91 Octane bad?
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Senior User
Joined: May 2007
Posts: 101
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From: Auburn, California
91 Octane bad?
Hi, I have a 1996 F-150 with the 300 (4.9). I have been putting supreme (91 octane) in it now for about 6 months because it helps it run cleaner, stronger and I don't have to worry as much about getting the crap gas out of the bottom of the tanks at the gas station. Also, when gas was about 4.50 a gallon, i was actually saving money because of the increased millage.
Then a friend told me that putting supreme in something that inst made for it is actually bad for the engine. He couldn't tell me what specifically that was and is usually wrong on about 75% of what comes out of his mouth so i wasnt too worried but thought if make sure.
Then a friend told me that putting supreme in something that inst made for it is actually bad for the engine. He couldn't tell me what specifically that was and is usually wrong on about 75% of what comes out of his mouth so i wasnt too worried but thought if make sure.
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
If this was an older truck I'd be telling you you're wasting your money, but since it's a '96 with OBD-2 it can take advantage of higher octane gas and deliver added milage and performance. And just for the record none of the widely available gasolines will hurt your motor, just don't put E85 in it.
Elder User
Joined: Nov 2007
Posts: 638
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93 BP with Invigorate, my quarter-million mile motor, all stock, burns rubber easily with 33x12.5 boots on and pulls decent gas mileage. Interpret this info as your friend is mistaken, my old beast loves high test.
More Turbo
Joined: Mar 2008
Posts: 658
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So it wouldn't make much of a difference in an OBD-1 truck? Just wondering, haven't tried it...
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
Shouldn't make any difference at all unless your motor pings on regular. Some of the OBD-1 systems with knock sensors can retard ignition if detonation is sensed, so if gas quality is particularly bad then you could see better performance with a higher grade fuel, but it's only because you're avoiding the bad fuel. OBD-2 systems will actually advance timing until detonation is detected so it makes the most of every tank full and that's where the increased performance comes from.
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Fleet Mechanic
Joined: Mar 2005
Posts: 1,603
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From: St Louis
Did the 351's ever get the knock sensor in OBD-II? Just curious because I know it wasnt given in OBD-I systems for 351 and 460 engines.
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I've got a 94 OBD-1 truck, and it runs markedly different on 93 and 89 octane. I always thought that EEC-4 used a knock sensor to adjust the timing, but from reading this thread, not? I guess I should check my ignition timing. I know for a fact that after running 300 miles towing my sailboat on 93, when I put 87 in it to run around town she pinged like crazy if i got on the gas for the next 20 miles (I took it easy to make sure that nothing got damaged)..Does this mean my spark timing is off?
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
Your timing may be a little too advanced for 89 octane, or you have too much carbon buildup in the combustin chambers. When I refurbished the 5.0 I put in my truck this summer I was surprised how much carbon I found in 2 cylinders, there were large deposits stuck in the squish area of these cylinders. I suspect this carbon was from the the PCV valve because it was on the back cylinders if I remember correctly.
Laughing Gas
Joined: Dec 2007
Posts: 959
Likes: 8
From: Imperial, MO
On the MAF equipped engines, a dirty MAF sensor will often cause detonation on regular grade gas. If you have a MAF, and you haven't cleaned it in awhile, I'd recommend it.
As far as the whole 87 vs. 93 octane debate, you'll only see a difference in performance and mileage if the engine is able to take advantage of the detonation resistance of the higher octane. If you're running a system without a knock sensor and you haven't advanced the ignition timing, or upped the compression, or added forced induction, etc., you're wasting your money. If you have a system that is able to control the ignition advance by utilizing a knock sensor to monitor detonation, then yes, you can see an increase in power/mileage.
-Rod
As far as the whole 87 vs. 93 octane debate, you'll only see a difference in performance and mileage if the engine is able to take advantage of the detonation resistance of the higher octane. If you're running a system without a knock sensor and you haven't advanced the ignition timing, or upped the compression, or added forced induction, etc., you're wasting your money. If you have a system that is able to control the ignition advance by utilizing a knock sensor to monitor detonation, then yes, you can see an increase in power/mileage.
-Rod