Octane / H5DI

[jjackkrash]

I was reading the manual and its says the H5DI (5.3l GDI) is good to run on 87. It also says the computer can adjust to higher octane. Is there any performance advantage to running higher octane gas? Or is running higher octane just throwing money away?

[vanhanbr]

I've always heard that if an engine can run on a certain octane, no need / nothing gained from going to higher octane. But if it were me, I wouldn't run 87 with ethanol unless I knew I was going to use up the entire tank that same day.

[jjackkrash]

I've always heard that if an engine can run on a certain octane, no need / nothing gained from going to higher octane. But if it were me, I wouldn't run 87 with ethanol unless I knew I was going to use up the entire tank that same day.

I have heard the same thing. But I also know from chipping cars that higher octane can have a HP advantage if the fuel management is programmed right. The PCM manual at least implies that the direct injection/programming can take advantage of higher octane, but it was not completely clear.

Fortunately, our corner store sells good gas without ethanol when he can get it, which is most of the time.

[JD ski]

The level of octane in fuel is for the control of the burn. Higher levels of octane create a slower burn pattern of gas. The higher the compression level of the motor the higher the octane must be in order to keep the motor from pre-detonating (this is where the piston is compressing the gasses into the combustion chamber and if the ratio is right the spark plug will ignite the mixture. However, if the compression ratio is to high, the heat of compression will detonate the mixture attempting to push the piston back down prior to the crank shaft pivoting over the center so the pre-detonation is attempting to push the piston down and turn the motor in the wrong direction). This is known as knock and can be heard as a knocking sound under acceleration. In pre-computer controlled motors at sea level a compression ratio of approximately 9.5 :1 will need a higher octane to keep the motor from knocking (damage). Modern computer controlled motors have knock sensors to change the timing to adjust to deal with minor compression and timing issues such as changing altitudes.

Some of the modern motors have fairly high compression ratios and are still designed to work in 87 octane. My car that has 10.2 :1 compression ratio runs fine at 5000 ft altitude (less oxygen) on 91 octane but at sea level it needs 110 octane (race fuel) to keep from knocking. So the answer is you need to run what the manufacture states. In your case it sounds like 87 octane is fine. If you chip the motor it will change timing and then you may need a bump in octane. Other wise you are just wasting money purchasing a higher octane fuel.

[jjackkrash]

Good info JD. I was just wondering whether the 2000 PSI that the fuel is injected at into the cylinder and the fuel computer in the GDI engines can take advantage of higher octane gas. I have no intention of chipping the boat.

[jjackkrash]

At least one Baller on ballofspray is reporting that the PCM GDI engines make noticeably more HP with 93 octane.

[DocPhil]

There are guys running 2016 G23s with that motor fully loaded with ballast. How much HP do you need in an SN200?

[vanhanbr]

There are guys running 2016 G23s with that motor fully loaded with ballast. How much HP do you need in an SN200?

Those guys are barefooting/jumping and need top end speed. Also, the 200 isn't as efficient of a hull as the older boats.

[jjackkrash]

There are guys running 2016 G23s with that motor fully loaded with ballast. How much HP do you need in an SN200?

"Need" is perhaps a strong word. I "want" all the HP the engine will reasonably deliver. Frankly, I was on the fence on getting the 6.2 with an extra 100 HP, just because.

[JD ski]

HP is something you can never have enough of. My 1975 Century Resorter is a sleeper. With a small block bored and stroked to 388 cu.in.. At the 4500 ft. elevation, fully loaded with gear and me it GPSs at 51 mph. I have never had it at a low elevation to know what it will run at sea level, HP can't have enough.