air fuel / injector duty cycle LED gauge

[Sonny]

I just installed one of these air fuel / injector duty cycle gauges with the two parallel rows of LED's. Why did I buy this thing? I never know what the heck it is telling me. I'm guessing I better not rely on this thing for setting boost levels. Do I need to hook it up to my leaf blower and buy a different instrument to install on the 951???

[Porsche944]

You should have installed a boost gage, but if you are going to be tuning the engine, I would recommend taking the car to a dyno shop. They will have much better test and measuring equipment.

[Sonny]

Oh I do have a boost gauge. Is seems that there is not much difference in the indications on the LED's with 5-7lbs as compared to 10-15lbs. ???? I think I 'll take your advise though and go to a dyno.

[Porsche944]

The LED gage is getting an input from the O2 sensor giving you an indication of where the fuel/air ratio is running. At idle or while cruising at a steady speed, the gage should cycle up and down to indicate that the DME is adjusting fuel flow to maintain a proper fuel/air ratio.

When you get on the throttle, the gage should jump to a rich reading and stay there until you either shift, or back off.

This is due to the fact that the Wide Open Throttle Switch in the Throttle Position Sensor tells the DME to use the WOT fuel mapping. When you shift to the next gear, or back off on the trottle the WOT switch opens and the DME shifs back to Partial Throttle fuel and ignition mapping.

Closing the WOT switch also shifts the DME to Open Loop Control which forces the DME to ignore the input from the O2 sensor.

Boost level has nothing to do with the gage indication unless you are having a fuel delivery problem.

[Sonny]

Thanks for the info. I guess I have alot to learn about the 951 tuning.

[Porsche944]

Sonny:

Yeah, tuning takes a lot of thought, study, patience, and proper tools/equipment.

In stock form these engines are designed to have a max boost of about 12 psi. The max boost level is controlled by a cycling valve. When the CV senses max boost it sends a vacuum signal to the waste gate. The waste gate opens and dumps exhaust pressure.

My '86 has what you could call a "Stage 1" mod. I have Guru 15 psi chips, a 3.0 BAR Fuel Pressure Regulator, and a ReliaBoost manual boost controller. I have boost and Fuel/Air ratio gages installed in a pillar pod.

The MBC takes place of the CV, and is set to limit boost to 15 psi. I have not dynoed the engine, but I figure I'm making about 250 HP at 15 psi.

For me this is just enough to give me that turbo kick without risking an engine failure. Driveability is very good, and the car gets about 30 MPG on the highway.

There are lots of resources available to get you up to speed. The main thing to keep in mind is that adding horse power costs money, and it is never a simple bolt on process.

Not to subvert this group, but you may want to check out the e-mail lists and message boards on RennList.Com.

[Sonny]

You do mean 250 HP at the wheels right?

[Porsche944]

Yes.