Fellow cz28.com member is helping me with the new motor. He is in another state so I haven't seen it run yet personally. He says he can hear piston slap and believes it is the cause of false knock showing up on the scan. The machine shop said they clearanced the pistons to .005 We had asked for .004 because of reverse flow cylinder head cooling. It was never actually checked by the mechanic who assembled the motor.

The fellow who's helping me says if it's the cause of the false knock I will probably just have to live with it or disable the knock system. I am definitely not comfortable disabling it. He said we may try loosening the sensor in the block a little (yes, it's an LT4 module on the ECM). I am afraid if we do that it may not be able to hear real knock either. Any way to get the sensor or module/ecm to differentiate between knock and slap?

How many of you who've rebuilt went with .005 piston clearance? Do you have the same problem?

 

I don't know how different the programming is between the TPI cars and the LT1 cars but I am getting into DIY tuning and there are some options I can change that can limit how much timing the ecm pulls due to knock. I can also make it less sensitive by telling it not to pull timing until X # of knocks is heard. I can also tell it to advance the timing that was pulled faster (attack rate vs. recovery rate).

It is a ghetto work around, but it keeps from disableing the knock sensor. I have false knock too from what I believe is the y-pipe hitting the oil pan. It pulls up to 16* because of that and that is loss of power I can feel. I am going to do the "ghetto work around" (only let it pull 8*) I explained above until I can get my headers put on and get a custom y-pipe built that has a little more room between it and the oil pan.

It might be something you want to look into.

 

Thanks, I thought there might be someway at least partially around it ....it will likely be a compromise no matter what though. I am pretty sure we can do the same things in LT1 Edit.

Is .005 really all that "loose"? Has anyone else gotten away with .005 clearance without having this false knock from slap problem? I'm just hoping it's really something else ........

 

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