Aeromod hopes dashed.

[superchow]

Jay,

If you run a lower octane fuel than recommended, and the engine begins to detonate, the "knock sensor" will retard the timing and stop the knock.

I agree.

Octane slows the burn rate in the combustion chamber, and doesn't "give" you anything.

I respectfully disagree.

It is true that one is trying to keep the air/fuel mix to self-ignite to avoid knocking. Wikipedia (I know, I know, don't trust anything you read on the internet...) states that Octane rating:

is used as a reference standard to describe the tendency of gasoline, petrol, or benzin fuels to self ignite during compression prior to the desired position of the piston in the cylinder as appropriate for valve and ignition timing.

As far as I know it does not affect the actual speed at which the gasoline will burn when it does ignite. (Please correct me if I'm wrong - THAT would be fascinating!)

Hm... how can I re-phrase what I am thinking. What about Diesel engines?

They operate without spark plugs based on the principle that the air/fuel mix self-ignites because of the high pressures within the cylinder. Compression ratios are higher (around 16:1 if I recall). Because temperatures are higher, and also because diesel contains more energy per unit of volume, the explosions are more efficient. Same amount of fuel + bigger bang = more efficiency.

By extension a higher compression ratio in a gasoline engine should yield higher efficiency. The only way I know of how to achieve this is to have a air/fuel mix that can be compressed more without self-igniting. And since the engine has a fixed displacement this can only be achieved by allowing more air without allowing more fuel to be injected. Therefore the valve timing adjusts to allow maximum air within the cylinder. This increases the air temperature as the air is compressed more. Now you have a lot of really hot air that needs a fuel that does not self-ignite - ergo - a higher octane rated fuel.

And since all cars I know have excess cooling capacity the excess heat is negligible in the life-expectancy-of-components scheme of things.

But please, (no sarcasm) correct me where I am wrong. I am here for the same reason as all of us: to save fuel and coin and improve efficiency.

[dennyt]

What's your instrumentation & driving strategy? I only have a vacuum gauge, and I'm committed to cruising at 15 in.Hg these days. Sometimes I go slow, if there's a headwind, and sometimes I go fast if there's a tailwind.

Aero will buy you more speed, but if you drive with load you can still get great mileage, just at a slower speed.

If you really want better aero, look into a partial kamm-back on the hatch. It shouldn't interfere with access.

Also, that 4psi you're giving up on the tires is low-hanging fruit. Tire "blowouts" from excess pressure are a thing of the past, 50-80 years past.

[brucey]

Fat Charlie:

Since we have a similar car, (although yours is a few years newer, and a turbo) I thought you might like to know that I've blocked my entire bottom grill out without any sign of increased heat. This includes driving around in today's 105 degree weather up and down mountains at any speed between 5 and 75 mph. The only time I've removed it in 3 years of use is when towing, as my mileage is going to be crap anyway and I'd rather not risk it. It also makes a noticeable difference in mileage with such a large bottom grill.

Also think it's funny you want larger mirrors, since I want smaller ones! They look like they belong on a truck. Want to trade?

[jkv357]

Originally Posted by superchow

(SNIP)I respectfully disagree.

It is true that one is trying to keep the air/fuel mix to self-ignite to avoid knocking. Wikipedia (I know, I know, don't trust anything you read on the internet...) states that Octane rating:

Quote:
is used as a reference standard to describe the tendency of gasoline, petrol, or benzin fuels to self ignite during compression prior to the desired position of the piston in the cylinder as appropriate for valve and ignition timing.


As far as I know it does not affect the actual speed at which the gasoline will burn when it does ignite.

(Please correct me if I'm wrong - THAT would be fascinating!)(SNIP)

No problem. That may be more accurate. My comment may be an oversimplification, and not technically correct.

I'll search around and see if I can find more specific info.

EDIT: a generally accepted description of what octane does is:The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. You're right - my comment about octane slowing combustion speed was not correct.

Jay

[graydonengineering]

Originally Posted by superchow

Jay,



I agree.



I respectfully disagree.

It is true that one is trying to keep the air/fuel mix to self-ignite to avoid knocking. Wikipedia (I know, I know, don't trust anything you read on the internet...) states that Octane rating:


As far as I know it does not affect the actual speed at which the gasoline will burn when it does ignite. (Please correct me if I'm wrong - THAT would be fascinating!)

Hm... how can I re-phrase what I am thinking. What about Diesel engines?

They operate without spark plugs based on the principle that the air/fuel mix self-ignites because of the high pressures within the cylinder. Compression ratios are higher (around 16:1 if I recall). Because temperatures are higher, and also because diesel contains more energy per unit of volume, the explosions are more efficient. Same amount of fuel + bigger bang = more efficiency.

By extension a higher compression ratio in a gasoline engine should yield higher efficiency. The only way I know of how to achieve this is to have a air/fuel mix that can be compressed more without self-igniting. And since the engine has a fixed displacement this can only be achieved by allowing more air without allowing more fuel to be injected. Therefore the valve timing adjusts to allow maximum air within the cylinder. This increases the air temperature as the air is compressed more. Now you have a lot of really hot air that needs a fuel that does not self-ignite - ergo - a higher octane rated fuel.

And since all cars I know have excess cooling capacity the excess heat is negligible in the life-expectancy-of-components scheme of things.

But please, (no sarcasm) correct me where I am wrong. I am here for the same reason as all of us: to save fuel and coin and improve efficiency.

I agree. In an engine designed with higher compression, lower octane fuel will have a tendency to pre-detonate because it will be under more pressure by the time the plug sparks. If the fuel does detonate early, it will not be at the correct "time" and performance will suffer. The computer will attempt to correct by adjusting the air fuel ratio. IF your engine is designed for it, the higher octane fuel will produce more power for the same engine displacement. More pressure means more potential energy. This is why race engines do not run on pump gas. The higher the compression ratio the higher the octane requirement to run with proper timing.

If your engine is designed with standard piston compression, the fuel will not pre-detonate, as long as you use regular or better. In this case, using higher octane fuel will offer no benefit.

Refer to the manufacturers recommendation for what octane you should use and don't wast your money on anything higher. As you engine ages (cylinder rings wear) the engines compression ration will gradually decrease to the point that it no longer runs at it's best on the recommended octane. A luxury car with a ton of miles often run great on regular even if it is supposed to require premium.

[brucepick]

Back to aeromods - how about smooth wheel covers?? And do be sure your tires are pumped up well. Not sure if you mentioned that already.

[Fat Charlie]

dennyt- My driving situation is the rat race. The Spaulding has 70-80 mph traffic and is a highway that's got everything bad on it: Dumb ramp and lane reduction combinations, local traffic, toll booths, construction, a bridge that people think is a pinball machine and basically no way to bypass any blockage. It's a high stress mad dash from the when I drop my kids off at daycare (the moment they open) through 45 miles of that to work. Now that I'm hypermiling I've given up the sprints, and I'm saving a lot by looking at the high speed gridlock as a great gliding opportunity. My main strategy change with hypermiling has been focusing less on shaving every possible second, waiting patiently for every passing opportunity. Shifting my efficiency focus more to fuel usage than time is much more rewarding and less frustrating- I can get high performance out of even the worst traffic. My driving tactics are P&G with DWL for getting up the hills.

Instrumentation is the car's mpg display and my phone stuck in the dash cubby running Torque. In an ideal world the car's display shows average mpg for the tank while Torque has dial gauges for load and throttle, digital displays for instant mpg and coolant temp and a graph showing the last 10 seconds of instant mpg. In the real world, shutting off the ignition shuts off the ECU and Torque isn't always happy about reconnecting after that. I'm left with the car displaying instant mpg and nothing else. I expect the kill switch will solve that problem.

I'm not willing to push my tires too hard. I'm a parts guy and I sell more suspension parts than you'd believe- the roads here in the northeast are brutal on cars. When Subaru calmed down the STI's engine and put it in the Legacy for '05, they gave it 225/45/17s. I guess they thought the 17s would make it look more sporty, but those tires are too low for the frost heaves and potholes we have here. With this tire size in this part of the country I just can't bring myself to go to max sidewall. I'm not worried about the tires failing on their own, I'm just worried about the tires not being able to handle the unreasonable abuse that our roads dish out. If I had 16s I'd be over sidewall by now.

A Kammback is a pipe dream for me at the moment. Not that I haven't given it a lot of thought, but it's far down the road. I don't focus on pretty, as such, but form follows function with aerodynamics. I'll start with aero where ugly can still help- underneath and inside. Those are smaller projects that have good shots at results; starter projects.

brucey- thanks for the moral support on the grille block, but there are things I have to get done first. I have to get a new thermostat put in because I've had the temp climb a few times in the last month. Then the kill switch so I don't lose my display every time I shut down the engine, then once I get used to a the lump's operating temp range I can start blocking air to the radiator.

[superchow]

Any chance there is a daycare closer to your work? I know I know nothing about your life situation, but that was the first thought I had in avoiding the rat race.

[Fat Charlie]

I can't pull the kids from where they are because what they're getting is too good. Commute-wise, it's less than half a mile off my route, maybe a mile from home. What's broken is working about 45 miles from home and working long hours. I've got three more years of that, then kid #2 is in kindergarten and I find something local with moms' hours.

In the meantime I've gone from burning four gallons a day to burning less than three. The my commute's biggest challenge is now the drive and not just staying awake. The next mpgs are going to be a fun challenge: I just had a 33 mpg tank, and 38 is double the car's EPA combined rating...

[dennyt]

Nice work!