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Wait... your first post's synopsis is this: You don't own a contour and don't know the compression ratio. (FWIW: the most SVT's are 10:1) You state that anything over 9.8:1 requires "race fuel"  , but somehow the engineers at SVT state premium is fine for our 10.1 (or higher!!) You finished up saying that we should always go by our "seat of the pants" feeling BECAUSE that MEANS its working. Never mention that just because something FEELS right, doesn't mean you don't need to tune, etc (granted fuel is not going to affect air:fuel, etc, but your general premise is flawed..) Nice.. but I think I'll file this away under "whatever..." Ray
'99 CSVT - Silver #222/276
In a constant state of blow-off euphoria.
Originally posted by Kremitthefrog:
I like to wear dresses and use binoculars to watch grandmas across the street.
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Originally posted by Port:
Is there a reason nobody has mentioned compression ratio in this discussion? You run a certain octane based on your compression ratio, not based on timing. Ocatane is the ability of the fuel to resist combustion under compression. If you run cheap fuel in a high compression engine the fuel will ignite before the ignition lights it strictly from being compressed. That is detonation. You can cause detonation by advancing your timing because it ignites the fuel before the optimal time in the engine's stroke.
9.8:1 is the cuttoff for running 91 octane pump gas at sea level. Higher compression than that you need to start thinkin about race fuel. I am not sure what these contours have, I don't have one, YET, but I doubt they require 91 octane unless you are right at sea level. The higher in elevation you are the less air going into your engine wich lowers compression in the cylinder, so you can use cheaper fuel.
If you run high octane where you don't have the compression to get it to ignite, you are wasting fuel and potential energy in the wasted fuel every time the cylinder fires. Resulting in lost power and fuel mileage.
If your car gets better mileage on the cheap stuff, and the "seat-of-your-pants" feel is better, run it, that means it works better. Higher elevation and lower humidity will allow you to run cheaper fuel.
Compression ratio is too simplistic of a concept to make a full determination about octane.
The most critical element is cylinder PRESSURE. Cylinder pressure is determined by much more than just compression ratio. Cylinder presseure is effected by compression ratio, valve timing, ignition timing, combustion chamber shape, port configuration, number of valves, any restrictions in both the intake and exhaust systems, fuel quality, fuel mixture, flame propagation, flame temperature, engine load, and so on.
Jim Johnson
98 SVT
03 Escape Limited
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Originally posted by Port:
Is there a reason nobody has mentioned compression ratio in this discussion? You run a certain octane based on your compression ratio, not based on timing. Ocatane is the ability of the fuel to resist combustion under compression. If you run cheap fuel in a high compression engine the fuel will ignite before the ignition lights it strictly from being compressed. That is detonation. You can cause detonation by advancing your timing because it ignites the fuel before the optimal time in the engine's stroke.
9.8:1 is the cuttoff for running 91 octane pump gas at sea level. Higher compression than that you need to start thinkin about race fuel. I am not sure what these contours have, I don't have one, YET, but I doubt they require 91 octane unless you are right at sea level. The higher in elevation you are the less air going into your engine wich lowers compression in the cylinder, so you can use cheaper fuel.
If you run high octane where you don't have the compression to get it to ignite, you are wasting fuel and potential energy in the wasted fuel every time the cylinder fires. Resulting in lost power and fuel mileage.
If your car gets better mileage on the cheap stuff, and the "seat-of-your-pants" feel is better, run it, that means it works better. Higher elevation and lower humidity will allow you to run cheaper fuel.
Wow. This is just a simple Ford forum.
Despite this fact, you'll get eaten alive here with random posts like this. On import car forums...  maybe, maybe not.
You can't just determine octane requirements based on compression.
BTW, I ran 91 octane on my 10.2:1 motor with 6 pounds of boost in 100 degree Arizona weather That was the end of May, early June to be exact. So as far as I'm concerned I've OWN3D your azz on that one.
What was that again.... oh yes, you were talking out your arse.....
By the way, fuel igniting before the spark plug ignites it is called Preignition. Yes there is a term for it and it isn't detonation. If it is just slightly off from the appropriate amount of timing then it manifests itself as pinging. If it manifests too early then it can detonate.
Detonation is caused when the gas pressure of the burning fuel elevates too soon before the piston reaches top dead center and overcomes the piston force that is compressing it when it is ignited before top dead center. This results in knock and damage. The gas pressure tries to overcome the piston pressure as it comes up...somethings gonna break.
This can be caused by preignition, high temps, or bad fuel. All of these possibilities are affected by high compression.
You can just plain set your timing too far in advance and cause detonation with no hint of preignition and still have nothing to do with compression ratio.
BTW, timing your car right means you initiate burn with the spark plug well before the piston is at TDC in such a way that the gas expansion creates the max pressure right as the piston passes TDC.
Former owner of '99 CSVT - Silver #222/2760
356/334 wHP/TQ at 10psi on pump gas!
See My Mods
'05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red
'06 Mazda5 Touring, 5spd,MTX, Black
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Originally posted by DemonSVT:
Originally posted by warmonger:
the burn rate of the fuel doesn't mean it has more power or in your terms energy.
Relatively speaking and definitely in low efficiency car engines, the small difference in structural changes affects available energy very little.
I doubt it affects the fuel economy at all!
For a car programmed to run on low octane (87) fuel running high octane fuel will lower efficiency.
The slower burn rate & later point of ignition coupled with the smaller amount of ignition timing (later to initiate the flame front) will give less time to adequately burn the fuel. (in effect it's like retarding timing) Therefore the engine has lower efficiency which means less power & mileage.
I'm adding that in case you meant it both ways instead of the SVT, for example, retarding the timing curve to run lower octane fuel. Mileage will drop a hair but you are right it will be very minor at best. (once the PCM adjusts) However the torque (and subsequent HP) drop off will be notable. A good tuned chip is proof of that. Timing curve is EVERYTHING.  
This is very sketchy. In principal you are correct for significantly different grades of fuel, however, in terms of available pump grade gasolines it isn't applicable. A car tuned for 87 octane will run the same on 87 through 93 and probably even upwards towards 100 octane. It definitely won't run less efficient, it just won't see any benefit from it.
Now when you talk about running gasoline and then later methanol or propane or a fuel that is VERY different in its octane and power capacity, then yes, I think you'd be correct.
Former owner of '99 CSVT - Silver #222/2760
356/334 wHP/TQ at 10psi on pump gas!
See My Mods
'05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red
'06 Mazda5 Touring, 5spd,MTX, Black
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Originally posted by Port:
9.8:1 is the cuttoff for running 91 octane pump gas at sea level. Higher compression than that you need to start thinkin about race fuel. I am not sure what these contours have, I don't have one, YET, but I doubt they require 91 octane unless you are right at sea level. The higher in elevation you are the less air going into your engine wich lowers compression in the cylinder, so you can use cheaper fuel.
SVT Contour:
Compression ratio: 10.0:1
SO are you saying that all of us NEED race fuel? get your facts straight buddy.
And why would you use 91 when you could get 93 or even 94 
~Ryan
1999 T-Red SVT Contour
41k Miles #1701 / 2760
B&M, Custom Y-Pipe, Alpine HU, Pioneer 6x8's & 6x9's
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Damn it. Everyone got to make fun of the ignorant fool before I could.
2000 SVT #674
13.47 @ 102 - All Motor!
It was not broke; Yet I fixed it anyway.
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Greg, I don't mind giving you the points.. You know I'm here for ya when you aren't around to jump on someone..
Ray
'99 CSVT - Silver #222/276
In a constant state of blow-off euphoria.
Originally posted by Kremitthefrog:
I like to wear dresses and use binoculars to watch grandmas across the street.
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I can see that you are some bench-racing fools. There is a saying we use in racing, "you can beat 75% of the competition in the shop", I think you guys just got your "arse" handed to you. I would love to get lined up next to one of you guys.
Run 14:1 in one of your contours with pump gas and see if you can get your timing to take care of that, and then tell me that compression has nothing to do with octane rating. Compression ratio is not too vague. You need someplace to start. Then you can take into factor air temp, elevation, humidity, barametric pressure, the whole bit. Did you miss that lecture when Vin Deisel was pulling a wheelie with only one rod left in the block?
I said at 9.8 you need to start THINKING about running race fuel. I didn't say your motor would shell if you ran pump gas with 9.9:1 compression. 9.8 and 10 are pretty close to the same number if you guys arn't too familiar with decimals and math. Why do you think ford doesnt build all of its motors with 12:1 compression? Because the proper fuel is expensive and a little hard to get ahold of. Most of you arn't ever at sea level, so you dont have enough air to get that kind of cylinder pressure anyway. And forget about engineers, they don't have a clue what is going on. There head is burried too far in their calculator to do any good. They probably can't screw in a light bulb without doing a few calculations on how much resistance the friction on the threads is going to give them.
And the comment about running boost in Arizona, the air is hot, which makes it very thin, and also has very little humidity, which makes it even thinner. So you running that "massive" boost you are so proud of is probably only making your contour run in arizona like a stock one would run at sea level.
Lay off the "Fast and Furious" movies, you might learn something and get a little faster.
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I happened to be AT sea level with my 10.25 (or whatever it actually is) for over 2 years. Does that mean that I am defying logic?
Personal insults directed at the entire community (familiar with decimel/math, etc) are not the ways to get us to listen to, accept, or even talk about your "theory" there.
I suggest returning to whatever "shop" you came from, reading up a little bit, and returning when you can intelligently discuss whatever thoughts you may have, instead of insults..
Ray
'99 CSVT - Silver #222/276
In a constant state of blow-off euphoria.
Originally posted by Kremitthefrog:
I like to wear dresses and use binoculars to watch grandmas across the street.
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Let's lock this thread. Nothing useful is happening now.
Jim Johnson
98 SVT
03 Escape Limited
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