SD 455 Engine Issues

SD455

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I have only had it ('74 Trans Am for two weeks). It was driving well at first, no issues. I would practice a few launches every day, just going to 60 with a shift from 1st to 2nd. I would open it up on the road occasionally, but only to about 110.

I have a philosophy of driving old muscle cars hard when I first get them because if something is failing I want to find out before I am 50 miles from home.

It has headers and the shaker scoop is open. The block (Y8), heads (16) and intake (LS2) are Super Duty parts, the block is matching numbers. The only thing that seems odd is the heads have a build date that seems more conducive to 1973, and the intake LS2 is a 1973 number as well. Transmission is TH400.

Anyway, I opened it up to about 120 the other day and when I got home it seemed to have a valve clicking on the drivers' side, one of the middle two valves. It may have been something else, but I could not find anything. We only have 91 octane here, so I added some octane booster and it seemed to stop clicking. I decided to drive out and do some launches. It felt like someone was kicking my drivers' door down low when I cruised over to where I launch. I did several launches, and it had no acceleration and seemed to be bucking a bit up to about 30MPH. I also noticed the when it did kick in, it would accelerate, then almost die, then accelerate again. It was bucking, revving, stalling, it sucked.

When it was bucking it seemed to be coming from below the middle right around the firewall.

I spent my life's savings on that car, I have to get it to run like it was 100% new.

Does anyone know what might be going on?
 
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SD455

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The engine is running fine now. No misses surges and no smoke in the exhaust. However, when I start it if I shift the (automatic) tranny into anything except park or neutral, the car starts knocking like somebody is under it hitting it with a sledgehammer. Very rhythmic......happens about once per second sitting still with my foot on the brake. If I start moving, it speeds up.

Flywheel?
 
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SD455

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Here you go. Engine sounds fine running up to 4000 RPM, you can hear the knock when I shift into reverse, back up, then drive, and move forward.

 
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SD455

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Do you think it is inside the TH400?

Nothing else makes sense.

I need to drive it 18 miles to the shop tomorrow.

Not sure what to do.
 
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SD455

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And we were all wrong.

The local shop swore from the start that it was an engine issue.

I had bent a pushrod.

It didn't make the knocking noise until I put it in gear because the engine wasn't under load.

They had to order a pushrod that was slightly shorter than the stock SD rods.

Does that mean that the camshaft has been replaced with higher lift and/or duration?
 
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