Ohh, that makes sense. So what about using low octane fuel in a low compression motor? Some people thought about lowering the compression of their engine before adding a turbo. I'm assuming they would benefit from low octane right?

 

Their pocketbook would certainly benefit as higher octane (ie more expensive) fuel is not needed with low compression as octane is only resistance to detonation. Less compression, less chance of premature detonation.

 

If you boost a low compression motor however, your effective compression ratio goes up.

Here's a formula for figuring CR at sea level.

E = C (( B / 14.7 ) + 1)

E = Effective CR
C = Static CR
B = Boost
14.7 = Atmospheric Air pressure


I actually wrote an editorial about this very thing for 350Zfrenzy last week. I'm assuming Jeremy is going to publish it for April. Haven't heard back from him though. I included a lot of good formulas in it. Such as figuring out altitude corrected HP, Atmospheric pressure loss etc.

I'll post about it when it's published.

 

Wow, that equates to 15.9/1 with 8 psi of boost!!

 

yep, it's 15.2:1 at 7 PSI. That's why you've got to pull timing.

 

ok, now how does timing factor in now. Oh, in case you havn't noticed, I know nothing about engines Maybe I should just go to howstuffworks.com

 

Timing is all about getting the most power out of each stroke without detonation. The spark plug fires a certain number of degrees before the top dead center position of each cylinder's compression stroke. If it fires too early you'll get detonation, fire too late and you lose power and burn up exhaust valves. It takes a some time for the flame to fully start burning all the fuel after your spark plug fires. So your car's timing is set up to fire the spark plug a certain number of degrees before top dead center(BTDC). It has these degrees preprogrammed into the ECU. At 1000 RPM it might be 20 degrees BTDC at 2000 RPM it might be 22 degrees BTDC. The reason your cars timing advances as RPM's go up is because the engine is spinning faster giving it less time to burn the fuel. It must fire the plug earlier to allow enough time.

The ECU preprogramming is figuring timing for 91 octane fuel. Nissan engineers know approximately how long it takes for that to happen with our compression ratio using 91 octane. If you are using a lower grade octane the fuel burns faster so there is a collision/detonation between the flame and the cylinder. The cylinder will be trying to go up while the combustion is pushing down. That is detonation and can destroy engines very quickly.

Raising your compression ratio or boosting does the exact same thing as running octane that is too low. The higher compression causes the fuel to burn faster, so your spark plug will be firing too early. That is why you retard timing when you go FI.

Hopefully it cleared things up.

 

So running 93 octane here in the Midwest does nothing?

 

No, not really sorry. Unless you're below sea level, or have raised your compression a bit. It's unecessary to run anything higher than 91. In fact, I could probably go to 89 if I didn't have a supercharger because I'm at such a high elevation.

 

That was a great explanation. Thanks for taking the time to help me understand

 

Real World Experience"

I bought a 5 gallon tank of 100 octane (over the 91 here) and it ruined my track day.

All trap speeds were actually lower, we figured after ruling everything else out the the ECU just didn't like it.

It did not help any , it actually cost me in lower traps.

Here is a pick of the 5 gallon can by my car. If it adds horse power I am sure I have probably tried it lol!

It's never fast emough!

Even some octane boost turns my 02 sensors a wierd red speckled color. I just run my 91. If I had decent 93 that would be great but we don't here just 91 which is currently about three hundred and fourty two dollars an ounce.

 

Gas prices suck.

I was talking to a guy at school today. We were talking about the future of cars, alternative fuel, etc. He asked me, "what about the Harley guys? I don't want to be buzzing down the highway, barely audible, sounding like an electric golf cart"

Imagine an electric Harley with a complex speaker system that makes it sound like it's an internal combustion engine

 

Informative thread!

Being that the ECU is tuned for 91, would a timing advancement (+2) on an NA VQ benefit those with access to 93 more (or at all)?