89 Octane runs terrible in truck

[renegade_987]

I dont know what octane's are sold in other states, but in Iowa it's 87,89, and 91. I usually use 87, a little pricier than 89 but worth it. Lately I've been putting in 89 because I've been traveling alot and wanted something cheaper. Apparently a big mistake. Everytime I put 89 in, truck develops a slight engine knock and gets about 10-13 MPG in the ciy, and 15-16 hwy. Normally I can get in the 20's with 87. I run the tank to empty, put in 87, and BAM, MPG's are back up and engine noise is gone. 89 also makes it lose power and acceleration. Any reason to this or is it normal. Truck has 4.6L V8, if that makes any difference.

Thanks in Advance!!

 

[dparm]

Isn't that truck spec'd for 87?

 

[renegade_987]

Originally Posted By: dparm
Isn't that truck spec'd for 87?


Probably, but then again, my grandparents Buick is too, and it runs fine on 89

 

[dparm]

You do understand that higher octane is HARDER to detonate, right? You are robbing yourself of power and spending more money.

 

[renegade_987]

Originally Posted By: dparm
You are robbing yourself of power and spending more money.


No kidding. Im filling up 3x as much as I used to. I'd better stick with 87, right?

 

[eljefino]

It's the ethanol. You "lucky" Iowans seem to get it in midgrade only.

Though the R+M/2 measuring is supposed to cure it, you apparantly have "octane sensitivity".

 

[dparm]

Originally Posted By: renegade_987
Originally Posted By: dparm
You are robbing yourself of power and spending more money.


No kidding. Im filling up 3x as much as I used to. I'd better stick with 87, right?


You should run the octane the manufacturer specifies.

Octane is a measurement of resistance to detonation. 93 octane is harder to detonate than 89 octane; some motors need this higher resistance because they are higher compression and early ignition would rob them of power.

Your truck, on the other hand, can't fully detonate anything above 87. That's why it runs rough and the gas mileage drops: incomplete combustion.

 

[Vikas]

Is 89 cheaper than 87 in your neck of woods?

 

[renegade_987]

Originally Posted By: Vikas
Is 89 cheaper than 87 in your neck of woods?


Ya, right now 89 is about $3.27 and 87 is $3.38, but the price varies depending on where you buy. 89 is always cheaper, though

 

[Bottom_Feeder]

Originally Posted By: Vikas
Is 89 cheaper than 87 in your neck of woods?

Yeah, it seems like he's got his 89's and 87's backwards. 87 should be cheaper and sounds like it works better in his truck, so...?

 

[Bottom_Feeder]

Originally Posted By: renegade_987
Ya, right now 89 is about $3.27 and 87 is $3.38, but the price varies depending on where you buy. 89 is always cheaper, though

If that's the case, and your MPG is taking such a serious hit using 89, then always using 87 is a no-brainer. It's what, an extra $2 per tank?

 

[renegade_987]

Originally Posted By: Bottom_Feeder
Originally Posted By: Vikas
Is 89 cheaper than 87 in your neck of woods?

Yeah, it seems like he's got his 89's and 87's backwards. 87 should be cheaper and sounds like it works better in his truck, so...?

I wish. In AZ where i used to live 87 was always cheapest, now it's 89 that cheapest

 

[Bottom_Feeder]

Originally Posted By: dparm
You should run the octane the manufacturer specifies.

Octane is a measurement of resistance to detonation. 93 octane is harder to detonate than 89 octane; some motors need this higher resistance because they are higher compression and early ignition would rob them of power.

Your truck, on the other hand, can't fully detonate anything above 87. That's why it runs rough and the gas mileage drops: incomplete combustion.

Exactly. Me and my old boss had to explain this to a guy we used to work with who bought a then-new GMC S-15 pickup. He was wrapped up in all the advertising saying that 89 octane was better, 91 was best, and 93 was like rocket fuel. He was complaining his new truck felt 'flat' and got terrible mileage. The manual specified 87 octane gas. He wasn't convinced until he ran it empty and filled up with 87 and the thing ran great after that.

 

[renegade_987]

So I have one more question. is running 89 'BAD' for my engine?

 

[Rhymingmechanic]

Originally Posted By: eljefino
It's the ethanol. You "lucky" Iowans seem to get it in midgrade only.

Though the R+M/2 measuring is supposed to cure it, you apparantly have "octane sensitivity".


That's your answer. The 89 is cheaper because it has ethanol.

 

[MrRPM]

so weird, around here everything has 10% ethanol and 87 is the cheapest and each grade above goes up by 10 cents.

 

[ltslimjim]

OP is indeed in an interesting area. I think Premium is cheaper than 87 at some stations. Of course, grade options vary and the contents of ethanol in the blend or not does seem to affect pricing.

 

[someguy]

That is very strange, I thought all of the fuel in Iowa had 10% ethanol. I know some stations in Iowa even have E85 and/or blender pumps. Maybe the 89 octane is more than 10% ethanol.

I would love to see a picture of the pump

 

[Whimsey]

Here in NJ we're stuck with 10% ethanol in all grades. Recently 89 octane is now over 20 cents a gallon more than 87 octane. It's actually less expensive to do 50% 87 & 50% 93 mixture. Plus you get a slightly higher octane rating by mixing them versus using a straight 89 octane mid grade gas. Both the mid grade and high test have risen substantially versus 87 octane. The spread use to be 10 cents more for 89 and 20 cents more for 93. Now it's more like 24 cents more for 89 and 40 cents for 93.

Whimsey

 

[ammolab]

Originally Posted By: Whimsey
Plus you get a slightly higher octane rating by mixing them versus using a straight 89 octane mid grade gas.


In many stations "89 Octane" gas is just a pumped mixture from the 87 and 93 Octane storage tanks. I don't know the ratio but I bet it is less than perfect at many retailers.