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Discussion Starter · #1 ·
So I've been curious after reading the arguments back and forth about premium vs regular fuel. So I decided to test it. Here's my setup.
2019 Subaru Ascent Premium
ODBLink LX car adapter
Samsung S9+ running OBDLink software.

87 Fuel in the tank, get the needle to 1/4 fuel left
Run 1/4 mile runs x2 on empty stretch of road near my house

Refill with 91, run tank empty and refill to 1/4 tank with 91
Run 1/4 mile runs x2 on same empty stretch of road near my house

First runs with 87 octane done in the morning when the temp was 68F.
1/4 mile = 15.23 secs | 0-60 = 7.39 secs
1/4 mile = 15.24 secs | 0-60 = 7.58 secs

Second runs with 91 octane done in the morning (3 days later) when the temp was 64F
1/4 mile = 14.85 secs | 0-60 = 7.08 secs
1/4 mile = 14.71 secs | 0-60 = 6.97 secs

Almost 1/2 second quicker!

Calculating hp from weight and 1/4 mile ET using this site (https://www.ajdesigner.com/fl_horsepower_elapsed_time/horsepower_elapsed_time.php) and using the worst time of the 2 yields:

87 octane HP rating: 256.9 horsepower
91 octane HP rating: 277.6 horsepower

17 hp boost from running 91 vs 87. That's not insignificant!

I'll probably still run 87 most of the time for the money savings, but when I plan on towing, I'll put 91 in.

The attached files are the 4 runs. They're actually html files, not text, so open them with a web browser.
 

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Do you know for sure it is 1/4 mile? how was that measured out.

Is there an elevation change on your 1/4 mile?

How accurate is your timing at start and finish?

Did you actually weight your vehicle on a scale?

I am not doubting here, I expect that higher octane may make a difference. I question the accuracy of the final numbers. I used the same calculator with my 15.92 run at mile high, my slip from Bandimere and my vehicle weight with me in it on the scale at the drag strip.



The numbers from the guy that ran his Ascent at the drag strip are slower, and lower. But I understand you are just looking to show the delta between octane and not necessarily looking foe accurate WHP.
 

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I'd question the numbers by themselves. OBD apps are pretty inaccurate for hp and 1/4 mile times.
17hp is not going to net you half a second. Generally 10hp is going to net you .1s. But 100lbs can also net you .1s.
 

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I'd question the numbers by themselves. OBD apps are pretty inaccurate for hp and 1/4 mile times.
17hp is not going to net you half a second. Generally 10hp is going to net you .1s. But 100lbs can also net you .1s.
Don't believe it. Going with 93 Octane from now on and beating all you 87 octane scrubs to 60 by 1 second.:cool:
 

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Looks like a solid agnostic test. Well done.

Your numbers show a clear win for premium under extreme conditions. For my uses, even while towing, 257 hp seems to work just fine. The price difference for premium has gotten excessive. I often see $.60 to $.80 difference between regular and premium or about 25% for an 8% increase in performance. Not worth it to me, but others are free to see things differently.
 

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https://www.ascentforums.com/forum/...al-discussion-forum/3729-ascent-1-4-mile.html

Above is my actual 1/4 mile run in which I did put 91 octane in it as a safety measure since I knew I'd run a few back to back and know it won't change performance but will help with the motor getting extra warm from hotlapping.

My 60' time and 1/4 mile MPH is very similar to what your text files show. But there's no way you're running a 14.7 to my 16.1 at the same MPH.
 
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Also,
Look at all your text files. 1/4 mph trap speed is the best identifier of HP. The fastest speed is your 87 octane run 1 at 89.5mph. the others are all even at 88.9.

I'm assuming your phone is using it's accelerometer to do the calculations but getting the MPH from the car? I bet your phone was in slightly different positions for all these runs.
 

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for now using mid or premium when towing, otherwise just regular old 87

Small turbos are fighting knock from the excess heat they make and from what understand premium helps prevent knock. Next round when towing in 7+ percent grade i'm want to check oil temp gauge to see what it reads to maybe give some indication to how much hotter the engine is getting under severe load.

I've also read some turbo engines dump more fuel in to help with cooling to reduce knock, so maybe running 87 under heavy load will result in more knock and therefore it dumps more fuel to bring temps down to prevent knock, but maybe this only hurts MPG and not performance???
 

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If you've ever owned a car with a small turbocharged engine, you may have noticed a difference in fuel economy between the advertised numbers and your real-world numbers. When I owned a Fiesta ST, I could never quite achieve the highway rating, despite my conservative driving. Well, turns out there's a reason for that.

Jason Fenske of Engineering Explained put together a short video showing us why exactly small turbocharged engines might not always achieve those EPA rated fuel efficiency numbers. You see, it all has to do with protecting the engine from knock. Compared to a naturally aspirated engine, there is significantly more pressure in the combustion chamber for a turbocharged engine. With enough of this pressure, the air-fuel mixture could ignite on its own without the help of a spark plug, causing knock, which could destroy the engine.

Turbocharged cars are programmed to prevent knock from happening by injecting more fuel into the cylinder, which lowers the temperature inside the chamber. While the engine makes more power this way, it also uses a lot more fuel, which brings down efficiency. Fenske explains the process in detail, in the video below.

https://www.roadandtrack.com/car-cu...urbo-engine-efficiency-engineering-explained/
 

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Discussion Starter · #11 ·
Do you know for sure it is 1/4 mile? how was that measured out.

Is there an elevation change on your 1/4 mile?

How accurate is your timing at start and finish?

Did you actually weight your vehicle on a scale?

I am not doubting here, I expect that higher octane may make a difference. I question the accuracy of the final numbers. I used the same calculator with my 15.92 run at mile high, my slip from Bandimere and my vehicle weight with me in it on the scale at the drag strip.

The numbers from the guy that ran his Ascent at the drag strip are slower, and lower. But Understand you are just looking to show the delta between octane and not necessarily looking foe accurate WHP.
It may be slightly downhill. I chose the flattest stretch I could find near my house. It was just for comparison, so as long as all runs were done in the same place in the same direction, the results can be compared with each other.

The OBDLink software handles all the timings. I ran several runs and they were pretty consistent, so the timing seems accurate, otherwise there would be a larger variance between runs.

No, didn't actually weigh it. Took curb weight + my weight + 3 gals of fuel. approx. Again it's relative.

When I plug the same numbers into a 1/4 mile ETA estimator based on weight and HP it gives .3 second delta for a 17hp gain. Given I might be slightly downhill, that might explain my .5 sec improvement.
 

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Discussion Starter · #12 ·
Also,
Look at all your text files. 1/4 mph trap speed is the best identifier of HP. The fastest speed is your 87 octane run 1 at 89.5mph. the others are all even at 88.9.

I'm assuming your phone is using it's accelerometer to do the calculations but getting the MPH from the car? I bet your phone was in slightly different positions for all these runs.
In a tall and wide car like the Ascent, wind resistance becomes a big enemy at speed. So yes, the trap speed is the same. On the 91 runs, the car gets up to speed quicker, but still can't punch through the air quick enough on that last stretch.

No, my phone is mounted on a holder that is glued to my "shelf" to the left on my steering wheel, so it was in the same position for all runs.
 

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In a tall and wide car like the Ascent, wind resistance becomes a big enemy at speed. So yes, the trap speed is the same. On the 91 runs, the car gets up to speed quicker, but still can't punch through the air quick enough on that last stretch.

No, my phone is mounted on a holder that is glued to my "shelf" to the left on my steering wheel, so it was in the same position for all runs.
Yeah being in a holder can be all the difference. The slightest off center from one to the next can cause a big difference. Just for giggles do a few more runs tweaking your phone between and post the results.

And yes wind resistance is proportional to the velocity squared but what you're implying is that there's this 90mph wall it just can't pass which is definitely untrue. Just keep accelerating and you'll see it easily creeps past 90.
 

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Discussion Starter · #14 ·
Oh I know. I've had it over 110mph. It definitely has the power.

I was just saying in the 1/4 mile timeframe the acceleration slows enough that it's not getting past the 90mph mark quick enough.
 

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Discussion Starter · #16 ·
Some more runs may also rule out wind variations on each of these days. Either that or we find someone who has a dyno we can use. :)
No wind on either day. Both calm mornings. Read 0 on my weather station :D
 

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Monitoring Knock

What would be more helpful for me is to monitor knock via an Accessport. That was the main reason I owned one with my STI aside from different tunes. Quite frankly I am surprised Subaru is even recommending 87 octane on a turbo engine. The STI forums were battling ringland failure at an alarming rate. I did a Crawford air/oil separator, lengthened my fuel lines to address fuel resonation issues, and was tuned with a Stage One tune to prevent the lean condition that was present to 4900RPM under full throttle. Even at that I was seeing knock and having to have my tuner constantly adjust using 91 octane which is all we have here in CO. I even had a summer and winter tune to adjust for the different winter vs summer blend of fuels. It is cheap insurance to run high octane and you can take better advantage of the timing curve without having to pull so much timing when you knock/detonate on lower octane fuels.
 

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Since COBB verified there is ZERO benefit to using anything more than 87oct as far as power goes, is it safe to assume there’s zero benefit for knock/load/towing protection as well?
Usually the 2 are tied together on a modern turbo engine.
 
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