I'm a bit confused about something. I've always thought that the higher the compression ratio the higher grade fuel required to prevent pre-ignition but as I've done more research I've noticed that a lot of cars out these days that require only 87 octane gas have rather high compression pistons some as much as 10.8:1. The 05 Mustangs have 9.7:1 & 9.8:1 compression ratio which I would have thought is better suited for the higher grade gas. I remember before the advent of computer controlled everything for an engine cars that used the cheap stuff would have compression ratios as low as 8:1, some even lower. It seems that most cars these days would benefit from the higher grade fuels with proper programming of the computers. I know that many people that have reprogramed their 05 Mustangs with either SCT or Diablo reprogrammers have not only gotten increased power but fuel economy also. Two mpg increase is not unusual and with the difference in price between low grade and premium fuel being only about 20 cents an increase in fuel economy of 2 or more mpg would make up for most of that price increase so people wouldn't really be losing much if any money by using the premium fuel. It doesn't make much sense to me that modern cars would be hobbled with low octane fuel when it's not necessary or significantly more economical to use.

If anyone can explain to me what the deal is with high compression pistons in modern cars being used with low octane fuel. There must be a reason it's done and some benefit to it. If so I must be missing something.

 

it's not the pistons that determine whether or not premium fuel is required. notice i said required versus recommended. the difference is enormous and you touched on it a bit. many ignition management systems have software to adjust the output of the engine depending on the grade of fuel used. the mustang engines, from what i have read, do NOT have this active management.

some auto makers, cadillac for one, decided about 5/6 years ago to redesign their northstar motors to run on 87 grade with no recommendatuion to run on the higher grade fuels. they did this by changing the compression ratio (a slight reduction), regrinding the cams and changing the cam timing, and remapping the ignition curve. the lst three are the most critical to the fuel grade required for the engine. cadillac did not reduce the output of the northstar, they just moved the power band around a little bit, the motors that run on 87 won't have as much throttle response as the older motors, but the difference is slight.

one last point; on my 86 honda interceptor motorcycle, the compression ratio is 10.5/1. i always assumed that the motor was designed to run on high test...when i checked the manual, there is no recommendation (forget about warnings about using lower grade fuel!!) to use the higher grade fuels. the engine even 20 years ago was designed and engineering to run on 87 with no knocking or pinging...and since the motor is liquid cooled, i would know if the thing was knocking. now, on the motorcycle, the the ability to run 87 is strictly the cam grind and timing...there is no electronic monitoring going on or anything like that. it's all done with the cam profiles and how the cams are timed. hope this helps somewhat. jackg 90seville 97k

 

I think the timing has a lot to do with it also. The programmers out can bump the timing up enough that you have to use 93 octane. It appears the v6 was detuned so as not compete with the GT.

 

I'd say it's all in the timing, since fuls are rated in their rate of burn. lower rate faster burning, higher rate slower burning.
But it does make sense to use higher grade on high compression to prevent a low grade fuel from igniting w/o spark.

 

the ignition timing is calibrated for certain octane requirments.

if the engine management computer is "intelligent", meaing it senses for knock and retards the ignition timing accordingly, then use of a higher octane can improve performance/fuel economy.

the engines in the 05 mustang do NOT, as far as i know, have knock sensors built into their engine management computers...so, the use of fuel with a higher octane than what is recommended by ford will not improve the performance of the engine.

in some cases, due to the chemicals that are added to 87 octane gas (all fuel starts out as "regular" and the chemicals are added to forestall knocking, that's how they get the higher octane ratings) fuel economy actually goes down and deposits can form in the combustion chambers!! this has been reported on some of the caddy forums i frequent..so...if your engine calls for 87, stick with it. jackg 90seville 97k

 

The 05 has two knock sensors on the V8 and one on the V6. This comes from the spec PDF at the offical Mustang site. A magazine claims the new Mustang will pickup 5 horsepower with premium. Car and Driver did a test on this maybe 5 years ago. The 4.6L Mustang GT did pickup a small amount of horsepower. Premium cars really need premium. A BMW M3 lost 15 or so horsepower.

 

This is really a complicated topic.

Just putting in higher octane and advancing the spark does not give optimum power because there is an optimum spark point regarless of the octane. The engine has to be designed for optimal performance with a specific octane fuel.

The higher compression ratio on new engines that run 87 octane are a combination of several things

Combustion chamber shape
Spark plug location
refinement of computer contols
knock sensors

The short of it is that newer engines have improvements in all those areas which allow them to run a higher CR with 87 octane gas.

But with the 05 Mustang GT the single factor that is most responsible for the ability to run higher CR with 87 octane is the more central location of the spark plug in the 3V heads.

 

Just got the August copy of 5.0 Mustang. According to the magazine, the 05 GTs are tuned for 91, the knock sensors/ECU will retard the timing for lower grade fuels. Like the previous post said, a complicated subject. I think the aluminum block helps the engine run a higher compression. I read iron is better for power, but aluminum is better for detonation.

 

thanks for the straight info...i stand corrected.

the most overriding, important aspect to this question to remember, is follow the advice of the manufacturer. if the manual doesn't state specifically that higher h/p or better performance will be the result of a higher grade of fuel, then don't waste your money.

here's an example, in the 05 toyota avalon with the twim cam 4 valve 3.5 v6 the engine is rated at 280hp and 260 tq WITH a * next to the ratings. with lower grade fuel, other than premium, it states that the output of the engine will be reduced. i don't believe that ford has done the same thing with the 4.0 and 4.6...what you see is what you get...no real improvement in using premium...and as i stated, it could cause deposit issues with the c/c.

the best advice to ANY engine use is drive it like you stole it...keep the revs up and max it out on a regular basis...once a month do a throttle braking procedure...take it up to redline in 2nd gear and back it down without using the brake opedal ..just let the motor do it's thing decelerating..do that 2/3 times..that is the BEST procedure for keeping your engine free of deposits and that is the number one cause for problems with engines as they begin to age....they need to be worked hard and allowed to rev free on a regular basis. jackg 90seville 97k

 

Great stuff guys. Really enjoying the read. I'll toss a little redundancy into the ring as well.

In the old days, the higher the horsepower, the more demand there was for premium fuel. on a 4v (4 barrel carburated) engine, you would most likely find a "Premium Fuel ONLY" sticker somewhere in the engine bay. As you increased the power of the engine, you had to increase the octane rating of the fuel to keep it from knocking.

The higher the octane, the more Hydrogen molecules there are in the chain. It's all in the refining you see. Originally, it was called OCT-ane because there were 8 in the strand. Over time, it's come to mean something more of a generic measurement (slightly off topic, but fun trivia, none the less).

In the old engines, you MIGHT be able to get the engine to run on lower grade fuels if you modified the timing. Just turn the distributor and you could find the sweet spot. If you increased the octane, you needed to advance the timing to compensate. You could now run much closer to TDC (Top Dead Center) than previously, while avoiding the knock. This makes the engine run more efficiently.

Find a buddy who knows this stuff and has an old car. they can show you with like 10-15 minutes of your time. It's really fun to watch how the engine responds to the varying timing adjustments.

What's more, you can also, like JackG says - adjust the timing VIA the camshaft. There's also the grinding to take into account, like you said Jack. The lift of the cam lets more air in. You tune the carb, and adjust the timing accordingly.

New cars do all of this for you. When I drive my classic, I keep a 1/2 in. wrench and a flat head screwdriver with me at all times (Timing and Carburetor). The higher the HP, the more "finicky" the engine gets. You end up adjusting the carb and the timing quite often. Heck, if my wife changes her mind, I have to make adjustments. It's a ritual that requires no fancy instruments if you've done it enough.

The beauty of these computerized systems is that it's like having a mechanic under the hood to constantly adjust EVERYTHING for you for optimum conditions.

Will Higher Octane help your MPG? Dunno... never looked into it. It's an interresting thought, and I see some reference material on here that's substantiating it. Will it help your performance? With these new cars... nearly instantly. You may not get huge numbers off of a dyno, but you'll certainly notice the car runs SMOOTHER and has a little bit more bite to it's bark. If I didn't drive 80 mi. each day, I would run Hightest. It's hard to justify the cash when you don't get big payouts, ESPECIALLY with gas prices being what they are. Still, I try and run a tank of hightest through every now and then. That keeps deposits cleared out and gives me a little pep to enjoy for at least one tank full.

 

StangerX you have a number of incorrect statements in your post.

1. The # of 'barrels' in a carb has nothing to do with an engnes octane requirements.

2. You have this backwards, higher octane allows the spark to occur farther before top dead center (not closer). Yes increasing the octane on an engine allows you to run more timing advance BTDC without knocking occuring, but there reaches a point where too much advance actually causes a loss in power because maximum burn pressure occurs while the pistons are sill going up in the cylinder.

The key to generating maximum power is that you want maximum pressure in the cylinder when the piston is approximately TDC. The A/F mixure has to be ignited BTDC because it takes time for the A/F mixture to burn and reach maximum burn pressure.

Just taking any engine, putting in higher octane and advancing the timing may not give you any HP increase, in some cases it may decrease HP for the reasons stated above.

3. Higher octane has nothing to do with making an engine run smoother. In fact it may make it run rougher because the higher the octane the shorter the burn time.

4. The octane rating of the engine has nothing to do with cleaning out deposits. This misconception comes from years ago when "Premium" gasoline was the first to add cleaning "degergents" into it. Today all gas has some sort of deposit cleaning additives. However oil company advertising perpetuates this myth becaue it is intended to get consumers to buy the much higher profit margin premium gas. So the advertising sort of infers (incorrectly) that only the premium grades have these detergents.

 

do you mean it'll burn slower, because the higher the octane the slower it burns.
By being a short burn thats the same as a fast burn, and thats what lower octane fuels do.