82 mm throttle body

[ike84]

Originally Posted by blue62

Ike
That all seems like pretty normal numbers to me.
A couple of things to keep in mind:
14.7 fuel air ratio is not what the engine needs under all conditions to perform best.
An example is WOT.. it should go rich. Heavy engine load requires a richer condition.

Foot off the gas your getting 22: exactly you don't need fuel in the air to coast. your getting 22 because you don't need fuel under that condition but the engine still pulls in air.
Hell you don't need spark. Some modern day engines cut off fuel and spark all together under off throttle conditions. An AFR sensor will see all air (or nearly so) under off throttle conditions.

Another thing: 14.7 fuel to air is the ideal ratio for Catalytic Converter performance.
Not engine performance under all conditions.
On a modern day computer controlled engine the primary function of the DME/ECU system is "Catalytic Converter performance, engine performance is secondary.

The DME/ECU under closed loop conditions constantly fluctuates the fuel air mixture above and below the 14.7 fuel air ratio. That is why O2 sensor signals constantly fluctuate in a sine wave. The DME/ECU sort of looks for an average under closed loop conditions.

Hey blue,

Thanks for the response. That's interesting about the sine wave pattern, which is exactly what I'm seeing. That seems a bit less efficient than a "hold steady" approach at control but maybe it's more effective to continually correct.

That's also interesting about the cutting off of spark while coasting. It makes sense to do so for fuel economy, as long as the alternator keeps spinning to keep up with current load demand.

The reason the lean tendency was surprising is just the excess heat and likelihood of detonation under those conditions. I see your reasoning though that if the motto is "emissions first, engine second" then it makes sense to do so.

With that being said though, the numbers on my drive this morning were much more centered around 14.7, especially while cruising under moderate load. I really wonder if the short term trims just needed 20 miles to adjust the mixture. I suppose 20 miles of lean isn't too bad considering the detonation sensors/ignition retard built in to the dme.

I think it's fascinating to learn more about how these computers actually do their job. Unfortunately, Bosch me 7.2 is kinda like this black box with very little info about the logic and processes involved. I don't think I've seen a single thread on this forum which is technically detailed regarding this (beyond basic principles of open/closed loop, short/long term fuel trims). I can't even find a good one on nefmoto, which is probably the best site on the web for that type of info.

Anyway, for me the enjoyment will have to be postponed. Looks like I'm leaking oil from the passenger side bank. Probably plug tubes, I should have replaced them when I changed the plugs and coils over the winter. Back on to the stands we go!

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[blue62]

IKE
Most people tend to think that the O2 sensor signal drives the DME/ECU.
But what really happens is the DME/ECU drives the O2 signal.
The O2 signal on these cars operates between a low of around .1 millivolt to a high of about .950 millivolts. Stoichiometric or 14.7-1 is at .450 millivolts.
So the DME/ECU goes a little rich to drive voltage down then goes a little lean to drive volt up. Constantly. The DME/ECU uses the response from the O2 sensor as feedback (closed loop) info to determine fuel requirements or fuel trim to drive the O2 sensor signal.. Which in the end is about Catalytic Converter performance.

Stoichiometeric or 14.7 lbs. of air to 1 lb. of fuel is the ratio at which there is exactly enough molecules of air to combust ever molecule of gas.
So 14.7-1 produces an exhaust that is most easily cleaned by the Catalytic converter.
To rich (excess gas molecules) Cats plug up. to lean(excess air molecules) Cats burn up.

But 14.7-1 it is not the ratio at which fuel air burns the coolest or produces the most power.
That is why you see a richer mix at WOT or high load and leaner mix at idle or very little load.
Engine performance needs vrs Cat converter needs.

Most of the time cars are operated in the lower end to middle of the cruise range as far as engine load goes. That's why you see the air fuel ratio closest to 14.7-1 in that range. 14.7-1 meets engine needs and Cat needs equally in that range.

The shutting off of fuel and or spark at off throttle conditions does save fuel but it is more about pollution.
With the constrains put on automobile manufactures by the EPA everything (as far is engine performance goes) is about pollution control. Performance (power) or fuel mileage is secondary.
EPA restrictions are the biggest reason you can get more power and sometimes fuel economy from an after market tune. It is also why after market tunes are illegal in some states.

[ike84]

Originally Posted by blue62

IKE

Most people tend to think that the O2 sensor signal drives the DME/ECU.

But what really happens is the DME/ECU drives the O2 signal.

The O2 signal on these cars operates between a low of around .1 millivolt to a high of about .950 millivolts. Stoichiometric or 14.7-1 is at .450 millivolts.

So the DME/ECU goes a little rich to drive voltage down then goes a little lean to drive volt up. Constantly. The DME/ECU uses the response from the O2 sensor as feedback (closed loop) info to determine fuel requirements or fuel trim to drive the O2 sensor signal.. Which in the end is about Catalytic Converter performance.



Stoichiometeric or 14.7 lbs. of air to 1 lb. of fuel is the ratio at which there is exactly enough molecules of air to combust ever molecule of gas.

So 14.7-1 produces an exhaust that is most easily cleaned by the Catalytic converter.

To rich (excess gas molecules) Cats plug up. to lean(excess air molecules) Cats burn up.



But 14.7-1 it is not the ratio at which fuel air burns the coolest or produces the most power.

That is why you see a richer mix at WOT or high load and leaner mix at idle or very little load.

Engine performance needs vrs Cat converter needs.



Most of the time cars are operated in the lower end to middle of the cruise range as far as engine load goes. That's why you see the air fuel ratio closest to 14.7-1 in that range. 14.7-1 meets engine needs and Cat needs equally in that range.



The shutting off of fuel and or spark at off throttle conditions does save fuel but it is more about pollution.

With the constrains put on automobile manufactures by the EPA everything (as far is engine performance goes) is about pollution control. Performance (power) or fuel mileage is secondary.

EPA restrictions are the biggest reason you can get more power and sometimes fuel economy from an after market tune. It is also why after market tunes are illegal in some states.

Blue, thanks for the knowledge brother. Always more to learn about this stuff. I've been working through an automotive textbook to get a better understanding of different systems and this convo definitely put the fuel chapter next on my list.

I will say that the last drive I took (15 miles home) the afr stayed 14.5-14.9 the whole time during closed loop operation. I guess the stfts really did play into that. Which I think is interesting because it implies that the dme is programmed to run lean until it can dial in 14.7 more precisely, or that this setup provides more air than the stock 996 setup (less likely since the MAF should be able to dial this in precisely without the help of the fuel trims). Thinking this through though it may just represent a difference in MAF housings since I am using a 3.25" diameter honeycombed pipe from a BMW V8 intake - same diameter as the 996 but maybe with fewer curves in the piping I am flowing air more efficiently.

I'll have the battery unhooked while replacing the plug tubes so I'll be curious if it goes back lean for the first drive after that.

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