91 vs 87, real difference?

[DesertFox]

Just hooked up my obdlink MX. 24psi on a WOT run.. damn. I didn't expect that.

Because that is not the boost pressure. The OBD2 reads MAP, not boost. You have to subtract atmospheric pressure, which is ~14.7 psi depending on DA. Your actual boost is about 10 psi. The gauge doesn’t depict fractions of a psi.



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[MJM]

So far we have been running premium in ours. Around here, that's 91 octane. According to the dash, it's gotten about 22 mpg over the 800 miles we've gone so far. At that mpg the cost difference isn't worth even trying "regular".

 

[YYCSahara]

So far we have been running premium in ours. Around here, that's 91 octane. According to the dash, it's gotten about 22 mpg over the 800 miles we've gone so far. At that mpg the cost difference isn't worth even trying "regular".

Must be regional. I'm the other way around. At my mpg it isn't worth me trying premium gas strictly looking at $. Around here it's close to $20 more per tank to fill it with premium.

 

[Dalingrin]

Must be regional. I'm the other way around. At my mpg it isn't worth me trying premium gas strictly looking at $. Around here it's close to $20 more per tank to fill it with premium.

$1+ more per gallon seems kind of crazy. It is usually $0.40 - $0.60 more per gallon in most places that I have paid attention.

 

[YYCSahara]

$1+ more per gallon seems kind of crazy. It is usually $0.40 - $0.60 more per gallon in most places that I have paid attention.

Yeah, it sucks up here. The spread between regular and premium has grown wider and wider over last couple of years.

 

[MJM]

Premium cost a bit more here as well. But the JLR only has a 17.5 gallon tank and with the mpg we're getting a few bucks a tank is worth keeping full performance. My wife loves to romp on it. Damn thing scoots when you do.

 

[turbosix]

Because that is not the boost pressure. The OBD2 reads MAP, not boost. You have to subtract atmospheric pressure, which is ~14.7 psi depending on DA. Your actual boost is about 10 psi. The gauge doesn’t depict fractions of a psi.



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So you're saying engine off it should read 1bar....

 

[DesertFox]

Believe whatever you want, but what does it read at idle? If it doesn't measure atmospheric, then you should see a negative value...vacuum. But you don’t, instead you see ~8 psi. Subtract atmospheric pressure and you are at 6 psi of vacuum...normal for idle.

 

[turbosix]

Believe whatever you want, but what does it read at idle? If it doesn't measure atmospheric, then you should see a negative value...vacuum. But you don’t, instead you see ~8 psi. Subtract atmospheric pressure and you are at 6 psi of vacuum...normal for idle.

Lol

 

[DesertFox]

LOL. I don’t know what to tell you, mine shows a positive 8 psi at idle, when I advance the throttle the value increases, it does not drop to zero then increase. This instrument is reading the MAP sensor, the PCM needs to know that value to control the engine at different altitudes and temperatures, not the boost value. Like I said - believe what you want.

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