Going by memory cuz I'm at work but I think our books say to time a 850 at 28' BTDC & Tri-Spark paper work just says follow the manual for the given bike but why do so many people in discussions of this say they time them at 30' BTDC???? I've gone through the effort to verify my timing marks with a degree wheel etc so since I can get it right I'd like to know should I aim for 28 or 30. Thanks
28' or 30' timing for a 850 with a Tri-Spark? (2015)
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lcrken
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Back in the '70s Norton supplied the Boyer transistorized system as an option for the 750 Production Racer, and recommended setting it at 30. Maybe that's where the 30 comes from. As far as I recall, the spec for stock Commandos, both 750 and 850, has been 28. The factory tuning notes for the PR recommended 28 degrees for poiints ignition and 30 degrees for "Transistor Ignition", meaning the Boyer at that time.
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pete.v said:gtsun said:
28 or 30 will be just fine.If you really need to split hairs, go for 29. Seriously, any one of those setting will is close enough. Maybe stay away from 27 and don't go above 31.
What he said. But you can't always trust the degree gauge on the cover.
I set mine to 30 and it pinged so I went to 28.
I need to put a wheel on it so I know how accurate it is but I'm sure it may be off some.
I just try it at different setting until it feels right but you said it's a race bike so I would degree it with a wheel.
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gtsun said:
Hi.
28°.
Ta.
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Best-correct full timing is based on the speed of combustion which is higher with higher CR so 28* is for 10 CR like Combats. Lessor CR engines should creep up toward 32* or just short of hints of backfire to start. Its hard to set TDC mark exactly so time light readings or mechanical measures can easy be a degree off which can be felt as slight sluggish response and less enjoyable exht note if not tested for a bit more by trial error by gosh by golly - then light it up to see where it lands and mark it. A single degree off can break your leg or give wider grin, at least in the higher CR engines. Octane matters too but my Combats at 28.5* do not ping on 87 if not being a heated wild man, otherwise better stick with 91-93 for kicking up heels much as ya like on best maximum torque ignition advance.
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hobot said:
Hi hobot.
Is the 87 you mention 'octane rating'? and 91-93 the better stuff?
In Australia, the figures quoted at the petrol pump are 91, 95 and 98 with about 10 cents per litre extra for each increase. Alcohol added petrol is available but I don't know anyone that uses it in their bike.
91 octane goes into cars and Norton riders usually opt for 95 or 98.
AFR & EGT to tune on road at present then off to the dyno to assess setting (inc. timing) and tweaks.
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Needing – different scales are used, AKI shown in Canada and the United States is 4 to 6 octane numbers lower than elsewhere in the world for the same fuel.
Australia: "regular" unleaded fuel is 91 RON, "premium" unleaded with 95 RON is widely available, and 98 RON fuel is also reasonably common.
United States: in the US octane rating is displayed in AKI. In most areas, the standard grades are 87, 89-90 and 91-94 AKI.
United Kingdom: 'regular' gasoline has an octane rating of 95 RON, with 97 RON fuel being widely available as the Super Unleaded.
Australia: "regular" unleaded fuel is 91 RON, "premium" unleaded with 95 RON is widely available, and 98 RON fuel is also reasonably common.
United States: in the US octane rating is displayed in AKI. In most areas, the standard grades are 87, 89-90 and 91-94 AKI.
United Kingdom: 'regular' gasoline has an octane rating of 95 RON, with 97 RON fuel being widely available as the Super Unleaded.
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gortnipper said:
Thanks for that gortnipper.
Aha, a variation in the numbering system rather than the fuel itself.
Seems we have a long way to go before we are truly a global village.
Australia has only relatively recently standardised the rail gauge, electricity voltage, some qualifications, etc but the speed limit around the country is still 100 or 110 kph depending on which state you are in.
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Hmm good point about what the numbers means for ign timing in various counties and elevations too. I accidentally got to check my TDC marking and points setting accuracy by a way you all might ponder too. Trixie stopped hints of kick back and sounded and repsonded best with both contact breakers set at 29* in stead of 28*, thinking I was getting away with a bit more spunk w/o detonation but when i leak down tested I got torque at my 0* mark so had to back up -1* to balance piston under pressure which meant I had really only set points to 28^ before TDC. On Peel with sealed vent system may just put compressed air in cases and let the push em up to functional TDC. Below adds octane tuning to attitude carb and spark time factor.
United States: in the US octane rating is displayed in AKI. In most areas, the standard grades are 87, 89-90 and 91-94 AKI.[40] In the Rocky Mountain (high elevation) states, 85 AKI (90 RON) is the minimum octane, and 91 AKI (95 RON) is the maximum octane available in fuel.[41] The reason for this is that in higher-elevation areas, a typical naturally aspirated engine draws in less air mass per cycle because of the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel. In some east coast states, up to 94 AKI (98 RON) is available.[42] As of January, 2011, over 40 states and a total of over 2500 stations offer ethanol-based E-85 fuel with 105 AKI.[43] Often, filling stations near US racing tracks will offer higher octane levels such as 100 AKI
United States: in the US octane rating is displayed in AKI. In most areas, the standard grades are 87, 89-90 and 91-94 AKI.[40] In the Rocky Mountain (high elevation) states, 85 AKI (90 RON) is the minimum octane, and 91 AKI (95 RON) is the maximum octane available in fuel.[41] The reason for this is that in higher-elevation areas, a typical naturally aspirated engine draws in less air mass per cycle because of the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel. In some east coast states, up to 94 AKI (98 RON) is available.[42] As of January, 2011, over 40 states and a total of over 2500 stations offer ethanol-based E-85 fuel with 105 AKI.[43] Often, filling stations near US racing tracks will offer higher octane levels such as 100 AKI
I haven't heard of AKI before. I understand there are 2 methods for calculating octane Research octane that's a mathematical formula and motor octane that uses some kind of standard test engine to measure octane. Research octane is higher than motor octane.
I believe that back when Nortons were new we used Research octane in the US (higher octane numbers). I could be wrong, i wasn't old enough to drive back then. Now we use the average of research and motor octane, which sounds like the AKI numbers you're using.
I ran 31 in my 850, that was the number I saw for a Boyer everywhere I read. Keep in mind that a Boyer doesn't reach max advance as early as a points system. They quote max advance at 5000 rpm, but I believe it keeps advancing after that.
If your Trispark finishes advancing at lower rpm (similar to points) then it might make sense to reduce advance to not be overadvanced at, say, 3500 rpm.
I understand why a points system would stop advancing at relatively low rpm due to the physics of the weights and springs. Any idea why a lot of electronic ignitions also do it that way?
My understanding about advance increasing with rpm is because the flame front inside the combustion chamber moves at a pretty constant speed but the engine is going faster and faster as it revs. Hence we want the flame front to reach the piston crown right after it's passed TDC so it can give a push on the way over. It seems like the advance should keep getting earlier and earlier with rpm, why shouldn't it?
Eric
I believe that back when Nortons were new we used Research octane in the US (higher octane numbers). I could be wrong, i wasn't old enough to drive back then. Now we use the average of research and motor octane, which sounds like the AKI numbers you're using.
I ran 31 in my 850, that was the number I saw for a Boyer everywhere I read. Keep in mind that a Boyer doesn't reach max advance as early as a points system. They quote max advance at 5000 rpm, but I believe it keeps advancing after that.
If your Trispark finishes advancing at lower rpm (similar to points) then it might make sense to reduce advance to not be overadvanced at, say, 3500 rpm.
I understand why a points system would stop advancing at relatively low rpm due to the physics of the weights and springs. Any idea why a lot of electronic ignitions also do it that way?
My understanding about advance increasing with rpm is because the flame front inside the combustion chamber moves at a pretty constant speed but the engine is going faster and faster as it revs. Hence we want the flame front to reach the piston crown right after it's passed TDC so it can give a push on the way over. It seems like the advance should keep getting earlier and earlier with rpm, why shouldn't it?
Eric
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ewgoforth said:
Not in all cases. Some engines get combustion speed help with good combustion chamber turbulence at higher rpm. In some cases, one benefits from backing off on the advance a bit for higher higher engine speeds while benefiting from greater advance at lower speeds.
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ewgoforth said:
No, both RON and MON use test engines. RdON uses on road results, which may be what you are thinking of?
AKI = (RON+MON)/2
see here - http://en.wikipedia.org/wiki/Octane_rat ... nt_methods
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Dances with Shrapnel said:
A couple of posts I read a while back that might be of interest:
timing-descrepencies-t12252.html
commando-timing-advance-curves-compiled-reva-t6488.html
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Good references. Note that most all ignition system plotted cap out at around 3,000 rpm. Combustion chamber turbulence taking over?
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ewgoforth said:
From the 750/850 Manual:
[Gortnipper’s note: UK uses RON, 99 RON =~93 AKL]
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