Running higher octane fuel now and then?

[ComancheKid45]

Random thought that I figured I'd throw out here to you guys. Do you see any viable benefit to running a tank of higher octane fuel every now and then? I'm usually 87 octane but have been known to fill up with 89 or even 93 now and then just for the state of mind that it may help "clean" the system after running a few tanks of "low grade" fuel. Idk if it even does a thing considering all fuels around here contain ethanol which is more corrosive than anything else in gasoline IMO.

 

Just wanted to bring it up and see what your opinions are.

[Minuit]

Nah. Premium (read high octane) fuel resists compression ignition better, that's about it. Some Seafoam in the tank will do what you want, but it's probably not necessary. Lower octane fuel isn't "dirty" - that's just what the oil company's marketing wants you to believe. It's also worth looking for a station near you that sells ethanol free gas, there's a few around here.

[Eagle]

Do you see any viable benefit to running a tank of higher octane fuel every now and then?

No.

[gogmorgo]

Unless your vehicle is tuned for the higher-octane fuel, if there's a noticeable difference in performance from one grade of fuel to the next, stop going to that gas station.

While it is typical to have higher-quality additives in the higher-octane fuel, it's by no means a rule, and just getting the higher-octane stuff at the local no-name discount gas station is only going to have the benefit of the higher anti-knock rating.

 

Annecdote time: I borrowed my dad's ancient Chevy van a few summers ago. He insisted on always running premium fuel in it because it ran noticeable better, even to the point of carrying around octane boosters to use when premium fuel wasn't available. When I got it, I ran a couple tanks of premium through it, decided I was using too much gas as it was to pay the extra 10% for premium, and switched back to regular. It ran like complete $#!&. I thought what the hell, it's just a 305, nothing fancy, and pulled a spark plug. Absolutely caked in carbon. The premium fuel was doing it's job and not getting ignited by all the soot left burning in the cylinders, but the regular fuel was igniting, and pinging like crazy because of if. I threw new plugs, wires, cap and rotor at it, drove it a bit harder (i.e. actually used more than the first quarter of the throttle travel... my dad drives like a grandma) and after a couple weeks the carbon had mostly burnt off, and despite driving much more aggressively than my dad, I noticed a 2-3mpg improvement, which is significant when you've only been averaging 10-12. I also noticed a decrease in backfires which might have more to do with the cleaner motor than anything else, but which I'll still attribute to more of the regular fuel igniting within the cylinder. So yeah.

[Megadan]

Nah. Premium (read high octane) fuel resists compression ignition better, that's about it. 

That is it.  Outside of additive packages, higher octane unleaded fuel gets its higher detonation resistance by having longer hydrocarbon chains.  Those longer chains resist ignition under the same conditions as shorter hydrocarbon molecule chains, thus requiring more energy input to achieve proper ignition.  That is why forced induction and high compression engines not only run the fuel to resist detonation which would be caused by the shorter molecule chains igniting on their own under the higher temps created in said engines, but tend to run better on it as well, the increased cylinder pressure creates said higher temps, and that energy creates more ideal conditions for said longer chains to ignite.  In fact, if you run a high enough octane fuel than an engine is designed for you can actually see a loss of power and economy as the engine will not be able to combust all of the fuel charge as the burn rate will be too slow.

 

That was my nerd moment, please continue. 

[Eagle]

On the flip side of anecdotes, if you have a Renix with the knock sensor, then there IS a benefit to running high test -- if you run it all the time and don't have a lead foot. When my '88 XJ was new, I ran high test all the time. My highway fuel mileage was phenomenal -- like 28 MPG on a road trip from Connecticut to Vermont, loaded down for a weekend with a lady friend, her daughter and a friend of the daughter, clothing for a winter weekend in Vermont, and "stuff" to keep the two kids occupied for a weekend.

 

Why? Because the Renix advances the spark as far as it can, then backs it off when the knock sensor detects ping. High test reduces ping, so at cruise throttle I was running with more advance than would have been possible on regular grade fuel.

[ComancheKid45]

Thats interesting. So if youre doing alot of highway driving the higher test fuel could be a benefit.

[Eagle]

It should increase your gas mileage -- more if you run a high-altitude CPS. But only for the Renix years -- the HOs don't have a knock sensor.

 

The real question is whether it will increase the mileage enough to offset the higher price of the fuel.

[Marine1Texas]

If you have a HEI Dizzy on the Renix you could advance in to get some extra mpg. Even on this new MJ I have that has the 4.6 HEI and carb I only use 87.

[ComancheKid45]

It should increase your gas mileage -- more if you run a high-altitude CPS. But only for the Renix years -- the HOs don't have a knock sensor.

 

The real question is whether it will increase the mileage enough to offset the higher price of the fuel.

I will give this a try with my 88 2wd. 70 mile round trip to work all highway. I'll let you know how it goes. Currently averaging 17.5mpg running 87 Oct.

[johnj92131]

Thats interesting. So if youre doing alot of highway driving the higher test fuel could be a benefit.

 

That is not what he said. 

 

Rather, the renix engine has a knock sensor that permits the ecu to advance ignition timing to the point where the engine knocks, then backs the timiing off.  Eagle put a high altitude Crankshaft position sensor on his truck.  The high alltitude CPS has a wider range of ignition advance than the normal CPS (octane requirement drops with altitude because altitude has the same effect as lower compression).  So because of the High Altitude CPS and the high octane fuel, his ecu advanced the timing further than normal which improved the highway MPG.  Without the High Altitude CPS, premium fuel would have done nothing at all for the MPG.

[johnj92131]

 

It should increase your gas mileage -- more if you run a high-altitude CPS. But only for the Renix years -- the HOs don't have a knock sensor.

 

The real question is whether it will increase the mileage enough to offset the higher price of the fuel.

I will give this a try with my 88 2wd. 70 mile round trip to work all highway. I'll let you know how it goes. Currently averaging 17.5mpg running 87 Oct.

 

 

You can also modify the stock CPS and physically advance the timing a few degrees for better highway MPG and the knock sensor would still pull the timing back in city driving when the engine knocked.  But again, it is the CPS sensor adjustment that improves the MPG.  This CPS sensor adjustment can even improve the highway MPG with 87 octane fuel under the right conditions.

[Megadan]

 

Thats interesting. So if youre doing alot of highway driving the higher test fuel could be a benefit.

 

That is not what he said. 

 

Rather, the renix engine has a knock sensor that permits the ecu to advance ignition timing to the point where the engine knocks, then backs the timiing off.  Eagle put a high altitude Crankshaft position sensor on his truck.  The high alltitude CPS has a wider range of ignition advance than the normal CPS (octane requirement drops with altitude because altitude has the same effect as lower compression).  So because of the High Altitude CPS and the high octane fuel, his ecu advanced the timing further than normal which improved the highway MPG.  Without the High Altitude CPS, premium fuel would have done nothing at all for the MPG.

 

The high altitude doesn't have a wider range.  It is literally, physically, advanced forward on the bracket so that the base signal timing starts more advanced.  You can take a regular CPS and make it high altitude by simply moving the sensor over on said bracket yourself.  Covered in Cruisers tips.

[johnj92131]

 

"The high altitude doesn't have a wider range.  It is literally, physically, advanced forward on the bracket so that the base signal timing starts more advanced.  You can take a regular CPS and make it high altitude by simply moving the sensor over on said bracket yourself.  Covered in Cruisers tips."

 

 

Impulse -

 

Absolutely correct!  I misspoke when I said the range of the CPS was larger. The high altitude CPS advances the timing because of the bracket as you have described.

[Eagle]

 

Thats interesting. So if youre doing alot of highway driving the higher test fuel could be a benefit.

 

That is not what he said. 

 

Rather, the renix engine has a knock sensor that permits the ecu to advance ignition timing to the point where the engine knocks, then backs the timiing off.  Eagle put a high altitude Crankshaft position sensor on his truck.  The high alltitude CPS has a wider range of ignition advance than the normal CPS (octane requirement drops with altitude because altitude has the same effect as lower compression).  So because of the High Altitude CPS and the high octane fuel, his ecu advanced the timing further than normal which improved the highway MPG.  Without the High Altitude CPS, premium fuel would have done nothing at all for the MPG.

 

That IS what I said.

 

Back in 1988 and 1989, I didn't know about the high altitude CPS, and I wouldn't have bothered replacing the original with one until it failed anyway. Just running high test fuel should result in an increase in mileage -- IF you have a light foot, so the engine isn't spending a lot of time under load so the knock sensor has to keep retarding the ignition.

 

The high altitude knock sensor should make more of a difference, but the difference is there without it. It does this because with the higher octane that reduces knock (ping, pre-ignition) the knock sensor doesn't retard the ignition as much or as far.

[Megadan]

 

The high altitude CPS(fixed) sensor should make more of a difference, but the difference is there without it. It does this because with the higher octane that reduces knock (ping, pre-ignition) the knock sensor doesn't retard the ignition as much or as far.

 

IIRC the high altitude CPS position vs standard gives 5 degrees more base advance timing.  While it does remove that from the back end, these engines are timed pretty far back from everything I have seen with my timing light.  Unless you run into some pretty crappy regular fuel at the pumps it shouldn't cause any issues.  Even with crap fuel, it shouldn't be a problem.  I have yet to run into one at least.

 

Honestly, advanced timing can help with highway economy, but most modern engine tunes will pull fuel and do a lean burn steady state mode, running AFR's well into the 15:1 range instead of kicking timing forward, and will work with the knock sensor to pull or add timing to keep things happy.  That's one reason a lot of modern cars and computer tunes get better highway econ.  Just like the advanced timing, it require a light foot, because the second any change id detected thoe AFR's drop back down.  With cruise control on flat ground though, it can make a big difference.

[mikekaz1]

For what its worth. "Midgrade" is a hoax. Only buy the lowest or highest grade. Thank me later.

[SVPete]

Unless your vehicle is tuned for the higher-octane fuel, if there's a noticeable difference in performance from one grade of fuel to the next, stop going to that gas station.

While it is typical to have higher-quality additives in the higher-octane fuel, it's by no means a rule, and just getting the higher-octane stuff at the local no-name discount gas station is only going to have the benefit of the higher anti-knock rating.

 

 

Concur with your first sentence....recently I started buying gas from the local Shell station. Cost is the same and I went from 18-19 mpg to 21.

[500 MJ]

I've been using my '90 as a DD lately due to other issues with my normal DD. I run premuim in the MJs because it's the only gas around that does NOT have ethanol in it. The trucks run noticeably better on it, hardly any engine noise compared to 87 octane with 10% ethanol... I use it for all of my power equipment as well, big difference there too compared to 10% ethanol gas... It seems to store for longer periods as well, I'm guessing that's a benefit of the non ethanol part...

[mikekaz1]

I can tell you from industry experience. Many fuel brands are identical... but (at least here in the north east) Shell IS one of the best fuels on the market due to its specific additives,fuel conditioners and general site maintainance/upkeep. Most of the fuel industry is "smoke and mirrors". But I find shell does have an advantage and a reasoning to its normally higher price.

[big66440]

I use 91 on mine, it pings on anything below that octane :dunno:, I may have carbon buildup but haven't verified.

[gogmorgo]

Throw a new set of spark plugs in there. If you've never checked it probably needs them.

[big66440]

Throw a new set of spark plugs in there. If you've never checked it probably needs them.

I did a full tune up on it, cap and rotor ( NAPA ) Taylor high Performance wires, new plugs ( factory recommended Champions) and I've done the 4 hole Injector upgrade and still pings though.