High NO. Not too bad at low load testing, but under heavy load, it was like 500 over the allowable.

So, after searching it seems to be that high NO is a indicator of being rich.

The plan:

dial back the timing

drop the fuel pressure a few PSI

replace the EGR valve (this might be overkill though seeing how it's not throwing a code and it's only 2-3 years old)

Now I told the guy is was a 305, which from the factory it was, but now it's a 350. . . . do yall think this would really have a serious affect on the numbers or is the difference small?

And as a secodary question, can too much timing be bad for performance, even if it's NOT knocking?

 

High NOx is typically from high combustion temperatures or high combustion pressures - they are formed under high temp, high pressure conditions (i.e., the combustion chamber).

It is rarely due to a rich mixture - that tends to cool the combustion. More likely it's due to a lean mixture or timing advanced too high. It can also be from carbon build up or a rough spot in the combustion chamber that is heating up and causing a hot spot.

Assuming the EGR is functioning correctly, that is. Verify that first.

As for the timing question, you want the cylinder pressure to reach the maximum when the crank has turned a certain number of degrees from TDC - 15 comes to mind. If you ignite sooner and max pressure is reached at say 5 degrees, it may not ping, but the piston won't be able to push as hard on the crank in a twisting fashion. So, yes, it can hurt performance, even without ping.

 

Well this is the first emissions test for this new motor, so the heads shouldn't have any spots in them.

So maybe I should just back the timing down to 0* and replace the EGR valve? They're not that expensive anyways and takes about 10 minutes to do, so no big deal.

Please tell me that the Cat is ok though. If I remember correctly, a high CO means the Cat is done.

EDIT: there are aluminum heads on this, but that shouldn't matter right?