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I just recently bought a 2017 focus st. When I went to fill up I put 87 in it ( the manual said 87 is okay, but now I know 93 is overall better for the car). Will the octane of the gas effect boost. Because ever since I filled up with 87 I have very little boost in first, between 1/4 and 1/2 boost in second and 3-5 all reach above 1/2. Any responses would be appreciated.

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All stock, sure run 87, if you aren't driving it hard no reason for 91/93.

You want all 252bhp you paid for? Run 91/93.

It's all in the ST supplement book. 87 aki limits you to ~230bph
 

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When I took ownership of my FoST from the dealer, it was filled up with 87 octane. Felt slower than when I test drove it. I emptied the tank and filled it with 91 and it was back to what I expected. I only would get 87 if you were going on a long road trip or something but even then, get a cobb AP from stratified and get a low boost setting for mpg increase.
 
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The turbocharger uses engine exhaust to produce boost. Less timing would result in less exhaust and therefore less boost.
 

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Fuel is a knock deterrent. Lower grade octane (87) will not inhibit knock as much 91/93. Hence, your engine will not hit the same boost pressure on 87 fuel like it would when you are running 93 octane. Typically, fuel is most made up of iso-octane and n-heptane. Iso-octane is the main ingredient to deterring knock.
 

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Yes. The ECU can use OAR to reduce the maximum load limit and thus limit boost. OAR is specifically designed to adjust for octane, and uses knock sensor feedback to learn.
In stock form, 1.92 max load @ OAR of 1, and 2 max load @ OAR of -1, but in some zones the difference can be up to ~.4 load which can be about 4-5psi of boost depending on conditions.
 

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There's also a common explanation for your boost/gearing observation - many turbocharged cars don't make full boost in first (or sometimes even second) gear. The lack of load on the engine means less available exhaust energy to spool the turbo.
 
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