: Premium vs. Regular Gas



sunygxc
02-14-2012, 09:26 AM
Hi guys,

When I purchased my Volt, the dealer said the reason I should use premium gas was because it's shelf life was longer and was necessary because gas tends to sit longer in a Volt then a normal car (obviously).

I typically go through one tank of gas in about a month. Is it necessary to use premium gas or doesn't it matter?

Just curious if the dealer was right or if there are other benefits of the higher octane gas that the Volt requires to function.

Thanks again for any advice/help!

scottf200
02-14-2012, 01:14 PM
Lot of discussions on this:

why-the-volt-requires-premium-gasoline/
http://gm-volt.com/2010/07/30/why-the-volt-requires-premium-gasoline/


Earlier this week GM released the 2011 Chevrolet Volt ordering guide for dealers to use. Included in the description about the gasoline generator were the words “requires premium fuel.” This led to specualtion and surprise from fans and skeptics as to why in the world the car would require premium gas.

According to Volt vehicle line director Tony Posawatz, there are two main reasons.

“The Volt is all about efficiency,” he said. “Premium fuel offers the opportunity to have a little bit more spark.”

“Ninety one octane fuel also offers the opportunity to be a little more efficient, he added. “So technically its a five to ten percent fuel economy improvement the few times that most people will run the range extender.”

Posawatz also claimed the increase is cost will be offset by the efficiency gains.

“Based on our calculations the fuel economy and efficiency gains you get will effectively compensate for the extra cost of premium fuel,” he said

Additionally, premium fuel is apparently slower to go stale.

If people are not using the extended range capability a lot, the premium fuel does last a little longer,” he said
“There will be a few that will have their gas go bad,” he added. “We have ways to address that as well.”

The AAA Fuel Gauge Report shows the current national average price for a gallon of regular unleaded gasoline is $2.74. The national average for a gallon of premium gasoline is $3.01.

DCFusor
02-14-2012, 01:18 PM
I did try regular in mine and the mileage goes down. They're running nicely high compression here that can take advantage of more octane - and it's not just spark timing they have to play with, but also cam timing (don't know if they sense and change that, but it's there and can affect net compression ratio of they do).

My SS Camaro would run on regular, just not as well, but then I live at 2200 feet altitude where the air is a bit thinner in the first place. Got about the same results there - premium was worth it.

The Volt didn't ping on roughly half and half 87 and 93, but it just didn't show the good gas mileage (lost about 3 mpg). Back to super for me.

Top_Speed1
02-14-2012, 01:31 PM
Premium.
They say you get back the added costs of the fuel with gained efficiency. On a recent road trip I fueled only with Sunoco Ultra 94. Averaged a bit over 42 in ICE mode driving w flow of traffic (speeding!). Was very happy with that mileage vs. some I've heard here.

bonaire
02-14-2012, 01:47 PM
What about mid-range - 89 octane? There is that third choice or even fourth if you go to a Sunoco where they have 87, 89, 91, 93 octanes. Each is .10+ jump between grades. 3 miles equates to 3/40 = 7.5% and if the jump from regular to premium is .40 then that is .40/3.89 or 10.2% - with the mileage difference, it actually is more cost-effective to use regular.

37 mpg at 3.49 = .09432 per mile (regular)
40 mpg at 3.89 = .09725 per mile (premium)

If I were going between two longer-distance cities and could get a fresh tank of regular or 89 octane, it may be more cost effective for that long drive than premium. But come on, though - we're talking a difference of 1.17 over a 400-mile drive. 38.90 for premium and 37.73 for regular.

So, my conclusion is that the "benefit" of cheaper regular is negated as both prices go up together. If we were looking at $1.00 versus $1.40, that .40 difference is so much more of a percentage increase than today.

So, premium seems to offer the best for all cases - more mpg and better engine spark management.

arveedee
02-14-2012, 01:47 PM
When you're dealing with advanced spark, it's really a good idea to stick with higher octane fuels anyway to eliminate any chance of pre-ignition which can, over time, damage the engine. Always fill with the premium.

Noel Park
02-14-2012, 02:28 PM
When you're dealing with advanced spark, it's really a good idea to stick with higher octane fuels anyway to eliminate any chance of pre-ignition which can, over time, damage the engine. Always fill with the premium.

Modern engines have knock sensors which will retard the timing to stop any pre-ignition. So the engine will almost certainly not be damaged. As DCFusor said, the Volt may even vary the cam timing. Alas, with retarded timeing comes more fuel consumption. Which is why Mr. Posawatz said that the increased fuel economy offsets the increased cost of the premium fuel.

Rusty
02-14-2012, 04:21 PM
They're running nicely high compression here that can take advantage of more octane - and it's not just spark timing they have to play with, but also cam timing (don't know if they sense and change that, but it's there and can affect net compression ratio of they do).

My understanding is the Volt has 40 degree variable valve timing, independent for intake and exhaust. So they certainly *can* modify the cam timing, and I suspect they do.


What about mid-range - 89 octane?

During my last cross country trip what I found that made the biggest difference in mileage wasn't octane, it was alcohol content. Running at 94 octane with higher alcohol that I saw near the big muddy got worse gas mileage than running at 91 octane straight up near the Rockies.

So when people compare mileage there's more than just octane to evaluate. But as others have said, in general I see enough better mileage with premium to make up the difference in cost between premium and regular.

saghost
02-14-2012, 04:27 PM
I did try regular in mine and the mileage goes down. They're running nicely high compression here that can take advantage of more octane - and it's not just spark timing they have to play with, but also cam timing (don't know if they sense and change that, but it's there and can affect net compression ratio of they do).


One of the GM reps pretty much told us a few months ago that in certain (not specifically described) portions of the power map the Volt uses cam timing to emulate an Atkinson cycle. I don't know if they change cam timing in response to the knock sensor, though (actually, if you got a knock event, going into atkinson would dissipate the knock while minimizing power loss, I think - lower effective compression ratio at combustion for the same power stroke (lower charge size.))

From what I've read, I believe that the main reason GM specified premium is additives/longevity for fuel that may sit for a year. However, if the car runs more efficiently on premium, as several people have tested, using regular likely makes no sense.

Noel Park
02-14-2012, 05:32 PM
#9 saghost,

I hadn't heard that about the Atkinson cycle before. Very interesting. Thanks.

saghost
02-14-2012, 05:46 PM
#9 saghost,

I hadn't heard that about the Atkinson cycle before. Very interesting. Thanks.

It came up about a month ago. I think it was in one of the threads about engine sizing, when forum members where trying to propose that a smaller dedicated engine would have given better CS economy. I wish we had a reliable bsfc map - the only one I've seen is the ecomodder one, and a few recent results have thrown it into question. Such a chart should show where the engine is stepping into such a mode pretty clearly. :)

silvercorvette
02-14-2012, 07:22 PM
I do not use premium, I make it a point to use only top tier premium. It is too much trouble to remember all the top tier gasoline companies so I make it a point to buy Shell premium exclusively.

Her is a link to a list ofTop tier stations (http://www.toptiergas.com/retailers.html)

DCFusor
02-14-2012, 07:43 PM
I don't think you can truly emulate the Atkinson cycle with cam timing, but you can sure do a few neat things if you have control over that - we old racers knew a few tricks here and there...And so do some new ones. The LS3 couldn't run on street gas if it didn't use cam overlap to lower the effective compression at low rpms. By the time the overlap causes greater than 100%
volumetric efficiency, the engine is spinning too fast for pre-ignition to occur before the piston is on the way back down! Neat little trick from the '70's that GM is now using themselves.

If we could really have a variable rod length to stroke ratio, we could go to town on some other things. Does the piston dwell more or less around TDC? This makes a crazy difference in how the engine works at various RPM - just look at the different torque curve on a 302 SS (old time 5 liter Camaro engine) vs a 350 (or a 305, totally different)...in fact, the old racers (and some guys today) still fool with tuning that for a particular track or car, even drag racers.

What's really new here is that the adder for octane has stayed nearly the same as it was when gasoline was in the $1 range. Back then, you couldn't make up with better mileage on super what you lost in cost quite so easily, though you could run higher compression and more spark and get more efficiency. Don't think racers don't obsess over this - one saved pit stop can be the race. We used to pay very close attention to it! And guess what - no racer runs regular. (there are other reasons of course)

Now that the adder for higher octane is a much lower percentage of the price, it really makes sense to have the high compression and use the better gas. The greenies used to object due to the nitrogen oxides produced by the higher combustion temperatures, but I suppose we've got something in the catcon to take care of that now. Living in a rural area, instead of LA where no city should ever have been built - I never thought that fixed nitrogen falling on my fields in the rain was a bad thing, myself, but that's me.

saghost
02-14-2012, 08:11 PM
I don't think you can truly emulate the Atkinson cycle with cam timing, but you can sure do a few neat things if you have control over that - we old racers knew a few tricks here and there...And so do some new ones. The LS3 couldn't run on street gas if it didn't use cam overlap to lower the effective compression at low rpms. By the time the overlap causes greater than 100%
volumetric efficiency, the engine is spinning too fast for pre-ignition to occur before the piston is on the way back down! Neat little trick from the '70's that GM is now using themselves.


Of course you could emulate Atkinson with valve timing, unless I'm missing something.

An Atkinson cycle is just a normal Otto cycle with intake valve closing delayed until ~30% of the compression stroke, right? (And thus gets its efficiency from a longer expansion stroke, at the cost of charge density - its effective displacement for charge volume being much lower than the geometric displacement.) So there's no reason you can't take any normal engine in and out of it, provided you have a wide enough actuation range.

Of course, it isn't as beneficial as a native atkinson engine, because you can't emulate the 13:1 geometric compression ratios, so you end up with a 7 or 8:1 effective compression ratio, which costs you some of the efficiency you gained in better expanding the exhaust. TANSTAAFL. :-/

leetownreb
08-05-2013, 05:46 PM
agree. BIG problems with non-Top tier gas in my Vette; it's very common. Too much sulfur. I try to find Top Tier stations when I drive on trips. On occasion, you can't. Find out what is in your area and buy it. Bite the bullet--it pays in the long run.