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Old 07-28-2008, 05:07 PM   #11 (permalink)
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Quote:
Originally Posted by BlackDeuceCoupe View Post
You didn't ask, but...

My ride has short gears for racing. I'm a little over 3500 RPM @ 60 MPH!

Doesn't *seem* to affect my gas mileage much though. It's the BSCF that matters, e.g. revs & torque NOT just revs!

The majority of my B16A2's torque comes in high in the RPM range, so... it's a perfect storm, if you will!

Engine efficiency varies with speed and torque, as can be seen by plotting Brake Specific Fuel Consumption.

Anyway, if you're convinced that higher octane gas makes YOUR car run better, have at it! I don't wanna burst any bubbles...
Well the idea of posting this WAS to burst my bubble if I was wrong, and hey I very well could be. We'll see next tank of gas

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Old 07-28-2008, 05:20 PM   #12 (permalink)
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my car used to ping like a mofo on 87 at part throttle but went away when it was floored. the 89 cured the part throttle pinging. if i drove very gradual which isnt as efficient and do not load the engine, it didnt ping with 87 but i do need to accelerate quite often. the price betwen grades is a dollar per tank give or take. if you have any audible ping with 87 you need to lower your timing or clean your combustion chamber. if you can hear pinging your engine was getting damaged way before that. i come from the boost side of fun so even 91 in my other car is substandard.

my mileage got better in the aspire with the lower octane but its because i had to change my driving practice to not detonate. no spirited driving. newer cars have sophisticated knock sensors that pull timing when knock is occuring but less timing means less power. your 'pulse' wont be as good as if the engine was operating perfectly.
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Old 07-28-2008, 06:12 PM   #13 (permalink)
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Here is my take...with 35 years of drag racing street driven cars:

If your car does not require premium and the timing is stock you can actually get worse mpg with higher octane, The higher the octane, the slower it burns....a lot of cars drag raced with premium fuel will actually run slower and have less power. Just run reg if your car calls for it.

As far as running 87 in a car that requires 92.....on my 05 SRT4 Neon (230 hp 2.4 turbo bone stock) I WAS running 87 when I first got it. On my 80 mile loop that I use for mpg comparisons I got only 27 at 55-60mph while it was rated at 31. I even fill up at the same pump and put it on slow speed till it clicks off...not a penny more...then with 92 oct I then got the 31. That car now gets 32.8 with a stage 3 set up that developes 285 hp!

As far as 10% ethenol is concerned.....you might get as bad as 20% WORSE mpg since more fuel is needed to obtain the same driving parimeters. I read in a magazine last year that E85 can be as bad as 27% worse than straight gas and on my flex fuel 01 Caravan it did get @ 25% worse on E85 compared back to back with reg 87 oct gas.

Guys that convert their drag race cars to alcohol must increase the jetting in their carbs by about double for the same power levels, but then they benefit from a cooler running motor and can mofify them to run more compression and or boost.
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Old 07-28-2008, 06:59 PM   #14 (permalink)
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That first tank was awful low at 18mpg as compared to the epa numbers. I bet you filled the previous tank on the uphill side of the pump and did not pump it as full as the current one. Todays gas tanks are pretty flat and it can make a big difference which side of the pump you fill from.

In my old corrolla, 87 and 89 were about the same MPG and 91 5% worse. I ran 89 cause it would never ping that way and I figured retarding the timing would be worse for MPG

In my current CRX HF the gas mileage at freeway speeds is pretty much the same for all grades, but at low RPM cruising, 45 MPH, 5th gear, 1400 rpm for example, there seems to be about a 5% increased MPG per grade according to my supermid and my last 3 tanks.
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Old 07-28-2008, 07:16 PM   #15 (permalink)
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I have seen octane make a difference while towing. With higher octane I get a little better FE if the weather is over 85 degrees. I think my '88 Ford does a poor job of adjusting the timing for hot weather, it senses detonation and pull sout 10 degrees or something, and really runs bad. The ECU might be dumping in 10% more fuel as to cool things off, as well? So on that particular vehicle, it makes a difference.

Just know that higher octane gasoline has the same BTUs as lower octane gas. Higher octane than required will actually reduce engine power.
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Old 07-30-2008, 09:50 AM   #16 (permalink)
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Quote:
Originally Posted by ATaylorRacing View Post
If your car does not require premium and the timing is stock you can actually get worse mpg with higher octane, The higher the octane, the slower it burns....a lot of cars drag raced with premium fuel will actually run slower and have less power. Just run reg if your car calls for it.
But thats at the high end. I doubt that I will see much of a difference on highway driving (infact my trip to Montreal showed that it doesn't make much of a difference, I got slightly over EPA). It might be a slower burn but its a bigger bang, I don't see how it would hinder low end MPG. Might muck with the timing but again, its the physics I know, not the engine stuff.

Quote:
Originally Posted by fritz View Post
That first tank was awful low at 18mpg as compared to the epa numbers. I bet you filled the previous tank on the uphill side of the pump and did not pump it as full as the current one. Todays gas tanks are pretty flat and it can make a big difference which side of the pump you fill from.
Yeah that was the worst fill I've ever seen, but i used the same pump as I did for these past fills so who knows what it was.
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Old 10-08-2008, 12:46 AM   #17 (permalink)
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Sorry to necro post but I wanted to update. While my fuel log is all over the map, I'm getting about 25 mpg with regular. I'm going to try 89 now, especially now that gas has come down.
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Old 10-08-2008, 01:42 PM   #18 (permalink)
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I used a tank of 91 octane in my '88 Escort that calls of 87 a few weeks ago because I couldn't get the 87. My driving habits and routes are about the same day in and day out and I didn't see any FE benefit although I could tell the car had more power.
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Old 10-08-2008, 02:03 PM   #19 (permalink)
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If you don't change the timing of your vehicle, I can see where some will say that increase in octane sees little results. BUT, if you were to use 91, then adjust timing to where it just stops pinging, then you should see enough of an increase in FE to justify the expense. I did the math and found that by doing just what I said, 91 octane is more than worth using for increased FE.

With 85 octane, I get 54 mpg (+5 deg timing). With 91 octane, I get 60 to 61 mpg (+10 deg timing). It costs me $.065 per mile in fuel with regular and $.060 per mile with premium. My break even mpg would be 57 mpg. Anything over 57 on premium makes premium fuel the least expensive.

Using regular 85 octane, it will cost me $142.19 per month in fuel over 2200 miles.
Using premium 91 octane, it will cost me $133.08 per month in fuel over 2200 miles.

87 octane puts the numbers about midway.

I can't make it any clearer.
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Old 10-08-2008, 02:11 PM   #20 (permalink)
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Quote:
Originally Posted by metromizer View Post
I have seen octane make a difference while towing. With higher octane I get a little better FE if the weather is over 85 degrees. I think my '88 Ford does a poor job of adjusting the timing for hot weather, it senses detonation and pull sout 10 degrees or something, and really runs bad. The ECU might be dumping in 10% more fuel as to cool things off, as well? So on that particular vehicle, it makes a difference.

Just know that higher octane gasoline has the same BTUs as lower octane gas. Higher octane than required will actually reduce engine power.
When I used to tow with the 6000 wagon, I would make it a point to put in 93 octane because it gave more power as well as reduced pinging. It made a difference without towing too, on the large hills that are on my way to and from work, but I would just use 87 usually because I didnt really need the power that bad. But the engine also had well over 200k on it. I figured that the added octane wont really give you more power than stock, but it will help compensate for the loss of power in a well worn high mileage engine.

Earlier this year I tried 93 octane for a few downstate tanks in the Celeb. I noticed no difference in mpg.

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