mark kibort

This goes for airplanes and cars.

These graphs give a representation of where the dangers can occure at different fuel air mixtures.

Notice, lean above stoich creates the same temps both , Exhaust gas temp as well as cylinder head temps. one of the reasons that a 928 has no issues running at near stoich, is that the engine has so much more thermal capacity, its like running it at 75% power setting if it was an aircraft engine. Between the heads, intake and valves, its pretty restricted. look how well that same block does when you get near double the hp at 6.5 liters and 480rwhp! obviously stoich with Andersons engine is going to be different than with your stock 84S.
however the chance for detonation is much greater at stoich as well!

Just a FYI for reference, especially when talking about NOS systems being dangerous. They are not dangerous in the sence of engines blowing up due to being lean, they are dangerous due to misuse, and carelessness

This graph shows the stoich (solid, straight red line) and the temp curves for different power settings and mixtures vs the chts and egts.

worf928

Best power is at 12.6:1 which is richer than stoich. That's why the LH goes to WOT maps and tosses the lambda loop out the window when the switch is triggered. Stoich is only for cruise and low emissions. Combustion leaner than 12.6:1 is hotter. Hotter can lead to detonation etc. What's your point?

mark kibort

All is true until the last comment. leaner than 12.6:1 is only hotter until stoich and a little past it. way leaner is way cooler! (but at the sacrafice of power)
so, all the concerns of NOS systems burning up engines, and detonation, is only true if the failure makes the engine run at stoich. A fuel failure of a 100shot NOS system will toss the engine into the the way lean range, and be way cool as well. (pun intended)

mk

Tony

seen it 1000's of times when leaning out an IO520. twist the ****...EGTS go higher...power picks up...then the EGTs drop...as does power. IRRC we ran 75' rich of Peak Egt? Its been ages. since ive burned Avgas.

My "aircraft Powerplants" books is still on my shelf here...scanned this.

mark kibort

Good stuff!

But the last paragraph talks about the possiblity of detonation going too lean. however, this directly contradicts the CHT and EGT graphs i posted above.
best fuel economy is lean of stoich and provides the coolest temps.
othewise, the old book is right on.

Mk

BrianG

My Av-gas days suggest that Tony's book is correct, Mark. I recall messing with the mixture control on long X-country trips and I can relate from experience that more power is produced rich of peak EGT. At cruise, in a trimmed aircraft, power output is very apparent. Even a slight increase in power will result in a climb from trimmed, and a small power loss will result in a desent.

The IO-540 I flew behind consistently saw best power at 50 degrees rich of peak EGT.

danglerb

Funny how so many pilots end up in a 928.

I seem to remember that vaporizing fuel is a significant source of cooling within the motor, and the problem with going lean wasn't generated heat as much as less heat absorbed.

Been awhile since I was twisting a throttle instead of stepping on one.

John Veninger

After several hours on the dyno, the stroker seems to like 13.2 best.

worf928

I'll bet that if you could have sensors in the cylinders that it would be closer to 12.6. That figure is what math/theory indicates. But, our ability to verify is based upon how well we can measure. In the end it is the dyno that matters since that's the only way we can measure and tune. So, if your testing says 13.2 is where you make the best power then that's where you should run. But, there's 0.6:1 offset somewhere in your measurements. That's find as long as it is consistent for your motor and not a variation in the devices you are using for measurements.

mark kibort

good point. the O2 sensors at the dyno are usually not calibrated . i had one run at the dyno where the operator knew something was off. my runs were all in the 13.5 to 14:1 range. when they adjusted things, it went back tothe usual 12.5 to 13:1. also, this is an average for all cylinders as was said, and its measured at the tailpipe. (more errors for doing it this way too)

Mk

mark kibort

Brian,

This is true, but not the point. we are not talking about power, we are talking about CHTs and EGTs at leaner than stoich levels. temp can drop MUCH further than just adding fuel (mike's post) and keep a decent power setting, near 80% of max. what this means is there is no need to worry about real lean conditions. its the stoich condition you need to worry about. sure, we all know max power is in the 12.5:1 range. but you can get best fuel ecomony running near 17:1, or just before the engine starts to miss, and then a tad richer. this is the coolest operation for that particular power setting. the curves posted above show this graphically.

Mk

John Veninger

We ran the dyno O2 sensor on one side and my calibrated wide band on the other. Both were within .1 of each other.

Were can I get an O2 sensor that installs in the cyl.?

worf928

Yup. There are two basic reasons why one would see the 'theory' of 12.6:1 as the maximum power point 'breakdown.' The first is errors in measurement due either to poor measurement devices or to inherent inacuracy or variability of the measurement method. The second is physical engine characteristics through either design or degradation.

mark kibort

tapping into the pipe just after the header (not in the hole in the tailpipe ), or mounted in each header tube would work. but since its cause and effect, EGT sensors at each tube could give you the same information.

mk

blown 87

Mark this chart is for 78% power or less (cruise power) we were talking about something different, or at least I thought we were.

Those of us that are or have been recip pilots in small GA planes, I know of no one that runs lean at 100% power, at least not twice.

We both know that you should not run N2O at less than full power.

Maybe that is part of the confusion on my part.

My statement still stands, I have never heard of a street driven , dry type supercharger of supercharger that would be put on a 928, exploding.

I have had and seen more than a few N2O explosions.