LuvmyC5

iTrader: (1)

I'm having an A&A Vortech SQ blower kit installed now and I spoke to the tuner and he said that the car is going fat at 3400 rpm with 18° of timing, but when it runs with 16°'s it's much better. It's going from commanded 12:1 to 11.3:1 in this range. He said that he'll probably leave it at 16°'s and just command the AFR to 12.1:1 to make up for the lost 2°. At 3400 rpm/18°, he said that he was getting 2° kr, which I thought was weird because wouldn't getting more gas stop knock? He was still getting slight knock at 16° though.

How can changing the timing effect the AFR? It's got me stumped!

Any help would be Greatly Appreciated!

jimmyblue

iTrader: (11)

Timing "lead" affects how completely burnt the mixture
is by exhaust valve opening time, how much pressure
is remaining in the cylinder to help scavenging (but cost
power, you left something on the table) etc. The nature
of the exhaust pulse affects how much residue is left behind
to dilute the next charge (and affect its burn-time) etc.

Short timing can result in an incomplete burn by exhaust-open
and then you push additional oxygen into the exhaust. This
does not affect the in-taken AFR but affects the AFR reading
because the WB senses exhaust oxygen and infers a mixture
from it,not directly measuring the air and the fuel going in.

Some people talk about "rich knock" in that you have a
sweet spot of no knock as you go up in mixture away from
too-lean, but then at some point it comes back in. Don't
understand that bit but I've seen it mentioned several
times.

LuvmyC5

iTrader: (1)

Thanks Jimmy for the reply!

What do you mean by short timing or timing "lead"? What can I do to prevent this or fix it?