The owners manual recommends the use of mid-grade gas when towing. I was wondering if anyone has tried the recommendation and if they noticed any difference.

everyone should use higher octane in summer or you are throwing money away

I saw in a different thread that altitude decreases the likelihood of any mileage increase with higher octane fuels. This seems to hold true for me, at ~6,000 feet; I average 20 mp/g with 85, and seems to average the same with 87. I just don't drop MP/G for the usual reasons as much with 87 (hard accelerations, stop and go, engaging 4-wheel drive).

I may not be a bad idea but more costly. These KJ's timing are tuned for about 85-87 octane from the factory (I think) so you may be wasting your money. I haven't noticed a difference in mine at all.

I towed a 3k trailer using reg unleaded (87 oct here in Houston) and it did just fine.

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2004 KJ Sport 4x4
L.O.S.T. #006901
Rusty's 2.5" Coil Lift & Diff Skid, 245/75-16 REVO's, K&N
****Grocery Rated****

wasting your money unless your engine is tuned for it, which from the factory it is not. If you have a aftermarket chip, then it probably is. Higher octane usually is only required in higher compression engines.

The KJ is rated at 87 I believe... use anything higher it's a waste of money, try it you won't see any gains - actually you may see a reduce MPG.

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2004 Silver Liberty Sport 4x4 (gone)
1996 Land Rover Disco I
L.O.S.T #000522 Life Time #JH053535

I know the KJ is tuned for 87-85 depending on the altitude, BUT the owners manual recommends midgrade when towing. I just curious on why

Maybe the extra strain on the engine from towing is enough to heat the cylinders just to the point where the extra octane helps the fuel detonate at the right time?

I know some fuel injected/computerized engines can adjust the spark depending on the fuel the vehicle is currently running... is the KJ's engine able to do this- maybe the fuel adds more range for the computer to play with?

Just some quick ideas...

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Frankenlift
Boulderbars
All J Tranny skid
31x9.5 SS TSLs!
Detroit Truetrac/Powertrax
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Higher octane fuel doesn't burn hotter; in fact, it burns cooler. Octane has to do with the amount of compression an engine produces not the temperature.

That's my understanding at least.....

Increased temperature increase the rate of most any chemical reaction including gasoline and oxygen.

I pulled a 65 Rustang from Topeka, Kansas to Lewistown, Montana 3 weeks ago using the low grade fuel most of the way. On the tanks that i did try mid grade, i couldn't tell any difference in how it pulled, operating temperature, and the mileage was the same. (calculated, but evic was within .5 mpg. and i checked each time i filled up)

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'04 Renegade
Many mods on,
Many mods to come.

Eddo - the higher ambient temperature would certainly bring the fuel-air mix to its flash point faster but the flash point is the flash point; for any given liquid the flash point temperature is a constant. No?

Again...octane is specifically to do with the amount of compression and the fuel's resistance to premature detonation (knocking). Higher octane fuels do not contain more energy than lower octane fuels.

Any ASE people on this board? Bueller? Bueller?

Well, flash point is flash point. But the burning of fuel and air would happen more quickly. Even though the burning of the fuel and air seems to happen instantaneously it is still has a rate and the rate is temperature depended. Hence the flame front would move down the cylinder quicker and possibly hit the piston before it reaches top dead center. A higher octane fuel actually burns slower.

I guess technically higher octane fuels could contain a slight bit larger amount of energy since it on average it has a slightly higher molecular weight(the reason it burns slower too), but probably not enough to make a difference. Also power is the amount of work accomplished in a set time. It requires energy to do work, but if it takes all day to accomplish then less power is produced.

Now how this effects towing, I'm not quite sure.

Yeah, I knew about higher octane having the slower (cooler) burn rate. And it makes sense I guess about the flash point being constant but the burn time being decreased by higher ambient temp......interesting....

Folks,

I'm only talking about running higher octane in (hot temps). Not winter time, not at 6K feet or any other altitude where air is cooler or oxygen is thinner. Cooler air does make a diff in the way gas burns, the few points higher octane does make a diff in summertime.

I'd say ONLY if during the summer (or when towing for that matter since that's what started the thread) you begin to notice ping or knock. Using higher-octane fuel when it's not needed will cause its own problems such as carbon build-up.

Here's a thought: could putting a heavier load on the motor temporarily alter the timing? 'Cause if that's the case......hhmmm.....

But doesn't the computer retard the timing if it receives a signal from the knock sensor. So it doesn't really matter what grade of gas you run, your computer will probably just turn down the timing a degree or two to compensate.

The octane level of the fuel helps determine when the gas is ignited. The higher the octane, the more pressure/heat the fuel can withstand before it ignites.

So, when the engine has an extra load on it, there is more heat generated because the engine is working harder: more resistance etc. Couple that with the fact that the air is warmer in the summer, and you have higher internal temperatures which make fuel more likely to detonate at a certain temperature and pressure. The computer can only adjust so far to compensate for the environmental variables.

If the KJ's computer compensates enough for knocking, it might compensate right out of the effective (or efficient range) of the low octane fuel, whereas using a higher octane fuel allows the computer to maintain the efficient balance since the higher octane fuel is less resistant to igniting prematurely due to the heat at a certain compression.

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Frankenlift
Boulderbars
All J Tranny skid
31x9.5 SS TSLs!
Detroit Truetrac/Powertrax
http://www.cardomain.com/ride/2062875

Calvin - that makes every bit of sense!

If the fuel and air mixture is detonating pre-maturely (prior to spark) as in the case described above, then the computer can't do a thing about it because all it can control is the spark.

However, normally the fuel air mixture does not start to burn till it is ignited w/ the spark plug. Its not a diesel engine. Yes we have all seen cases of run-on with the older carburated cars, but that detonation could be happening after the spark plug would have normally ignited the mixture.

At high RPMs you can has as much as 30 degrees of spark advance. This means you ignite the mixture when the crankshaft is still 30 degrees away from pushing the piston to its highest point (top dead center). You actually must ignite the fuel/air mixture prior to TDC (top dead center) otherwise the flame front will be chasing the piston down and you lose a lot of power. This is why timing is advanced(igniting sooner) with RPM because the piston is now moving faster and you must ignite it sooner so they meet as close to TDC as possible.

The knocking you hear is the flame front meeting the piston before it is top dead center. It ignited too soon. The piston is still coming up and the flame front is pushing it down. This can cause the piston to shift and make the ping against the cylinder wall.

The thing with higher octane fuel is it burns slower and it less volatile. So it has less of a change of pre-detonationg (igniting before the spark) and when ignited it burns slower. Higher tempatures and pressures can cause the rate of the reaction to increase or the pressure and tempature can be large enough to cause the mixture to ignite by itselft w/o a spark. Either one of these can cause pinging or knocking.

Great thread!