost. I've had mine for 2 years now and it makes all the difference in the world.

With boosted cars it's easy to run them lean if you;re just rolling into the boost. This eliminates that. As soon as I see any boost, the AFR drops to 12's. It's a beautiful thing.