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AFR Question

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This one's for you, Andre. So i'v noticed that there are a lot of "tuners" out there who like to tune RICHER than 11.5:1 on high boost engines... to stave off detonation.

I'v read that 11.5:1 is the richest you want to go with Gasoline. And, that if you need to go richer... it means you have too much timing. So, a 10.8:1 AFR with x amount of timing will get the same results as 11.5:1 AFR with x-3 degrees of timing, for example. Is this true?

I have similar question on this too. We tried install haltech plug-in pro on stock subaru v8 (with light modification) , the reading AFR was rich as 9.8 - 10'ish WOT on the stock MAP provided by haltech. As the result, the engine slightly jerking at certain rpm eg(3600rpm) where the wideband afr shows 9.8. As the RPM increase, the AFR slightly leaner but not smooth too. I tried email a haltech support on the website, i can't seem get response too. The owner seem frustrated as our location don't have any 4wd dyno and a professional tuner.

Similarly , i tune several Mitsubishi (4G18) engine with ecu reflash method on NA engine. The AFR value was believe to be AFR 12.8 to ideal. I tried to push slightly leaner AFR to 13.1 , the engine seem gives fuel economy and power on WOT. However, certain engine with adjustable fuel regulator pressure is set to different pressure , mild racing camshaft and etc gives different characteristic and response for example, richer at 12.4 to 12.6 at WOT on higher load. We datalog the AFR, RPM , loads and knocks several times before increasing the ignition. It is nice if have external knock detector, might can help eliminate knock (pre-detonation).

There are possibility, increasing the fuel richer to avoid detonation, i have seen tuners out there adding more fuel by adding excessive fuel, Similarly , i believe haltech stock map is richer to avoid any detonation. Also, too much fuel doesn't make a lot different, but adding ignition does.

My question: what are the safety factor AFR value for TURBO and NA? Just a standard or rules of thumb. It would be helpful before goes into fine tuning it.

1) On Idle

2) Partial throttle

3) WOT (Wide open throttle)

The Haltech Plug and Play with the Base maps, altho pretty close is not intended to just be fitted and away you go. They intentionally make the base maps Rich and with low timing so you can start with a Safe tune (if its a stock engine) They need a full dyno tune.

AFR's for Idle will depend on if its batch fire/full sequential, cam agressiveness, intake design etc... But around 13:1 - 14.7:1 is a good starting point... Just play around up and down to see where it idles the best (where it pulls the highest vacuum) some engines will like as rich as 12:1

Partial throttle a good aim is 14.7:1, Can go up to about 15.5:1 if you are chasing economy

WOT for a N/A engine aim for about 12.5:1 - 13:1 but once again is different from motor to motor, Some like it rich to keep detonation away, others can be quite happy at 13.5:1 with no ill effects.

For a Turbo motor 11.5-12.5 depending on boost levels and type of motor.


at 9.8 jerking is imo 99% sure a rich misfire

If it was a rich misfire then it would show lean not rich? As the o2 Sensor will read all the unburnt oxygen.

I find that the rich misfire will lead to an erratic AFR reading but since some of the combustion charge is still being burned I would not recommend to gauge the AFR/Lambda reading as THE important data to troubleshoot in this particular instance. Given, assumingly, holydevil is running the stock ignition system, not a high energy capacitor discharge ignition, it is fairly safe to assume that the spark energy is insufficient to ignite the combustion charge completely. A 9.8 is, IMO, overly rich and does more harm rather than good in this particular application with either pump gas or e85. More advanced timing may lessen the rich misfire but it would be a bandaid only and potentially, if the engine is knock limited, very dangerous.

Referring to Mareks original question, some super tuners trade in a couple of degrees of timing for more (overheated) boost pressure to impress customers. Once holes in the pistons are being found, it is the injector to be blamed (the uneducated customer is then being told and believes that the destructive knock was caused by a lean mixture), this sounds sarcastic but it is what really happens in the market.

Im pretty sure the O2 sensor can't respond that quick.

The problem is that when it comes to AFR there is no single 'correct' AFR that you need to achieve on every engine. In comparison tuning ignition timing is quite easy - You either can achieve MBT timing or reach the knock threshold and either way you know when the timing is optimal. With the fuel delivery however there is likely to be a wide range of AFR over which the torque is reasonably constant. Go massively rich or massively lean and torque will drop off at both ends, but the 'sweet' spot can be anywhere from say 12:1-14:1 on an N/A engine for example.

Of course I have my own target AFR for particular engines, fuels or combinations that I know from experience are a safe starting point, but then I will move richer or leaner to see what a specific engine wants to run at.

The key is to understand the effect that the AFR has on the engine's performance. If you approach every tune with a predetermined AFR in mind, you may end up leaving some power on the table. For example I find in general that a naturally aspirated engine will perform well and be reliable with an AFR of 13:1 at WOT. This works well for a Honda B16A or K20A for example. Try and apply that same mentality to an unmodified LS2 though and you will be fighting knock at high rpm. If on the other hand we tune the LS2 at a richer AFR of about 12.5:1 at high rpm, it will accept 2-3 degrees more advance and make more power. This just highlights why you can't always rely on one number.

To answer your specific question Marek, I've seldom seen a turbocharged piston engine running on pump gas that wanted or needed to run richer than 11.5:1. Almost without fail, a richer mixture will start hurting your power/torque. There are many tuners who target richer but in my experience it's unnecessary.

I've had a number of cars that I've retuned over the years that had been running high boost on pump gas (28-30 psi) with mixtures in the 10.5-11.0:1 region and very little timing. Usually I find that I end up with the same power (or better) by dropping the boost back to a more reasonable level (22-24 psi I often find is the practical limit for pump gas), leaning out the mixture to perhaps 11.5-12:1 and adding some timing back in. The side affect is that the engine/turbo are also under less stress and produce less heat.

All of this is dealt with in detail in our 'Understanding AFR' course, so as a shameless free plug (to myself), feel free to check it out. It's only $89 USD and as always it comes with our 100% money back guarantee. https://www.hpacademy.com/courses/understanding-afr/

Just to address another point raised in this thread, if your engine is running very rich then you risk a rich misfire (normally under high load/boost). A misfire of any nature will show as lean on a wideband meter as the unburnt oxygen in the exhaust will be sampled by the sensor - It's not the fuel the sensor is interested in.

You can still experience poor, jerky or hesitant performance though when the mixture is excessively rich without actually suffering from a misfire. The excessive fuel in the combustion chamber degrades the combustion event resulting in poor or partial combustion. The combustion even with a very rich mixture will also be slower and hence the chances of achieving peak torque are reduced.

I hope that adds some clarity to the situation.

I don't understand why a misfire would cause a lean reading?. Isn't the O2 sensor measuring the CONCENTRATION of O2 in the body of gas?

Wouldn't the concentration of O2 be the same regardless of whether its combusted or not?

Even if an incompetent tuner is running way outside the compressors efficiency range (overboosting)... my question is... won't he get the same torque (in every instance) running at 10.8:1 with x degrees of timing as he would with 11.5:1 with

Is there ANY scenario where you would want to tune richer than 11.5:1 ??

Most of the oxygen is being "consumed" by the combustion, having said that oxygen that is left over after the burn is less than prior to the spark event and the subsequent burn if the air fuel charge.

Should the combustion not take place or only partially take place, the O2 sensor sees more oxygen, makes sense?

Yeah i get it now... the O2 is bound, or even broken up and bound, to other elements... so the sensor can't detect it.

HPA's comprehensive course "Understanding AFR" explains in great detail as to how and why a certain engine needs / wants to run on its own AFR. The quintessence is that there is no fixed value and one has to elaborate, based on the knowledge conveyed by the courses content, one can make his own decision to run the best power whilst still be on the safe side.

I never asked the question "what AFR should i run". My question was far more specific than that!.

Yes there are some engines where you have to tune richer, than what would be ideal for max power. That's likely on turbo setups with high specific power output and low octan fuel or engine with alot of backpressure (restrictive turbo, high boost). Due to the high cylinder presures, we have to run quite retarded ignition timing, to prevent the engine from knocking. This leads to very high combustion and exhaust temperatures ( which again leads to detonation). Consequently you have to richen the mixture up, to improve the internal cooling, bring the temperature down and add an additional safety margin. But the cooling effect on pump fuel is not that big, so adding fuel is not always the right way. You have to find the sweet spot between, boost, AFR, compressor effeciency and timing. As Andre mentioned, it's a balance act between a few things.

I really like to have a pre-turbo EGT gauge to find the right AFR's. Especially on cirquit or highspeed race engines which sees very high load for long periods, it's often necessary to go a bit richer, which allows a bit more timing and consequently we can keep the exhaust temperature in an acceptable range. With the time you will get a feeling what works on which engines and situation.

I second what Adrian mentioned about race engines, especially on a customers road race car, I noticed that with the ideal AFR the car performed very well on the dyno.

A couple races in the season I noticed that the knock sensor was picking up more noise (possibly light detonation) the longer the car was racing, so adding in a bit of extra fuel cured that right away.

While I still had to leave a couple of ponys on the track, it sure helped to safeguard the engine.

Teardown 3 months ago to check on everything -> all in perfect condition!

BMW s54 motors like 13:8 afrs at WOT. I couldn't believe it. Every motor is different

13.8 at peak torque??... sounds like the injectors are blocked! :P

Or its been tuned by an idiot.

Marek0086 are you sure that your wideband reads right? That's enormous lean. I would be worried about EGT and combustion temperature during long periods of high load.

thanks guys for the fast reply :D. Truly resourceful and informative.

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