Idle RPM whilst coasting

[LS1 Cobra]

Hi All, This is my first post.

I've been reading here for a while and there is some great info on here.

I'm building an AC Cobra with an LS1 under the hood. I also help out a bit with some other guys LS1 powered Cobras.

One thing we've noticed with the LS1 is that when you are coasting along at speed and you release the accelerator and press in the clutch the idle sits up around 1200 RPM. Once you slow down and stop it drops back down to normal 800 RPM or so. I figure this is a function in the tuning to make the car smoother to drive on the road when shifting gears etc. By the way it dropps of when you slow down to a certain speed it makes me think it's something the ECU is doing with the IAC relative to the Vehicles speed sensor.

In a lightweight 1100KG Cobra this is down right anoying since that 1200RPM is almost enough revs that you don't slow down when you back off the throttle. It means you have very little engine braking especially when the motor is supercharged and makes 550RWHP.

Is this a feature that can be disabled or a table that can be adjusted? It would be good if the idle stayed the same regardless of the speed you're doing as it would make the car feel a lot more natural.

Cheers

Michael

[GMPX]

Table B4309 will be your friend. It holds the IAC out above it's park position based on road speed, so also make sure your park position is not too high (table B4308).

Cheers,
Ross

[LS1 Cobra]

Thanks Ross!

[jfpilla]

Check one other thing. If the idle airflow is too high it will act like the throttle cracker and give you that cruise affect. I would lower the idle airflow value at the temp. it cruises and use the Cracker for finer adjustments.

[mistermike]

I've kind of wondered what the logic of tracking road speed on the idle is, because my car does it too.

Cobra Man, 550 RWHP in an AC Cobra? Probably on one of those low-reading Aussie dynos too. You're a sick individual :wink:
That thing's gotta be like getting shot out of a canon. What kind of blower are you running?

[jfpilla]

mistermike, I'm not sure I understand what you're saying?
To clarify a point. Idle only occurs when the car is stopped. As soon as it starts to move the Throttle Cracker takes over. If Idle Airflow is to high the car will cruise because the TC is additive to IAF. IAF is often called Running Airflow because of that affect, I think. This part of tuning can be frustrating, when tuning a big cam car, since it may be necessary to have some cruise, just as it's necessary to have a high idle.

[SSbaby]

I changed both my IAC and IAC Parked values to 9.0 g/s for all values above 60C. The engine so smooth at idle it feels like a stocky if you ignored the lopey sound. :)

I found that my IAC Parked values were set too high (at 10.0 g/s), which slightly elevated the revs by around 300-400rpm before settling at the normal idle speed of around 950rpm.

BTW, I have a 224/224 110 lsa cam.

[jfpilla]

SSbaby, Did you lower the Idle Airfow values and Idle Airflow Parked at the same time? The reason I ask, is that 60C is a high IAT. Is it that hot there? If your IAT is not exceeding 60C the change in IAParked might not have done anything. Also, lowering IAParked should increase the aiflow.

[LS1 Cobra]

The blown Cobra belongs to a friend of mine. My motor is basically stock at the moment. The Blower on my frineds car is a Procharger and it's producing the numbers with surprisingly little boost (around 7-8 PSI). He's running a set of full house Nathan Higgins heads, a 224 cam, 55 pound injectors and 2 Bosch 044 pumps. It accelerates like youre going into warp drive and it's the scariest thing I've ever driven in my life.

Here's the dyno sheet after the heads went on and it got a MAFless tune. Since then it's got a new CAI stup as the normal Cobra bonet scoop just couldn't flow enough air. It later made 552RWHP on a Dynopac dyno with the intake mods.

http://www.clubcobra.com/photopost/data/500/16256Dyno_1a.JPG

[SSbaby]

SSbaby, Did you lower the Idle Airfow values and Idle Airflow Parked at the same time? The reason I ask, is that 60C is a high IAT. Is it that hot there? If your IAT is not exceeding 60C the change in IAParked might not have done anything. Also, lowering IAParked should increase the aiflow.

Yes, I changed the values at the same time. :)

Trust me when I say that by doing the changes described it also eliminated the annoying symptom of an escalation in revs before settling to the normal idle speed.

I thought the idle air values corresponded to ETC not IAT temperatures? :? In which case my ETC temps would approach 80C+ and would therefore be influenced by the changes I made.

PS Sorry to hijack your thread LS1 Cobra. :)

[jfpilla]

SSbaby, Did you lower the Idle Airfow values and Idle Airflow Parked at the same time? The reason I ask, is that 60C is a high IAT. Is it that hot there? If your IAT is not exceeding 60C the change in IAParked might not have done anything. Also, lowering IAParked should increase the aiflow.

Yes, I changed the values at the same time. :)

Trust me when I say that by doing the changes described it also eliminated the annoying symptom of an escalation in revs before settling to the normal idle speed.

I thought the idle air values corresponded to ETC not IAT temperatures? :? In which case my ETC temps would approach 80C+ and would therefore be influenced by the changes I made.

PS Sorry to hijack your thread LS1 Cobra. :)

I trust you. The reason I ask is that you are not sure what the affect of each change is, when you change both together. The idle airflow "parked" table is based on IAT. Idle airflow is ECT.

[SSbaby]

I trust you. The reason I ask is that you are not sure what the affect of each change is, when you change both together. The idle airflow "parked" table is based on IAT. Idle airflow is ECT.

Ah-ha... I wasn't aware of that! No wonder I had to fiddle around with it a few times - I did change IAC values from -8C upwards as well, although I assumed that the 60C+ readings were of most relevance in this topic hence why I quoted for IAC & IAC Parked temps > 60C... I must have just 'fluked' it this time!

But it idles a lot better than it did. Thanks for the heads up, jfpilla! 8)