wspohn said:
1 - unless you have a modern engine that has 'search forward' programming, that is the ignition advances until it 'hears' light ping before stopping,
That's not the way it works, at least not in any computer I've played around in. There is a map for mean best torque, and the computer uses that as a hard limit for advance. There is not only no gain to advancing past MBT, the engine actually makes less power, even if it's not detonating. So, a well calibrated system won't go past that point no matter what.
If your engine can run at MBT on 87, it is not going to benefit from higher octane fuels. My S60R is one of these.... I get slightly better fuel economy on 87 than 93. Have touched 30mpg on 87, barely make it to 28 on 93. (I'm sure having only 8.5:1 compression is the main factor... you probably couldn't get it to detonate under cruise if you tried) The real gain is saving roughly 20% at the pump.
OTOH, I've had cars that got wildly better economy on 93. As with anything, if you aren't testing, you don't know.
Knurled. said:
wspohn said:
1 - unless you have a modern engine that has 'search forward' programming, that is the ignition advances until it 'hears' light ping before stopping,
That's not the way it works, at least not in any computer I've played around in. There is a map for mean best torque, and the computer uses that as a hard limit for advance. There is not only no gain to advancing past MBT, the engine actually makes less power, even if it's not detonating. So, a well calibrated system won't go past that point no matter what.
A lot of modern cars are kind of half way in between.
For sure, there's a MBT spark table for every engine out there. Most engines have a based spark table for 87, and then will use the knock detection to advance to MBT if it's ok. It will add spark really slowly, but take it back out really quickly.
wspohn said:
3 - although in is counter-intuitive that intake valve deposits would be a factor on DI engines where the fuel charge goes straight into the cylinder, in fact it is a significant problem and requires periodic rectification with shell blasting on some DI engines. Detergents in fuel bypass the valves and spray treatments are usually counter indicated as detrimental to turbo seals. See https://www.aa1car.com/library/intake_valve_deposits_gdi_engines.htm for sources of the deposits
Not every DI engine has intake system deposits, and some are a lot worse than others. Oddly enough, as i've seen competitive data, the same OEM's that struggled with intake system deposits back in the 80s are the same ones today with DI.
In reply to alfadriver :
GM looks at the fuel level sender to increase by at least a certain percentage between key-off and key-on, they they will start out at the high octane maps. If detonation passes a certain threshold, it will switch to the low octane maps and will stay there until the next detected refueling event.
This actually caused them some problems early on in this strategy because they didn't account for people who left the car running while they refueled. I forget the term they had for it but it was charmingly pretentious like "mid flight refueling". So, they had to rewrite the code to accomodate it while also accounting for normal sender motion due to fuel slosh.
Anyway, it always made me wonder if one could refill with 87, then GINGERLY accelerate out onto the Interstate and run that tank of 87 out at light throttle, without triggering the low octane maps. Never had long-trip access to a GM (that wasn't loaded down with a trailer) to try it.
In reply to Knurled. :
Interesting- to me, if you start at the high octane assumption, you also have to label the fuel filler "high octane recommended"- which goes ok for Caddilac and Lincoln, but not GM or Ford.
In reply to alfadriver :
The way I interpret the way that they do things, everything has a high octane and low octane map, even if the vehicle is rated for 87.
I don't know if this means 91 vs 87, or 87 vs. Bob's Route 66 Mystery Dinosaur Squeezins.
On the truck tunes, the difference is usually only two or three degrees, incidentally. Nevertheless, it's important for someone who is doing an engine swap to remember to tune the "high octane" map, AND copy all that data to the "low octane", since if you are not implementing the fuel level sender, you don't know what map the PCM is using, so you tune for good fuel and let the knock sensor sort it out. Which is not IDEAL, of course.
Hmmm..... For S&Gs, I just dusted off my old version of HPT and loaded one of the tunes that they give you to poke around in. (I no longer have access to the tune repository, or the wealth of different tunes that I'd personally scanned in, so this is all I can see) Looking at the tune for an '02 Camaro LS1. The main low octane table is generally 5-8 degrees less advanced than the high octane. But the confusing part, to me, is that the high octane table is also somewhat higher in many areas than the "Max Torque Timing" table.
I love it when things don't work the way I thought they did. Means I get to learn something new.
