Hi there ,
My car is a 1994 Lt1 with no catalyc convertor , my problem is that it must pass the emission test here in the Netherlands.I have tunercats and a custom tune but i do not know what i need to chance to run the car leaner en maybe pass the emission test.I have a true dual system and long tube headers and two little mufflers. I have tried it with two little catalyc convertors behind the mufflers , but the effect was not so good.
Does anyone have some sugesstions ?


Thanks ,

Michel

What emissions stadards do you have to meet? Do they test for NOx? If they test for NOx, you may not gain anything by leaning it out. As you lean out the A/F ratio, CO and unburned HC's drop, but NOx will increase. That's why they hard-coded the stock PCM to always run at 14.7:1, except on cold start, and when you go to wide open throttle (power enrichment, or PE mode). Its extremely difficult to change the stock closed loop air fuel ratio with programming. You have to set the PCM up to run in open loop, and then play with the parameters that control the open loop programming target A/F ratio.

If you don't have to pass an NOx test, one way to produce a leaner exhaust is to use a fuel with a very high alcohol content - ethanol or methanol.

Other than that, the best solution is to put cats on it. My 94 was set up with dual 3" CarSound cats, and would pass NJ rolling emissions while still capable of producing almost 500 flywheel HP. But the engine was managed by an aftemarket ECU that could be programmed for any A/F ratio. The cats do not really hurt performance, paricularly when you have true duals and can use dual high flow cats.

Hi Injuneer ,

We don't test for NoX , is it possible that i can fake the PCM by telling it has very big fuelinjectors ( 36 lbs ) so that the injection time be shortend ?
I saw you have also true duals , do you have good ground clerance ?
The carsound cats , what type of brand is this , i am looking then for a small cat for a good ground clerance.

Thanks ,

Michel

If you set the injector constant to a larger than actual injector size, it will ony run lean on intial startup. As soon as the O2 sensors see its running lean, the PCM starts to adjust the short term and long term fuel corrections to bring the A/F ratio back to 14.7:1. The long terms get stored in the PCM, so from that point on, its running at the same A/F ratio is was before you put in the incorrect injector constant. Just an idea, but doing something to the O2 sensor signal wire to make the signal appear higher than it really is would cause the PCM to think its running rich when it isn't, and drive the A/F ratio leaner.

Why not just install cats?


It's pretty much a track-only car. The exhaust consists of a 3" Borla XR1 shorty muffler bolted to each collector header, and a 3" pipe and turndown near the axle. No significant clearance problem.





The Carsound cats are made by MagnaFlow.

http://www.car-sound.com/04basics/04converter.asp

The ones I had were a bit thinner than the Borlas in the photo above. My original exhaust was with the 3" CarSound cats bolted to the header collectors, then a custom 3" Y-pipe from Mufflex, and a Mufflex 4" catback with the 4" FlowMaster Race-Series single out muffler. The only time the exhaust hit anything was when the car was lowered 1.8". But then a lot of stuff under the car hit when it was lowered. The exhaust was set up with bolted flanges, so that at the track, the cats could be removed and replaced with the Borla mufflers. This was not done because of power considerations, but because with the 300-shot of nitrous, I have to run highly leaded C-16 from VP fuels, and I didn't want to poison the cats.

Ok , thanks for the answers Injuneer , i think i have to buy a pair of cats but then i have to change my exhaust system.Does it hurt performance that you run the exhaust pipes with no x-pipe or H-pipe ?
I have driven with a seperate system and my toughts were that it was a little faster as the setup i am now driving.
This problem comes every year back , so i need a good solution for the exhaust system.
I saw on the Internet a very small catalyc convertor , do you think that they can do the job ? It is a Dynatech power cat , and a magnaflow spun-cat.
I have very little clearance under my car and in every city in the Netherlands there are speedbumps.

Thanks ,

Michel