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Old 12-27-2008, 10:36 PM   #1
bnaccs
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Default Gas octane rating

I have been running 85 octane in my 08 OBS. Will this hurt the car? It seems to run fine, or should I spring for the extra 87 octane. Is there any difference for my car? My car seems to get about 1 mpg better when I use 87 for a few tanks. Any thought , or what octane others use would be helpful.
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Old 12-28-2008, 12:38 AM   #2
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run, 93 oct. with an ispeed reflash.
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Old 12-28-2008, 03:06 AM   #3
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i can tell you from experience that the ECU is running more max timing advance than 87 or even 93 can handle, right from the factory. So it pulls timing until it gets a no knock timing advance that it is happy with, and stays there until something changes requiring more timing pulled or allowing more to be added (advanced). with 85, it is pulling at least as much as with 87, and should technically be pulling a bit more. On 87, factory ECU image i was running about 65% of my max timing advance and on 93 i was running about 77%. So besides the ****ty factory ECU timing/AFR tables, 85 probably wont kill your engine anytime soon, but it wont help it, will rob a tiny amount of power and potentially cause more knock events which is hard on pistons and rod bearings over time. If you really want to run with 85, you should tune for it. I tuned for 87 and 93 so i have an acceptable map for either.
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Old 12-28-2008, 09:48 PM   #4
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i have to disagree . the stock timing maps for the most part are right on for 87 octane.

yes running 85 is no good .

if your car calls for 87 run it . they dont even sell 85 where im from .

race face is talking about stock advanced timing maps which only advance timing in high rev high load cells if it has better gas . but the base tune is tuned for minimum best timing running 87 octane.

the problem lies in your base map which is tuned for 87 . most cells on the base timing map have no value for advance in the advanced map. it doesnt try to advance anything . but might knock when using a lower octane.
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Old 12-28-2008, 11:44 PM   #5
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RaceFace is right and watchung is wrong.

I have never yet seen a new-world (05+) NA subie that wasn't knocking like a mofo on the stock timing maps. In fact, the stock tune will even knock on 94 oct on the cars I've had the opportunity to study long-term.


To the OP: let your fuel mileage be your guide. The only way changing octane can change fuel economy is by allowing the engine to run more timing (which is more efficient). Run a couple of tanks 85 and get a baseline for fuel economy. Run a couple of tanks of 87 to get an average mileage and see if it's any different than with 85. Try the same thing with 89, 91, 93, and 94. At some point, the fuel mileage will quit getting better. That's the "optimum" octane rating for your car. Now, the question is wither or not that's worth the money. Just compare your mileage gain to the increase in cost for each octane rating. For example, if you're spending 10% more to get 91oct, but you're getting 15% better fuel mileage, then it's a good deal. However, if you're spending 20% more to get 94 oct, but only getting 15% better fuel mileage, then it's a waste of money.
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Old 12-29-2008, 12:02 AM   #6
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I second Williaty on this one. I've been seeing the same things.
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Old 12-29-2008, 01:52 AM   #7
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how can anyone say im wrong here? if your car is knocking then you have a lemon and need to be in touch with some people .

where are you finding knock on the timing stock map?

are you finding knock with the base map or in higher cells where the ecu uses advance?

are you finding knock with stock afr? or with mods ?


if you car is knocking you should not be running 85 octane period!

i am a relative noob at ecu tuning but have tuned timing once before mechanicaly, like 15 years ago.

i think wility in a rush to be the man you may have jumped the gun here with your figurtive red pen . i'm pretty sure that everyone would agree that you know your subarus and most cars pretty darn good.

but i can say that my motor does not knock anywhere running the stock base timing map.

tis true though that when the ecu tries to use the advanced map there may be some issues but to tell people i am wrong? where is your proof?


the only thing i disagreed with race face on was how badly it was tuned from the factory.it did seem in my op that i disagreed with all of his statement which is unfortunate because i definatly dont.

its only in the advanced tables i have seen any problems with knock . and the ecu has statagy built in to deal with that. so i think thats good for most people. personall i am not satisfied with that. but if the dude is running 85 to save a few pennies i doubt he cares about advanced timing tables . really just trying to convey to the dude that he should run at least 87.

but this guy running 85 with the base maps is no good . he may cause knock in more areas then just the advanced.

i do see here wility where you say

"The only way changing octane can change fuel economy is by allowing the engine to run more timing (which is more efficient). "

that is certainly not true . the motor should only advance timing until mbt is achieved ! if your advancing timing past that point you are not creating an efficiancy you are practicing reduntancy.
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Old 12-29-2008, 02:13 AM   #8
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Originally Posted by watchunglava View Post
how can anyone say im wrong here? if your car is knocking then you have a lemon and need to be in touch with some people.
I can say that you're wrong because I promise you that I have more experience tuning NA subies than you do. I've spent SUBSTANTIAL time on 4 cars now (meaning worked with their owners over the course of months to get them perfect) and have logged and/or provided basic tunes for more than 20 others (actually not sure how many more than 20 at this point, could be above 40 by now). All of them to a greater or lesser extent, are knock-limited and will not use the full timing advance the stock tune allows.

From comparing many of the cars, from reading several of the patents FHI and Denso hold, and from a MASSIVE amount of experimentation, it's VERY clear that the stock tuning strategy is designed to knock and to target timing advance that it cannot possibly reach. It's actually a pretty clever strategy. It allows the ECU to constantly tune itself for maximum fuel economy and minimum emissions while giving a very high probability that it will be out of warranty before it burns down.

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where are you finding knock on the timing stock map? are you finding knock with the base map or in higher cells where the ecu uses advance?
Probably about 30% of the cars knock only in cells with advance. Another 30% or so knock in the advance cells and knock very, very lightly in cells without advance. About 40% of the cars knock in basically every cell.

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are you finding knock with stock afr? or with mods ?
Both. Bone stock cars knock like a mofo. Modified cars usually knock just as much unless they have headers, in which case they knock just a little less. EL headered cars knock least of all.

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if you car is knocking you should not be running 85 octane period
The problem with that statement is that if you continue to apply that logic, you shouldn't be running 94 oct either, because you'll knock on that too. Are you suggesting that the requirement for the 2.5i be running C16 race gas?

Quote:
i am a relative noob at ecu tuning but have tuned timing once before mechanicaly, like 15 years ago.
Due to the knock control strategies available to modern ECUs, modern timing and mechanical timing tuning bear almost no relation to each other.

Quote:
but i can say that my motor does not knock anywhere running the stock base timing map.
If haven't altered the Base Timing, Maximum Advance, or any of the knock control strategies, then I call bull**** on this. Well, either that or your knock sensor is fubared. As I've said, I've noticed every car I've ever logged knocking. Every single other person who's tuned a 05+ NA subaru has, if they're logging knock correctly, noticed knocking. It's incredibly unlikely that your one car doesn't knock when a hundred others do.

Quote:
just trying to convey to the dude that he should run at least 87.
Now that I agree with, simply because Subaru calls for 87 oct. That, and I know it needs more than 87 oct

Quote:
that is certainly not true . the motor should only advance timing until mbt is achieved ! if your advancing timing past that point you are not creating an efficiancy you are practicing reduntancy.
What I said wasn't wrong, but you're right that I should have been more specific. However, in the case of a stock car with the stock ROM in it, the car is in absolutely no danger of being able to exceed MTBT. In fact, in a lot of the map, it can't get anywhere close. Due to the way the ROM is set up, if the car is pulling any timing at all, it's not hitting MTBT and more timing is therefore better. If you start increasing the timing beyond the maximum allowed by the stock tune, you certainly run the risk of exceeding MTBT.
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Old 12-29-2008, 02:22 AM   #9
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Quote:
Originally Posted by watchunglava View Post
i have to disagree . the stock timing maps for the most part are right on for 87 octane.

yes running 85 is no good .

if your car calls for 87 run it . they dont even sell 85 where im from .

race face is talking about stock advanced timing maps which only advance timing in high rev high load cells if it has better gas . but the base tune is tuned for minimum best timing running 87 octane.

the problem lies in your base map which is tuned for 87 . most cells on the base timing map have no value for advance in the advanced map. it doesnt try to advance anything . but might knock when using a lower octane.
Quote:
Originally Posted by watchunglava View Post
how can anyone say im wrong here? if your car is knocking then you have a lemon and need to be in touch with some people .

where are you finding knock on the timing stock map?

are you finding knock with the base map or in higher cells where the ecu uses advance?

are you finding knock with stock afr? or with mods ?


if you car is knocking you should not be running 85 octane period!

i am a relative noob at ecu tuning but have tuned timing once before mechanicaly, like 15 years ago.

i think wility in a rush to be the man you may have jumped the gun here with your figurtive red pen . i'm pretty sure that everyone would agree that you know your subarus and most cars pretty darn good.

but i can say that my motor does not knock anywhere running the stock base timing map.

tis true though that when the ecu tries to use the advanced map there may be some issues but to tell people i am wrong? where is your proof?


the only thing i disagreed with race face on was how badly it was tuned from the factory.it did seem in my op that i disagreed with all of his statement which is unfortunate because i definatly dont.

its only in the advanced tables i have seen any problems with knock . and the ecu has statagy built in to deal with that. so i think thats good for most people. personall i am not satisfied with that. but if the dude is running 85 to save a few pennies i doubt he cares about advanced timing tables . really just trying to convey to the dude that he should run at least 87.

but this guy running 85 with the base maps is no good . he may cause knock in more areas then just the advanced.

i do see here wility where you say

"The only way changing octane can change fuel economy is by allowing the engine to run more timing (which is more efficient). "

that is certainly not true . the motor should only advance timing until mbt is achieved ! if your advancing timing past that point you are not creating an efficiancy you are practicing reduntancy.
the stock maps knock anytime the IAM creeps back up and then a high load is applied to the engine. especially a high load/low rpm combination causes knock in my experience. The knock will dissipate and stop as soon as the ECU knock control strategy has time to pull timing. its designed like this for some reason.. i suspect so that the engine will extract max power from the gas it has to burn, allowing for some headroom for better gas/lower IAT's and lots of padding for crappy gas?high IAT's. the only way you can accomplish both having extra timing for better gas and padding for ****ty gas is to tune it to knock and then allow the ECU to adj timing to stop knock. just my theory.

watchung - you dont understand how the base and advance timing maps work it seems from what you say in the first post i quoted. the ECU always uses advance unless the IAM is 0, then it is only running the base timing and knocking so bad it wont last long unless ur really lucky and the knock stopped right when the IAM went to 0.

The ECU is tuned fine from the factory if you understand that knocking on a regular occasion is ok, but not ideal. its not knocking severly or for long but its still knocking.
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Old 12-29-2008, 03:14 AM   #10
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wait dude , the ecu advances timing in all cells or just with values in the advanced?? half the advance map is 0 . thats because minimum best timing has been achieved with out any knock at 87 octane.

the reason the advance map is there is so you can potentially run minimum best timing in those cells where knock was shown to the origional designer. the base timing # is a safe # that will run with out knock on 87. the advance is there so if you are running good gas the ecu can creep you up closer to mbt , until it sees knock of course.

the ecu advances things in such small increments too. and pulls at the first little sign of knock, so i dont see how the stock timing tune is bad in anyway for the average driver looking to run ""85"" to save pennies . it does what its supposed to do.
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Old 12-29-2008, 03:23 AM   #11
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Is this just an im right your wrong thread or is there a point?
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Old 12-29-2008, 03:36 AM   #12
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Originally Posted by watchunglava View Post
wait dude , the ecu advances timing in all cells or just with values in the advanced?? half the advance map is 0 . thats because minimum best timing has been achieved with out any knock at 87 octane.
Not quite. The Maximum Advance map is 0 where the engine is unlikely to experience severe enough knock to destroy the engine the first time it tries to run in those cells AND the engine is also typically able to reach MTBT (or at least come close) on crap gas. We can prove that it's more than just where it can reach MTBT on crap gas due to the point that it will knock in cells where the MA map is 0. Think of it this way: the MA map is non-zero where the engine both can't run MTBT and it's able to knock severely enough to blow the engine in a hurry. Where the MA map is 0, the engine can reach MTBT on crap gas and if it does knock, the knock will be mild enough not to destroy the engine before FLKC and FBKC can tame the knock. Keep in mind that the ECU can still pull timing via FLKC and FBKC even where the MA map is 0. And, on a stock car/stock tune, it does.

Quote:
the ecu advances things in such small increments too. and pulls at the first little sign of knock, so i dont see how the stock timing tune is bad in anyway for the average driver looking to run ""85"" to save pennies . it does what its supposed to do.
I agree that the stock timing control strategy is quick to retard timing and slow to advance it, giving an impressive amount of protection from knock. However, the system is always trying to increase the timing in order to be as efficient as possible. This means that the engine, in the areas where timing is less than BT+MA, is frequently knocking. If it weren't frequently knocking, it would be advancing timing until it started knocking.

Now, the really unknown, and really critical thing, is just exactly how bad for the engine this is. Obviously Subaru feels, at the very least, that it's not damaging enough to have a substantial number of engines blow within the warranty period. However, that doesn't shed any light at all onto the long-term health of the engine. The amount of knock inherent in the stock tune could be perfectly harmless over the long run. It also could turn out that the 05+ cars will end up with a 20% engine failure rate by 100kmi. We just don't know.



BTW, the point of this is that there's dozens of people who read these threads as reference material. It's critical to be pedantic not because you and I are trying to reach an understanding, but because people will use this as a reference.
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Old 12-29-2008, 10:48 AM   #13
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Originally Posted by williaty View Post
BTW, the point of this is that there's dozens of people who read these threads as reference material. It's critical to be pedantic not because you and I are trying to reach an understanding, but because people will use this as a reference.

And I for one thank you for the dissemination of info here. Reading this thread has improved my understanding of knock management on our engines. Now to look up what all those acronyms mean.
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Old 12-29-2008, 10:58 AM   #14
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I got some tuning to do haha.
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Old 12-29-2008, 12:25 PM   #15
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If bnaccs can find 85 octane gas then it's because he lives in mountainous area out west. Lower air density there means lower loads and less likelihood of knock. He can run 85 octane there without problems.
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Old 12-29-2008, 12:57 PM   #16
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Originally Posted by Cocoa Beach Bum View Post
If bnaccs can find 85 octane gas then it's because he lives in mountainous area out west. Lower air density there means lower loads and less likelihood of knock. He can run 85 octane there without problems.
I may be wrong, but isn't this mentioned in the owners manual?

Also, I would like some clarification:

All this talk about "every engine knocks" implies that subaru engines are designed to knock. However, in MANY other threads people talk about how bad it is for your engine to knock at all.

So are you guys saying that some knock is OK? Are there varying degrees of knock? I run 87 octane and have had zero problems so far on my stock engine. Are you guys saying that I need to be running a higher grade gas for fear of blowing my engine up?
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Old 12-29-2008, 01:01 PM   #17
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Originally Posted by mslstixx View Post
I may be wrong, but isn't this mentioned in the owners manual?

Also, I would like some clarification:

All this talk about "every engine knocks" implies that subaru engines are designed to knock. However, in MANY other threads people talk about how bad it is for your engine to knock at all.

So are you guys saying that some knock is OK? Are there varying degrees of knock? I run 87 octane and have had zero problems so far on my stock engine. Are you guys saying that I need to be running a higher grade gas for fear of blowing my engine up?
Well, yeah! There's a difference between knocking slightly before TDC and knocking midway during the compression stroke.
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Old 12-29-2008, 01:12 PM   #18
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yes there are different degrees of knock. knock can be called pinging as well. its detonation of the air fuel mixture instead of an even burn.

the only reason i replied to this eleentary thread was the fact that this guy was going to potentially ruin his car running the 85 . im not here to argue every point.

tuners are more anal about knock then most .

a little knock shouldnt kill your motor as long as its tough enough to handle it.

we arent talking about a huge knock that grenades the motor.
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Old 12-29-2008, 02:11 PM   #19
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I may be wrong, but isn't this mentioned in the owners manual?
Yes, owner's manual recommends 87 AKI, and that is what you should use if you're at sea level. But, if you lowered your engine's compression ratio, then you could run 85 AKI at sea level without problems. That is effectively what happens when you operate normally aspirated engines at high elevation, due to much lower atmospheric pressure. So you can use 85 AKI if you are above, say, 4,000 feet in elevation (i.e. if all gas stations in your area carry 85 AKI gas).
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Old 12-29-2008, 02:40 PM   #20
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.....so reading through this thread.....correct me if I'm wrong.....

The engine starts. The computer looks for knock. If it sees none, it starts to slowly advance timing. It continues to sense. It continues to advance. At some point, it senses knock and quickly backs off. It then takes recorded values for where knock was sensed and puts in a padding and sits there? Or will it again slowly advance?


In looking at this (my understanding from this thread)......it appears to be octane independent, so long as a "non-pinging" timing value is in the range of the computer. I'm also assuming a stock car and stock computer without some screwing around with factory values whatsoever.

(oh.....and to the original poster....please fill in your profile.....blue stripe at the top...user cp)

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Old 12-29-2008, 02:52 PM   #21
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It's not nearly as simple as that.

1) Engine starts
2) You do something with the throttle
3) Your actions cause the preconditions for one of the knock evaluation methods to be met (this is the tricky bit)
4) The ECU begins to listen to the knock sensor
5) Knock is heard
6) The ECU retards timing
7) No knock is heard
8) The ECU advances timing
9) No knock is heard
10) The ECU advances timing
11) Knock is heard
12) The ECU retards timing
13) No knock is heard
14) The ECU advances timing
15) No knock is heard
16) The ECU advances timing
17) Knock is heard


Repeat steps 3 through 17 constantly as the car is running throughout all the different load and RPM ranges. The ECU is constantly advancing timing until it causes knock, then pulling timing until the knock stops. The only time the engine is ever going to stop knocking is if the ECU can advance the timing to the sum of Base Timing+Maximum Advance without encountering knock. This absolutely cannot happen on pump gas. Someone who's decatted could experiment and see if it can even happen on C16. So most of us who tune these reduce the values in the Base Timing table until we get the timing down to what the engine can actually run without knock on the pump gas available in our area simply to reduce the stress on the engine.
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Old 12-30-2008, 12:00 AM   #22
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I live in the Denver area, and have 85, 87, and 91 octane fuel.
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Old 12-30-2008, 08:59 AM   #23
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Hey.. doesn't this imply that the ECU is capable of running on 93 and actually taking advantage of it?

P.S. What ECUs are we talking about here? All Suabru ECUs? Or just the opensourceable 32-bit Denso ECUs?

Last edited by Skidd; 12-30-2008 at 11:34 AM.
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Old 12-30-2008, 10:05 AM   #24
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Quote:
Originally Posted by Skidd View Post
Hey.. doesn't this imply that the ECU is capable of running on 93 and actually taking advantage of it?

P.S. What ECUs are we talking about here? All Suabru ECUs? Or the opensourceable 32-bit Denso ECUs?
Yes, that is exactly what it means. the first couple tanks of 93 i ran, i did extensive logging on, and found a very slight increase in mileage. my IAM climbed about 0.150 so i was using a bit more timing advance but it was not any difference I could feel.

I'm confident that this is the case on all 05-07 2.5i's, have a feeling that the 08-09's are going to be very similar if not the same. And would guess that even the pre-05 OBD-II stuff would be similar.
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Old 12-30-2008, 11:33 AM   #25
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Hmmm... I think I'll start loggin my IAM and try some 93.. see what happens.

<start of speculation mode>
But... one thing that makes me question the 93 (at least in my car) is doesn't the ECU have a max advance too? If my ECU is programmed for 87, I assume it's going to try and advance the timing to the point of knock, or to the point of it's MAX advance. Is it not likely that the amount of advance that "could" be used when full of 93 is beyond what the ECU considers max advance? Therefor 93 would not get used to it's "full" potential. Perhaps.. 1/2 of it's potential? Meaning that if anything 89 would be better than 93, and perhaps still 87 over 89. Again, all dependent on this mythical "max advance".

I base this speculation on this post
http://forums.nasioc.com/forums/show...7&postcount=40

Last edited by Skidd; 12-30-2008 at 12:37 PM.
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