John Veninger

Mark,

I have a welded bung on both header collectors. We screwed the dyno's wideband right into the collector.

mark kibort

Just some information we spoke about off line. Keep in mind the entire discussion was running the engine at lean of stoich and analysing the risks, should your NOS system fail to produce fuel flow. reall lean, passed stoich is not a danger, as shown, as chts and egts are at lower temp levels. you cant get 100% power from the engine, due to this NOT being the best power setting. the point is, there is usually no danger runninig real lean as most folks would have you to believe. NOS explosions are due to a gas/NOS fog present in the intake plenum (like a bomb) that gets ignited before it can enter the engine's heads. thats why i like the dead pedal NOS switch. the flow of air is now moving at WOT and then NOS is activated after you let go of the clutch.

MK



"Now, lean of Stoich. Carl Marbach, founder of AVweb writes about the Wright Aeronautical Division (who produced the DC6,DC7, Constellations) recommended these engines be operated on the lean side of peak to lower cht's, AND to avoid the possiblity of dentonation at high power settings. thousands of these planes flew for MILLIONS of hours using lean of peak operation. GAMIjectors has done this too and their engines at overhaul time look better than those that dont use this technique. it makes sense if you can see the CHTs and EGTs of an engine as you go lean of stoich. the temp falls dramatically, and so does power. at near the max lean point, you will get a rough running engine, go a little richer, and you have found the best mix of fuel economy, power and temps. max power will be found at near 12-13:1, but only at stoich (or near it) will you see high cht's, egt's and the propensity for the intake charge to detonate.
Mk

John Veninger

Kibort wrote:
You can get a lot more that 480

mark kibort

then you got it right. we are only talknig about 12.6 to 13.2:1. however, i would be worried about a cylinder that could be closer to the 14:1 range without you knowing it now! i did hours of testing too, and found 12.6 was 3hp better than 12:1 and was also 3hp better than 13.4:1. so, i set it at 12.6:1 and that gives a level of safety. did you know what your EGTs were at that setting? it may not be worth the 5hp you gained if the temps are much higher. plus, things change at the track, under WOT for long durations and higher engine temps.

mk

blown 87

Mark I think we are in agreement on most of this, I really do.

People get into problems, as you have said from being careless with N2O, myself included.

I just feel there are more ways to get into trouble with N2O than there are with a well made and thought out blower system.

When you get into talks about N2O you are talking about max power, right?

Even with a lab grade wide band and a Innovate wide band I am still very afraid of going lean when at 100% power.

What kind of F/A are you running when you are on the dead pedal? Are you rich or lean of 14.7-1?

worf928

Hold that thought Mark. While it is true that leaner-than-stoich conditions do not generate higher exhaust gas temperatures it is not true that 'there is no need to worry.' Lean combustion can lead to detonation. Detonation is bad. I'm trying to formulate a simple explanation of why and am in the process of checking references. But, essentially:

It isn't the temperature that kills the motor. Detonation - knock - is the effect of the combustion flame speed approaching the speed of sound with the consequent and associated super-high pressure wave inside the cylinder.

docmirror

Folks, As for the stoich results, the perfect mix of aviation grade fuel to Std Temp & Pressure dry air is 0.067. Avgas is made that way, and it always and forever will be(I hope). This is a pretty simple volumetric equation of x moles converting to y and z products of combustion using STP air(62F, 1 Atm). Car gas, is a different animal. It changes quite a bit over the year, and the addition of Ethanol makes things much, much worse. I think the best stoich is around 0.070, but that is open to change of course.

There are variations in efficiencies based on RPM, throttle setting, cam profile, flame front, and a host of other engine parameters that make a difference in the Brake Specific Fuel Consumption. Aircraft engine mfg don't like lean of peak for the most part because the potential for disaster is exposed, and the added efficiency when gas was 59 cents/gallon(1970) is only a few gallons per hour.

Now that avgas is in the $4-5/gallon region, lean-of-peak operations have made a comeback. As stated above it's been known about and done for a long time on big radials and also on fighters in WWII. Lindbergh went to the Pacific front to teach the pilots there about lean-of-peak so that the planes could get from airstrips in the Solomons to the Japanese lines and return with fuel to spare.

Hard to accept truths:Once peak EGT is reached, leaning the engine further will result in cooler operation. HP is HP, namely, that 300HP generated at LOP is the same as 300HP generated ROP. Detonation cannot happen lean-of-peak(in a clean combustion chamber). Detonation and/or pre-ignition can occur rich of peak to peak EGT. Highest internal cylinder pressures occur about 50-75F rich of peak EGT. This is the region that most NOS systems are set to run at. This also results in highest CHT, and most blow-by.


After taking a few classes in LOP operation, and flying a Continental plane LOP for quite while with six-probe EGT, I'm sold on LOP operations for aircraft if done correctly. For cars, it's a different ballgame. Not a lot of 928s are run in the 55-75% rated power category. Most cars actually run a variance program on the stock fuel injection system to accommodate the cats. Next time you're at the emission station you might be able to detect it, but some cars have a long variance period that may not be captured by the machine. It seems, the life of cats can be extended significantly if the F/A ratio is varied around peak EGT to actually coat, and then 'burn-off' the excess products of combustion. The variance is supposedly quite small, but helps greatly in cat life. Someone with more experience in cats can chime in with better info.

Also, many cars, including the 928 have a 'characterization' period when the injection control actually learns the normalization of the sensors. Some O2 probes are not that well calibrated, mass air sensors also aren't perfect, and temp sensors although fairly accurate have to accommodate both Wisconsin(-31 last night) and Aledo, Texas(+111 on Aug 26). If you disconnect the battery, then reconnect it, and watch your fuel economy, you can see the characterization at work. Economy will start out marginally bad, and slowly increase as the brain 'learns' the profile of the various sensors in the car.

In my plane, I operate regularly at 60% power, or 138HP and burn right at 8.8 gallon per hour. This provides 0.383 Lb/HP/Hour of fuel LOP(better than Continental says it will do). the best I can do rich-of-peak(ROP) is 172HP and fuel consumption of just under 13 gallons/hour for BSFC of 0.443. when there are no speed limits, your speed is basically a function of how much do you want to spend? The higher HP will get me there sooner, but only save about 30 minutes on a six hour stage. To me this proves the old adage that time really is money.

<edited for spelling>

Doc 90GT, Beech Bonanza B35

John Veninger

3-5hp is "noise" on the dyno at my power levels.

FlyingDog

The stoichiometric ratio is based on the fuel blend and air blend. It has nothing to do with what engine you are running.

blown 87

That is correct, but for most of us running street gasoline it is close to 14.7-1

Nirtomethane is about 1.7-1 if I remmember right.

worf928

Docmirror's description of lean operation is from the context of steady cruise. Or in other words - the engine load is constant or not changing quickly. The automotive context is more dynamic yet Doc's comments still apply for steady-state operation. The difference is that Lindbergh's pupils didn't have to worry about the EPA sniffing their tail pipes. Lean combustion produces more NOx than Stoich. Since the EPA doesn't like NOx, the compromise - assuming the original technology of our 928s - for emissions is to run at Stoich.

FlyingDog

Ethanol is 6 or 7 which is (part of) why 10% blended ethanol fueled cars burn more gasoline... so much for less dependance on the Middle East.

docmirror

yep, some guy at Univ of Wash did a study on the life cycle cost of sowing, growing, cultivating, processing, and distributing one gallon of Ethanol to the distribution point. IIRC, it came to 1.6 gallons of dino fuel to 'make' that one gal of Ethanol. Geeeeeezzzzzz. Pilots and boaters really hate the stuff. It does very bad things to vintage fuel systems.

Doc

a4sfed928

I thought the reason EGT temps drop above 14.7 was the fact that light detonation was starting to occur disturbing the combustion chamber boundry layer. This disruption of the boundry layer removes its insulating properties and therefore more heat is transferred into the surrounding metal thust removing heat from the spent gases end result lower EGT temps.....Scotty shields up!

mark kibort

You missed the point here. the ratio for stoich doesnt change, but the effect can. think about it. 50% part throttle, a 84S can run at stoich and not even ping, let alone create a dangerous and destructive high temp, that might be at 120hp level. Then, you go full throttle, and suddenly stoich can be an issue at 240hp. now, increase displacement, add double the amount of fuel and air , double the HP to 500, and now you have a much more sensitive situation to stoich. the reasons our stock 928s last so much, is that the forces even at WOT, are like running a stroker at 50% throttle. the power capacity of the 928 engine is way under rated and restricted by design.

Mk