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accleration Enrichment

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Hi Guys,

i am having some minor issues with the accel enrichment on tip in, that maybe the collective knowledge of the forum could help me with.

engine details:

4.0l 911 engine

ITB's

ID1000 injectors

when i step in the gas i get a lean peak around 16 to 17 then the afr goes rich at around 9.3. of course when this happens i get a stumble from the engine. i understand that the itb's when you crack them dumps a ton of air so it needs the fuel to compensate however the fueling seems to responds too slowly and then gives too much fuel.

Any suggestions you can provide would be greatly appreciated.

the accel enrichment settings are as per the attached images.

post-1216-143450215481_thumb.png

post-1216-143450215485_thumb.png

post-1216-143450215486_thumb.png

post-1216-143450215487_thumb.png

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Hi Trophy,

Firstly is your fuel pressure regulator MAP referenced, fuel pressure should increase as the throttle is cracked and Manifold pressure drops, to give initial enrichment.

What fuel pressure are you running, min for an ID INJ ECTOR is around 43 psi static.

Are the dead times setup correctly for your ID 1000 injectors and the fuel pressure you are running.

What RPM's do you suffer this fault.

My experience also with multiple butterfly or even single butterfly engines with lean accelerator enrichment issues is that they will, what I like to call"" LEAN SNEEZE"".

They will give off like a chuff type sound or if lean enough a good backfire out through the intake.

Which being a Porsche would I imagine be hard to distinguish, as everything is in the rear.

Anyway the fact that you go momentarily lean and then it reads very rich to me, unless you have a LEAN SNEEZE or Backfire, indicates your ACCEL ENRICH is set up to deliver to much fuel.

The AFR meter will read momentarily lean due to excess fuel and there being a greater unburnt oxygen content in the exhaust, once it clears itself a little and starts to burn the mixture a little better the mixture then begins to read rich.

Remember an AFR meter Lambda meter what ever is measuring oxygen content not fuel content in the exhaust.

You have ACCEL SENSITIVITY at 150 for a start, I have never seen a setting this high before not even on 6 throttle big cammed cranky GTR's.

Is your fuel table and timing table tuned correctly.

There is no point chasing your tail and trying to tune out an issue with a trimming input if the base tune is not correct.

The base fuel table being incorrectly tuned can lead to way out there numbers such as 150 for sensitivity etc.

I suggest you data log a run with smooth throttle transitions and check your AFR against the Target, if they are close, then check you don't have a HOLE in your timing curve with values to low for what the engine needs to produce torque to accelerate away without the bog down.

Finally, I set the ACCEL SENSITIVITY back to a realistic value of say 30 to 40, I would then adjust the ACCEL LOAD CORRECTION table, those numbers look to clinical to me to be correct for your setup.

Remember take small steps and don't change 16 tables at once.

Keep it simple , adjust sensitivity rate and Accel Load Correction and see how you go.

Make sure you read all of this and understand it.

From the regulator and the fuel pressure all the way to the end.

Once again the accel settings will not in most cases help poor transitional tuning in the main fuel table.

Regards

Dave.

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Thanks Dave...

I don't have a map reference for the regulator, that will be the first thing i will address.

The fuel pressure is running at 55psi which is the stock pressure for 993's. all the dead times are spot on as per Injector Dynamics numbers for 55 psi.

This usually happens at lower RPM's, anything below 4000.

The tune has been done professionally, by a guy i trust and respect. He is has been doing this for many,many years so the ignition table and fuel tables are perfect (or pretty damn close).

I am thinking the condition you describe being unburnt fuel and air showing what is essentially a false lean and then it going rich is what i am experiencing. i can hear every possible noise from the intake in this car as there is no sound deadening, and it doesn't sound like your "Lean Sneeze"..

This are the steps i will take:

Connect a vacuum line to the FPR

and reduce the sensitivity to 30 and do some logging

Thanks Again Dave, much appreciated!!

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Hi Trophy,

anytime, no problem, that is what this forum I about everyone helping each other.

I would advise checking for the strongest vacuum signal with a vacuum gauge for the vacuum source for the fuel pressure regulator.

I am not familiar with you engine so I don't know the inlet manifold design or layout with the individual throttles and getting a vacuum reference strong enough for the reg.

Sometimes on multi throttle applications I make up what I call a vacuum log, I run a hose from each vacuum source at the ITB and common them to an aluminium rail i have made up.

I then run a vacuum hose from the vacuum log to the regulator and / or MAP sensor if required.

Regards

Dave.

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I have a Vacuum line running from each throttle body to a common Manifold, I need this for the Brake booster.

We did try to tune the engine with the Map sensor however the map signal jumped around quite a bit which made the tuning almost impossible, I have some very big cams on this car with a fairly narrow Lobe Center so the vacuum signal even with the Vacuum manifold can be challenging.. We ended up tuning the car with Alpha-N.

I did connect a vacuum line to the FPR, I an not currently logging fuel pressure so I need to buy a sensor this week and connect it up. The AFR's went very rich, especially at idle with the vacuum line connected, need to see what pressure it is now running at idle. The lean spikes were not as pronounced which is great, the over fuelling still happened so need to look at the load enrichment now.

Thanks again.

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