startup instability issue

[DSteck]

Ok, a friend of mine just got his C6Z running with the built motor. Specs:

Huge cam
Ported heads with 12.9:1 static compression
LSXR 102mm manifold
NW 105mm throttle body
RPS LSX BC2 clutch

When you start it up, sometimes the idle sits there and swings back and forth until it goes into closed loop. My car doesn't do this, but I also have a stock throttle body, despite a very similar cam and the same clutch. I don't want to force closed loop to come on sooner as a fix for this. Anybody have any ideas? I've tried playing with the minimum idle airflow and statup airflow to try and smooth it all out, but have had no luck. If you let your foot off the clutch while in neutral, it'll steady up (probably from the inertia of the transmission).

[Sam88gta1]

Your idle airflow tables are 30% low.
Also need to set the Idle Adaptive Spark Underspeed tables. Your problem is a airflow and timing issue. Once those tables are set the idle will be stable.

Im providing a stock 2007 file with the changes you need to make to the idle for you to compare.

[DSteck]

I'll look into what you said, but the underspeed is supposed to be the same as my QM600 car, unless my buddy didn't properly copy all the values for his base file that I sent him. I didn't think to check and make sure he properly applied everything. On big cam cars, I bring the underspeed correction in much sooner and much more aggressive.

The idle airflow table was set up by forcing idle at 650, 800, 1000, and 1200 and recording what the idle airflow was, then taking just a little bit out.

[BERSERK]

Please try this:-

VCM Editor> Engine Diag> Airflow>MAF sensor>MAF Fail high13500????
should be at least 15500 with this CAM

[Con]

Just finished tuning my 427 with very similar specs, 412RWKW.
Idles at 950rpm with a 30rpm plus or minus deviation.

However because of my cam size etc, my tune is quite different to your tune.
If you are interested in trying something new please let me know.

[DSteck]

Please try this:-

VCM Editor> Engine Diag> Airflow>MAF sensor>MAF Fail high13500????
should be at least 15500 with this CAM

That's got nothing to do with it. Even at WOT, the MAF is only hitting about 11000 at the most.

Just finished tuning my 427 with very similar specs, 412RWKW.
Idles at 950rpm with a 30rpm plus or minus deviation.

However because of my cam size etc, my tune is quite different to your tune.
If you are interested in trying something new please let me know.

Post it up, and I'll check it out later. I've never had a car give me crap like this with startup idle, and I've done setups with this exact same clutch and huge cams before.

[Con]

I know your pain, the joys on working on cars lol.

I tuned mine in SD only, I found big cams are definitely less efficient and set my AFR at idle to about 15.8-16.3. You can actually put a vacuum gage on and find max vacuum while you adjust the mixture and timing.

Both under speed over speed only correct the timing by plus or minus 1.5 right up to 50rpm deviation. If you the correction is too much the idle becomes too erratic.

I also tune cars by TPSvsRPM, as with big cams the vacuum is so low. E.g. 50% throttle and the reading can be 105KPA. By tuning by TPS even at 50% throttle you can hit 7000rpm and adjust the AFR. This makes for a very responsive tune. I also set the open loop AFR to the maximum torque of the motor. By running it on the dyno, you can hold the car at 50mph and adjust the AFR until a max torque reading is achieved. In most cases it is between 13.5-13.8. This again makes the car feel very strong, before it goes into PE mode with a commanded AFR of 12.8
In my opinion closed loop tuning is only for mild set ups. All race cars we tune using Motec are in open loop and tuned by TPSvsRPM.
Hope this helps

[HNK]