Is 91 octane really over kill on a 3rd gen?

[Roc Doc]

The ethanol actually helps lower detonation. Its why e85 works so well and makes such good power.

The newer cars run a lot better because of all the tech, direct injection and really efficient turbo setups that dont produce much heat. I was never able to make more than 350whp on 91 octane. But ethanol never stops making power!

Understood. Have a buddy that runs a BBC Corvette (body) and is making over 1200hp with E85, it helps in cooling the intake and combustion chambers, etc., it just requires a really, really big carb...and it's 1/5th the price of real race gas. But damn does it burn your eyes when it's running.

The problem with E85 or even E10 in daily drivers it it's hygroscopic and can cause corrosion issues with cars that are not driven a lot, and/or in high humidity areas.

 

[AntleredRuin]

I've even seen alcohol injection kits for old carbureted engines that supposedly allowed you to advance the timing significantly on pump gas without any pinging.
(I used to frequent the Jeep FSJ forums a lot)

Yeah, the age of the engine technology makes a big difference in all of this as well.

There are basic rules of thumb, but they are not all-encompassing rules.

Like the whole 'no replacement for displacement' rule. For the most part, bigger is more powerful, but a more efficient engine design helps a whole lot as well.

 

[JayRolla]

Understood. Have a buddy that runs a BBC Corvette (body) and is making over 1200hp with E85, it helps in cooling the intake and combustion chambers, etc., it just requires a really, really big carb...and it's 1/5th the price of real race gas. But damn does it burn your eyes when it's running.

The problem with E85 or even E10 in daily drivers it it's hygroscopic and can cause corrosion issues with cars that are not driven a lot, and/or in high humidity areas.

Yes, not running you car regularly on e85 is not a good idea. That being said we have been running it off and on for over 15 years. Took about that long for my brothers car to finally eat all its SS lines and fittings. He ended up spending around $400 on fuel lines and fittings rated for nascar that can run 100% alcohol.

We have to run 2-3 fuel pumps, 2150cc injectors and dual fuel rail setups when hitting 1000+whp levels on our 4 bangers. Im in the process of building a 1000whp 4 banger as we speak.

The e85 smell is so good. I miss it so much. With fuel injection you only smell in burned which is not so bad. I can actually tell driving down the road when an e85 car is in front of me. I will look around and see an Evo, Subie or some type of high modified car and get all jealous. Miss the smell so much!

 

[JayRolla]

I've even seen alcohol injection kits for old carbureted engines that supposedly allowed you to advance the timing significantly on pump gas without any pinging.
(I used to frequent the Jeep FSJ forums a lot)

Yeah, the age of the engine technology makes a big difference in all of this as well.

There are basic rules of thumb, but they are not all-encompassing rules.

Like the whole 'no replacement for displacement' rule. For the most part, bigger is more powerful, but a more efficient engine design helps a whole lot as well.

BMW is releasing a car with an alcohol injection system I believe her very soon!

 

[AntleredRuin]

BMW is releasing a car with an alcohol injection system I believe her very soon!

I'd be interested to see the numbers for that!

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[JayRolla]

I'd be interested to see the numbers for that!

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I lied, its water injection!

BMW M Power World

 

[tommyd1nes]

Yes it is overkill, it just burns slower, but there is no reason for higher octane fuel in a "stock" motor.


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[jeremyc74]

What's really amazing is the high boost, high specific output engines like in the GTR or Porsche Turbo, that are running relatively high CR, and really high combustion chamber PSI (under boost), and they run just fine on pump gas...even with that GD ethanol.

And it's not that the 5z will advance for premium, but it might under the right circumstances have to retard for 87 (or 85 like they have in some high altitude locations). Semantics, I know. :nod:

The use of direct injection paired with advanced knock detection helps a lot in that regard. My car is at 10.2:1 compression ratio, and I'm at 23psi from about 2500RPMs up.

One of the cool things the advanced ECUs can do is individual timing curves on each cylinder, and they use hyper sensitive knock sensors so they're basically riding right on the edge of detonation all the time. Modern technology FTW! :surfer:

 

[gamefreakgc]

This one's beat to death, but sometimes you see mileage gains with 91+ over 87. My old toyota gained over 1 MPG on 91, making it more economical than running 87.

I can't say though if it's the same on my 4Runner though since I have to run 91 with the supercharger. I have got upwards of 19+ MPG's on highway trips though at times so maybe it does. And in Oregon I got 22 MPG's on 93 octane - not recorded in Fuelly though.

 

[AntleredRuin]

But like I said earlier: Were you actually saving money, though? You got 1 mpg higher but paid a decent amount more per gallon for the 'premium' fuel. You paid a premium for the premium.
MPG gains alone might not be worth it.

 

[gamefreakgc]

Yeah my cost per mile went down enough that it paid for the extra $.20 per gallon. But that was when gas was almost $4 a gallon. Now that it's dropped, you'll expect to pay about 7-8% more for premium gas. If you see a 7-8% increase in mileage then it's worth it from an economical point of view.

 

[AntleredRuin]

That makes sense. It really can fluctuate like crazy depending on gas prices, climate, gas station, etc. And I guess you have to constantly be measuring as well. Have a basic idea where your 'break even' line is.

There's almost a full dollar difference between regular and premium here in Houston right now. I just filled up with 89 at $1.95/gallon, and the 93 was $2.87. I haven't done the math, but pretty much, unless I get a full 1/3rd more MPGs, it's probably not worth the 1/3rd price addition.

 

[gamefreakgc]

That makes sense. It really can fluctuate like crazy depending on gas prices, climate, gas station, etc. And I guess you have to constantly be measuring as well. Have a basic idea where your 'break even' line is.

There's almost a full dollar difference between regular and premium here in Houston right now. I just filled up with 89 at $1.95/gallon, and the 93 was $2.87. I haven't done the math, but pretty much, unless I get a full 1/3rd more MPGs, it's probably not worth the 1/3rd price addition.

YIKES that's awful! Yeah don't bother with that! Your MPG's would have to jump from about 18 to 24 to make that even break even (which is impossible lol). Out here in CA where all the oil refineries are the spread between Regular 87 and Premium 91 is $.20 to $.25. Definitely worth a try out here.

 

[AntleredRuin]

That sounds a bit more like it!
Although I HAVE gotten 22.8 before! 99% freeway going 60 the entire time. Haha.

 

[jeremyc74]

Yeah my cost per mile went down enough that it paid for the extra $.20 per gallon. But that was when gas was almost $4 a gallon. Now that it's dropped, you'll expect to pay about 7-8% more for premium gas. If you see a 7-8% increase in mileage then it's worth it from an economical point of view.

Most of the stations here keep the same price difference between the different grades ( about .20 for mid grade and .40 for premium )no matter what the base price is. I don't know if that's the case everywhere though.

 

[gamefreakgc]

Most of the stations here keep the same price difference between the different grades ( about .20 for mid grade and .40 for premium )no matter what the base price is. I don't know if that's the case everywhere though.

Yeah, it is the same here. Except it's more like $.10 and $.20-$.25 between grades. Seems like CA's Regular gas is overpriced but the Premium is cheaper than other states out there (except for Oregon... I should move there).

 

[I_bRAD]

I've been running 91 in mine since that's the only gas around here I can get that doesn't have ethanol in it. Yes, the 91 octane is overkill, but the lack of ethanol definitely is not. When they really start loading up on the ethanol around here for "winter" gas, then the mileage really takes a hit on regular. I'm talking 350kms/tank vs 420 on super. It more than pays for itself.

 

[jeremyc74]

I've been running 91 in mine since that's the only gas around here I can get that doesn't have ethanol in it. Yes, the 91 octane is overkill, but the lack of ethanol definitely is not. When they really start loading up on the ethanol around here for "winter" gas, then the mileage really takes a hit on regular. I'm talking 350kms/tank vs 420 on super. It more than pays for itself.

Down here they all have it, but we don't have seasonal blends so maybe that's the difference.

The ethanol has less energy content (but higher octane), so there's no doubt that pulls mileage down. If they're using different level of ethanol in the different grades that would certainly explain why some people are getting better or worse mileage depending on what they're buying.

My car gets worse mileage than the 4Runner on the E85. With a 13 gallon tank that's a real pain in the ass. :fish:

 

[nc-grayson]

I'll throw a wrench in the argument. Anyone care to shed some light?

I have a 1998 Lexus LX470. The sticker on the gas door and the owners manual state "Premium Unleaded Fuel Only, Min 91"

Take for the sake of argument, the exact same truck in a 1998 Land Cruiser. Same engine, same drivetrain, same everything except for suspension and badging. The Land Cruiser calls for regular fuel, min 87.

What gives there??

FWIW, I run 87 in my 3rd gen. I run 91 (or higher) in my tuned, turbo'd euros. And I have 5,000 miles of strictly premium in the LX, and also 5,000 miles of strictly regular fuel (back to back testing) in the LX, and by my math, no notable differences, other than extra pocket money for a pack of smokes or a case of cold ones.

 

[jeremyc74]

I'll throw a wrench in the argument. Anyone care to shed some light?

I have a 1998 Lexus LX470. The sticker on the gas door and the owners manual state "Premium Unleaded Fuel Only, Min 91"

Take for the sake of argument, the exact same truck in a 1998 Land Cruiser. Same engine, same drivetrain, same everything except for suspension and badging. The Land Cruiser calls for regular fuel, min 87.

What gives there??
.

Different tuning. The Land Cruiser was designed to be driven in places where premium isn't an option. The Lexus has US specific tuning in the ECU to meet EPA requirements, so it wasn't a concern.

They'll both run fine on 87, they just won't run as much timing advance, which means less power.