Chris`s85Z28

i`m pretty sure my car is suppost to be run on 91 oct. but i run it on 89, i`ve never put 91 or sunoco ultra 94 in it.

will running regular gas bring my gas milage down?

i don`t want to put 91 or 94 in it unless it`ll give better milage.

car is a 350 TPI in a 1990 GTA

Saigon_Bob

iTrader: (2)

octane doesnt affect fuel milage at all.. period

run the lowest octane that the engine doesnt ping with

Chris`s85Z28

k, cool thats what i thought, but i`ve heard otherwise.

just wanted to make sure

Viprklr

iTrader: (5)

Bingo. Higher octane fuel is actually harder to burn.

Salem13

That's actually the simplest way to put it, regular is more volatile... but wait there's more...

High test will actually garner better fuel mileage in a properly tuned car, now the question is will that translate into $ savings?

You see higher octane fuel actually has additives that cause it not to burn unless ignited, it has a higher flash temperature. Dieseling or knocking and pinging are cause by the fuel either burning without spark by the heat of the piston and/or compression after being turned off (dieseling) or burn before spark during a stroke (knocking and pinging) a higher octane will prevent these as the heat and/or compression is less likely to cause ignition, it must wait for the spark.

If you're hearing pings or getting knocks with 89 I'd suggest a tune up first and a look at how hot your engine is running then maybe a change of brands... crap no-name gas many times is exactly that... crap.

Without getting into detergent levels which are also important lower tier fuels are many time purchased as tank remixes, tank bottoms and leftovers... these are all activities that take place on the tank farm.

Now why use premium?

Well if you have at all adjusted your factory compression ratio or intake volume you should use premium or a higher octane than 89, both these adjustments can cause incorrect timing of the explosion of fuel due increased volume of fuel/air in the cylinder or higher compression above factory specification designed to burn 89 as a low point. You may not get and audible or noticeable degradation on the road by your knock sensor's correction but it is definitely not at it's highest performance possible for your engine build.

In short raising octane insures you get all of your fuel burning when it is supposed to, not before or after.

It will not add power via a more powerful explosion as many think... if you want a big flash/kaboom use low octane, again, it's more volatile.

Higher octane can translate into better performance through efficiency only.

Now about the $$$

The only real way to find out is to burn some of each.

Say 10$ worth gets you 2.5 (@$3.99) gallons of regular and premium gets you 2.35 (@$4.25) gallons of premium the increased efficiency may balance the cost with a slight increase in mileage or even actually save you a penny or two... nothing really but... if you can burn the higher octane without it actually costing you more, why not?

Add to that brand name fuels such as Shell/Exxon/Chevron/Sunoco add more detergents to their high tier fuels thus keeping your injectors/fuel system and valves cleaner.

To end this, the lowest cost per gallon fuel you can run is not always the best for the short run dollar per mile or for the long run life of your car.

And yes... I work for Shell.

rmmstnr

Does anybody by any chance know what effect changing the compression ratio has on the mileage of our beloved V8 3rd Gens? I did some math and heres how much each grade of gas costs to run per mile as of today's prices (in Missouri anyway, rounded up a couple cents for ease of use):


________/ Reg. (@ $3.50/gal) / Med. (@ $3.65/gal) / Prem. (@ $3.80/gal) /
__________________________Cost Per Mile (In Cents)_________________
@ 20mpg /_______17.5______/________18.25_____/________19________/
@ 22mpg /_______15.9______/________16.6______/_______17.27______/
@ 25mpg /_______14.0______/________14.6______/_______15.2_______/
@ 27mpg /_______12.96_____/________13.5______/_______14.07______/


As you can see (hopefully, it's not a very good chart, hope you can read it), a car built to run on Premium would have to make at least 2mpg more than one built for regular to be cheaper in the long run. But, the question is, would upping the compression on a Regular-spec engine to Premium-spec gain at least that much mileage? Anybody got an idea?

JulieGTA

I've been using shell/chevron 91 almost exclusively, it seems to ping less. My mileage is cruddy though.
(Coincidentally I have not had any luck keeping a motor in my car and I am thinking of switching to another brand.)

I have used 100 octane from some other place, and seen the difference at the dyno. I also used at the drag strip. (It's over $8.00 a gallon!!!) This is a higher compression motor though.
I once used a full tank of 100 octane on a road trip and it didn't help the milage any. Maybe did more harm than good?

I talked to an attendent at the Shell I go to, and he said the 76 station down the street uses more ethanol.
All the gas stations around here get their gas from the same source, so the difference is additives.
(I was interested to learn that they refill the ground tanks almost every day, so my theory of stale gas was debunked...)

I can see how additives would help an older motor but what effect does Ethanol have on the octane? And or/mileage.

On the other hand I know some people whose non-stock cars are able to run on lower octane, because they have less compression or maybe aluminum heads? And they seem to get better milage too. I'm confused about the whole thing.

Guest

If its not knockin, dont bother wasting your money on anything other than low octane. Other than since I've been running a supercharger, I've never had an issue running low octane. Thats an LG4, LB9, LB9, and L98, and in different climate situations as well. My dads car before the last cam change and head change would run on 87 but it wasnt ideal. It wont run on it now. When you raise the compression there's much more chance of the fuel igniting itself. Aluminum heads helps this a little because they pull heat from the chamber faster than cast iron. The cam also has an effect, as well as the intake and how well the cylinders fill. Thats why you see newer engines with better head and intake designs running higher compression, its all about swirl.

Ethanol at 10% will raise the octane rating slightly, if nothing else was different... but I can assure you it is different, and you're still getting 91 when it says 91. Ethanol also has less energy than gasoline, so your mileage will suffer accordingly.

Blue_Thunder

iTrader: (1)

I use Shell V-Power. I have noticed a difference in how the engine runs since I switched 2 months ago. Not miliage wise though. I get 14-15mpg in the city and 19-20mpg on the highway.