[ultraclyde]

I'm running a JLT intake with SCT written tune. I recently switched from the 93 oct down to 89 oct. On hard accel in 2nd and 3rd I'm getting some valve rattle above 4k rpm. The datalogging shows the spark advance bouncing up and down at that time and the knock sensor picking up some noise. I've tried taking out 2 deg of timing above 4K and richening WOT 2%, but there's not much change.

Any ideas?

Also, can somebody explain what the short and long term fuel trim numbers mean?

Thanks....

[BlueStreak03]

By valve rattle do you mean detonation? If so then you're just going to have to keep pulling timing until it goes away. Adding fuel alone won't fix it. It could in fact cause larger problems down the road ( fouled O2 sensors ). What is your ignition timing set at right now? When mine was N/A and tuned for 93 Octane, I think it was around 14-15 degrees. If you're anywhere near that and running 89, you are BEGGING for problems. I would look at the initial ignition timing setting and yank 4 degrees out from that number.

[helicfii]

I agree. Most tuning tool manufacturers recommend global timing reductions with a reduction in octane.

My own experience: I noticed that a performance 93 Octane tune on my car yields an increase in mileage (about a 2-3mpg- that's about 10%). Even though 93 octane costs me more, the difference in mileage pays for the difference in cost of the 93 octane fuel. I get more power for the same $ per mile- a good deal!

[kj_cinci]

Yes, go back to 93 oct!!

[ultraclyde]

I'm not sure what the base timing is set at. I'll have to check. Including all spark advance, the timing during these incidents ( which I'm pretty sure is detonation) is a little over 20 deg. You can see the advance hunting up and down over about a 5 degree range, which I'm sure is the computer trying to back down enough to correct the problem. Unless I'm really pushing it high up in 2nd and third, it runs fine. I'd prefer to pull some timing just on the top end and preserve the bottom end.

As soon as the wife finds another job, I'll bump it back up to the 93 tune.

Oh, and it's actually the 87 tune I'm running, not 89. My mistake.

[kj_cinci]

Hmm yea... the 87 and this heat doesn't help... the ram air should help some though...

20* timing sounds extreme... dunno... I'm blown and safe but max out at 14* at the peak... Will change that to ~18 here soon...