Black smoke/soot out of exhaust (tainting rear bumper)

Hi All,

I have a NA k20/24 engined tuned with 725cc injectors. Injector data was keyed in as per datasheet given by manufacturer Denso. Car is tuned on a dyno at 13.00 AFR. But whenever I go for a spirited drive, black smoke/soot comes out from the exhaust to the extend that it blackens the rear bumper. Mid-throttle are all tuned to 14.7AFR slowly transitioning to 13.00 moving towards WOT. I can't figure out why the excess in fuel still exists. I've also reduced the tip-in throttle fuel compensation to only 3% at WOT and 50% throttle.

Anyone encountered this before? Car is tuned using Hondata Flashpro.

is the over run fuel cut active? it could be dumping fuel on over run, or it could be oil burning

This could be simply related to the spray pattern and atomisation of the injector. Fuel delivered as liquid droplets into the cylinder won't combust as well as finely atomised fuel or fuel vapour and this can lead to the fuel passing through the combustion chamber unburned and your black smoke issue. It could also be related to your injector timing so it may be worth trying adjustments here to see if you get an improvement.

Chris250: Yes, i notice that it is as the datalogs showed the injectors go to 0% immediately at 0% throttle.

Andre: The spray pattern is determined by the type of injectors used am I right? As for the injector timings, I've entered as per the datasheet provided by the vendor. I've tried searching for the Denso injectors online but i can't find any information related to it. Possibly an obsolete model as I had to add-on an adapter kit to connect it to my car. Since it is rich now, what should i do when adjusting the injector timings? increase or decrease the timing?

I've just spoken to the workshop, apparently they supplied me with the wrong injector chart (it was for the 1000cc type!). And I googled the actual part number, it's actually Raceworks injectors and not Denso. Will try to input the correct timings and update you guys again later.

On the other hand, if I use new injector timings, i would have to retune the fuel maps, so if i just let it be, should i be worried about the black smoke?

The correct deadtimes appears to be lower (faster) than the previous numbers that I used, 20-25% lower. My AFR at idle went leaner immediately with the new injector timings and i had to add 25% fuel to make it stable around 14.7. So if I were to retune the fuel map, is it wise to also add 25% across the fuel map to start as a base?

Changing the injection timing isn't usually that sensitive of an adjustment. You might get some slightly smoother operation in some areas, and perhaps a small power bump that you can only see on the dyno.

You can add 25 percent but changing dead times is a non linear thing so expect that number to be significantly off in some areas.

Hi All,

Went for a few pulls on the road last night, and adjusted the fuel maps. Ended up with 12% additional fuel only for the WOT areas and 15% in the mid throttle areas. So Arghx7 is dead on about the non-linearity. Car feels smoother and no more exhaust popping. Hope the black smoke is gone too. However, the injector duty cyle is still 35% tops at WOT. I understand that it has lots of room for more power in the future, but is that a concern that the injectors are running so low a duty?

Thank you so much for the help guys!

Found a short description that helped me understand further and would like to share it here. I shall quote the author Albuquerquefx from Honda-tech forum:

"Here's the simple reason: injectors also have a minimum duty cycle, not in percentage, but in milliseconds. If your motor at full power needs only a 50% duty cycle, then at idle (where it makes only ~5% of full power) is going to need about 2% duty cycle. If that 2% duty cycle is below 1msec, then your car either won't idle, or in order to make it idle at 1msec or better dwell times, it will be pig-rich.So, injectors that hit ~80% duty cycle at full throttle mean you've got a solid chance of being able to idle correctly. If it needs more duty cycle, you're likely to start having problems with them sticking open. And if they're TOO big, then you're likely to start having problems with idle."

Which explains my sometimes unstable idle.

What is the injection pulsewidth at idle? Is it under 2 milliseconds?

Arghx7: No idea. The Hondata Flashpro only shows the duty cycle in the logs.

I think i've mixed up with words used here, I was actually making adjustments to the injector dead times and not the injector timing (as described in Andre's webminar).

I have noticed that almost all R35 GTR produce black smoke when fitted with aftermarket injectors.