Retarding Ignition Timing

[KiwiAlfa156]

Yesterday I retarded the ignition by a few more degrees to the 750 and performance dropped off. The bike felt flatter in power delivery in a short test ride and I didn't do a pull chop. I did pull a plug, but it really hadn't run long enough to show any difference. I have returned the timing to the prior position, as I think I might have hit the sweet spot with my first adjustment. I'll do a few more experiments when I get the time to fine tune and post some pics and what the (subjective) results are.

 

[rphillips]

I dun'no, Hemmings.com, not sure how they measure, Higher octane gas is processed through additional steps that farther refine the blend and cause it to burn more slowly than lower octane... No idea how they measure it and pretty sure they aren't talking racing fuel, think just pump gas. Also no way to know if Hemmings is telling the truth or not, nor if the folks telling me the same for yrs. and yrs were telling the truth. I can be sure LO octane pings and knocks (more like an explosion) and HI doesn't ping nor knock, seems less like an explosion.... Waaay too much for me to try to figure out.

 

[GregT]

Interesting you mentioned the FZR750 Yamaha. That head is one of the very worst I've ever come across for getting the charge burned. Not helped by the factory tolerances on the squish band meaning it's usually around 1.5mm clearance. The OW01 ignition has two advance peaks - 42 deg around 6500 rpm and 48 deg around 8500 to about 11000rpm where it drops to 30 deg. Presumably a rev limiter.
When the Loctite Yamaha team came here for the WSB rounds, I watched the team manager - Steve Parrish - blending fuel. Roughly 50/50 organiser supplied 100 octane and local 91 bought from the service station across the road from Manfield. I got talking to him as you do and told him I was aware of the poor combustion chamber shape and had a fair idea of why he was doing the blend - basically to speed up combustion and raise it's temperature for more complete combustion. He just laughed and said the dyno told them to do it.
Can't argue with that. I believe they were using Tony Scott engines so the squish would have been set up very tight.

 

[KiwiAlfa156]

The 5-valve Yamana Genesis engines were a bit a problem child. Cylinder filling was maximized to the detriment of combustion efficiency by virtually non-existent squish in the early engines and marginally more in later iterations. Kevin Cameron described them as follows:

"In practice, the five-valve FZR750 of 1985 experienced slow combustion; it needed quite early ignition timing of 45 degrees BTDC. Tuners noticed that this engine could be built either to accelerate or to deliver top-end, but not both. Strong acceleration requires high compression, but preventing knock may call for “squish”: areas of the piston crown which come very close to matching areas of the head at TDC. As such areas come together, with a typical clearance of 0.7mm, the mixture between them is rapidly “squished” out, creating jets that give the air-fuel charge a last-moment stir, helping accelerate combustion.

There was clearly some difference of opinion within Yamaha at the time they hired Valentino Rossi to ride their M1 MotoGP bike for the 2004 season. Masao Furusawa, the engineer assigned to provide the best possible ride for Rossi, had four bikes built for Vale to test; two with five valves, two with four valves, two with a flat (180-degree) crankshaft, and two with a crossplane (90-degree) crankshaft. Rossi liked best and went quickest on the four-valve/90-degree combo, so that’s how future M1s were built.

Later, Yamaha’s 1,000cc R1 production bike also switched from five to four valves, a design which continues to this day....

When Eddie Lawson won the 1993 Daytona 200 on a five-valve Yamaha, its valves had been relocated radially inward by clever welding and re-machining to form a tighter cluster around the central spark plug, possibly resulting in a faster-burning combustion chamber."

https://www.cycleworld.com/story/bl...eaders-yamaha-five-valve-engines-and-chatter/
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[KiwiAlfa156]

I can be sure LO octane pings and knocks (more like an explosion) and HI doesn't ping nor knock, seems less like an explosion.... Waaay too much for me to try to figure out.

No, your're on to it. The reality is the burn speed isn't all that important, the ability for gasoline to take high compression without pinging/knocking is the important bit. That's what the octane number tells you.

I think I know why the octane number got associated with burn speed. Gasoline octane ratings are assigned to the fuel due it's knock resistance compared to a reference fuel. That reference fuel has two components, the hydrocarbon n-heptane which has poor knock resistance, and is assigned a 0 octane number and the hydrocarbon iso-octane which has excellent knock resistance and is assigned an octane rating of 100. These are the standards anti-knock resistance (the octane rating) is measured against.

If a gasoline has the same anti-knock characteristics of a reference mix of 91% iso-octane and 9% n-heptane, that gasoline is rated as 91 octane. If a gasoline has the same anti-knock characteristics of a reference mix of 89% iso-octane and 11% n-heptane the gasoline is rated as 89 octane.

What has been conflated or connected in folks minds is that the high reference hydrocarbon iso-octane rated at 100 octane burns slower than the low reference hydrocarbon n-heptane with 0 octane. Not because of their octane rating but because of the chemistry of the substances.

Indeed if the gasoline was only made from these two hydrocarbons (instead of the hydrocarbon soup that makes up the gasoline fraction of volatiles distilled at the refinery) then burn speed would slow as octane went up. But while it is true for the reference substances that are used to give octane ratings it isn't necessarily for the fuels that are quantified against them.

And that's the thing, a lot high octane gas may burn slower than lower octane gas. Or it might not... But the octane rating won't tell you for certain if they do.

 

[rphillips]

Kind'f wish Hemmings hadn't made it so simple, that just makes it more confusing... They never mentioned sometimes HI will burn slower and sometimes faster.