Exhaust Recommendation for 90' 7M-GTE

[Arlo444]

First let me say these forums have been a great source of valuable information. I am in need of exhaust recommendations for my daughter's 90' 7M-GTE turbo. A "friend" replaced the stock down pipe with an aftermarket 3" pipe and removed the front cat. From there it's stock to the rear cat then cat back is 3" HKS. The problem is it fails NOx inspection here in Texas. I've confirmed no vacuum leaks, checked EGR, VSV and recently performed tune up. Need to get it road legal but still want the performance. Suggestions?

[Green7mgte]

hi, welcome to the bord.

NOx is caused by high combustion temperatures. The EGR reduces the combustion temperature by mixing inert gasses into the air/fuel mix.
retarding the timing will decrease the NOx. You may want to even retard it more than spec by a couple degrees or more. Then after inspection, put it right on spec.

Also fill up with premium. High octane burns slower and should help too. (Less chance of detonation)

Maybe try one step colder spark plugs too. This reduces the chances of a hot center electrode causing pre-ignition.

A lean fuel mixture also burns hotter and may increase NOx. If one injector is spraying lean, that could cause it to burn hotter in that cylinder.

If all else fails, weld in a second universal catalytic converter (in series, not parallel).

(taken from http://answers.yahoo.com/question/in...4113417AA6uoEz

best of luck