Understanding AFR

Discussion and questions related to the course Understand AFR

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so how the afr and ignition timing being affected when the mivec system kicked in? i really need some clarification as im currently having mivec + turbocharged setup. thanks all in advance.

sorry i am not sure what you are asking in this question

Sorry for bad english. My car is an old mitsubishi colt with 4g92 engine (mivec). So, i rebuild the engine with a higher stroker kit, and turbocharged it. And im wondering how to tune the fuel map and ignition timing map when the mivec system is activated as there were no explanation in this though out the course. Fyi, the engine is fitted with forged internal, gt35 garrett and currently on Haltech PS1000.

I'll assume you're referring to the EVO 9 MIVEC system? In that case it doesn't really 'kick in' as it is just continuously variable cam timing. MIVEC doesn't really affect your AFR targets and you'd still optimise the ignition timing in exactly the same was as you would for a fixed cam engine. With cam control on the inlet cam we usually run in the fully retarded position (0 deg advance) at idle before advancing the cam to near the maximum range of movement around 3000-35000 rpm. As the rpm climbs towards the limiter we retard the timing back to zero (or close to).

If you are referring to a switched cam control style (more like Honda's VTEC) then you simply want to find the rpm point where the cam profile/timing will switch. This is easy to do with just two ramp runs - one with the cam switching at very low rpm (2-3000) and one with it switching at very high rpm or not at all. The point that the lines cross is the optimal changeover point. With regard to fuel and ignition timing this doesn't really change but you may see a step in your fuel map at the changeover point in order to maintain a consistent AFR.

Thanks for the reply Andre, and yes im referring to switched control style like Vtec as im currently using a 4g92 mivec head (not a continously cam control evo9). But the point here, i need some sort of guidance, or preferably rough estimation on the lambda target and ignition timing for my base configuration because i am going to be tuning on the road (i feel too intimidated to rent a dyno because im a newbie in this world). Once i gained enough confidence, then perhaps ill rent a dyno to fine tune everything. And lastly, what do u mean by 'The point that the lines cross is the optimal changeover point.' Is it referring to the line cross of torque and hp on the dyno graph? If that case, surely im not be able to get those data as i mentioned before that i will be doing on the road tuning. Hope u can shed some light. Thanks again Andre.

The cross over point Andre is referring to is where the two cam profiles over lap... so to find this you would need a print out or be able to view the run on a dyno screen.

Firstly you would limit the system from “engaging” mivec and do a run; then engage mivec lower down the rpm as suggested above at around 2500-3000rpm.

The two graphs will intersect each other; this is the area that you would want the profile change from the cams to happen; when Vtec/mivec is tuned properly you loose that initial kick as many people come to love! the power will become a lot smoother and pull much better from lower rpm.

As for ignition have a look at the courses offered here in regards to ignition tuning; and how to go about road mapping your car. Make sure you have the right tools and equipment to correctly monitor what’s going on.

Same with lambda targets; have a look at the courses as there is a lot of information on how to set and achieve your target number. Just because there is a variable cam system doesn’t mean that there would need to be different targets for fuelling; just a different set up slightly as mentioned when the cam changes profile to meet up with the new higher lift profile.

Thanks Dave for the simple explanation. Just wondering, is there any way for me to find the ideal rpm for switching the mivec other than as mentioned earlier? Is it safe for me to do trial and error for this? And i get a bit confused here as per explained in the course, Andre did mentioned that the compression ratio will increase as we go into positive boost, thus we need richer afr and ignition retard to prevent knocking. So with a higher cam lift (in this case the Mivec enganged), what is actually happening inside the combustion chamber (in term of the requirement for the afr and ignition timing)? Need some clarification to digest the correct info properly so i dont end up blowing my engine. Haha. Sorry if my question is a bit dumb, and thanks again for those who going to reply.

Dyno. It's the right tool for the job. You are making this so much harder by not using the right tools.

As least you don't have to buy a dyno, you can just rent time on one.

If cost is a concern, then have a professional tuner use the dyno. That is the most efficient way to get this done correctly. If you are willing to pay to learn, then pay to use a dyno, and learn how to tune this.

I agree with David - You can't effectively tune a VTEC/MIVEC crossover point without a dyno. Personally I don't change my AFR target for low cam/high cam at WOT - There's no need. For an N/A engine on pump gas you'd likely be in the vicinity of 0.88-0.92 lambda. For a turbo application this will depend on the fuel and boost but you're likely to be 0.75-0.82. The timing is just whatever the engine wants to achieve MBT or stay away from knock. As D.C has mentioned above, the techniques of finding optimal ignition timing are covered in our Practical Standalone Tuning course.