Burning question for Duke, 87, 89,or 91 octane

[Ray G.]

Hello;
Always interested in your informative posts. Have not seen any, lately, on octane.
"Premium Unleaded Fuel Only" is the label on the fuel inlet.
Wondering if "91" is necessary in all but high heat and towing situations?
Was told in the late '90s by a GM employee, testing fuel milage in the Denver area, engines could run on 85 octane w/ no ill effects.
What is your opinion.
Thanks for your help.
Ray

[Joe C.]

Re: Burning question for Duke, 87, 89,or 91 octane

Hello;
Always interested in your informative posts. Have not seen any, lately, on octane.
"Premium Unleaded Fuel Only" is the label on the fuel inlet.
Wondering if "91" is necessary in all but high heat and towing situations?
Was told in the late '90s by a GM employee, testing fuel milage in the Denver area, engines could run on 85 octane w/ no ill effects.
What is your opinion.
Thanks for your help.
Ray

All modern engines equipped with electronic engine controls, specifically, knock sensors, will safely run on fuel with less than the "recommended" octane levels. The reason for this, is that when the sensor detects detonation, it will send a signal to the PCM, which will then retard the spark timing until the detonation ceases. This is usually a "step function" and often the degree of retard will overshoot that which is necessary.

The question should be: "what is the trade-off in engine performance and mileage if I save money by using lower than 'recommended' octane fuel?" Your engine will be noticeably down on power. Surprisingly, you will save money by using the "recommended" octane fuel, because your fuel mileage will increase.

You might ask: "But Joe, how do you know this?"

Funny you should ask me that. Back in 1985, when my Corvette was brand new, I was in the (anal) habit of calculating my fuel mileage every time I took a highway road trip with the car. This was before the MTBE and/or ethanol levels were increased, and it was just plain, good ole (?) unleaded. I always use "hi-test", or 93 PON octane gas. At the time, with my 700R-4 and 3.08 gear, I was getting better than 26MPG! I restrained myself as best I could, set the cruise control @ 62, and left it there. As an experiment, I did this same calculation on a couple road trips (yes, same trips, and very similar weather conditions, also, made certain that they were ROUND TRIPS). The fuel mileage decreased, significantly. In fact, the calculated price of these trips was LESS, when I filled up with premium fuel.

These results will vary, depending on your region, because fuels are formulated slightly differently depending on altitude and prevailing state laws.

Joe

[Clem Z.]

Re: Burning question for Duke, 87, 89,or 91 octane

i have seen the same thing on the new corvettes. i made a trip to bowling green with a fellow corvette owner and he use regular and i used hi test. he always used a couple of gallon more at a fill up. his was a manual 6 speed and mine was a 4 speed automatic with the optional 3.15 rear gear. if mine was a 6 speed manual also i bet the difference would have been greater.

[Duke W.]

Re: Burning question for Duke, 87, 89,or 91 octane

As previously stated modern cars since the early to mid-nineties, have detonation sensors that will cause the engine management system to select a less aggressive spark advance map if significant detonation is detected, and this primarily affects high load spark advance rather than cruise, so typically fuel mileage is not affected, and few people can recognize the slight loss of acceleration performance that occurs with less high load spark advance.

Note that for new Corvettes the owner's manual says "91 octane recommended, but not required" except Z06 which is "91 octane, minimum."

In a high altitude environment an engine can tolerate lower fuel octane without any detonation on the most aggressive spark advance map because air density is lower for any given temperature than at sea level or low altitude, so there is less tendency to detonate on lower octane fuel. In fact, some high altitude areas may have lower octane fuels - like 85 instead of 87, etc.

Some years ago Car and Driver did a test using both premium and regular fuel on "premium fuel" engines. The mileage results were similar, but IIRC on the Honda Accord V6 it actually got a little better mileage on regular than premium, and I also got some indications from various sources that some regular blends had slightly higher energy than some premiums, but fuel blends have changed a lot since then. Typical oxgenated fuels have 2 - 3 percent less energy than straight gasoline of similar octane, even though they cost more to produce.

Of course, "your mileage may vary", so it's up to each individual with a "premium fuel" engine to test the effects of regular by keeping accurate fuel consumption records, listening for detonation, and monitoring any performance changes with the SOTP.

One final note. Back when regular was about a buck a gallon, premium was about $1.20 - a twenty percent difference. Nowadays, the 20 cent difference remains, but if you take into account that regular is about $3.50, which it is in LA, the $3.70 price for premium is only 5.6 percent more.

I can share my experience with my '88 Mercedes-Benz 190E 2.6 five-speed. It only has a 9.2:1 CR, but is placarded for premium fuel, and it does not have a detonation sensor. Back in the first few years of ownership I could detect a little detonation on premium fuel with the AC on in hot weather traffic when the coolant temperature got up to about 100C, and this was on our 91 PON California premium.

Now I only drive it in the cool months from November to May, and despite modifiying the electronic "centrifugal" advance to be more aggressive by changing a trim resistor and using regular fuel, I rarely hear any detonation as long as the outside temp is below 70F and the coolant is below 90C.

I do get a little transient detonation on upshifts if the temps are higher than listed above, but I can "drive around" this by shifting at higher revs or easing into the throttle a little slower in the next higher gear.

The more aggressive centrifugal advance has considerably improved the low end torque on what was a torque shy engine, exacerbated by tall, widely spaced gearing, and around town mileage is up ten percent from high teens to low 20s, but highway mileage remains in the mid to high twenties range.

I sometimes refer to the engine as "half a L-79" because it has a very similar torque curve shape
and idle characteristic (700@15") to the L-79, but only half the displacment.

It now has good low end torque and pulls strongly to the 6650 rev limiter, which is easy to hit in the lower gears because it wants to keep revving.

BTW, it was radar-gunned at 132 MPH by the Nevada Highway Patrol during the 1989 Silver State Classic Challenge.

Duke

[Terry M.]

Re: Burning question for Duke, 87, 89,or 91 octane

Just a little FWIW information.

I like used police cars, which I have purchased from North Carolina. The 2001 Camaro, with LS1 -- the B4C in my sig, ran in service on 87-octane fuel. I'm sure that fuel mileage was NOT on their radar gun. Lest you think the NC Highway Patrol babied that car -- by 90K miles they had replaced ALL the valve springs, and up to that point they had rebuilt the rear axel twice. Bouncing that thing off the rev limiter will do that. It is a hand full running on Premium, and noticeably smoother on the better fuel.

ALL C6s are factory started (three gallons I think) on 87-octane, or so I was told on the buyers tour.

[Clem Z.]

Re: Burning question for Duke, 87, 89,or 91 octane

Just a little FWIW information.

I like used police cars, which I have purchased from North Carolina. The 2001 Camaro, with LS1 -- the B4C in my sig, ran in service on 87-octane fuel. I'm sure that fuel mileage was NOT on their radar gun. Lest you think the NC Highway Patrol babied that car -- by 90K miles they had replaced ALL the valve springs, and up to that point they had rebuilt the rear axel twice. Bouncing that thing off the rev limiter will do that. It is a hand full running on Premium, and noticeably smoother on the better fuel.

ALL C6s are factory started (three gallons I think) on 87-octane, or so I was told on the buyers tour.

do they have 2 fuel filling systems because i would not think they would send ZO-6s out of the factory with 87 in the tank. JMHO

[Terry M.]

Re: Burning question for Duke, 87, 89,or 91 octane

I watched them put the only fuel filler they had into my Z06. One size fits all. That makes the first fill-up that the dealer is supposed to do all the more important.

[Clem Z.]

Re: Burning question for Duke, 87, 89,or 91 octane

I watched them put the only fuel filler they had into my Z06. One size fits all. That makes the first fill-up that the dealer is supposed to do all the more important.

the reason i wondered is because they run ZO-6 at full throttle in the checking dyno at the end of the line.

[Terry M.]

Re: Burning question for Duke, 87, 89,or 91 octane

Maybe it was because I was in the passenger seat, but she only ran mine to 4K, but gosh she shifted so smooth. She only took it to 80 mph or so. I'm still struggling to get the smooth shifts she got -- I just need more practice.

[Clem Z.]

Re: Burning question for Duke, 87, 89,or 91 octane

Maybe it was because I was in the passenger seat, but she only ran mine to 4K, but gosh she shifted so smooth. She only took it to 80 mph or so. I'm still struggling to get the smooth shifts she got -- I just need more practice.

me also as mine 08 is the first stick shift corvette since my new 66 427 vette. try not taking your heel off of the floor and just push the clutch with your toes.unless i run it to the red line i skip shift,go from first to third and third to fifth when just cruising around.

[Tom S.]

Re: Burning question for Duke, 87, 89,or 91 octane

When driving a stick I very rarely use the clutch except take off and stopping.Just shift when you let up on the fuel and slide it into the next gear. Now this will not work under full throttle very well but very well but otherwise it works very well!! Try this and I will bet with a little practice you can make it as smooth as an automatic! Tom

[Patrick H.]

Re: Burning question for Duke, 87, 89,or 91 octane

When driving a stick I very rarely use the clutch except take off and stopping.Just shift when you let up on the fuel and slide it into the next gear. Now this will not work under full throttle very well but very well but otherwise it works very well!! Try this and I will bet with a little practice you can make it as smooth as an automatic! Tom

I used to do this on my VW Rabbits, but as a teenager didn't care if it ruined the trans. I would have had to fix it myself anyway. Haven't really tried in on my Corvettes much, and definitely not on my 08.

With only 98 miles on it, I haven't tried much at all.

Patrick

[Duke W.]

Re: Burning question for Duke, 87, 89,or 91 octane

Upshifting without the clutch won't hurt or wear anything as long as you get it dead nuts on everytime, which is indicated by slipping into the next higher gear with virtually no effort and no "grunch".

Practice makes perfect, but it's tough to get more than 90 percent or the time.

On some cars you can actually up shift faster with the clutch because the revs are slow to drop off and you have to do the clutchless shift rather slow to avoid grunch.

The same applies to downshifts, but it's tougher to master. You have to blip the throttle passing through neutral and rev match perfectly to avoid grunch.

I'm lazy, so I alway use the clutch up and down, but all my downshifts are double clutch using heel and toe if I'm also braking, and I rarely "miss" shifts - either up or down.

Duke

[Nick M.]

Re: Burning question for Duke, 87, 89,or 91 octane

Tom

How are you able to shift w/o the use of the clutch ? Don't you get grinding gears?

[Michael F.]

Re: Burning question for Duke, 87, 89,or 91 octane

On my 03 ZO6 I get the same fuel mileage with mid grade 89 octane or 91 premium here in texas and on my trip back to carlise last year. When I bought the car I tried a measured freeway trip of 180 miles with 91octane and then with mid grade(89) octane and then same trip, same road, same weather with reg (87) and the fuel mileage with reg was down 2-3mpg and power drop was noticeable. Of course my 67 435 and 70LT-1 both need the good stuff