Diablosport Forums

My gearhead mentality goes back to the days of playing with ancient, highly tuned BMW's with mechanical distributors and crazy complicated sidedraft carbs. Then I fell out of it for decades, now to return to the world of programable tuning - amazing and wonderful!

Anyway, here's my topic:
On this board there have been a number of posts that the high octane tunes deliver more power but don't do much (if anything) for mileage compared to the 87 tune. This seems oddly counter-intuitive to me. Back in the day you'd run the highest octane you could find and then start fiddling with spark advance. If you wanted balls-to-the-wall performance you'd find the max advance under WOT that doesn't knock. If you wanted mileage you might go even farther but just keep your foot light enough to avoid knock - no WOT, no tire chirping, etc.

There's lots of discussion on high octane tunes here about advancing spark, reading the knock sensor etc. But it always seems to be about maximizing power, chirping tires, 1/4 mile times and stuff like that.

So I guess the question is, does the power gain from more advance come from greater efficiency? ( Or, if not, then where does it come from?) And if so, wouldn't greater efficiency also translate into better mileage, just like in the old days, if the driver is trying to be maximize mileage?

...which leads me to the knock sensor. I used to have a Saab 9000 turbo. Supposedly the computer on that was always adjusting timing to suit the driving conditions based on knock detection - low load, part throttle and it would be way advanced for economy. You could put in any gas you wanted and not worry. But with 93 octane the thing was a rocket. So isn't the purpose to detect knock and back off timing a little to the point that the knock disappears? If so, then wouldn't the optimum mileage setting be the point where there's occassional knock detection/correction under load? In the old days if you knocked going up a hill you just backed off the gas a little. Reading between the lines it sounds like the GM computer isn't dynamically changing timing like the old Saab. But that's hard to believe....

Sorry this is long, but the more I learn the more I don't understand and it drives me nuts. I think this is fundamental stuff to understanding what I'm doing. Maybe it will help some other folks too. Thanks in >>advance<<
for any comments or clarifications!

you will still get better mileage with the higher octane tunes than stock...

and once you were to get your fueling dialed in correctly, you would get even more....

I went from 18.5 mpg to 21.5mpg in my Monte SS... that's with a 93 octane tune and getting my fueling adjusted correctly...

here's info on getting your injector slope adjusted correctly in order to get your LTFT's dialed in to the correct range of 0 to -2:

http://diablosport.com/index.php?name=P ... opic&t=917

I have a perfect example. I was towing the trailer with 91 octane in. Getting 11.5 average MPG (according to truck compuer). I was forced to fill with 87 octane (only option in the back woods). My average dropped to 9.5, the instant mileage was down to about 7-8.

raueda1 wrote:

So I guess the question is, does the power gain from more advance come from greater efficiency? ( Or, if not, then where does it come from?) And if so, wouldn't greater efficiency also translate into better mileage, just like in the old days, if the driver is trying to be maximize mileage?

I guess the basic answer is: let's assume that the heat release of the fuel and the flame speed remain relatively constant, for this discussion. One can assume that the amount of temperature rise resulting from burning a specific mass of fuel within the cylinder volume (volume of the charge) is relatively constant. Fuel gives you roughly 18,000-20000 btu/lbm. It is a property of the fuel. Thus, the temp rise is what it is for a given mass.

However, if the spark comes in LATER (when the combustion volume is smaller) the resulting pressure rise due to the fuel's heat release (temp rise) will be higher (smaller volume). This results in a higher pressure, and, thus, a higher combustion efficiency, and, of course more power.

Therefore, if you are always injecting the same amount of fuel, only setting the charge off at a later time, the mpg will remain relatively constant, but producing more power. This, of course, assumes that the fuel doesn't experience pre-detonation (knock). To avoid knock, a higher octane fuel is required, which allows you to compress the mixture MORE prior to spark w/o pre-detonation.

Now, your engine doesn't run WOT all the time. If the control is dialing a given POWER for a given condition via the throttle opening, the result would be less required throttle at a given condition (as you are producing more power with advanced timing).. and maybe the NET result is a mileage improvement.

For sure there are dozens of variables and this thought here is simplistic. But, on the big picture scope of things, MORE specific power, HIGHER efficiency with an advanced spark tune.

Maybe this helps a bit as a "thinking it thru" explanation.

EXACTLY what I was thinkin'...

Grumpy must be a damn engineer! Those guys make my life hell sometimes!

05Z71 wrote:Grumpy must be a damn engineer! Those guys make my life hell sometimes!

don't spread viscous rumors like that...ESPECIALLY if they're true.....

Grumpy_Aero_Guy wrote:

raueda1 wrote:
>>>>>snip<<<>

Maybe this helps a bit as a "thinking it thru" explanation.

YES!! I suffer from an engineering/chemistry background so the theory appeals to me. It also helps me figure out what I want to do -- if I go with a custom tune (which I certainly will) then I'd like to be clear on what the objectives are and the tradeoffs are. This helps me wrap my mind around it. Thanks, big help.

I don't know what Grumpy just explained but here's mine:
I drive a 2005 Corvette C6 with an LS2 motor. I am averaging now 21-23.8 mpg and getting between 26-28 mpg on the highway cruising at 80mph.
I am running the 93 octane performance tune and have dialed in my LTFT's and my PE's thanks to Lew.
How's that? This ain't no economy car.
I even changed my fan settings and am keeping my vette cooler so she lasts a long time.
I love my Predator.

justasheet wrote:I don't know what Grumpy just explained but here's mine:
I drive a 2005 Corvette C6 with an LS2 motor. I am averaging now 21-23.8 mpg and getting between 26-28 mpg on the highway cruising at 80mph.
I am running the 93 octane performance tune and have dialed in my LTFT's and my PE's thanks to Lew.
How's that? This ain't no economy car.
I even changed my fan settings and am keeping my vette cooler so she lasts a long time.
I love my Predator.

All good. Economy car, maybe not. But, the reality is, at 80mph cruise in a skateboard with a 6.2l, that engine is barely fainting a yawn. What are you turning at 80, 2100 RPM?

Grumpy_Aero_Guy wrote:

justasheet wrote:I don't know what Grumpy just explained but here's mine:
I drive a 2005 Corvette C6 with an LS2 motor. I am averaging now 21-23.8 mpg and getting between 26-28 mpg on the highway cruising at 80mph.
I am running the 93 octane performance tune and have dialed in my LTFT's and my PE's thanks to Lew.
How's that? This ain't no economy car.
I even changed my fan settings and am keeping my vette cooler so she lasts a long time.
I love my Predator.

All good. Economy car, maybe not. But, the reality is, at 80mph cruise in a skateboard with a 6.2l, that engine is barely fainting a yawn. What are you turning at 80, 2100 RPM?

Yep, 2100rpm and it is a 6.0l motor, lol. You are right, but none the less, my 2.4l Solstice never saw this performance or mpg nor do many vehicles that have a V8.

Come on, give credit where credit is due. The Corvette is a well engineered AMERICAN auto and the Predator high octane tune added another 3mpg to it's consumption along with a kick in the ass on full acceleration.

You've got a happy camper here so Grumpy, stop being so grumpy.
Jeff

I think when you get everything dialed in and the engine is running its best giving you all that nice power, some people will drive harder which ends up costing more gas then if they had all that power and just drove "normal". I'm one of those people who drives harder then they should at times but I still get better mpg with the 93 tune then with out it and the truck is sure a lot more fun to drive.

So my gigantic Av gets about the same gas mileage as a vette? WOOT.

Edit: Because someone was gonna ask; my 5.3L 2007 Avalanche gets 20-21 MPG, 22-23 with perfect throttle on a straight, flat road. And that's on the 87 octane tune running 89 octane.

I'm getting my annual birthday "time to play" fillup of 93 octane next week tho

Hopefully after I get Cammed and Headed sometime within the next year or so I'll pull off a solid 23-25 MPG so that after I get the dubs put on her it'll only drop to 21-22 and I can still piss some people off when I tell em I get equal or better gas mileage than their cars

justasheet wrote:

Grumpy_Aero_Guy wrote:

justasheet wrote:I don't know what Grumpy just explained but here's mine:
I drive a 2005 Corvette C6 with an LS2 motor. I am averaging now 21-23.8 mpg and getting between 26-28 mpg on the highway cruising at 80mph.
I am running the 93 octane performance tune and have dialed in my LTFT's and my PE's thanks to Lew.
How's that? This ain't no economy car.
I even changed my fan settings and am keeping my vette cooler so she lasts a long time.
I love my Predator.

All good. Economy car, maybe not. But, the reality is, at 80mph cruise in a skateboard with a 6.2l, that engine is barely fainting a yawn. What are you turning at 80, 2100 RPM?

Yep, 2100rpm and it is a 6.0l motor, lol. You are right, but none the less, my 2.4l Solstice never saw this performance or mpg nor do many vehicles that have a V8.

Come on, give credit where credit is due. The Corvette is a well engineered AMERICAN auto and the Predator high octane tune added another 3mpg to it's consumption along with a kick in the ass on full acceleration.

You've got a happy camper here so Grumpy, stop being so grumpy.
Jeff

Wasn't trying to be grumpy. Never implied that it wasn't a good car. Just making the point that with that Power/Weight Ratio and the low Cd, it will get good MPG. I find it almost amazing that it can do that.

As far as your solstice, that engine is cranking much harder to maintain speed and doesn't have to Torque of your V8.

Why do you think I was not giving credit? I mean, I drive an american truck, and wouldn't have it any other way. My truck averages 15mpg city. For what it is, I find that remarkable. I have gotten 20mpg on the highway in a rolling barn door. Doesn't get any better than that.

I was just making the point that your V8 is just sitting there laughing at you cruising at 80. No criticism intended.