Bonneville America Forums Forums Technical Forums Performance Mods 270° Ignition Surface Maps

Hi Guys, for those that are interested, here are the standard 270° 790/865 ignition advance surface maps.

Definitely interesting, but difficult to get my head around it!

Hello Mike,

Could you shed some light on what we're looking at? I noted the 5 different TPS sensor points and the differing colours probably indicating (Correct me) advance timing. If this is so, could you give us the amount of timing each colour represents in degrees?

Does 100 TPS indicate full throttle or the other way around?

The 790 appears to have a much wider range of advance maps then the 865.

Interesting

Sorry, maybe some explanation is in order.

Back in the day most vehicles used points ignition with either vacuum or centrifugal weights to increase ignition advance as revs rise. This was effectively a 2D system which could be shown in a graph with an ignition advance axis and a RPM axis.

Ignition advance is necessary due to the speed of the flame front and the combustion process in the combustion chamber. There is an ideal point of maximum pressure in the combustion process that produces most power, this point is between 12°-15° ATDC in the power stroke. There are four main areas that influence the amount of ignition advance needed these are chamber design, cam design, compression ratio(CR) and piston surface area.

If you had a bad chamber design and low CR for example, the combustion process/flame front would be slow and would need more ignition advance to ensure peak pressure occured at 12°-15° ATDC. On the other hand if the chamber design was very good and you had a high CR the combustion process would be much quicker and require less ignition advance.

With the advent of electronic ignition and engine management systems this 2D graph can have another axis added for engine load measured by the throttle position sensor (TPS). Engine load also has an effect on the speed of the combustion process, full throttle is more efficient than part throttle and so needs less ignition advance.

The igniter unit on our twins has five ignition advance maps stored in memory, one each for 0%, 25%, 50%, 75% and 100% throttle openings. The surface maps in the graphics above show how the ignition advance is varied depending on engine load and RPM. Unfortunately I cant show the advance figures as they are Triumphs design and their copyright.

Sorry that was lengthy and I hope I have explained the process well enough to be understandable.

So what does that mean for those of us with higher compression big bores and "upgraded" cams?

So, if you take a motor with a wilder cam, higher CR ( i.e. 38mm), more piston area (i.e. 904), and good chamber design, would the result be no change to the advance?

I'm curious because that's essentially the setup I will end up with, and wonder what kind of re-map I should go with.

Thanks for posting that. Wish I understood it. Why does the 790 look incomplete? Should I re-map (if thats possible) if I go big bore, keeping stock 790 cams?

If I understand this correctly and assuming that each colour range represents around 1 Degree of advance with initial advance set at 4 to 6 degrees BTDC, the 790 at 100% TPS load has about 3 degrees of less advance at max rpms that the 865? I thought the throttle maps for the 790s were a bit more radical and they make their power at higher rpms. Higher rpm usually mean more advance?????

What's the usuall max advance at max rpm....35 degrees BTDC?

Also sent by PM:
The ignition mapping stuff is fascinating and I had a look at the web site linked at the bottom of your post - very interesting.

On a practical matter, the igniters on Americas/Speedmasters seem to fail more than you would think. I have heard of probably a dozen cases out of 1000 or so regular members. I wouldn't have imagined these weree servicable pieces but you are actively modifying them. Would it be worth someone shipping you a dead one to see what is up?

Second but related point, there are after-market igniters for 360 degree engines available quite inexpensively (like us$100 vs us$600), would you be able to re-map one for the 270 engine? I understand that remapping is probably software and the failures are hardware but I thought I'd ask.

Thanks for any response.

Sorry I haven't got back to you before.

I believe the America/Speedmaster igniter problems relate to positioning of the unit. Under the tank, hot and open to the weather as opposed to the Bonnie/Scrambler, under the seat, cool and protected from the weather. This is only what I think and have nothing to qualify this. Maybe some sort of heat/weather guard around the igniter would sort the problem.

Usually the dead ones are dead and there's nothing that can be done. Although recently, a complete map upload of all the maps and rev limiter re-programme has bought a 270° back to life, but I couldn't guarantee an igniter would be bought back to life if it had a problem.

I have tried to access Procom units, but they use different comms, so no access, sorry. But worth asking.