AFR readings help?

[YamaMTX]

Anyone with a turbo kit, especially the Ulmer turbo kit if you could help me out that would be great. When I start it up and it gets warmed up a bit the AFR is between 14.8 and 16. When you gas it or cruise around 50 mph it runs between 11-12 When you stab the throttle and let off, the engine almost sounds like it misses a beat and when you let off super quick the gauge quickly hits 18 for a split second and then goes back to idle (15 or so) I am new to all this stuff so maybe this is normal but I believe the idle is lean and the mid range is fat? Any info or light that can be shed on this for me would be great. Thank you!

 

[pro116]

Sound a little lean to me.Have you contacted ulmer racing since it's there kit.

 

[YamaMTX]

I have emailed them about a clutch kit for the past two weeks and have gotten zero responce. That being said I have emailed about this issue and I will call Monday as they are not open on the weekend.

 

[FLYBYU]

I think their traveling with the snow shows.

 

[bottlerocket]

I would leave a message and he call you back.

 

[Srxspec]

I returned your email just a few minutes ago, all perfectly normal and explained in detail in the email.

 

[YamaMTX]

Thanks SRX!

 

[Dimebag]

Would anyone care to explain about the AFR readings?

Which numbers do we want to see? What to expect?

When does it get critical ?

I installed a innovate a/f gauge on my mcx 180 nytro this summer,
but don't know for sure what readings to expect.

Thanks

 

[macnytro]

Would anyone care to explain about the AFR readings?

Which numbers do we want to see? What to expect?

When does it get critical ?

I installed a innovate a/f gauge on my mcx 180 nytro this summer,
but don't know for sure what readings to expect.

Thanks

yes an explanation would be good so i now what to look for when i start mine this year thanks

 

[nate007]

14.7:1 AFR is the standard air fuel ratio for a naturally aspirated motor. That means 14.7 parts air, 1 part fuel. When you add boost, a number of things have to change. Since you're increasing the ammount of air in the motor, you have to increase the ammount of fuel too. This isn't just a straight across the board adjustment, because there are far too many variables to explain in detail here, but things like engine heat, outside air temp, engine load and rpm, etc all play a part in what the increased ratios should be.
Typically, a boosted engine can run between 11:1 AFR and 12.5-13:1 AFR. That meaning now you'd have less parts air to more fuel, but in reality since you've now packed the cylinders with more air, you have to add lots more fuel to get to those ratios. Adding extra fuel can and does help to cool the compressed air charge and prevent detonation. This is partially why a boosted motor will get less fuel economy in many cases.
So with 14.7:1 being the normal (stoiciometric) ratio, and 11(ish) being on the rich side, a boosted motor can also run on the lean side at 15.3+ during light throttle cruise as long as engine heat and correct timing control can help keep the motor from detonating.
The problem is that with these sleds, although they are light years ahead of where they were years ago, from an EFI standpoint, they are prehistoric, and very simple. They don't have the capacity to make changes to fuel and timing beyond a couple percentage points, and even with the mainstream control boxes for boosted applications, they monitor near enough functions to properly correct for boost. They simply add a certain ammount of fuel based on a boost reference signal and at certain rpms. Someone correct me if I'm wrong, but I don't think they will correct for engine heat? This is why there are many people with boosted motors who complain about crappy mileage. The systems simply dump more fuel in to control detonation, sort of like trying to flood a fire with gas..
The few boxes that have more power (intelligence) and can monitor and adjust for everything are quite a bit more expensive and are much harder to tune, but are still only part of the equation of properly setting up a boosted motor.
Sorry for the long read, hope that explains AFR a little?

 

[Dimebag]

Sounds great nate. Thanks a lot.

I will note down the values you mentioned and come back with a thread when i get a test ride on my sled.

 

[Essarex]

I will add to what Nate wrote.

14.7:1 is the theoretical perfect ratio of air to fuel. In other words, if you were to look at the chemical equation for the oxidation of air and fuel, it shows that it takes 14.7 pounds of air to completely combust (oxidize) 1 pound of fuel. At that ratio, the entire hydro carbon amount is converted to carbon dioxide and water (plus heat which is what provides the pressure to push the piston down).

No extra air or fuel is left over in the process.

The reason that high performance 4 cycle engines make max power at 12.5:1, is because as Nate put it, these engines are still relatively primative. We cannot get perfect air and fuel blending given the very high rpm and limited time for combustion. Therefore, we throw enough fuel at the motor to take advantage of every last molecule of air that the motor can inhale. As you go lower than 12.5:1, you begin to quench the flame fromt and lower the heat of combustion and hence the pressure (pushing power of the expanding gasses).

The beauty of E85 is that alcohol allows a HUGE window of air/fuel ratio before quenching starts to become a power robber.

 

[Dimebag]

Okay guys,

I have just been out on the first ride this season. We finally got some snow

AFR values are as follows;

Idling: 12.8 - 13

Cruising about 50 km/h: 12

I can see it bounce to about 14++ sometimes when i release the throttle.

I did not get to test it in climbs with full throttle, as we dont have that much snow yet.. What readings should i expect while climbing?

I also noticed that i dont have enough boost at the moment, have to make some adjustments there. Should the afr change when i get the boost levels correct?

Does the readings sound good?


Thanks.

 

[cannondale27]

Couple things account for lean spike at throttle close.
When you close the throttle the injectors only deliver enough fuel for idle.The time it takes for that to happen is instant.Now the time it takes for the large volume of air left in intake to be used is longer.Add to that the Nytro has a feature that slightly opens the IACV to lessen engine braking and I think it explains lean spikes at throttle close.At cold startup you will see same thing till IACV closes and idle goes down to normal.Its a induced vacuum leak.

 

[Dimebag]

That sounds great. I was hoping there was a logical explanation to the lean spikes.

From other AFR posts on this forum the values i listed seem to be pretty normal.

I'll take a closer look when I get some real pow to test it in, and get the boost levels adjusted properly.