does anyone know if vtec engagement effects ignition timing?
and if it does, in what way?

i've been trying to research about ignition timing lately, and have got the very basics of what effects ignition timing.
there just doesn't seem to be much info on vtec and i-vtec's relationship to ignition timing,

 

Optimal ignition timing is definitely different for the high cam lobes, but I wouldn't call that "effecting ignition timing". You just have to tune the ignition timing for whatever the engine wants. If I advance ignition timing beyond about 15 degrees at 3500 on the low cam, I start getting knock, but once VTEC engages at 3600 RPMs, ignition timing goes up to almost 30 degrees advance.

 

so the high cam in relation to the low cam would want more advancement in ignition timing?
or would it be different depending on what rpm you are at?

 

ignition is different depending on your rpm. It's a function of mean best torque when you're tuning. On the low cam at low rpms it's more based on advancing until knock and backing off.

You must know from looking that there are multiple tables for ignition values in some software, 1 for the low cam and one for the high cam.

 

The dynamic conmpression is lower on the high cam than it is on the low cam, so optimal ignition advance is automatically increased because of that. Ignition timing on the high cam is less dependent on RPM compared to the low cam, but very dependent on volumetric efficiency.

 

you're doing it wrong

 

Please elaborate. Would like to learn how you attack the low cam timing..

 

I'd like to hear how Brian does it too, but I'll tell you how I do it. After doing a very good fuel tune for a very flat AFR, I do several "good" street dyno pulls, and average them together, and compare the shape of the torque plot to the injector pulse width. In areas where the torque curve goes down while the injector pulse width goes up, I add some ignition timing (maybe 1 degree). Then I do more street dyno pulls. If the dip gets smaller, I add more timing in the same area, otherwise, I go the other way with it. Once my fuel and torque curves match as good as possible, I start moving the whole timing line up or down to find the maximum curve. If I start getting knock counts somewhere, I start altering the slope of the ignition curve, and continue. Once I hit peak torque or hit the knock limit, I back off a bit.

 

Thanks for sharing your method of street tuning, that's great information. How close do you think this gets to a dyno tune? Also how do you tune partial throttle?

How much knock count do you consider too much, and how do you alter the slope of the curve? Once you hit peak torque or knock limit, do you back off the entire curve, or just that spot and keep going with the rest?

 

What do you mean automatically increased, you would have to tune it to add advance in the lower rpm's on the high cam after lowering VTEC, right?

Also, what affect on fueling does lowering VTEC have, I'd guess it leans out the mixture, which should also add power? Is it likely that lowering VTEC alone without tuning for it would cause a loss in power?