Real-world AFRs at WOT...

[Enthalpy]

Leaner with less timing means more exhaust heat, and it won't mean more power unless you are VERY rich. Set the AFR for something safe as suggested and set the timing for either MBT (Minimum Best Torque) or a few degrees shy of detonation onset, whichever comes first.

[mspiegle]

I always thought that if you want maximum power, you have to find the AFR that gaves you the max power, and most of the time, it's around 12,5.

Pierre

No necessarily. You might get max power, and insanely high EGTs that melt stuff at around 12.5:1, for example- so you would need to go richer, and sacrifice power to save your engine. For most NA engines, 12.5:1 is a really good guess. If one had super-high compression or something, or perhaps was air-cooled, you may need a richer mixture.

Also- timing comes before fuel, because advancing timing results in lower EGTs, meaning you can run leaner (but still rich), wich makes more power and saves gas. Too advanced timing results in reduced power, or engine-destroying knock.

I have read this thread through a couple times and find it very valuable. It is helping me solidify some of my tuning beliefs, but I had a question about the above statement.

Doesn't knock/ping happen when your combustion temps/pressures are too high at TDC causing the mixture to become unstable? If that's the case, then how can you say "advancing timing results in lower EGTs" AND "too advanced timing results in reduced power or engine-destroying knock". Won't that only be true if your timing was heavilly retarded to begin with, or if you bring the point of complete combusion closer to TDC?

Is there any way (via math) to determine how long a mixture will take to burn at any given RPM (based on density and mixture composition) and attempt to target the timing of complete mixture burn to the point of greatest mechanical advantage?

Edit:
Also, if I understand correctly, EGTs rise as timing is retarded because you are pushing the peak pressures/temps closer to the opening of the exhaust valve. Is this correct? If so, this might be a dumb question, but wouldn't that mean the peak pressures/temps would be the same in the combustion chamber regardless of when it occurs? And if that is true, what's the point of lowering EGTs?

Edit2:
We all know that higher octane gas has a higher resistance to knock (the AKI measurement). I've also heard that higher octane gas burns slower. If this is true, then wouldn't that mean that a higher octane gas can change the power put out by an engine? If it burns slower, then it could move the point of maximum combustion temperature closer to (or away from) the point of greatest mechanical advantage. Is that true?

[Enthalpy]

I have read this thread through a couple times and find it very valuable. It is helping me solidify some of my tuning beliefs, but I had a question about the above statement.

Fantastic questions! I will answer them as well as I can.

Doesn't knock/ping happen when your combustion temps/pressures are too high at TDC causing the mixture to become unstable? If that's the case, then how can you say "advancing timing results in lower EGTs" AND "too advanced timing results in reduced power or engine-destroying knock". Won't that only be true if your timing was heavilly retarded to begin with, or if you bring the point of complete combusion closer to TDC?

Yes, knock happens as the pressure reaches too high. If you aren't knock-limited (further timing advance yields more torque without detonation), then more advance will also result in lower EGTs. The reason is overly-retarded timing doesn't allow the piston to extract work from the mixture as well, and so the mixture has more energy (heat) as the exhaust valve opens and the blowdown phase begins. Sometimes, you can actually have timing that's so retarded that combustion is still occuring in a major way as the exhaust valves open.

As you start to detonate, the PPP (Peak Pressure Point, or the point of peak pressure during combustion) will suddenly go from way too late to way too early in the mechanical cycle. You will see a drop in torque from this.

Is there any way (via math) to determine how long a mixture will take to burn at any given RPM (based on density and mixture composition) and attempt to target the timing of complete mixture burn to the point of greatest mechanical advantage?

Make no mistake about it: You can model EVERYTHING that's going on inside an engine. When you get to the real world, however, there are a few problems, which is why tuning is required.

First, cycle-to-cycle variations are inevitable. Second, we don't measure everything with an EMS (or factory system for that matter) that we need to calculate exact burn rates. This is why there are only rules of thumb, and you really need to tune the setup properly and not rely on these factors. You use the theory to make sure the values you have make sense.

Edit:
Also, if I understand correctly, EGTs rise as timing is retarded because you are pushing the peak pressures/temps closer to the opening of the exhaust valve. Is this correct? If so, this might be a dumb question, but wouldn't that mean the peak pressures/temps would be the same in the combustion chamber regardless of when it occurs? And if that is true, what's the point of lowering EGTs?

No. If the cylinder's volume is expanding too early in the combustion cycle (retarded timing), then this will hamper the peak pressure that's achieved during the cycle, which of course, has an effect on the temp.

Edit2:
We all know that higher octane gas has a higher resistance to knock (the AKI measurement). I've also heard that higher octane gas burns slower. If this is true, then wouldn't that mean that a higher octane gas can change the power put out by an engine? If it burns slower, then it could move the point of maximum combustion temperature closer to (or away from) the point of greatest mechanical advantage. Is that true?

Absolutely right! This is why the common wisdom of pump octane is to use the minimum octane that prevents knock. This is because the EMS and ECU were optimized for a specific octane, and changing that octane moves the peak pressure point.

If you have someone come to you wanting to tune using either old gas or a hodge-podge of octanes and mixtures, drain their tank and fill it with the fuel they intend to use going forward.

Some people will report mileage loss when putting high octane gas in their street-driven car. This happens because the timing was optimized using faster-burning, lower-octane fuel.

Wow! I wrote a book. Sorry about that.

[mspiegle]

I have read this thread through a couple times and find it very valuable. It is helping me solidify some of my tuning beliefs, but I had a question about the above statement.

Fantastic questions! I will answer them as well as I can.

Doesn't knock/ping happen when your combustion temps/pressures are too high at TDC causing the mixture to become unstable? If that's the case, then how can you say "advancing timing results in lower EGTs" AND "too advanced timing results in reduced power or engine-destroying knock". Won't that only be true if your timing was heavilly retarded to begin with, or if you bring the point of complete combusion closer to TDC?

Yes, knock happens as the pressure reaches too high. If you aren't knock-limited (further timing advance yields more torque without detonation), then more advance will also result in lower EGTs. The reason is overly-retarded timing doesn't allow the piston to extract work from the mixture as well, and so the mixture has more energy (heat) as the exhaust valve opens and the blowdown phase begins. Sometimes, you can actually have timing that's so retarded that combustion is still occuring in a major way as the exhaust valves open.

As you start to detonate, the PPP (Peak Pressure Point, or the point of peak pressure during combustion) will suddenly go from way too late to way too early in the mechanical cycle. You will see a drop in torque from this.

Is there any way (via math) to determine how long a mixture will take to burn at any given RPM (based on density and mixture composition) and attempt to target the timing of complete mixture burn to the point of greatest mechanical advantage?

Make no mistake about it: You can model EVERYTHING that's going on inside an engine. When you get to the real world, however, there are a few problems, which is why tuning is required.

First, cycle-to-cycle variations are inevitable. Second, we don't measure everything with an EMS (or factory system for that matter) that we need to calculate exact burn rates. This is why there are only rules of thumb, and you really need to tune the setup properly and not rely on these factors. You use the theory to make sure the values you have make sense.

Edit:
Also, if I understand correctly, EGTs rise as timing is retarded because you are pushing the peak pressures/temps closer to the opening of the exhaust valve. Is this correct? If so, this might be a dumb question, but wouldn't that mean the peak pressures/temps would be the same in the combustion chamber regardless of when it occurs? And if that is true, what's the point of lowering EGTs?

No. If the cylinder's volume is expanding too early in the combustion cycle (retarded timing), then this will hamper the peak pressure that's achieved during the cycle, which of course, has an effect on the temp.

Edit2:
We all know that higher octane gas has a higher resistance to knock (the AKI measurement). I've also heard that higher octane gas burns slower. If this is true, then wouldn't that mean that a higher octane gas can change the power put out by an engine? If it burns slower, then it could move the point of maximum combustion temperature closer to (or away from) the point of greatest mechanical advantage. Is that true?

Absolutely right! This is why the common wisdom of pump octane is to use the minimum octane that prevents knock. This is because the EMS and ECU were optimized for a specific octane, and changing that octane moves the peak pressure point.

If you have someone come to you wanting to tune using either old gas or a hodge-podge of octanes and mixtures, drain their tank and fill it with the fuel they intend to use going forward.

Some people will report mileage loss when putting high octane gas in their street-driven car. This happens because the timing was optimized using faster-burning, lower-octane fuel.

Wow! I wrote a book. Sorry about that.

Thank you for the detailed response! All of your answers made perfect sense to me, but i'd like to get some clarification on the last question regarding octane.

I understand that if you have higher octane and advance the ignition more, you can definately extract more power - but I was curious what would happen if you didn't change the ignition advance. If you take a car that was tuned for 87, then you put in 100 octane, the 100 octane will burn slower and move the PPP. This can change the power output of an engine, correct?

[Enthalpy]

If the engine was tuned for 87 octane and if it wasn't detonating, then putting higher octane in the engine will REDUCE the engine's performance in most every way (mileage and peak power/torque).

[fscott]

No, peak power will occur in the region of 12.5:1 - 13.0:1. Your exhaust gas temperature will be dangerously high running stoichiometric mixtures at WOT, you may even lose an exhaust valve or two.

How hot is dangerously hot? I've never seen numbers for this.

Fred

[PSIG]

...Compression is definitely one factor, but not the only one. You can have impressive compression and boost and not be knock-limited. Examples include the S2000, 3S-GTE and quite a few high-flow EGR Volvo engines of late. As a result, making generalized compression ratio statements have also been proven to be false as of late.

Volvo has demonstrated that high-flow cooled EGR allows for 13.5 AFR without power reduction in the least. I am reproducing this on my MR2 right now with great results. Talk about your fuel savings!...

I've enjoyed your statements in this discussion, but this one may need some clarification. Maybe it's the way you said it - but 'generalized compression ratio statements ' are still true - all else being equal.

Although I don't have the specifics the Volvo, for example, is an unequal apples-to-oranges situation where they appear to be using the EGR carbon monoxide detonation suppressant technique. If so, that certainly makes compression ratio a new issue - but doesn't appear to make other rules-of-thumb false. Would you mind qualifying this statement so it is less confusing to the subject for me?

Along the same issues - how are you implementing EGR on your MR2? I'd like to get some better mileage with my Suburban where even a tiny improvement saves big bucks at the next fillup. Gasoline is on the rise again!

Thanks!
David

[saqmaster]

No, peak power will occur in the region of 12.5:1 - 13.0:1. Your exhaust gas temperature will be dangerously high running stoichiometric mixtures at WOT, you may even lose an exhaust valve or two.

How hot is dangerously hot? I've never seen numbers for this.

Fred

You want to keep the EGT between about 750C and 850C.. that is without a cat.. with a cat you need to keep it in the 750C region.. mapping a car with a cat is a very complex task..

[Enthalpy]

...Compression is definitely one factor, but not the only one. You can have impressive compression and boost and not be knock-limited. Examples include the S2000, 3S-GTE and quite a few high-flow EGR Volvo engines of late. As a result, making generalized compression ratio statements have also been proven to be false as of late.

Volvo has demonstrated that high-flow cooled EGR allows for 13.5 AFR without power reduction in the least. I am reproducing this on my MR2 right now with great results. Talk about your fuel savings!...

I've enjoyed your statements in this discussion, but this one may need some clarification. Maybe it's the way you said it - but 'generalized compression ratio statements ' are still true - all else being equal.

Although I don't have the specifics the Volvo, for example, is an unequal apples-to-oranges situation where they appear to be using the EGR carbon monoxide detonation suppressant technique. If so, that certainly makes compression ratio a new issue - but doesn't appear to make other rules-of-thumb false. Would you mind qualifying this statement so it is less confusing to the subject for me?

Along the same issues - how are you implementing EGR on your MR2? I'd like to get some better mileage with my Suburban where even a tiny improvement saves big bucks at the next fillup. Gasoline is on the rise again!

Thanks!
David

I was trying to list other factors that creates a knock-limited engine. Compression ratio is really secondary to combustion chamber design (in my mind). The old hemi and wedge chambers don't resist knock very well at all and are quite inefficient.

Cooled EGR is being used as an ON-BOOST knock supressor, allowing much leaner mixtures without engine output sacrifice. The main goal here is fuel savings.

In case that didn't give you the answer, I guess I need you to ask a more specific question.

My implimentation makes plenty of flow available if I cool it and recirc it before the turbo. My valve doesn't have to be monsterous at all.

Suburban?? LOL. If you want better mileage, buy something that doesn't weigh 16,000 lbs. Short of that obvious statement, you might be able to get away with a lean burn mapping of that engine, provided you are careful and your engine's cooling system is in good shape. With an engine that pulls a load like that, you really have to watch coolant temps as you map it.

Then there's the old standbys, which net you quite a bit of economy savings:
-proper tire pressure
-harder, lower-wear and skinnier tires
-clean injectors (you would be surprised how many suffer from this)
-proper alignment
-change the oil with a synthetic
-engine properly maintained
-drive gently! (This one is huge for such a honking tank as a Suburban)
-Don't vary the throttle all the time on the freeway. Either be steady or use the cruise control.

[mspiegle]

No, peak power will occur in the region of 12.5:1 - 13.0:1. Your exhaust gas temperature will be dangerously high running stoichiometric mixtures at WOT, you may even lose an exhaust valve or two.

How hot is dangerously hot? I've never seen numbers for this.

Fred

You want to keep the EGT between about 750C and 850C.. that is without a cat.. with a cat you need to keep it in the 750C region.. mapping a car with a cat is a very complex task..

How close are EGTs and combustion chamber temps? Corky Bell specifies (although somewhat arbitrairilly) a combustion chamber temp of 1075 degrees. Has anyone validated this to be somewhat correct? Way off? Right on? Or just some imaginary number that you don't have to worry about as long as you aren't knocking?