Octane
09-25-2007 | 06:34 AM
#16
Drifting
Joined: Dec 2004
Posts: 2,722
Likes: 0
From: Munich
That unattributed article is hugely misleading and sets out misleading formulae, for example, it should be -
(R + M) / 2
However given its origins, I would expect nothing less.
There is another side to the octane question and that is the quality of the fuel station. Many gas stations have diry leaky tanks, sometimes by accident or poor maintenance, sometimes its deliberate.
There is ample empirical evidence to support the hypothesis that the gas supplied by newly established gas stations performs better than possibly higher octane rated gas form older and more distant stations.
In my experience, the modern ECU does adjust to higher octane gas
R+C
09-25-2007 | 11:16 AM
#17
Formula One Spin Doctor
Rennlist Member
Rennlist Member
Joined: Jan 2005
Posts: 20,448
Likes: 0
From: RPM Central
"higher octane burns slower. this allows you to advance timing ---> HP."
This is not exactly a simple nor accurate statement. Increasing the ignition timing may give you more HP, IF, and only IF you can move the peak cylinder pressure to 12-14 degrees ATDC (whatever is optimum for an engine combination), AND assuming that it is not currently there on your given fuel. Simply advancing the ignition timing INCREASES the pumping losses because it starts the combustion process while the piston is still moving towards TDC. This is not desirable...
This is not exactly a simple nor accurate statement. Increasing the ignition timing may give you more HP, IF, and only IF you can move the peak cylinder pressure to 12-14 degrees ATDC (whatever is optimum for an engine combination), AND assuming that it is not currently there on your given fuel. Simply advancing the ignition timing INCREASES the pumping losses because it starts the combustion process while the piston is still moving towards TDC. This is not desirable...
Dependant on engine combination , BY 20 deg you should already have achieved peak cylinder pressures , you state 12-14 , i guess you are talking about your experiences with 911 engines ..
09-25-2007 | 12:07 PM
#19
Instructor
Joined: Aug 2007
Posts: 200
Likes: 1
From: North Palm Beach, FL
what is the difference in Exhaust Gas Temperature and Air Fuel Ratio between 93 and 100 oct. I am assuming you ran AFR when on the Dyno and that should be the key to if you need 100 octane or not.
09-25-2007 | 12:10 PM
#20
Drifting
Joined: Dec 2004
Posts: 2,722
Likes: 0
From: Munich
I received the following private message from A Wayne - does he work for Blackwater - should I be worried?
the text of a threatening private message has been removed at the request of the web master.
R+C
the text of a threatening private message has been removed at the request of the web master.
R+C
Last edited by Nordschleife; 09-26-2007 at 07:46 AM. Reason: I published a private message threatening me, apparently that is 'wrong'
09-25-2007 | 12:42 PM
#21
Formula One Spin Doctor
Rennlist Member
Rennlist Member
Joined: Jan 2005
Posts: 20,448
Likes: 0
From: RPM Central
09-25-2007 | 01:10 PM
#22
Nordschleife Master
Joined: Dec 2001
Posts: 8,305
Likes: 12
From: Kingston, NY
EGTs will vary with ignition timing, AFR, and cylinder pressure. AFR has little to do with determing the need for higher octane fuel. The ignition timing and cylinder pressure as well as where the peak is will give you an idea of what octane may be ideal for a given engine combination. Just as an aside note, I run my 12:1 compression RSR engine with very large camshafts on 93 octane Mobil street fuel I get at my local fuel station. The engine produces within 4hp of the best factory RSR sprint engine with slide valves I have ever tested and that engien ran on 110 octane and was higher compression than my engine.
09-25-2007 | 11:40 PM
#23
Rennlist Member
Joined: Sep 2003
Posts: 9,170
Likes: 10
From: New Jersey
Just as an aside note, I run my 12:1 compression RSR engine with very large camshafts on 93 octane Mobil street fuel I get at my local fuel station. The engine produces within 4hp of the best factory RSR sprint engine with slide valves I have ever tested and that engien ran on 110 octane and was higher compression than my engine.
09-26-2007 | 09:20 AM
#24
Nordschleife Master
Joined: Dec 2001
Posts: 8,305
Likes: 12
From: Kingston, NY
My engine has technology that is 10 years newer than the factory had at the time they produced the RSR. It has heads that are more efficient in their construction and ports that have a much better design, and a combustion chamber that carefully controls the piston to head clearance. This makes it more efficient. I have camshafts that I designed which have much better valve control and therefore can run lighter springs. If I ran 100 octane, I would be able to pick up power for sure, so running 93 is a compromise in power vs running cost. I still have areas of improvement like running the factory RSR headers over the cheap stuff I have in there now. I'm also working on a new piston and connecting rod combination to reduce weight and reduce the rod/stroke ratio and fix the terrible designed Mahle RSR pistons.
I hope this helps...
I hope this helps...
09-26-2007 | 12:52 PM
#25
Rennlist Member
Joined: Feb 2003
Posts: 29,956
Likes: 180
From: saratoga, ca
no, the lower octane gas has a higher energy potenial. the higher octane allows for higher compression, advanced timing and other things to more than make up for the lower energy potential. the octane, as was mentioned, reduces detonation, and by causing a slower burn rate.
mk
mk
This is what I understand about gasolines. The higher the octance the higher energy content, lower octance ignites at a lower temperature and this is why running a lower octane in a higher compression car causes that pinging you hear. Its predetonation. A higher octance takes more heat to ignite thats why it is more suitable for higher comression cars...not that it burns slower. Burn a higher octane (with higher BTU content) more completely (with a higher compression engine) - you get more energy (more power to the rear wheels). I had asked the same question previously and this the answer I got with regards to running 100 compared with 93 in a car that is rated for 93.
The BTU content varies from station to station and even to load of gas to load of gas. To ensure that you are getting every bit of BTUs your car can use, run the 100 octane. All it does is eliminate a small variable in your equation. Will you see a difference? Probably not and for the extra 3 dollars per gallon, ($1200) that's 6 new tires...think of all of the grip that will buy! A whole lot more performance that what ever negliable benefit you might get from a higher octance.
just my $0.02
The BTU content varies from station to station and even to load of gas to load of gas. To ensure that you are getting every bit of BTUs your car can use, run the 100 octane. All it does is eliminate a small variable in your equation. Will you see a difference? Probably not and for the extra 3 dollars per gallon, ($1200) that's 6 new tires...think of all of the grip that will buy! A whole lot more performance that what ever negliable benefit you might get from a higher octance.
just my $0.02
09-26-2007 | 04:28 PM
#27
Lifetime Rennlist Member
Joined: May 2001
Posts: 4,929
Likes: 4
From: KC
Some higher octane fuels have more energy, some less. Some burn faster, some slower. That is not a function of Octane.
One reason there is so much misunderstanding about octane is that one person does a test and then tries to generalize the results to octane rating.
09-26-2007 | 04:34 PM
#28
Rennlist Member
Joined: Feb 2003
Posts: 29,956
Likes: 180
From: saratoga, ca
\
09-26-2007 | 04:39 PM
#29
Race Director
Joined: Oct 2001
Posts: 10,033
Likes: 1
From: Houston, TX USA
It's been a while since I've looked up the engineering/scientific definition of detonation, but that is NOT it. It's similar but not the same.
09-26-2007 | 04:45 PM
#30
Lifetime Rennlist Member
Joined: May 2001
Posts: 4,929
Likes: 4
From: KC
Burn (rate) is when you have a flame front that progresses across the combustion chamber. Detonation is when all (or some) of the A/F charge explodes - there is no flame front in that case.
A faster burn rate will tend to case higher pressures and may lead to detonation, but there is more to it.
BTW - I am probably the only person on this forum to have operated the test engine and done octane ratings. The original test had a certain set of operating conditions (air temps, water temps, etc). When more vehicles started using automatic transmissions, those conditions were not appropriate so another set of test conditions were developed. The original was Motor Octane Number and the latter was Research Octane Number (MON and RON). What we have not is an average of the two. You can have two fuels of the same "Octane" (RON + MON /2) that perform differently in different cars because those cars do not operate very close to the test conditions.
A faster burn rate will tend to case higher pressures and may lead to detonation, but there is more to it.
BTW - I am probably the only person on this forum to have operated the test engine and done octane ratings. The original test had a certain set of operating conditions (air temps, water temps, etc). When more vehicles started using automatic transmissions, those conditions were not appropriate so another set of test conditions were developed. The original was Motor Octane Number and the latter was Research Octane Number (MON and RON). What we have not is an average of the two. You can have two fuels of the same "Octane" (RON + MON /2) that perform differently in different cars because those cars do not operate very close to the test conditions.