The octane thread below has my tumblers jumbleing a bit. If I was to tune my vette to take advantage of 100 octane, what would be the possible upside? Car is in mid 400's at the wheels now. How driveable would it be if it was tuned for 100+ octane and there was only 93 available. Not my daily driver, so having to park it for a while wouldn't reduce me to pedestrian status.

 

You would make some extra power, but if you're at mid 400s WHP, there won't be *that* much to be gained; a tuner would be able to lean the air/fuel ratio towards 13:1 for SOME power... Maybe another 10-15WHP? (A look at your AFR ratio now will tell the whole story)
Timing will also be advanced, but not as much because it can, but also because it must; the high octane fuel burns slower, and without a global timing advance, the engine will feel more sluggish and less responsive on High Octane until that happens. So, on the upside, you make a little bit more power and you do it burning a little bit less fuel. But it will do that burning MUCH more expensive fuel (what is 100 octane at now; $6/gal?).
Now, when you put 93 octane in there, it will detonate, and it will detonate a lot. Eventually the engine will pull back timing until that stops, assuming the low octane map accomodates the change. The trouble with that though is that it can't learn UNTIL it detonates. Given that factory LSX engines all run Hypereutectic cast pistons, which are notoriously brittle, you could not pay me to purposely cause detonation on my motor... But would it work? Yeah, it would, for some time.

Now, if you want to do this the right way, you could get yourself a small laptop and load it up with HPTuners; you could keep a very agressive map in it for pure high octane racing usage, and another one for pump gas... Even 87 octane if you wanted to, and you could flash the ECU between the two...
A lot of imports have the option to run a little gizmo called an "Accessport", by a company called "COBB". The AP stores multiple (a dozen +) maps in it and can flash them on the fly; race gas maps, pump gas maps, valet mode, stock tune, etc... Unfortunately we don't have that for our cars, but the laptop with HP Tuners is the next best thing...

 

I've had programmers on my last two diesel trucks. Plug it in, choose how much extra power you want (current one for my duramax goes from 20 to 120 added hp), wait a few seconds for it to download, and you're ready to go. Something similar to that for the vette with a few mission specific tunes would sure be nice. I'm no high tech redneck, so teaching this old dog how to run HP Tuners might be reaching a little too high. Plugging and playing I can do.

 

Sam, I'm going to take you to task a little bit because it sounds like you're a little out of your area of expertise AND the "high octane/slower burn" BS is one of my pet peeves that I've posted about many times with multiple links in an effort to dispel this "old wives tale". I'll start by saying the octane rating of a fuel has no correlation to the burn rate of a fuel or the flame speed in the combustion chamber. In fact, a slower burning fuel increases the tendancy of a fuel to detonate because as the flame front travels across the combustion chamber and the pressure increases, the temperature of the end gases has a longer period of time to increase to the self-ignition temperature increasing the chance of another flame front starting resulting in detonation. This is counter to the goals of a high octane fuel. Rockett Brand has a 118 octane fuel formulated to burn very fast that's used in NHRA Pro Stock because the 500 cu in engines require the fast flame speed due to the big bore (think over 4.6") and 10,000 RPM engine speeds. NHRA Stock and Super Stock racers use the same 118 octane fuel to take advantage of the faster flame speed to REDUCE timing and make more HP. How??? Because by starting the combustion event later in the pistons' compression stroke, the negative work done on the piston is reduced but the faster flame speed enables peak cylinder pressure to still occur at the optimum 11-15 degrees ATDC. So while we have the same BTU content, the same lbs/hr of fuel going into the engine, and the same IMEP, we've improved BSFC by reducing the height of the curve during the compression stroke on the pressure ordinate of the PV diagram which reduces FHP (BHP=IMEP-FHP)...in laymans terms, the engine makes more HP.
http://www.repairfaq.org/filipg/AUTO...l#GASOLINE_001
From the end of paragraph 6.3:
"The antiknock ability is related to the "autoignition temperature" of the hydrocarbons. Antiknock ability is _not_ substantially related to:


1) The energy content of fuel, this should be obvious, as oxygenates have lower energy contents, but high octanes.
2) The flame speed of the conventionally ignited mixture, this should be evident from the similarities of the two reference hydrocarbons. Although flame speed does play a minor part, there are many other factors that are far more important. ( such as compression ratio, stoichiometry, combustion chamber shape, chemical structure of the fuel, presence of antiknock additives, number and position of spark plugs, turbulence etc.) Flame speed does not correlate with octane."

Expanding on the discussion of the flame speed of the 118 octane fuel above as related to timing, just because you run higher octane fuel does not mean you can make more HP by advancing ignition timing. If you already are at best timing for peak cylinder pressure at the optimum crank angle with 91 octane fuel, running 100 octane may allow more ignition timing but you'll make less HP. Higher octane fuel is not always the path to more HP in a given engine.

I've seen you post several times about never letting the engine get to the point of knock/detonation because the piston will fail or never use lower than 93 octane gas because even one KR event will cause engine damage. This is like "Chicken Little" running around saying "The sky's falling, the sky's falling". There are many forms of knock ranging from "good" to "inconsequential" to "destructive". Yes, there are some forms of knock that are good...it's often referred to as "ping" or "light knock" and occurs just as all the end gas is about to be consumed. A new flame front is started but doesn't have enough time to grow and cause a large collision with the flame front associated with normal combustion. The most efficient form of combustion would be if all the gas was to burn at once after the piston crosses top dead center but that would be explosive in nature and the metal parts would not survive the temperatures and pressures associated with that type of combustion. "Ping" or "light knock" is the closest we can get to the most efficient combustion without causing damage to the engine. The knock sensors are really accelerometers/vibration sensors that measure the frequency at which the engine block is vibrating at. The frequency associated with ping/light knock causes the ECM to start pulling timing. Even if you get heavy knock/detonation, hypereutectic cast pistons can tolerate a few in succession with no damage. While pressure is ultimately what causes failure, the problem is more related to heat than pressure. During normal combustion, there is a boundary layer at the surface of the piston top and the combustion chamber that protects the aluminum that melts at ~1200*F from the 1500+*F temperature of the flame. During abnormal combustion, the "shock waves" from the collision "scrub" the walls of the combustion chamber and the top of the piston and eliminate the boundary layer. That's why sustained detonation will cause HP loss and overheating...because there is more heat being transferred to the coolant through the walls of the combustion chamber. On the piston side, there is no coolant to carry the extra heat away and the temperature of the piston increases...as the temperature increases, strength decreases to the point that the piston fails due to the pressure. Hypereutectic cast pistons are more susceptible to this failure mode than forged pistons due to the vastly different thermodynamic properties of the two pistons. The top of the hypereutectic piston runs much hotter (about 550*F) than the top of the forged piston (about 450*F) due to its' inability to transfer as much heat as a forged piston.
http://www.mre-books.com/sa21/sa21_5.html
The lower heat transfer rate combined with the higher temperature at which the hypereutectic piston operates at is what makes them much less tolerant to sustained detonation events than forged pistons, but they will still survive a few hits before KR kicks in.

PS Not all LSx engines have hypereutectic cast pistons...the LS9 and LSA have forged pistons.

 

Thank you, thank you, thank you. This should be made a sticky. Everyone should ink to this post whenever anyone talks about octane and flame speed.

 

Glass_slipper, you know you are the one person I respect the most on these forums, so please allow me to show you where I got that "BS" from just so you know I am not pulling it out of my hat, so to speak. I may well be out of my area of expertise; my M.E. Degree included only one class specific to Internal Combustion engines and it was by no means focused on combustion dynamics. THAT SAID, I don't make stuff up, so here is my source for what I said:

"Engine Management: Advanced Tuning", by Greg Banish, BSME, professional Engine Tuner for both OEM vehicles and performance shops.
Chapter 7 on "Ignition", page 58:

"Fuel Octane is a number representing the ratio of Octane to Heptane in Gasoline. Higher octane numbers indicate a stronger concentration of the slower, more stable-burning octane molecules than the more volatile heptane. Since high-octane fuels have a slower burn rate, larger amounts of ignition lead can and should be used. Increasing the ignition lead results in an increase in torque, so higher fuel octane levels allow for more power resulting from the necessary ignition advance increase.
Keep in mind that fuel octane is a global change to the engine's ignition advance needs; Running higher octane fuel means more spark advance at idle and part load as well as WOT. Changing from 93 octane pump fuel to 110 octane racing fuel may allow 4 degrees of additional spark advance at WOT (with an accompanying increase in power) but may idle poorly and exhibit reduced fuel economy and throttle response unless all other spark tables are increased by a like amount."

If you've ever read any of Corky Bell's books ("Maximum Boost" and "Supercharged!", or A. Bells' "Forced Induction Performance Tuning", or Jeff Hartman's "Engine Management Performance Tuning", you will find all of them say something along the lines of higher octane fuels requiring more spark advance. I'm out of town right now, but if you would like I can post some relevant paragraphs when I return to my company's engine research laboratory on Monday.
You are right; time and money constraints mean I do not have first hand experience on this, so unfortunately I am limited by what I can learn from books and from others. All the tuners I spoke with mirrored the books in saying higher octane fuels burn slower and require more ignition timing. If you happen to know of any good books on the subject that you could suggest to me, please do; this is one of my favorite subjects and I would be more than happy to learn more about it. Again, not trying to contradict what you are saying; just telling you where I got my info from


This is very good info. To be honest with you I never really think of fuel in terms of combustion velocity; I take Octane rating to be what it is tested to be: a measure of a fuel's resistance to detonation. With that knowledge, it makes sense to me that with more resistance to detonation you would be able to run more timing and make more power since MOST engines are knock limited well before MBT... Clearly that is not the case for all engines; thank you for pointing that out.

Now show me a 10.9:1 compression engine that can run best timing with 91 octane fuel... I know on my car alone a switch from 91 to 93 octane fuel would lead to 10-15HP gain because it was knock limited on 91 octane fuel.

Yes, but the knock I am talking about is full throttle, knock. You are correct in pointing out that some pinging is acceptable and even tuned for under very low load conditions, but knock occurring at full throttle and full power can get very destructive, very quickly. I resent the chicken little analogy; I have lost 3 engines due to knock; all 3 were professionally tuned on a load bearing dyno, and all 3 times by the time I realized knock was occuring it was too late, despite knock monitoring, ECU controlled knock retard, etc... Maybe I am too cautious, but when it comes to my cars I always tend to err on the side of caution because of much much it costs every time you blow a motor...

All knock sensors I have seen were piezo-electric transducers; I.E. A microphone bolted on to the engine block.


Interesting. If this was the case, then it seems like running a ceramic coating thermal barrier would make Hypereutectic pistons less prone to failure. A lot of the breakages I see though are shattered pistons; in Hypereutectic castings the percentage of silicon in the metal is so high as to make it very brittle; it was my understanding that these pistons tend to break apart when subjected to detonation. Either way, point taken. I guess it all depends on how much detonation you are willing to accept in your engine; I will wholeheartedly admit that any motor can tolerate some before anything happens, but at the same time I know that it doesn't take too much to damage bearings, crack ring lands or blow head gaskets, and since my cars are all daily drivers, my posts reflect a much more conservative stance towards things that can potentially lead to their destruction. I really appreciate your input, feel free to correct me any time; you know a lot and I am happy to learn from you, but please no chicken little analogies next time


I knew that (I mentioned it in another thread where the OP said the LS9 would never make over 600whp because it had cast pistons), but thanks for pointing it out

 

Whew....good stuff guys. I too have lost an engine from knock. I thought there would be some warning or perhaps the PCM would pull timing and protect me a little. Not so in my case. Never heard it or felt it, just ended up having to bore a turboed 5.3L to 5.7. Went with forged pistons this time around though.