Harley & petrol octane

[rok-the-boat]

You are right, it shouldn't need a higher rating but Evos do run rather hot. My compression is 8.9:1 but since coming to NZ it is a little uneven. I'll try a higher octane rating next time just as a test.

[Headbanger]

The octane rating is a load of crap but I always get the best the servo has to offer, Whether its for my HD, my dirt bike or the lawnmower, Hell, whats a few cents?

[skidz]

I use 91 in the roadking and runs well. Have tried higher octane and seemed to play up a bit.

[bernardf]

I have a 1988 Lowrider EV27 cam , mikuni carb , Hypercharger aircleaner , Crane single fire ignition Edelbrock heads and Wiesco forged pistons with about 9.5cr or so i need to run 98 or she pinks a little under full throttle but does go well all the same

[paulmac:Me]

I have an 89 1200 and alternate fuels depending which pump I've pulled up next to !!

[peasea]

But seriously though, notwithstanding poor Honda cylinder heads with cracks , as near as I can work out the Evo engine has about an 8.8-8.9:1 compression ratio. Now, according to everything I've read, octane rating is purely a rating to ascertain the anti-compression-ignition properties of a given fuel. Now, I understand that some cylinder head designs are more susceptible to compression-ignition (or detonation) than others -- I just recently read some stuff about cages with Ricardo heads, with `squish points' and stuff like that, designed to avoid detonation -- but the biggest factor in detonation is the amount of compression the charge is under.

So if a CB250RR with an 11.5:1 compression ratio can run 91, why can't an Evo with an 8.9:1 compression ratio?

How long have you got? After 30yrs of engine building and trying to extract the maximum performance from countless pushrod engines this isn't an easy question to answer. Compression is only part of the picture, as is squish, but something so many overlook is swirl. With four valves per cylinder you will move more air/fuel mix for a given port area and smaller cylinders make more power, more efficiently than larger ones. If those smaller ports are correctly configured and the chamber is closely matched to the flow characteristics of the ports then swirl will be optimized and you can put more compression in for any given fuel, which will in turn be better atomized. More compression allows a more aggressive cam lobe and on it goes. I have eight different publications just on Chevy engines and the largest word count comes in the cylinder head sections. Factory HD combustion chambers and ports have been slow developers. Put a 16-valve 1300cc DOHC engine up against an Evo and you'll soon get the picture; the torque figures are interesting though. Basically it boils down to efficiency and I'd suggest anyone uses the best fuel available for their pride and joy. As an aside; our '03 V8 Holden Berlina is supposed to run fine on 91 but it doesn't like it, neither does my Evo.

[peasea]

What petrol do you use for your Evo engines in NZ - 91 or 98 octane? My engine temp is 175-210, depending on the weather. In Asia where I lived before coming here, we only had one choice. I'm running mine on 91 at the moment but it is running a little erratic, which could be the jetting being so close to sea level - plugs look fine though. I don't think it's pinging but I might try 98 next time.

You might want to consider the actual content of the fuel. When NZ switched to unleaded a while back the local fuel contained high levels of solvents, such as Toluene, to regain lost octane. These solvents don't so much 'burn' as go bang, resulting in a lot of engines pinking or running erratically. Fuel injected cars (read imports) were less suceptible to the problems thanks to on-board computers adjusting the ignition timing at the onset of the 'knock'. Hence everyone flogged off their Holdens and Falcons and bought cheap imported Toyotas that could run on weasel's piss, which is what Marsden Point produces and available at the 'big four' retailers. I don't know what the solvent figures are now but when Gull came on the scene (with Asian-sourced fuel) their fuel had the lowest levels of solvents but good octane figures and our 302 V8 Ford (at the time) ran well on their super, even though it had a compression ratio of 9.5:1. I use Gull premium (imported) or BP ultimate 98 (also imported) in my Evo, anything less and the smoothness drops off. On 91 it's a lumpy bag of bolts. Furthermore, unleaded fuel should only be used with a catalytic converter, otherwise carcinogens are pumped directly into the air; don't run your bike in the shed! (Get your mother-in-law to do it.)

[Divot]

But seriously though, notwithstanding poor Honda cylinder heads with cracks , as near as I can work out the Evo engine has about an 8.8-8.9:1 compression ratio.

The twin Cam I am riding at the moment is 10.5:1

[xwhatsit]

The twin Cam I am riding at the moment is 10.5:1

But that's not standard, though, is it? I mean it stands to reason if you're going to plane heads and put in Wiseco pistons, you're engine is going to want to pink unless you feed it slower fuel.