911SC - EFI Conversion - Maintaining Steady Idle After Giving It Some Revs...
 

Hi,

OK, this is a bit of an open-ended question. Please bear with me.

I've been working on an EFI conversion for my SC project car some time now. The physical side of the conversion is more or less complete.

The cold start tune is OK too - the car idles nicely:

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I struggled for some time with an oscillating idle issue for a while, which turned out to be a combined problem with a faulty idle motor and some settings in the MicroSquirt related to idle advance.

An issue I recently faced was that when I gave the throttle a little blip (not too much, or too aggressively, it's just using a calculated base VE map) - the revs would rapidly rise and then quickly fall - often below idle revs.

The engine would then struggle to return to idle speed (perhaps oscillating a bit before getting there) or simply cut-out. Very untidy!

I have a solution in place right now to use a significant amount of timing advance added on top of the base timing (5 degrees, as stock) for a total of 20-25 degrees when the revs drop below 800rpm in these low-load conditions - and this works rather well.

I actually like the snappy throttle and the quick return to idle.

But is this the right way to approach it? Is there harm in using so much advance at low-revs?

I've read here that the Carrera 3.2 Motronic system would use it's idle control valve to softly return the revs to idle speed. I'm only using the idle motor in open-loop mode (i.e. for warming-up.)

http://forums.pelicanparts.com/porsche-911-technical-forum/812418-why-do-my-rpms-drop.html

How do engines without EFI control this behaviour? (CIS, carbs, etc.)

Regards,

Daniel

I have been doing it that way for many years

If I recall correctly, the MFI system uses vacuum retard. So maybe it takes a fraction of a second to build up enough vacuum to pull the timing back. Then you get that super low idle speed.

I recall having the same prob as you, under certain conditions. Sounds like this is the right way to handle it.

Thanks for the replies guys.

That gives me a bit of confidence to proceed with a bit more basic tuning before I try and find a dyno operator to flesh out the rest for a solid tune.

Coincidentally, I just watched a video from Garage Time where he explained why the 356's revs were relatively slow to drop after a blip of the throttle - the weights from the centrifugal advance mechanism take a moment to "come back in."

One I assumes it's done in a similar fashion on an SC with CIS as well (either the vacuum or centrifugal advance comes into play.)

Thanks!

Yes, A timing bump will help catch. You also want a timing increase off throttle to help slow the engine down. This, along with a fuel cut, will make great driving throttle response.

BTW, I would not put too much into the timing just yet. Make sure you have reasonable AFRs before digging deep into timing. The default AFR target and VE table curves in most ECUs are often WAY wrong.

Ultimately, you will catch idle by both timing and fuel.

timing bump removed the issue for me too. I have stable idle and it never drops below. But it took some fiddling. I increased timing advance at idle below 900 rpm and that helps. You can experiment in your driveway. Also make the VE 'flat' in that idle area to avoid fuelling changes with minor RPM changeds. Make certain that there is no accellleration enrichment happening as it hunts, if you are using MAP based tuning. Turn off EGO control at idle.

Yep, I'm working on the AFR and VE tables!

The idle range is good, but beyond that I'm working with what has been generated.

I haven't got acceleration enrichment on yet - it's recommended to have the VE tables "dialed-in" before fiddling with that, so that's how I'm proceeding.

I think at this point I need to find a tuner (a little difficult where I live now), or muster up the courage to do some "auto-tuning" in my driveway.

Wish me luck...

Daniel

no need to use auto tuning at idle. Not a good idea in my experience. More productive to adjust VE values at where the cursor is, and immediate cells around too manually until it idles stable at a sensible AFR, 12.5 or so works well for me. Another thing that can make it difficult is the IAT sensor heating up and thereby wrongly leaning out mixture. I disabled this function entirely by making the curve a flat line instead. It was a pain to deal with getting out of car parks after shopping when the ECU leaned out because it thought the IAT was higher. Once engine starts the cooling fan and induction gets fresh air at ambient temperature. But sensor may still be heat soaked and it causes leaning out. I found no good way to deal with this but to disable it entirely and it works fine with more stable idle

Yeah, the idle range is good (dialed-in manually at 13.5:1) and I'm happy - but I meant autotuning above and beyond that. Not sure how well that will work... But I will try.

Thanks for the tip on the IAT sensor, I'll keep that in mind!

Hi Daniel,

JP Novak has graciously shared a technique for focused tuning with several of us here and Julian was kind enough to write it down and post it on my thread, here is the link and it is post #361 on page 19.

http://forums.pelicanparts.com/porsche-911-technical-forum/1067600-2-7-x-faktory-pmo-efi-upgrade-19.html

Good luck

Cheers for that!

What a great forum!

Daniel