Just curious, what other risks are associated with the symptom of slowdown shudder?
Rev hang is almost an advantage to rev match when upshifting..
When my motor is cold the revs drop a little "too" fast which makes rev-matching the upshift from 1st to 2nd a little different and rougher than when warmed up.
Your response has me wondering what other repercussions there might be caused by slowdown shudder.., care told elaborate?
slowdown shudder can be cause both by "IQ" too low or too high - and remember, when you change "IQ", not only are you changing the fueling, but you're changing the effective advance. the SOI (start of injection) has not changed, but the EOI (end of injection) has. higher IQ means more effective advance. lower IQ means timing is effectively retarded.
for most stock/typical tunes, it would be because IQ is to low - durations are too long per the IQ amount. if you're at IQ of 2.0mg instead of 4.0mg, the pump map is off by about .130v (on a stock pump map!!) - that's a decent amount of duration to be off by. that would mean, for example on a stock 10mm/.185 fueled car, 15mg @ 2000rpm would actually be about 17mg worth of fuel. a stock pump map actually calls for a tiny bit of fuel injection when 0mg is requested starting around 1250rpm and increases down to the lowest of rpms. with correct fueling, this will only be enough to keep itself idling along nicely. if fueling is too much, it will want buck a little down close to idle rpm. this might be partly because timing is now too retarded. maybe a little too much "torque", or poor combustion? most tunes just use a pump map very similar to a 10m/.185 pump map (just fudged up towards 51mg to max out the duration), so the behavior at 0mg is there.
slowdown shudder can also be caused by too much advance. it's a subtle difference. too much advance getting a little too much negative torque, can cause a fluttering.
another probably reason why the shudder can happen is because IQ being off from reality, the ecu's idle stabilization program/heuristics work. if IQ isn't real/correct, it's simply not going to work as well as it should during the very low rpms when it is in play.
all of that should point to the importance of IQ numbers being real or at least based in reality, rather than how most tunes work, with IQ numbers being "just a number" - depending on the tune, "IQ", pump/nozzles, 40mg @ 2000rpm might mean 45mg, or it might mean 65mg. 10mg @ 1500 might mean 8mg, or it might mean 20mg.
rev-hang happens because the fueling is too "strong"- durations are too long per the IQ amount - that is, 5mg is actually more than 5mg worth of fuel. because the go pedal isn't an on/off switch, as you let up on the pedal, it catches some % throttle input - it will still call for some fuel, as determined by how fast the pedal comes up and the rpms... look at the driver wish below. if you rev up to 3500rpm, and let off, you'll catch a few mg worth of fuel ~3000rpm. on a stock, or correctly calibrated tune for fueling, you won't notice much. but if fueling is too strong, those few mg might be actually 8mg fuel - enough to be noticeable and thus causing the rev-hang.
here's a stock driver wish map. virtually all tunes use this map without any modifications other than fudge up the last couple columns so it goes to 51mg:
the correct way to increase "rev-hang" if one wants a little bit for shifting purposes, would be to modify the driver wish map a little bit, rather than throw off the fueling. generally, IMO, if the fueling is correct, the stock driver wish map for the lower % throttle columns works perfectly.
Freakin' beautiful.
