Short answer: higher octane fuel burns slower than low octane fuel, which is not good for HP, most of the time! How much power does "too much" octane cost you? So little it would be difficult to measure.What makes you say it'll run worse?.
Still more control than a carbureted engine has to adjust to knock. Retarding the timing can still have a big effect on knock. I’m not sure if the ECU will alter the amount of fuel flow or not, the throttle cable only allows more air in. Does the ECU control the DBW position, meaning if I floor it and the computer disagrees I’m not getting full throttle?As for running 87 and letting the ECU pull timing, I've always wondered how safe it really is - sure, the ECU will alter the timing (octane scaling), but only after it's sensed knock. And in a drive-by-cable supercharged car where the ECU doesn't control maximum boost pressure OR throttle position, I don't see how the ECU could compensate for preignition if you put lower octane fuel in.
Obviously in a modern DBW + turbocharged car with an ECU controlled wastegate things are quite different since the ECU can play with another variable (boost pressure) in addition to timing.
I agree with you about carbs. It's obviously safer to run a car that can adjust dynamically on lower octane gas than one that can't. I'm just not convinced that it's totally safe or a good idea routinely, since the ECU doesn't know the octane upfront and thus is stuck being reactive, as in knock still has to happen before the ECU can start doing anything about it.Still more control than a carbureted engine has to adjust to knock. Retarding the timing can still have a big effect on knock. I’m not sure if the ECU will alter the amount of fuel flow or not, the throttle cable only allows more air in. Does the ECU control the DBW position, meaning if I floor it and the computer disagrees I’m not getting full throttle?
This might not be true. It depends on the sensor technology used.I'm just not convinced that it's totally safe or a good idea routinely, since the ECU doesn't know the octane upfront and thus is stuck being reactive, as in knock still has to happen before the ECU can start doing anything about it.
That is SWEET and a really cool idea. I had no idea anyone was actually doing that, and from some reading around online I'm not sure anyone other than Saab actually does. They always seemed ahead in engine management - if I recall correctly they also had the first wastegate management / electronic boost control system that would scale boost when it detected knock.
This is NOT true!BTW. ECU can sense knock quickly, but still only after it happens and starts damaging the engine. It slowly returns the timing back and knock and damage happens again! and again! So using 87 when the car was designed/tuned for 91 is possible, but not very good.
Pat,This is NOT true!
While it is true that the ECU does not retard timing until it detects knock, it is not true that this knock is damaging the engine.
During normal combustion, the fuel/air mixture in the cylinder burns at a relatively constant rate from the spark plug out to the cylinder walls. When the timing is advanced too far for the fuel quality/pressure/temperature, then larger portions of the mixture burn simultaneously, which causes a fast rise in cylinder pressure. In some cases this fast rise can cause peak pressure to rise above 'normal' levels. This 'spike' in cylinder pressure is what we refer to as knock. It also causes the cylinder walls to expand quickly and 'ring'. This ring is what the knock sensor is tuned to detect.
This knock does not cause any damage at all unless cylinder pressure gets high enough to exceed the yield strength of one of the components, or pushes down on the conrod hard enough to exceed the film strength of the oil in the bearing. In other words, you have to knock HARD to hurt anything.
The whole point of a knock sensor system is that it detects knock LONG before it gets bad enough to damage anything. In fact, many ECUs are set up to advance the timing until it sees a little bit of knock, regardless of what the base timing is or what fuel you use, so it won't knock any more on 87 than it would 93, it will just run more retarded and make a little less power. Obviously there are limits to how far it can go, which is why it probably can't advance the timing far enough to make use of 100 octane fuel.
That said, I don't know much about the Lotus ECU specifically and how its knock routine is set up, so the easiest thing to do is just run the fuel the car is designed for and call it a day. As mentioned above on an NA car the power difference is negligible anyway.
EDIT: regarding oxygen content, the linked GT260 says it has 3.7% oxygen by weight. Pretty much the whole country is stuck with E10 at the pump now, which is ~3.5% oxygen by weight, so no real difference there. The GT260+ that Phil mentioned above is 4.5%. Without going into the math, that 1% extra oxygen means about 1HP on a 250HP engine, so not much advantage. Every little bit helps though....