LT1 poor performance
 

Just seeing if anyone has a theory on this engine.A customer brought in a 57 chevy nomad refitted with a iron headed lt1.It ran fine he just wanted a little more power.I added 1.6 roller rockers and full length headers,nothing crazy just a little old school hot rodding.Touching it off it sounds great,road testing wow it pings so bad you really cant get on it,and mid throttle roll ons it feels like it now has barely enough power to pull itself???Ive done similiar mods on camaros and corvettes all benefitted but this is new and unpleasant.Any ideas?

sometimes a tune is in order with long tube headers and/or is there an adjustment needed for any knock sensors? best wishes

Rockers are a little to tight? How does the vacuum look at idle? Pinging indicates lean and that the knock sensor is not working. You should have codes if it pinging? Maybe you bumped something or did something to restrict the fuel flow?

Maybe you are very unlucky and something happened to fail while you are working on it? How was the conversion done? is it a factory OBD1 or 2 system? How old is the opti?
Is the MAF connected? If the MAF connector is original it will be VERY delicate. Any damage to that connect would cause the symptom you described.

it does sound like lifters may be a little over tightened. did you go with 1 or with 1/2 turn. also did you change the pushrods, if it didnt have rollers befor it will need different length for roller lifters, maybe get some better springs on her.

Thanks for the suggestions.I kept the stock pushrods,which has always worked in the past.The rockers were zeroed and 1/2 turned.I did have to put a 90 degree elbow on one knock sensor for proper clearance of the headers.They make headers for 57 chevys and headers for lt1s,but apparently not 57 chevys with rack and pinion steering and a lt1 lol.I should add i swapped out the springs for ones suggested for 1.6 rollers.I would not doubt it needs a different tune,the one in it is obviously custom and by a company i can not locate, go figure.

you using a 93 ecm, dyno is always best, solomon would be a good mail order tuner. if you had the plugwires off better double check, one off or not on correct or the right ones crossed can do this. dont ask how i know

Is this a GM system, Painless? Holley? Megasquirt? or one of 20 others out there. Can't really help if we have no idea what we are working with.

Try unplugging the MAF sensor and see if the car runs better. Try unplugging the Knock sensor. There really needs to be a way to get codes out of the car or at least see data in the computer. It just a matter of time before something goes wrong. If it was my customer that what I would tell him. "I might have bumped a wire but this needs figured out for when something goes wrong".

As you have described your upgrades they are minor, you should have no trouble with a stock tune. You would not get the full benefit of the upgrades but the car would certainty be drive able because the O2 sensors would be able adjust your fuel mixture as need. Your headers do have both O2's in the collectors? It may take a little while for the block learn to get use to new O2 locations but that would not be as bad an issue as you described. I am talking slight shake at idle and maybe a sputter on acceleration the kind most people would not notice. I have not done Lt1's but I did a lot of L98 TPI cars back in the day. If is almost the same fuel injection system. Even with a CAM upgrade I never had one run as bad as you described on a stock tune.

You could back off the valves a little just until you hear a slight tap and see if the car runs better. Based on your first description if sound like a car starving for fuel.
I have no idea your back ground but when a GM computer sees WOT (Wide Open Throttle) it is considered a safety issue and it no longer has the emission issues. It will dump extra fuel. Some racers will take advantage of this with a small dry NOS hit at WOT. If you have spark knock at WOT with known good fuel somehow you are swinging for 11;1 fuel ratio to an 18+:1 fuel ratio. That is dramatic, and well beyond what a stock O2 sensor can adjust.