Ls1 swap sputtering under wot and just cruising

[rb20hatch]

So i just bought this car a couple days ago and ran great. I pulled the muffler off and ran straight pipe all the way out. Now the car is starting to run awful. It idles smooth but once you get going it starts to sputter and backfire and then wot it will do the same then clear up and light up the tires no problem.

So far i have pulled the valve covers to check for bent pushrods or cracked valve springs and note the motor only has 18k miles. I put bra d new plugs in and has msd wires. I ha e searched and searched for an answer on ls1 forums and nothing. It is a super clean swap with wiring specialties harness. Somebody please help im lossing my mind and trying to get it running right for a drift event this weekend!!!

[thegr8one013]

The previous exhaust you had could have been so restrictive and was tuned with that restriction. Or the old exhaust back on, if it gets better then you need to return with your new exhaust

[lunchmeat]

Try leaving the battery disconnected for a few hours if you're still using the gm ecu. Had to do that to my truck when I cleaned the throttle body. Idled erratically and hesitation when I punched it. The reset cleared up the hesitation, but it took almost a week for the ecu to relearn the idle.

[rb20hatch]

Yea im going to have the muffler put back on today. Ill update and let yall know if it helps.

[rb20hatch]

Put the exhaust back on and still doing the same thing. Seems worse today. On the way there to exhaust shop and back its like its missfiring at all rom now but its not throwing any codes

[lunchmeat]

Have you tried pulling spark plug cables and checking for bad coil packs? Ecu reset?

[onehundredoctane]

Have you tried pulling spark plug cables and checking for bad coil packs? Ecu reset?

THIS.

The LS engines having the spark plugs and wires right there at the exhaust manifolds crap.

I put heat sleeves on my spark plugs and wires, if that's your problem I would suggest the same.