E: Engine Modern Fuel & Ignition Advance

[Vince Farrell]

I have been staggered by the alterations possible by a programmable ECU when Ben gets on the rolling road with his k100, ignition point and mixture are altered in situ on the laptop with the engine pounding away. Back on the Vincent a lambda sensor plugged in the exhaust and the slots in the BTH body make some running changes possible

This is a very good technical thread, but doesn't really answer the question that's niggling me and probably a few more owners of standard Vincents. The Riders Handbook tells the average owner how many degrees before TDC at full advance was recommended. Disregarding worn ATDs and all the other variations that alter the fully retarded position, what is now (if different) the best setting for a standard bike for general riding using lead free petrol as supplied in the UK? References to shape of modern combustion chambers, high compression, twin sparks, fast burn, slow burn, racing, sprinting etc. are interesting, but most bikes are pretty standard. There appears to be some disagreement among all the technical talk as to how modern petrol behaves.

If it was possible for the factory in the `40s to provide a good compromise, surely with so much knowledge in the VOC someone can tell us how many degrees we should be using now?

Vince Farrell

 

[Chris S]

Vince makes a good point as most bikes are relatively standard.
I suggest setting somewhere near the factory 39 degrees at full advance and then trying a bit more or less advance.
Less advance may provide better high rpm performance but at the cost of low rpm and starting behaviour and vice versa.
Bearing I mind that if the mag cover is off, it takes a couple of minutes to alter the timing a degree or two either way and then try it.
No two bikes are identical so don’t be scared to try and find the best for yours by trial and error. You may not find the answer written down for you in a book.

 

[Oldhaven]

I wonder, as Pete suggests above, why the full advance number is not currently given as XX degrees at xxxx RPM. With an ATD or a D distributor of course this does not matter as much since the advance RPM is not normally or easily adjustable. With current programmable ignitions, especially for user programmable ignitions, there is almost unlimited adjustment available so the experimentation with occasional access to a Dyno someone Like davidd does can be very valuable. The actual shape of the ignition curve and its relationship to RPM is now at least a bit more important for anyone looking for better performance. I do remember someone quoted on the forum saying that for a Vincent getting the advance "all in early" is what is required, but whose Vincent in what state of tune was that advice intended for?

 

[davidd]

If it was possible for the factory in the `40s to provide a good compromise, surely with so much knowledge in the VOC someone can tell us how many degrees we should be using now?

If we set the timing like the factory did we would go to the dyno, start at 40 degrees and keep retarding until we found maximum power.

Despite the very good technical discussion, I still maintain that if you want the timing on your bike to be correct, you have to do what the Factory did. So far, few if any have done so. This certainly does not bother me, because it takes a lot of effort and some money to do dyno testing, but this is why the Factory had a dyno.

My understanding is that 34-36 degrees seems to be the area where most "regular" bikes have found good performance. I don't see anything wrong with this, except my own experience that seat of the pants analysis is often wrong regarding tuning performance.

My racer runs on premium pump gas straight out of any gas station pump here in the US. The two points I find interesting are first, the idea that you advance the timing to get more power vs. retarding the timing to get power; and the idea that if your Vincent engine is modified from stock, how do you determine what the advance curve should look like.

David

 

[Cyborg]

Despite the very good technical discussion, I still maintain that if you want the timing on your bike to be correct, you have to do what the Factory did. So far, few if any have done so. This certainly does not bother me, because it takes a lot of effort and some money to do dyno testing, but this is why the Factory had a dyno.

My understanding is that 34-36 degrees seems to be the area where most "regular" bikes have found good performance. I don't see anything wrong with this, except my own experience that seat of the pants analysis is often wrong regarding tuning performance.

My racer runs on premium pump gas straight out of any gas station pump here in the US. The two points I find interesting are first, the idea that you advance the timing to get more power vs. retarding the timing to get power; and the idea that if your Vincent engine is modified from stock, how do you determine what the advance curve should look like.

David

All the technical discussion is above my pay grade as well, but for what it's worth, I agree with David. If one wants to determine the perfect scenario for a given bike/fuel, then a dyno/analyser is the best route. As for seat of the pants analysis... an example, so the story goes was Nicky Hayden's first outing on an RC51. He had kind things to say, but wasn't all that enamoured with the power... until he was shown his lap times. Even his posterior didn't make a good dyno.
Taking a modern electronic ignition and using a dyno is something that I would love to do, but as mentioned elsewhere, I don't think I could stand dealing with 20 or so full pulls on the old girl. Another thing to consider is that it seems most electronic ignitions on the market are multi spark, so if I were to establish the best advance curve for my Comet engine with the PowerArc (programable) ignition, I don't know how much help that would be to someone using a mag. I guess total advance would be of interest, but then with differences in fuel and engines.....

 

[davidd]

I recently talked to a Vincent owner that had his bike on the dyno many months ago. He wanted more power and changed the cam. He ran the bike and said it was as powerful, if not a little more powerful. He was very disappointed when the dyno showed a loss of 4 bhp. A quick switch back to the original cam and some exhaust work gave him the 4 bhp back with an addition of 2 bhp more. It is a real talent to tune a bike by the seat of ones pants.

The DynoJets that are run as inertial dynos cause very little wear. I think I did about 40 pulls to 7000 on my racer during two all day sessions and ran it all that season after the tuning. I then sold it and it ran another 5 seasons with a piston and ring change in the 5th season. The Vincent dyno was a load dyno. That will wear a bike out in no time.

David

 

[Bill Thomas]

I used to tune my bike by doing standing 1/4 mile sprints, I have said before how the length of exhaust can make a lot of difference, Also what is in the silencer.
But what ever change you make can effect all the other settings.
For me, My two into one exhaust worked far better than L/ning pipes.
I see road Vin's, Where the end finishes are some inches different.
Cheers Bill.

 

[greg brillus]

One point that no one has mentioned on here is the effect of modern fuel and ignition timing on other makes of British bikes...........Or certainly any old bike with hemispherical combustion chambers, cars included. The facts are that most all need their overall advance reduced by around 4 to 5 degrees. With an ATD the range it operates over changes little apart from normal ware. Reducing the timing at full advance will result in the retarded position being at or after TDC. This is why the fixed stops need to be modified to reduce the operating range of the ATD to a travel of 15 degrees. So a retarded engine with 4 to 5 degrees BTDC plus 30 degrees in the ATD will give you 34 to 35 degrees full advance. Most road going Vincent's set up this way even with 8:1 pistons and Mk 2 cams will be fine.

 

[Pete Appleton]

David mentions 'doing what the factory did' That would indeed be an interesting exercise to repeat with modern fuels and would go a long way towards answering the question posed by the original post.
I suspect that the factory were working around the problems of using mechanical advance systems. Perhaps they just picked a final advance figure that gave the best compromise at all rev ranges.
Today I set up a series 'D' distributor on my electronic ignition test rig ( alright, it is an old food blender and an oscilloscope) the purple line on the graph below is what I found. In fact the curve is so different from what I expected that I did it all again to check my measurements.
The three plots are very different. At some rev ranges there is a difference of 10 degrees. Assuming that the published figures for bth and pazon are accurate then someone ( or everyone) must have it wrong. My interpretation of the information provided by Grey One makes me think that the advance curve should ideally flatten out at the top end, or even head back down again. That would all be very difficult to do mechanically.
I also tried to do the graph for a magneto with ATD. Unfortunately that wouldn't go on my state of the art analyser so I had a go at doing it on the lathe. The maximum speed achievable with that was 2000 crank rpm but there was almost no detectable advance movement at those speeds. I will have to build a better mag test rig. Has anyone done the degree wheel/strobe test with a mag?
As per OldHaven I have been working with programmable ignition systems. They are very successful but what curve do you program?
Of course we could find that it makes very little difference and we should just set it to 36 degrees and get on with riding it.

 

[Oldhaven]

My interpretation of the information provided by Grey One makes me think that the advance curve should ideally flatten out at the top end, or even head back down again. That would all be very difficult to do mechanically.

I am also above my comfort level here, so I hope to learn something I can understand from those who grok this better than me, but looking at what very high revving engines are doing with advance, I think that sounds right.



With our low RPM engines we seem to only be using the first half of what is possible with an ignition advance curve that runs into the high teens. If davidd was running up to 7000 it looks like starting going down in advance compared to a leisurely 4000-5000 RPM's plateau might be the right direction to be going. (Though whether it means more or less power, detonation risk...?????) Not an easy task for bob weights. For most of us the three curves above or something like that seem to work pretty well.