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      10-10-2008, 03:47 PM   #23
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Quote:
Originally Posted by J08M3 View Post
As far as I know this is false info. Higher octane will not cause any damage to your engine. It will only cause damage to your wallet. It does have the possibility to improve performance since higher octane fuel burns slower. The slower burn of the fuel is good because it will continue to burn for a longer period during the power stroke of the engine. This is a benefit because power will continue being created during the expansion of the burning gasses for a longer period of time instead of a quick explosion then nothing more for the rest of the power stroke. For best performance you the most complete and slowest burn of the fuel.
Are you saying higher octane results in slower flame propagation? Also, there is limited amount of time for combustion to be completed, and it is possible to end up with incomplete combustion, so there must be some kind of trade-off there.
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      10-10-2008, 05:00 PM   #24
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hey guys, never used octane booster so can't comment on it.

however, fifthgear actually did a test in a research lab where they dynoed an engine using various octane boosters. The result is, at least under test lab conditions, the engine actually lost power...

nonetheless, they did say that perhaps if you use it for an extended period of time, if might help. but not much.
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      10-10-2008, 05:57 PM   #25
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I understand we discussed this before. All I was asking was when the engine is cold, its "erky-jerky" until it warms up, however, with a higher octane, this disappears.

AM I right that I am not the only one who engine is "not happy" til it warms up?
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      10-10-2008, 06:39 PM   #26
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Quote:
Originally Posted by lucid View Post
Are you saying higher octane results in slower flame propagation? Also, there is limited amount of time for combustion to be completed, and it is possible to end up with incomplete combustion, so there must be some kind of trade-off there.
Yes higher octane results in slower flame propagation. It is true there is a limited time for combustion to take place. What happens with low octane is it burns too fast and is all burned before the piston hits BDC (bottom dead center). Therefore there is a period of time the piston is still on the power stroke and the flame has finished burning. If you have a fuel that will burn for more of that time you have the potential for more power. This is just one variable associated with different octane. Another important one with higher octane is it allows for higher compression, which heats the gas more and causes more of it to be burned during the power stroke. Of course all of this needs to take place in a combustion chamber that has the right shape. You want the most gas burned completely for the longest possible time to achieve the most power.
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      10-10-2008, 06:50 PM   #27
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Quote:
Originally Posted by lucid View Post
Guys, we discussed this multiple times. Although the compression ratio is obviously mechanically fixed (there are experimental engines that have variable compression ratio btw), there some kind of margin for adjusting the timing, and if you put in higher octane fuel, to a certain extent, you get some benefit. After a certain point, increasing the octane does not make a difference. We have seen data on the E92 M3, and the E46 CSL that proves this.

I also remember seeing one of the British TV programs testing a bunch of off the shelf "octane booster" in a proper engine lab. If I can remember correctly, none of them increased output, and some actually decreased output. I don't remember which products they tested exactly though or what the test engine was.
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Quote:
Originally Posted by J08M3 View Post
As far as I know this is false info. Higher octane will not cause any damage to your engine. It will only cause damage to your wallet. It does have the possibility to improve performance since higher octane fuel burns slower. The slower burn of the fuel is good because it will continue to burn for a longer period during the power stroke of the engine. This is a benefit because power will continue being created during the expansion of the burning gasses for a longer period of time instead of a quick explosion then nothing more for the rest of the power stroke. For best performance you the most complete and slowest burn of the fuel.
Agreed, higher octane would not damage your engine at all!!!! My bad!! It just won't make, for the most part, any difference above the point where your car's ECU was tuned.
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      10-11-2008, 03:33 AM   #28
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I don't know if this is relevant, but I knew a guy in high school who put octane booster in his crappy old mazda. I don't know exactly what it did, but he ended up having to flush the fuel system.
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      10-11-2008, 07:54 AM   #29
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Quote:
Originally Posted by J08M3 View Post
Yes higher octane results in slower flame propagation. It is true there is a limited time for combustion to take place. What happens with low octane is it burns too fast and is all burned before the piston hits BDC (bottom dead center). Therefore there is a period of time the piston is still on the power stroke and the flame has finished burning. If you have a fuel that will burn for more of that time you have the potential for more power. This is just one variable associated with different octane. Another important one with higher octane is it allows for higher compression, which heats the gas more and causes more of it to be burned during the power stroke. Of course all of this needs to take place in a combustion chamber that has the right shape. You want the most gas burned completely for the longest possible time to achieve the most power.
Good to know about octane and flame propogation. I am aware that octane raises the auto-ignition temperature of the fuel though. I still don't about making a general statement about slower being better. There are many variables, and I must assume the optimal flame propogation velocity is different for different engines. I follow the scenario you outlined about the combustion potentially ending way before BDC with low octane fuel, but I don't know that it is the case for every engine. I'll look this up.
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      10-11-2008, 08:55 AM   #30
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This is the theory I was taught. I never saw the inside of a combustion chamber while and engine was running in real life. Although I believe compression and combustion chamber shape have a bigger effect in producing power than the speed of the burn. Many lower performing engines don't completely burn all the fuel that is injected in the cylinder. I believe burning all or as much of the fuel as possible is the most effective way to gain power, more so than slowing the time down it takes to burn. Anyone else have any input on this?
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      10-13-2008, 10:28 AM   #31
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Quote:
Originally Posted by J08M3 View Post
This is the theory I was taught. I never saw the inside of a combustion chamber while and engine was running in real life. Although I believe compression and combustion chamber shape have a bigger effect in producing power than the speed of the burn. Many lower performing engines don't completely burn all the fuel that is injected in the cylinder. I believe burning all or as much of the fuel as possible is the most effective way to gain power, more so than slowing the time down it takes to burn. Anyone else have any input on this?
Yes I think your assumptions are correct, but today’s engines for the most part burn most of the fuel inside the cylinders, especially since the advent a fuel injectors and ECU that control the amount of fuel/air in the cylinder. Fuels today also have additives such as MTBE or Ethanol (the case in most US states) to burn fuel more efficiently and reduce contamination. As a set rule, if more fuel is pumped into the cylinder and not matched with more air (that is only about 21% oxygen), then you would be wasting fuel, and vis-versa. The stoichiometric relationship of fuel and air molecules must always be met or exceeded to obtain more power. Slowing the timing when there is equal fuel/air mixture would simply reduce power. Burning fuel more efficiently by perhaps changing valve timing to allow more air in would increase power for instance. So I suppose in this sense you are right on.

Cylinder/piston design is more to do with how much air you want in the cylinder (naturally to be matched with sufficient fuel etc…) or whether you want more torque or horse power. For instance, a longer stroke will produce more torque but lower hp than the opposite design. There is also the delicate matter of forced induction whereby pistons may be switched or designed with a concave (low compression piston) head rather than flat or even convex in order to reduce the compression ratio and then compress more air into the chamber without premature detonation issues. Finally another important consideration in engine design is rotating mass/weight of the piston/crank assembly and here is a great ballgame regarding metallurgy and thermodynamics. But his is a different matter altogether, though fun as hell.
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      10-13-2008, 10:47 AM   #32
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Originally Posted by BMW-M-Mexico View Post
Cylinder/piston design is more to do with how much air you want in the cylinder (naturally to be matched with sufficient fuel etc…) ..... But his is a different matter altogether, though fun as hell.
This is true, but piston design (concave, flat, convex...) also changes the shape of the combustion chamber. So does the design of the cylinder head. This greatly affects how the flame travels through the fuel. This is an interesting topic because you realize how many variables are included in producing power. Important to note there is compromise with every change made. It is a constant balance of all the variables to get maximum power.
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      10-13-2008, 10:52 AM   #33
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Quote:
Originally Posted by J08M3 View Post
This is true, but piston design (concave, flat, convex...) also changes the shape of the combustion chamber. So does the design of the cylinder head. This greatly affects how the flame travels through the fuel. This is an interesting topic because you realize how many variables are included in producing power. Important to note there is compromise with every change made. It is a constant balance of all the variables to get maximum power.
Yep, totally agree with that!!! It does get really complex and lots of fun. At the end of the day, you never really know how these thing will affect the engine etc... untill you build it and test it.
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      10-13-2008, 11:01 AM   #34
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Yep, totally agree with that!!! It does get really complex and lots of fun. At the end of the day, you never really know how these thing will affect the engine etc... untill you build it and test it.
Yep, hence the wonderful industry that has be born from this issue, and the reason there is always room for new ideas and improvement.
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      10-13-2008, 11:12 AM   #35
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Yep, hence the wonderful industry that has be born from this issue, and the reason there is always room for new ideas and improvement.
Couldn't agree with you more!! Reasons why I have my car in Lake Forrest, CA with Racing Dynamics (RDSports to others) where they are rebuilding the S65 engine with a very nice stroker set up. Keeping it all NA (as I already have FI engines) and with at least 534bhp and 400+ lbs/ft of torque. Sort of my own CSL I suppose.
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      10-14-2008, 07:01 AM   #36
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Back to the original topic... Octane booster. So what exactly is in the little bottle they call Octane Booster? Does it really change the octane of your fuel? What is the difference between fuel that has had the octane raised by Octane Booster, and just buying fuel that is already a higher octane?
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      10-14-2008, 08:33 AM   #37
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After reading a lot of different threads I am suprised to find that the average fuel seems to be 91 or 93 octane there in the States and that you have trouble getting 93 sometimes is that correct?

Here in Australia althought our fuel is dearer overall but we have a choice from 91,93,95 & 98 and the higher octanes also have ethanol up to 10% added. We did have the Shell V-Power Racing 100 octane fuel but shell has stopped making it as of July 08.

Mobil here has S8000 which is 98 which everyone seems to think is better that the Shell normal V-Power 98. So I run both my cars the M3 & AMG on 98 (even with 10% ethanol sometimes) and don't have any problems. The AMG has been running for over 20 months on it.

Most of the fuels in Europe have ethanol added anyway. As from 2009 the main V8 racing car scene here in Australia will be running E85, that's a 98 octane with 15% ethanol I think. They have stated that during testing there has been no noticable difference in revs or HP.

Fuels with ethanol here in Australia have tax relief so they are usually cheaper, on the right day you can buy a 98 for 93 price if you hunt around.
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