3.7 87 octane?

hooperswish · **Posted:** Fri Oct 26, 2007 1:25 pm

Hello all,
I can get by with the lowest octane gas available (85 octane here in the higher altitude of Colorado, which is comporable to 87 octane at sea level) and was wondering if anyone runs the more expensive higher octane stuff. My buddies toyota tacoma only likes the midgrade stuff...I have no probs at all with the cheap stuff...how about you?

jerbacher · **Posted:** Fri Oct 26, 2007 1:29 pm

well, the manual recommends 87. i've never used anything else. seems to run fine!

tjkj2002 · **Posted:** Fri Oct 26, 2007 1:30 pm

hooperswish wrote:

Hello all,
I can get by with the lowest octane gas available (85 octane here in the higher altitude of Colorado, which is comporable to 87 octane at sea level) and was wondering if anyone runs the more expensive higher octane stuff. My buddies toyota tacoma only likes the midgrade stuff...I have no probs at all with the cheap stuff...how about you?

Mine knocks like crazy with the 85 octane,does so-so with the 87 as long as I baby it,really likes the premium,and it really really likes the 93-94 octane premium back home is SD.But then again I'm running the JET stage II chip and 1500lbs over stock curb wieght so I need the higher octane fuel.

tommudd · **Posted:** Fri Oct 26, 2007 1:33 pm

mine has always ran well with plain old 87, have tried clear up to 93 but could not really tell any difference

detroit_doc · **Posted:** Fri Oct 26, 2007 1:42 pm

It comes down to how aggressive the calibrations (PCM maps) are. If a computer is not designed to take advantage of higher octane (by running leaner and with more ignition advance), you will not see any improvement.

In my experience Japanese vehichles are much more willing to continually attempt to increase ignition timing and lean the fuel mixture until a knock event occurs. Because of this, there are gains when higher octanes are used. American vehicles tend to be less aggressive and thus there is generally no gains from using a higher octane.

The one caveat to this is a highly loaded vehicle (i.e. towing). So let's say the Jeep PCM will only advance to a maximum of 27 degrees of ignition advance. Under normal load it can reach 27 degrees of ignition advance with 87 octane. So even if you put 91 in it, there is no gain because the PCM will not advance the ignition any more. Now in that same jeep, you hook up a 5000 pound trailer. Now because of the increased load, with 87 octane and 27 degrees of ignition advance you encounter a knock event. The PCM detects the event and keeps backing off the timing (and increasing injector duty cycle) until the knock events stop. You are now making less horsepower. Had you been running 91 octane that knock event would never have occured and you would still be running at 27 degrees of ignition timing.

So in a nutshell.... A jeep will make peak power on 87 octane. However under a large load a knock event may cause the PCM to drop back to more conservative settings causing you to lose some power. under those circumstances, running a higher octane will prevent the knock from occuring preserving the same amount of power normally made on 87 octane in an unloaded vehicle.

BeepBeepJeepJeep · **Posted:** Fri Oct 26, 2007 1:49 pm

mine seemed to make pinging noises under acceleration with 87. I put 89 in it and it seems to have subsided

Pote · **Posted:** Fri Oct 26, 2007 1:53 pm

mine runs great on 87 and use that most of the time. Once this summer when it was in the upper 90's temperature-wise, I filled up with high octane. I did notice in the high heat it ran a bit better and I actually saw a slight bump in MPG. Back to normal temperatures and the premium did nothing in the way of power or gains in efficiency. I'm now running 87 through all the cool weather and it hums along.

_UnLiMiTeD_ · **Posted:** Fri Oct 26, 2007 3:52 pm

detroit_doc wrote:

It comes down to how aggressive the calibrations (PCM maps) are. If a computer is not designed to take advantage of higher octane (by running leaner and with more ignition advance), you will not see any improvement.

In my experience Japanese vehichles are much more willing to continually attempt to increase ignition timing and lean the fuel mixture until a knock event occurs. Because of this, there are gains when higher octanes are used. American vehicles tend to be less aggressive and thus there is generally no gains from using a higher octane.

The one caveat to this is a highly loaded vehicle (i.e. towing). So let's say the Jeep PCM will only advance to a maximum of 27 degrees of ignition advance. Under normal load it can reach 27 degrees of ignition advance with 87 octane. So even if you put 91 in it, there is no gain because the PCM will not advance the ignition any more. Now in that same jeep, you hook up a 5000 pound trailer. Now because of the increased load, with 87 octane and 27 degrees of ignition advance you encounter a knock event. The PCM detects the event and keeps backing off the timing (and increasing injector duty cycle) until the knock events stop. You are now making less horsepower. Had you been running 91 octane that knock event would never have occured and you would still be running at 27 degrees of ignition timing.

So in a nutshell.... A jeep will make peak power on 87 octane. However under a large load a knock event may cause the PCM to drop back to more conservative settings causing you to lose some power. under those circumstances, running a higher octane will prevent the knock from occuring preserving the same amount of power normally made on 87 octane in an unloaded vehicle.

Now that was a great description that even people that are not that mechanically inclinded can understand too.

Very good info thank you!

snowsport · **Joined:** Tue Oct 18, 2005 11:52 pm **Posts:** 1211

^ x2. Thanks!

Jeepjeepster · **Posted:** Fri Oct 26, 2007 9:12 pm

Ive never heard knocking with 87 octane but I run 89when towing anything.

Rush345 · **Posted:** Sat Oct 27, 2007 3:25 am

Yes and except for the muffler and air fliter mechanically it's stock I might run 87 one tank load every other month , but from day one I read the owners manuel and it said basically use the cheap stuff it likes it. Now my '95 Dakota 4x4 5.2L , If I ran 87 it got better gas milage but was slugish coming off the line, and if I used 94 octaine it's gas milage took a nose dive, But coming of the line it would throw you back into you seat.

Tokyojoe · **Posted:** Sat Oct 27, 2007 11:14 am

If going into the mountains, I like the premium. There is a difference in fuel economy and power. I normally run the 85 around metro Denver.

Eddo · **Joined:** Thu Jul 07, 2005 7:01 pm **Posts:** 1214

85 octane can be used at higher altitudes. It even says so in the owners manual if I remember correctly. The reason is because the pressure is lower at higher altitudes which reduces the absolute pressure in the cylinder (this is one reason why power is lost at higher altitudes). Lower pressure in the cylinder means the reaction will occur at a slower rate, hence no need for higher octane. Higher octane fuel actually burns slower than lower octane. Higher octane is in a simplistic idea contains longer chain molecules that take longer to be completely oxidized (burned).

With higher octane you can increase the compression ratio (higher cylinder pressure) and put more air and fuel in the cylinder for more power. However, the more pressure you have the faster the reaction will run. Thus you need to either decrease ignition advance or use a slower burning fuel (higher octane).

You can benefit from higher octane under heavy loads because the cylinders heat up more which increase the reaction speed.

You engine really shouldn't ping with any fuel, unless you use some farm gas with octane of 78 or something. The knock sensors should be able to detect the knock long before you can really hear it or feel it and decrease the ignition advance to eliminate the knock. Believe me if your engine is knocking you will know it. It's an unmistakable knack, knack sound.

bearmedic · **Posted:** Sat Oct 27, 2007 2:14 pm

hey if you exchange the throttle body or add an airaid spacer to it and an intake or just a k&N filter will that make a cause to change your horse power
drew

tjkj2002 · **Posted:** Sat Oct 27, 2007 10:58 pm

Quote:

unless you use some farm gas with octane of 78 or something

The farm gas I have run back home in SD is 94 octane with 15% ethonal,my KJ likes this stuff

.

Quote:

85 octane can be used at higher altitudes

That is true but when offroading premium really helps for all the slow crawling alot,with all my wieght I need premium cause I do ping with 85 octane and if offraod it pings with 87 octane.

3.7 87 octane?

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