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      08-05-2018, 06:07 PM   #1
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gas mileage, octane impact and spark plug impact

I keep log book in my car and write down each fill-up. My bimmer is a e88, n52. For the first 58K+ miles, I used at least mid-grade gas. I used premium if running an autocross or something but used mid-grade for regular use. My mileage averaged 26.69 mpg.

Recently I started using my bimmer as my daily driver. I switched to regular since BMW says it is the minimum acceptable octane. My plan was to switch back if it pinged at all or showed any other sign of an issue. I have detected no issues so I have now used it for over 13K miles. My mileage over this time has averaged 27.4 mpg. If I run an autocross again or otherwise anticipate needing maximum power, I will fill up with premium. But for my daily commuting, I think regular is working fine.

I also changed plugs recently - at over 70K miles. The gap was surprisingly large on the ones I removed. I should have measured it but did not. On the next 4 tanks (about 1200 miles) I averaged 25.79. On the next three tanks I measured 27.4 mpg - exactly my overall average since I switched to regular. Do you think it's possible our cars learn to change the spark advance or something to adjust for the gap in our spark plugs?

I get frustrated at times with my bimmer but I should also give the BMW engineers credit for the spark control technology. I got rid of my Suzuki SUV to get down to one car. It was supposed to be able to burn regular but it pinged in really hot weather and when I was pulling a heavy trailer. So I had to use higher octane fuel occasionally. I detected a small mpg benefit in that vehicle but not enough to justify the price of the higher octane fuel. I don't haul anything really heavy with my bimmer but it has a hitch and I pull a trailer occasionally. I have never detected pinging. I believe BMW has better logic controlling the engine to make it work this way.

BMW recently ran a silly article in their weekly email saying higher octane is better for our cars and would justify itself with the mpg savings. In my car, that just isn't true. He cites better cleaning but that is a function of the brand, not the octane. I think it is good to test higher octane, especially if you detect pinging, but if the performance doesn't change, I believe it is a waste of money. The higher heating value, the measure of the energy contained in the fuel, is actually higher with lower octane. So if your engine burns it OK, there is a reason mileage could be very slightly better on lower octane. But not if the engine computer has to back off on spark advance.
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      08-05-2018, 07:59 PM   #2
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Under normal circumstances, you will never hear a modern BMW engine ping or knock. The sensors are very sensitive and will back off the timing long you would be able to detect anything.

Octane is simply resistance to ignition; higher octane fuel is more difficult to ignite, lower octane fuel has (marginally) more potential energy. In theory a modern ignition system will advance the timing to improve power and efficiency if the fuel and other conditions (ambient air temperature etc.) allow. In practice factory systems are pretty conservative and often leave something on the table rather than pushing the envelope. There are also a lot of other variables (ambient again, driving conditions, traffic, climbing/descending, etc.) that play into it as well. Without a larger and more controlled sample, I'd say the variance in your results is within the statistical noise.
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      08-05-2018, 08:27 PM   #3
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What octane are you using when you say premium vs mid grade vs regular?

It varies a bit from region to region. I try to run 90 or 91, will run 89 if that's all I can find.

From the Owners Manual:

Required fuel
Super Premium Gasoline/AKI 91
This gasoline is highly recommended.
However, you may also use gasoline with less AKI. The minimum AKI Rating is:
> 128i: 87
> 135i: 89
If you use gasoline with this minimum AKI Rat- ing, the engine may produce knocking sounds when starting at high outside temperatures. This has no effect on the engine life.
Do not use any gasoline below the speci-
fied minimum fuel grade. Otherwise, the engine could be damaged.<
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      08-06-2018, 12:01 PM   #4
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FWIW I only us 93 octane.

Its only a few more dollars a tank.
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      08-06-2018, 12:07 PM   #5
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If I was worried about how much fuel costs, I would have bought a Pious.

Interesting info none the less.
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      08-06-2018, 07:23 PM   #6
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Quote:
Originally Posted by JimD View Post
BMW CCA* recently ran a silly article in their weekly email saying higher octane is better for our cars and would justify itself with the mpg savings.
JimD I am the author of the article cited in your original post.

As others in this thread have pointed out, which I also noted in my article, modern engine control systems are continuously making a vast amount of adjustments to ensure a ping and knock-free experience depending on the octane level of fuel being combusted. They're perfectly willing and able to acquiesce to different octane levels, and over the long run, the systems adapt and trim to what's most often being used.

I think what people might be failing to connect is the fact that regardless of brand, premium octane gasoline contains more additives and detergents across the board. Top tier choices like Shell, BP and others get away with charging a slight premium because on average, their highest available octane contains even more additives and detergents than that of premium gasoline from other non-top-tier brands, regardless of the fact that those will still always have more than regular (87 octane in my area).

I might understand the cost argument from the side of towing with a large displacement engine, or driving an economy car from new to 250,000 miles, but even then, according to my own repeated personal experience, premium pays for itself in the way of increased fuel economy, which I can only attribute to increased performance and efficiency across the board. In your case, I can't help but feel perplexed. It looks like you special ordered your convertible and have since become a long-term owner (nearly 10 years), all while being meticulous enough to log fuel receipts the entire time, all of which I admire.

But with all of that in mind, from my standpoint, I just can't understand the goal in trying to save what can only add up to a truly negligible amount of money over the life of the car, at the expense of running what is largely referred to as the minimum level octane gasoline for the engine. Regardless of your fuel economy numbers, I would be willing to bet the engine develops less horsepower and torque when running 87 as opposed to 91 or better--the change might not be apparent after one tank, but over the long run, I would expect the appropriate trims and maps to be adjusted to make full use of premium octane. Off topic, but in my case, I shelled out good money for Dinan S2 software for my 135i, which claims significant gains with 93 octane. The best I can do is 91, but if anything higher were available, I'd use it without question.

The same thing applies to the cleaning properties of premium, owing to what I noted above regarding additives and detergents. While the N52 is a port-injected engine that is more than capable of adjusting to lower octane, the cleaning properties of premium gasoline are actually most apparent on port injected engines because fuel is allowed to be sprayed onto and drip down the intake valves which combats the formation of carbon or what's commonly referred to as gunk. For someone who's owned their BMW as long as you, I fail to understand the interest in wanting to gamble with running lower octane fuel when the owner's manual recommended premium octane is absolutely proven to make for reduced longterm buildup of carbon deposits, with some cases even demonstrating an ability to reverse accumulation.

The amount of the thought given the subject is another aspect. Is it really worth it to ponder, consider and experiment, when one can rest assured that premium is a safe choice that, more likely than not, is doing more good than harm?

In my case, I like to experiment, and having a few extra cars to mess with allows for the opportunity. It's also part of why I wrote the article; people clearly have passionate feelings about this subject.
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      08-07-2018, 12:07 PM   #7
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The OP really confuses me. He says there is no difference between octane ratings in his experience, yet he uses premium when he competes so he must perceive a performance difference. For him gas mileage appears to be the same with regular but I guaranty he is sacrificing HP. Dyno tests prove that HP and torque decrease with lower octane.

If you check the web sites of top tier gas suppliers, they state that premium has more additives than regular. Mid-grades are merely a blend of regular and premium at the pump, so the extra additives of premium are diluted by the regular.

As previously mentioned, what the OP is not monitoring is deposits in the combustion chamber and the cost to shell-blast those deposits away in the overall cost equation of using regular. However, if you use top tier gas of any grade that is not likely to be an issue.

To answer the OP's question - yes the engine ignition system takes into account spark gap since it is integral to the overall ignition process.

Finally, as others have mentioned, the "savings" of using regular are negligible. If you drive 10000 miles a year with an average of 25 mpg and premium is 30 cents more than regular, you save a whopping $120 a year!!!!! You can save a lot more $ by buying one less Starbucks coffee a week.
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      08-07-2018, 12:29 PM   #8
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^^ Not just conjecture either; Hassmachine, who tunes these cars, has mentioned they will run 100% fine on 87, at the expense of power.
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      08-07-2018, 08:20 PM   #9
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Once again Tock172 makes a baseless clam, this time that higher grades of gasoline contain more detergents. This is from Shell's website:

"The Nitrogen Enriched Cleaning System gives all three grades of Shell gasoline the ability to clean up gunk on intake valves and fuel injectors. Industry data indicate and car manufacturers recognize that a clean engine is more fuel-efficient, produces fewer emissions and allows vehicles to perform at their best."

That is not all they say, it is simply the most applicable. If Tock has some different information to cite, I am interested. But when the manufacturer says all three grades has the same detergent package, I am going to believe that.

I believe that I will get more spark advance and thus power from higher grades of gas. But on my daily commute, I do not need more power. I haven't tested this theory but for my occasional track use, I take the chance this is wrong. I don't think it is. But it is my untested theory - an opinion.

The thing that Tote claims that he provides no meaningful data to support is that higher grades of gas pay for themselves with gas mileage. I don't think my data proves my car gets better mileage with regular. What I think it proves is that the mileage is close enough that there is no way higher grades are cost effective. I don't regret using mid-grade - 89 octane - for most of my car's life. I do not think it hurt it at all. I probably had more power then but it was power I wasn't using.

My data says nothing about what a 135i or other turbo motor needs or what benefits higher octane would provide in a turbo motor. I am interested in that data. It could easily be different. It is also possible that other n52s would get different results. I would be more surprised by that but it is fully possible. My one engine suggests but does not prove that n52s get about the same mileage on 87 octane when not being driven hard.

Tock is a good writer but he expresses his opinions as fact. That is common with journalists these days and some other people. It is annoying to me. As an engineer, I was taught to test theories to see if they are correct, not make things up and claim loudly that I know best.

It does not bother me at all if Tock or others want to use higher octane gas than their car really needs. There is at least a good chance it gives you more power. It is your money to spend any way you want. The only thing I suggest is if you think it provides you meaningful benefit that you see is more valuable than the money, then develop some data that supports your theory. Or simply state it is your opinion that is unsupported by facts. You could still be right. If that was the way Tock wrote his article, I wouldn't be criticizing it.
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      08-07-2018, 08:59 PM   #10
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Quote:
Originally Posted by JimD View Post
Once again Tock172 makes a baseless clam, this time that higher grades of gasoline contain more detergents. This is from Shell's website:

"The Nitrogen Enriched Cleaning System gives all three grades of Shell gasoline the ability to clean up gunk on intake valves and fuel injectors. Industry data indicate and car manufacturers recognize that a clean engine is more fuel-efficient, produces fewer emissions and allows vehicles to perform at their best."

That is not all they say, it is simply the most applicable. If Tock has some different information to cite, I am interested. But when the manufacturer says all three grades has the same detergent package, I am going to believe that.
You're cherrypicking statements from the Shell website, specifically marketing copy from the Nitrogen Enriched Gasoline page which discusses their entire range of gasoline fuels.

Here's a statement which applies directly to Shell V-Power NiTRO+ Premium Gasoline from the FAQ:
Quote:
Originally Posted by Shell
Shell V-Power NiTRO+ contains the highest concentration of the Shell Nitrogen Enriched Cleaning System and cleans up faster than Shell regular. In fact, Shell V-Power contains seven times the amount of cleaning agents required by federal standards. The Shell V-Power NiTRO+ formulation was engineered with triple-action protection for optimal engine performance, delivering unsurpassed protection against gunk, unbeatable protection against corrosion, and our best protection against wear.
If that's not enough, here's some more marketing from Shell:
Quote:
Originally Posted by Shell
  • It is the only gasoline that contains the patented and exclusive Shell Nitrogen Enriched Cleaning System
  • Actively cleans performance-robbing gunk from intake valves and fuel injectors
  • No other gasoline protects better!
  • The Shell Nitrogen Enriched Cleaning System uses a patented molecule, which cleans and protects intake valves and fuel injectors. Enriching the molecule with nitrogen makes it more thermally stable and therefore more effective in tough engine conditions with higher temperatures and increased pressures. This is particularly important in modern fuel-efficient engines to prevent the breakdown of cleaning agents.
Quote:
Originally Posted by JimD View Post
Tock is a good writer but he expresses his opinions as fact. That is common with journalists these days and some other people. It is annoying to me. As an engineer, I was taught to test theories to see if they are correct, not make things up and claim loudly that I know best.

The only thing I suggest is if you think it provides you meaningful benefit that you see is more valuable than the money, then develop some data that supports your theory. Or simply state it is your opinion that is unsupported by facts. You could still be right. If that was the way Tock wrote his article, I wouldn't be criticizing it.
I went to school at a university with a strong reputation for its college of engineering. I am not an engineer. In my experience, engineers have difficulty envisioning the grander scheme beyond the single component they've been tasked with developing, improving or fixing. I think the same thing might be happening here, with a fixation on debating the properties and benefits of premium fuel stemming from you not agreeing with the tone or presentation style of my article.

The world is full of baseless statements (see above examples), but I stand by everything I wrote in this thread and in the article. I never once portrayed any of my own personal experiences to be taken as fact, and in the article, relied almost exclusively on data and literature directly from Shell outside of my own shade-tree experiment, which is fully divulged. I'd encourage you to make an inquiry regarding the content of Shell V-Power NiTRO+, but can warn you that these companies are expectedly coy with their proprietary formulations, as the contents are commonly protected as a trade secret.
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      08-09-2018, 05:54 AM   #11
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The reality is, the N52 is not a high compression engine. It has knock sensors, it can play with timing and spark, and as a result, the engineers at BMW determined that it will be fine with 87 octane. The reason the mpg hasn't changed switching from one to the other shows he isn't driving the car hard enough for a higher octane to make any difference, and proves that there is no issue with pre ignition on 87.
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      08-09-2018, 09:32 AM   #12
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Tock,

Thanks for posting the Shell comment that their premium has more cleaning agents than their regular. When I looked at their website I did not notice that. If I had, I would have worded my previous comment differently. So more cleaning agents are an advantage of premium.

My guess is that would be more significant for the dual turbo motor BMW makes (forgetting the codes at the moment) that tends to build up carbon in the intake. I'm not sure how significant it is for my normally aspirated motor.

My personal theory, based mainly on a post on the "Bob is the oil guy" site, is that using gas from different suppliers at least occasionally is important to reducing deposits. This thread was from a guy that claimed he used a brand exclusively for 10k miles or something, tore the engine down, looked at it, then put it back together and tried a different brand. He found different deposits for different brands but not grossly different amounts of deposits. He believed that one brand tended to remove the deposits of other brands so now he rotates. I use Shell mostly but justify the occasional "off brand" fillup as potentially removing deposits from Shell gasoline. This is just an untested theory that makes some sense to me, may not to others.

The main point of my original post was to supply some data from my vehicle suggesting that others see what the difference is for them. I still disagree with the premise I got from your article which I read as saying we all should use premium. I don't think the unquantified greater amount of cleaning agent justifies the significant premium in cost to me. But I respect other's right to spend their money as they choose. I also suggest that before you or anyone else assume it will help your gas mileage you try it and see. You might be surprised. I expected my mileage to be a bit better with mid-grade (89 octane) and it was not.
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      08-09-2018, 10:12 AM   #13
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I was thinking the same thing, a babied car will get same MPG on any grade gas.

Quote:
Originally Posted by captain slowly View Post
The reason the mpg hasn't changed switching from one to the other shows he isn't driving the car hard enough for a higher octane to make any difference, and proves that there is no issue with pre ignition on 87.
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      08-09-2018, 12:01 PM   #14
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Quote:
Originally Posted by bagekko View Post
I was thinking the same thing, a babied car will get same MPG on any grade gas.
That has been my experience.

I also understand, that up to the point of detonation, regular gas produces my BTUs than premium, as the knock protection tends to reduce the available BTUs. Or something to that effect. I actually get slightly better mileage on regular than premium, but I do not drive hard, and if I do, I bump up to premium.
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      08-09-2018, 01:32 PM   #15
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I do not generally drive hard as I have implied in my previous posts. But I do occasionally feel the need to accelerate more briskly and I still don't have any detonation. The car adjusts to the octane well. I may be down slightly on power but I cannot detect it.

I just signed up for some spirited driving in the mountains this fall and will put mid or premium in for that. I'm sure 87 would work but I may use a bit more power depending on whether I decide to stay near the front or drop back and thus I think a little higher octane is indicated.
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      08-09-2018, 02:53 PM   #16
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Based on my short knowledge(if you say "shut up" because of this, then, well....), gasoline detergents are not so effective to clean up the intake valves in these days' *direct injection* engines since the gasoline doesn't go through the intake valves.

Also N52 needs minimum 87 while N54 needs minimum 89 due to turbo charging which means hotter air-fuel mixture inside of the cylinders which leads to premature ignition. Higher octane will prevent this.
To lower the temperature of the air-fuel mixture inside of the cylinder, ECU adds more fuel into the air and that leads to bad fuel economy. By using higher octane gasoline, less fuel is needed to prevent premature ignition and it will help a bit of fuel economy.

That's why mileages of many turbo charged engines don't match to the manufacturers advertised numbers if you drive hard - well.. I mean much worse than NA engines.

However, consensus(?) is that the difference is not that much and that's why, if you don't care about power when you need it and care more about $120 a year fuel cost, it's OK to use minimum recommended grade fuel.

my 2 cents.

By the way, in Canada, if you drive 10,000km a year and my 135's mileage is 16L/100km, cost difference between 87 vs 91 is about CAD $0.17/L, so it's about CAD $272 = US$208.

Last edited by edwardmagichands; 08-09-2018 at 03:00 PM..
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      08-09-2018, 04:07 PM   #17
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A little story... last year coming back from the Swiss alps... I couldn't find any SuperPlus 98 octane(Premium) fuel... so I filled my tuned 135i with ten liters of Super 95 octane. I never heard any knock... but since my car was tuned for 98 that is what I always run. I was being a good little boy, until I hot the German Autobahn with no speed restrictions.. hammer down and I was cruising around 135 mph. Came into some slower traffic speeds were down to 85 mph-ish... which seems downright slow when you are use to driving at Warp 8!! So... as the traffic cleared out... I accelerated, thru the gears, WOT, third to fourth... I feel a bug hick up... SES light comes on... I thought I blew my charge pipe... a slow down and feather the throttle a few times feeling out the engine. The engine miss fired for a few seconds then clear out.

I drove the car slowly(130 mph) until I found some top tier SuperPlus(98 octane) fuel. I filled her up.... back up to Warp 8 speeds(>250 to 280 kph) then had to refuel again... just to burn up what ever was left of that crappy low octane fuel. Never had that problem again.

I never heard any ping. I only felt the misfire at speed, which feels like you ran over some logs in the road!! Really. The whole car shakes like no other car I've driven. Anyways... I've done numerous V-Max runs since then... not a problem since. I'll never fill up with low octane fuel again! lol

Over here in Germany you can even buy 102 octane fuel at Aral or Shell. I have tried 20 liters of those fuels when I am low on fuel... I don't feel any gains in power... but I do feel the engine's torque curve feels smoother to me as the engine revs build. I like it.

edwardmagichands as far as carbon buildup goes... one thing that reduces carbon build up is using a BMW approved oil and frequent oil changes. You're correct in that premium fuels can't clean the intake valves on DI engines like ours... but the top tier fuels can help keep the injectors in good condition(spray patterns). I think frequent oil changes are a good idea.

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      08-09-2018, 04:44 PM   #18
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Nice story, but you are tuned for 98 octane. Of course anything lower will not go well. The OP's car is "tuned" for 87 by BMW, and does just fine putting around at 65-70mph. Then a journalist expert comes to tell him that he will get better mileage if he uses 93, which he did/does not, and a bunch of other arm chair mechanics proceed to tell him to buy a Prius...
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      08-09-2018, 04:58 PM   #19
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Quote:
Originally Posted by captain slowly View Post
Nice story, but you are tuned for 98 octane. Of course anything lower will not go well. The OP's car is "tuned" for 87 by BMW, and does just fine putting around at 65-70mph. Then a journalist expert comes to tell him that he will get better mileage if he uses 93, which he did/does not, and a bunch of other arm chair mechanics proceed to tell him to buy a Prius...
Well... I guess my point was, that you will never hear any detonation with modern BMW like our cars.

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      08-09-2018, 05:08 PM   #20
edwardmagichands
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Exactly.

Understand what exactly happens inside of your engine and decide what to use.

You can use whatever you want.
You will not get much better gas mileage even if you use higher octane gasoline.

I'll keep using AKI 91.
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      08-09-2018, 06:37 PM   #21
JimVonBaden
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Quote:
Originally Posted by edwardmagichands View Post

By the way, in Canada, if you drive 10,000km a year and my 135's mileage is 16L/100km, cost difference between 87 vs 91 is about CAD $0.17/L, so it's about CAD $272 = US$208.
Sure, but I get virtually nothing for that extra $208, and I can get about 7 more tanks of gas, or a couple of pretty nice dinners for that.
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      08-09-2018, 09:02 PM   #22
edwardmagichands
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That's the point. If you care more about 7 more tanks and a couple of nice dinners, that's your choice and no one should argue with that.
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