Built up a 355 with a mild cam and TPI about 2 years ago, and i've got about 9000 miles on it, i had it tuned perfectly 0 knock counts from 0rpms to 5500rpms any gear any speed...

Decided I wanted to see what it sounded like with open headers the other day and i took my exhaust down, and fired it up and hooked it up to my computer to datalog, and saw it was going lean, so i tuned it richer and took off, and i noticed that once I hit 1900rpms or higher i would get knock counts and they'd keep going up and up the faster I went.

Went home and parked it and threw my exhaust back up. Fired it up with the original tune, blms and ints perfect, took off and it happended again, knocking about 1900rpms....

No mater which gear, 1st-overdrive it'll knock above 1900rpms?

any idea what is going on???

I reduced the timing 10* and it'll keep knocking.....had the timing around 17* when I hit 1900rpms in 3rd gear and it kept knocking...

Thought maybe it was some bad gas, siphoned it out and put in some fresh stuff, same thing....

 

Have you verified that it's combustion related and not mechanical?

 

Where is a good place to start looking?

 

Took out all the spark plugs, found alot of carbon/flakey build up on the #3, replaced it, took all the plug wires off and tested them, 117-138ohm, took the cap off and noticed that one of the terminals was busted, replaced the cap/rotor, put everything back together. Fired it up, same thing except now I noticed that:

1st gear: knocking above 2500 rpms
2nd gear: knocking above 2200 rpms
3rd gear: knocking above 1900 rpms
park: Knocking above 2800 rpms

I thought it you shouldn't get knock counts in park? Could my knock sensor be failing? Or where should I start looking, pull a valve cover off and look for what? Any input would be greatly appreciated.

 

prehaps you might have a small exhaust leak when going back togther with the exhausr causing a lean mixture + carbon = detenation??

What are the LT and ST BLM after 1900 RPMs

 

if you put the exhaust back on does it still knock?

 

Yeah, it knocks with the exhaust on and with it off.

Not sure what would have caused carbon to build up with a motor that is only 8000 miles old. But it def isn't lean, the blms and ints are +or- 4 throughout the whole powerband.

 

You pulled sufficient timing and still knocks. so next step is to add 50% race gas with 93 octane and try that. If still knocks I will suggest mechanical. I get KC every day when accellerating moderately fast with manual trans. I reduced timing 2200-2600 40-60MAP(per log of K) and still occurs. I am at around 30SA when it happens. It does not occur(logs) above 3600 RPMS. I believe it is the manual trans(clunk) and or the real duals maybe rapping on something behind collector.

Also I understand R rockers add to noise but mine are quieter than the prior stock units to ear?