Before I get flamed it should be understood that I'm aware of the commonly accepted practice of running richer AFRs as boost increases. I understand that running on the rich side seems to have a cooling effect that helps prevent detonation. I came across an article that advocates a different idea. Instead of running 11.5 afr, 15deg. advance and 15 psi of boost they advocate something more like 12.5-13 afr and 10-5 deg. advance on 15 psi for example. The idea is that running a rich afr has more of an effect on burn speed than any kind of cooling effect. According to the article a rich afr only delays the combustion event so that there is less chance of multiple flame fronts/detonation. I guess the only way to know for sure would be to do a before and after and monitor egts and check for Knock. Has anyone had any experience with an approach like this? Please don't reply if you're just going to use a "NO because NO" response but I would love to hear any ideas about it.

 

I know people that take a similar approach when running N2O. They insist it is not running lean that does damage but too much timing that does.

 

I never thought of doing it with nitrous but the same line of reasoning should apply. Do you have any info on what they run or how long those engines last? I know of some guys running x275 that said they run kinda lean under spray but they said it was "dangerous" and wouldn't tell me anything else about it so I never really thought about it much after that. I might just try this out at some point. I would love to hear more about this strategy.

 

Weather is be boost or spray it all come down to cyl pressure. And making peak cly pressure at 15*ATDC to get the most mechanical advantage on the crank. All timing should be made to achive this (for max power). If you take fuel out of the equation and assume no knock/detonation. Leaner is better power wise. 11.5 is safe for high cyl psi but you would make more power at 13afr if the rotating assembly ie piston for sure could take the heat. The richer you are the less heat the piston must soak up. Piston spray jets help tremendously i belive the new lt1 has them as well as the LS9 i belive. Oil helps dissipate the heat.

 

I'm not debating that peak cylinder pressure should occur at 15 - 20 deg ATDC. What I'm talking about are different methods of achieving that end. It is a well know fact that AFR does effect how long a combustion event takes to complete. If the approach I'm talking about actually ends up working and not destroying pistons, it just means that the the cooling effect plays less of a role than a rich AFR delaying a combustion event.

 

The only way i could see you proving this would be on a dyno and running a spark/AFR hook. Dial in your spark at a given AFR and then start leaning/riching it and see what power does. Id suggwst and eddy current dyno as you are able to maintaining a given speed under max load.

 

I know they make pressure transducers built into spark plugs. If I could data log something like that with efi live I could get the answer I'm looking for. A steady state dyno would really help too.

 

Subscribed.

 

The N2O guys I was talking about set the A/F they want to run then read the plug ground strap for setting their timing.

 

I don't know why I didn't think of just reading the plugs. I would still like to measure cylinder pressure and tune the car on a dyno but in the end, if the ground strap isn't too far down then the chamber and pistons aren't too hot for too long. As soon as I get my welded chamber heads on the 4.8 I'll try this approach on a 175 shot with COS5.


I'd like to hear joecar's thoughts on this. Don't you tune cars for a living?

 

No, I tune for hobby (for myself and friends) altho I have done a few professional jobs, including CNG tuning....

(and pretty soon looks like I'll be looking for a new daytime job...)

I've seen the results of insufficient fuel, friends bringing their car home from Irwindale on a flatbed.

There's more to it than just flame speed and cooling... when not sufficiently rich the air/fuel charge can explode instead of burning... the extra fuel reduces the probability of this.

 

you're asking for trouble with that mindset. Start rich and on low timing, and gradually work your way up. Have a set of spark plugs ready so you can pull em after a run to see if your fueling and ignition timing is spot on. A dyno and or track run helps in this case.

this is what keith duckworth from F1 cosworth has to say about boost and fueling..


8:05-9:05 timeline..

 

I don't think your data stream with EFILive or HPTuners will be fast enough for that...I think you probably need some more expensive equipment.

A friend in college had one of those plugs...but after all he invested in trying to work with it, I don't remember him ever really getting great data because of the limits of what he could try to log it with.

 

You would need an oscilloscope.

 

Somthing i found on another site.


"Lower octane gas burns more easily. It also means that the flame front moves faster. So, if you get a tank of crummy, low octane gas, you're more likely to get pinging, even though your timing and compression are the same. To compensate, you could retard the timing some amount (less advance). Likewise, the flame front moves slower in higher octane gas, which requires more ignition advance, but also allows higher compression.

If the spark plug fired exactly as the piston was at the top of its stroke (top dead center or TDC), then you're not being as efficient as possible. The instant after the spark plug fires, the piston starts moving down, and the flame front has to catch up with the piston. The distance the piston moves, after is starts moving down but before the flame front reaches it, is wasted since it's not extracting power from the process. Only after the flame front catches up does it start pushing down and giving power.

The trick is to actually have the spark happen before the piston reaches TDC. That way, when the flame front reaches the piston, it's exactly at the top of its stroke and ready to start moving down, and you'll be getting power out of the fuel's burning during the whole power stroke."

"So where does the knock suppression of richer mixtures come from?

If the mixture gets ignited by the spark, a flame front spreads out from the spark plug. This burning mixture increases the pressure and temperature in the cylinder. At some time in the process the pressures and temperatures peak. The speed of the flame front is dependent on mixture density and AFR. A richer or leaner AFR than about 12-13 AFR burns slower. A denser mixture burns faster.

So with a turbo under boost the mixture density raises and results in a faster burning mixture. The closer the peak pressure is to TDC, the higher that peak pressure is, resulting in a high knock probability. Also there is less leverage on the crankshaft for the pressure to produce torque, and, therefore, less power.

Richening up the mixture results in a slower burn, moving the pressure peak later where there is more leverage, hence more torque. Also the pressure peak is lower at a later crank angle and the knock probability is reduced. The same effect can be achieved with an optimum power mixture and more ignition retard.

Optimum mix with “later” ignition can produce more power because more energy is released from the combustion of gasoline. Here’s why: When hydrocarbons like gasoline combust, the burn process actually happens in multiple stages. First the gasoline molecules are broken up into hydrogen and carbon. The hydrogen combines with oxygen from the air to form H2O (water) and the carbon molecules form CO. This process happens very fast at the front edge of the flame front. The second stage converts CO to CO2. This process is relatively slow and requires water molecules (from the first stage) for completion. If there is no more oxygen available (most of it consumed in the first stage), the second stage can't happen. But about 2/3 of the energy released from the burning of the carbon is released in the second stage. Therefore a richer mixture releases less energy, lowering peak pressures and temperatures, and produces less power. A secondary side effect is of course also a lowering of knock probability. It's like closing the throttle a little. A typical engine does not knock when running on part throttle because less energy and therefore lower pressures and temperatures are in the cylinder.

This is why running overly-rich mixtures can not only increase fuel consumption, but also cost power. "