×

Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Rich AFR/high rpm idle on 'aggresive' cams?

Practical Standalone Tuning

Forum Posts

Courses

Blog

Tech Articles

Discussion and questions related to the course Practical Standalone Tuning

= Resolved threads

Author
1561 Views

Hi,

Recently my car got tuned by another tuner as I don't feel confident in doing it (yet). It is a 4 cylinder engine 16V, 1.8l with light flywheel, 262° cams. Speeduino based ecu.

So here is the thing, the car idles around ~12.0 AFR and tuner told me not to worry about it and that the car will now idle on 1300RPM when hot because otherwise RPMs can surge (it definitely surges sometimes on electric load).

To me both values seems a bit odd, 12.0 seems rich, 1300RPM seems high when hot, it idles smoothly on this RPM, I would be okay with lumpy idle.

Can anyone share some insight on that? Can car in anyway benefit from close loop pwm in such situation? Currently it is not configured.

Does the car not have drive by wire throttle or an idle air motor? If it has neither, then you have no control over idle air, and setting a fixed throttle opening that results in excessive idle RPM is sometimes required to avoid stalling. How high it must be varies with engine setup.

AFR is rich, but I don't know enough about that ECU, your fuel system, engine, or fuel used to judge it. If the engine stumbles or sometimes stalls when run leaner, that's likely why they did it. If you feel ready to tinker on your own, you can adjust idle fuel in small increments so the result AFR gets closer to stoich, observe idle behavior, and see if it degrades or the engine sometimes stalls at a richer AFR.

It depends on camshafts specs a lot and engine displacement. Duration, overlap, valve lift, lobe separation angle, advancing or retarging them... More information is required about them to give you the exact answer. But speaking out of my experience the idle is too high and AFR is too rich. For very aggressive camshafts AFR is usually the best at 13-13.5 at idle (providing optimal ignition timing set) whilst RPM is about 1100-1150.

As Shota said, there are many things that will affect idle quality and as Mike said, a high idle may be a required condition to compensate for them - and you haven't provided much useful info'.

You may be able to drop idle a little, and lean it out a little, if you're prepared for a rough idle and possible stalling when cold. If you've a clutch plate without a Marcel spring, you can expect it hard to move off.

You don't mention what type of air mass metering it uses - with some common meters reversion can be a big problem at idle and low rpm, this may be an issue the higher idle speed is trying to avoid?

How are you measuring the AFR - if it's a lambda, pre-cat', that suggests idle quality is basically good as a poor idle quality usually shows up as a "lean" condition because a misfire, or excess exhuast dilution, can sometimes how up as that because of the unused oxygen. If it's a tail-pipe lambda, and you use a cat', that may not show up.

Thank you all for your input. I will try to play with AFR and see how the car reacts.

I am also gonna give additional input, here is my cam timing: https://docs.google.com/spreadsheets/d/1PFJInhn7sOsHwqv-euhsF417-aVGpTfvuwedF-gkzFo/edit?usp=sharing measured mounted vs crank degree. It is BMW M42B18 engine (NA) - with linkage (no DBW).

The car is using ECU onboard map sensor (Speeduino) (so the vacum hose is like 2m long), the engine has idle air motor and ECU is controlling it (Open Loop PWM).

ECU has LSU 4.9 controller and I have O2 sensor in the downpipe, car was tuned with his sensor in the tailpipe, I got the information that my O2 downpipe sensor is almost spot on (with his). And AFAIR they both showed ~12AFR on idle.

Fuel is 98 octane gasoline.

If anyone has additional input I will be grateful

If you haven't already watched it, HPA has specific info on big cam tuning for idle within the reflash tuning course. While your software may look a bit different, the principles remain the same and will help you make adjustments including injection timing.

A lot of times you'll find that tailpipe based sensors will not provide a accurate reporting of AFR especially at idle when there isn't much airflow, I saw a lot of times but not always.

12:1 seems extremely rich, I'd be concerned with over diluting the oil with fuel and also causing plug fouling. Even on huge cam stuff I feel like 13 range is generally rich enough but like others have mentioned it could be what's needed. You can probably reduce the idle down as well if your open loop then you should be able to drop it down by playing with the pwm duty. Just find the sweet spot between drivability, stable idle, and response. Don't always get hung up on a number if the car likes it. From my experience with older bmw engines, they tend to like a bit more rpm at idle and a slightly richer mixture than lambda.

Keep in mind too that lightweight flywheels tent to be trickier to tune sometimes because of the difference in acceleration/deacceleration.

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?