Tuning question - cold start & decel

**YLD-16L** · 28-01-2009, 09:45 AM

Originally Posted by oldcorollas

hmm, i never realyl played with having 100% cut of injectors on decel.. partly because with the cam overlap i didn't develop that much vacuum. (30kpa max?, ie 70kpa absolute.. or maybe 60?)

what vacuum do you have on decel?

Don't know who that was directed at but just for the discussion mine sees 27" vac at decel with throttle closed. Idles at 21 - 22"Hg.

Cold start on my "simple" microtech is great, probably has to run a little richer than an ECU with better control but it works fine and is driveable straight off the turn of the key without any stumbling or hesitation, I retained the wax pallet on the throttle body to provide increased cold start idle rpm. It's fully open at anything below about 35 degC and is fully closed above 78 degC coolant temp.

**TurboRA28** · 28-01-2009, 09:51 AM

I don't seem the kind of vac on decel that you guys are seeing.. Does cam timing have an affect on this?

**andrew_mx83** · 28-01-2009, 10:01 AM

Originally Posted by oldcorollas

because you sound like a fuckwit with an untuned car??

not really......anything with lots of valve overlap will leave unburnt fuel in the exhaust to go pop

**oldcorollas** · 28-01-2009, 04:39 PM

Originally Posted by andrew_mx83

not really......anything with lots of valve overlap will leave unburnt fuel in the exhaust to go pop

err.. no. not if it is tuned correctly.
soem race engines are tuned to have fuel cooling for the cylinders etc during overrun (if they don't have fuel limits etc) but this is not ideal for a street car.

what do you call "lots of overlap"?
(on the hotrod i helped a little with, it only had 5-10kpa vacuum = maybe 1-3"Hg vac, due to big cams and ITB's.. but it didn't have popping on decel???)

previous question was directed generally..
YLD are they vacuum or absolute figures? if absolute:
21-22" = 27kpa abs
27" = 10kpa abs
thats heaps! not much overlap? i would have expected minimums (on decel) of maybe 30kpa = 23"
29.92" Hg = 100% vacuum

hmm, 60-70kpa abs = about 10"Hg...

**YLD-16L** · 28-01-2009, 05:00 PM

Originally Posted by oldcorollas

previous question was directed generally..
YLD are they vacuum or absolute figures? if absolute:
21-22" = 27kpa abs
27" = 10kpa abs
thats heaps! not much overlap? i would have expected minimums (on decel) of maybe 30kpa = 23"
29.92" Hg = 100% vacuum

hmm, 60-70kpa abs = about 10"Hg...

Vacuum figures (gauge) cross referenced from ECU, 3 bar map sensor used only for logging and a regular analogue gauge all read pretty similarly or the same if you only read whole numbers. They could all be wrong though as the ECU is cheap, the 3 bar map sensor was calibrated off the best gauge I could lay my hands on and the analogue gauge is an Autometer unit

None are at the pinnacle of accuracy.

Stock cams and cam gears. It wasn't that long ago it idled at 19-20"Hg and would only pull about 25"Hg on decel. I know why it's changed at idle but no idea why its changed at decel.

**andrew_mx83** · 28-01-2009, 05:02 PM

fair enough. anything i have worked on with lots of overlap has popped and burbled. but im a fan of lots of fuel so maybe that explains it lol.

**jeffro ra28** · 28-01-2009, 05:05 PM

Originally Posted by andrew_mx83

not really......anything with lots of valve overlap will leave unburnt fuel in the exhaust to go pop

Sorry you are wrong.

Plenty of engines with very big cam overlap AND using shithouse mechanical piss and dribble injection systems still dont go pop on decel if they are tuned properly.