Fuel Injected X Computer is Amazing!

AZDrifter

True Classic
I installed an AEM wideband oxygen sensor while putting in the Allison exhaust so I could see how bad the air/fuel ratio is with all of the modifications I have done. Modern OEM computers compensate for unexpected additional air by running the motor extremely rich. We have to buy "piggyback" computers and tune out the extra fuel and get them back into a reasonable range.

The X computer is 34 years old. You would expect it to be a mess after trying to compensate for an aftermarket cam, a bit more compression, and a free-flowing long tube exhaust. Plus I have new fuel injectors. Quite the contrary. Once it got up to operating temperature, it maintained an ideal Air-fuel ratio for fuel economy. It bounced between 14:1 and 15:1 for the whole test drive, except during throttle lift off, where it went a tad rich. I can't wait to start tracking gas mileage with the new header. I think I can top 35 mpg. :headbang:

I am guessing that early computers were a lot less complex and just stuck to the task at hand. Plus they didn't have to deal with ignition timing. The ideal ratio for making power is 13.5:1, but the benefit of getting there is not worth the cost.
 
Well, not really a "computer" as such, more like a series of opamps implementing a PID controller. Nevertheless, very elegant (even if completely undocumented).

Dom.
 
That's quite a wide (14.1.-15.1) AFR sweep for a steady state (idle or cruising) . .5 would be more efficient (14.7-15.2 running LH2.2 on my X), but no way you're getting that from L-Jet :)
 
One thing I found out about these sytems is if you try turbocharging an L-jet system, no matter what you do, the engine will still ping unless you break the signal between the ECU and O2 sensor under boost.

I installed a rising rate fuel pressure regulator on my Spider and I knew I had enough fuel and enough ignition retard. It took me awhile to figure out what the problem was.

It seems the ECU lambda system will not allow the fuel mixture to go above a specific point under any condition unless you distrupt the signal. I'm not sure what that limit is.

The solution was to place a pressure switch between the ECU and O2 sensor so as soon as the switch detects pressure it breaks the signal and allows the ECU to run blind.

I suppose this is some sort of fail safe to prevent the engine from running too rich due to high fuel pressures or any other false readings for that matter. Where in my case I wanted the engine to run richer than normal under boost.

Here is a photo, not the best shot. I have three pressure switches mounted on a homemade manifold out of brass tees connected to a hose after the throttle body. One pressure switch goes between the ECU and O2 sensor. One activates my "dirt cheap" boost retard system using a 5 pin GM HEI control module and the other one kicks the AC compressor off under boost.

RebuiltT34.jpg
 
Back
Top