I'm still trying to digest the "art" of tuning a truck and have learned from this forum that octane is not a measure of power in a fuel (whereas btu is). Also, it's rather obvious that there are different tunes for different octanes. Could someone explain the advantage of a higher octane tune over a lower octane tune (assuming you're using the proper fuel with the proper tune) ? IOW, I can run an 87 octane with an 87 tune or I can run a 93 octane with a 93 tune at a higher fuel expense. What do I buy in terms of HP and fuel economy with that premium for 93 octane?

 

I will limit my answer the part you are missing and that is hardly ever addressed here.
You need to understand that you cannot change your ignition timing to take advantage of higher octane fuels. This makes you stuck to 87 octane with no gains by using any fuel of higher octane.

What the tuning chip systems do is allow change in ignition timing within the computer so it will render higher octane fuels useful in producing more torque and horsepower with each combination change that is applied to the computer.
There are other changes that are also applied to enhance the performance you are looking for such items as transmission shifting and others, but there are limits to how much extra can be produced.
Taken in combination, these changes are quite noticable.

 

Very well said bluegrass

 

Sorry to sound **** about this but I'm just trying to understand the variables. So the higher torque and HP comes with the higher octane and the ignition timing change, regardless of the btu content, or are you assuming there is an increase in btus with the increase in octane?

 

No icrease in BTU content of the fuel.
Octane rating is a measure of anti knock qualities of the fuel and results in a slower flame, more controlled travel during combustion.
This being the case, the ignition has to start the flame earlier in the compression cycle so peak cylinder pressure ocurrs at the same point as with lower octane fuel.
That point is somewhere between 10 and 20 * after top dead center, of crank/ piston travel.

 

Mostly yes, some kinda-sorta.

For example - while there is not always an automatic increase in BTU content with higher octane pump fuels, it is in fact rather common to have a higher BTU content in some premium fuels over the same brand's lower octane fuels - again, this is not a "global" thing, it's simply something that we do see with some fuels. So no, you don't *automatically* get more BTU's with higher octane - but we do see that does in fact happen in some cases.

With regard to "burn rate" of higher octane fuels - the deal is that it takes longer (both in real time and in number of degrees of crankshaft rotation) to *ignite* the higher octane mixture - meaning to get it to start burning - to *initiate* the combustion event. This is why not only *can* you add more timing with higher octane fuel before experiencing pre-ignition, but for the engine to run correctly with higher octane fuels you *must* use more initial spark lead - so that the engine literally has enough time to burn enough of the A/F mixture. This is why your vehicle's owner manual for these trucks usually tells you not to use premium gas - because in stock trim, the engine simply is not properly tuned for it, and using premium gas in these engine can cause anything from no noticeable symptom (other than a decrease ion MPG n some cases) all the way to very noticeable symptoms of reduced power and even a check engine light on the dash, due to the emissions increasing to more than 50% above the legally allowable limit for that vehicle's model year.

However, once the higher octane mixture is lit (and here we're talking about comparing 87 octane to say, 91 or 93 octane fuels), then the flame front basically propagates itself across the combustion chamber at about the same rate of speed - albeit in sometimes with more stability in that burn rate.

Octane ratings are indeed used as the way to express a fuel's resistance to a specific form of detonation - pre-ignition. Interestingly, that is not determined in a "normal automobile engine" - instead, the octane rating is determined by testing in a specially made variable compression ratio single-cylinder octane rating engine. The compression ratio is increased more & more until pre-ignition occurs - looking for the maximum amount of effective cylinder pressure that can be tolerated on a given fuel prior to onset of pre-ignition.

These are just minor points - don't mean to "nit pick."

PS - There is no point or inherent reward in using significantly more octane than an engine is tuned for - I think that might have been the bottom line that was being wondered about in the original post? By law, all levels of octane coming from the same gas pump are required to have the same basic quantity of detergent additives, for example - though in some cases we will still see some "premium" fuels having a bit better overall effective detergency compared to the same brand's lowest octane fuels.

I hope that helps more than it confuses, & my apologies if it didn't!

 

And to confuse things even further... I was buying 89 Octane at a no name station thinking the higher octane rating equated to better performance and ultimately better gas mileage. The station sold 89 octane as Regular, 90 Octane as Mid-Grade, and 93 octane as Premium, at the same price other stations were selling 87, 89, and 92 octane fuels, so I thought I was getting a good deal!

Then after reading another post expressing good results with gas from 7-Eleven (Citgo), I switched (it was the same price) and guess what... My gas mileage has gone from 15.6 MPG to 16.1 MPG in just one tank! I'm no scientist or engineer, but it sure seems the quality of the fuel your buying weighs in more than the octane rating! At least in this case anyway!

Don't know if this has anything to do with the discussion at hand, just my 2 cents... Thanks for reading...

 

Thanks for your response Mike, you seem to be able to explain things well.<BR><BR>
I'm <b>STILL</b> negotiating on my trade value (but getting closer), but plan on ordering an Excalibrator when we close the deal. I'm trying to figure out what kind of tunes I'd want, and what the various benefits are. From what I gather here, the tune will give me more horsepower, regardless of the octane I'm using (provided the tune be appropriate for the octane). But, the higher the octane, properly tuned, will return even more HP. Since the mileage is a function of btu, not octane, neither the tune, nor the octane will increase mileage, except to the extent the mfg puts a little boost in the btu content with it's more premium fuels. <BR><BR>
With a 10% +/- variation in the cost of fuel between 87 and 91/93 octane, it would seem to me that for a road trip, with nothing in tow, with no load, a tuned 87 octane consuming truck would respond "well" and give me great economy. If I find myself in a situation where I need better performance, such as towing, I bump up to the 91 or 93 tune and buy appropriate fuel. <br><br>
Am I finally getting it?

 

May I suggest getting an 87-octane performance tune, a 93-octane toe-preferred tune and a 93-octane max-performance tune. With these three tunes (four counting your stock 87-octane tune) you will have all your bases covered. I say this because you’ll have an 87-octane & a 93-octane performance tune. Then for towing you’ll have a 93-octane tow-preferred & your factory 87-octane tune which is also very capable of towing. So no matter which octane fuel is in your tank you will have performance and towing capabilities. It only takes a few minutes to switch tunes with the Xcalibrator.

 

That's exactly where I was heading, subject to Mike's suggestions when I order. I just wanted to get a feel for the interplay of the various criteria. I think I may finally be there, lol

 

octane = (R+M)/2

 

I was searching to see if there were any threads that could tell me if one tune is better than another for fuel mileage. I'm running a Superchips 1714 from Troyer's (thanks Mike - LOVE it!). Am I better off running the 87 octane tune, 91 Perf or 91 tow for the best fuel mileage?

I ran 2 tanks with 87, and I'm finishing my second tank of 91 (perf tune). Mileage has remained nearly constant (15.3 with mixed driving), anthough the weather has been changing quite a bit. Does anyone have any good MPG numbers with the 1714?

Thanks!

 

I'm not sure what the point of "nunley_e's" post was now that it has been edited - I didn't see it before it was edited. Currently, it simply shows the method of *expressing* the octane rating we use here in America - which is simply the average of both the Research and the Motor methods, hence the equation (R+M)/2, which can be seen right on the gas pump.

For UnConundrum - (great screen name) Yes, you sure do get the point, and very well! Jpdadeo's suggestion makes a lot of sense, as that setup gives you a program for just about everything (other than maybe an all-out drag strip program, which isn't applicable for most truck owners) and every level of octane - I'd say that would make an *excellent* choice of tunes.

Now me personally, while I do like to spend less on fuel if I reasonably can, I will NOT give up fuel quality or power or performance to try to save a few pennies by dropping back to 87 octane - now that's just *me*. Even if I'm driving 1000 miles straight thru, I want all possible power on tap all the time, and the premium gas tunes also deliver better cruising MPG, too - but then I'm a hot-rodder, and perhaps not really a good representation of how "the average" truck owner feels!

Good luck with your deal,