8 replies to this topic

[timmy850]

Hi All,

 

I'm after a few ideas on the ignition box I'm using. It has a programmable timing curve vs RPM and also dwell vs RPM. My current dilemma is working out what amount of dwell is appropriate to get the best overall starting and running. 

 

I have a 25D distributor that's been locked so there is no mechanical advance. Vacuum advance still works in addition to the rpm based advance

Accuspark electronic module (that is used to send the signal to the blackbox)

Standard 3 ohm coil

I also have a 1.5 ohm coil spare

Accuspark blackbox unit (which is capable of running a >1.5ohm coil)

 

The options I can think of are below:

Option 1: have a constant 60 degrees of dwell across the rev range and use the 3ohm coil the same as a points setup

Option 2: have a dwell curve from the recommended minimum of 20 degrees at idle to 70 degrees at around 5500rpm using a 3 ohm coil.

Option 3: have a dwell curve from the recommended minimum of 20 degrees at idle to 70 degrees at around 5500rpm using a 1.5 ohm coil.

 

My thoughts are:

Option 1: same as points and there shouldn't be any problem. 60 degrees at idle will be oversaturating the coil, and potentially it could be higher at higher RPM to keep the coil power up.

Option 2: this only seems to be of benefit up top, and would potentially make it harder to start with less dwell angle

Option 3: I believe this would give the best result for both starting and top end running? Is there a standard 4 cylinder dwell curve that's suitable?

 

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The original Accuspark instruction video used an oscilloscope to check for coil saturation at various RPM points, but they have since removed the video. I think this would ultimately be the best option, or if I knew what the optimum coil dwell time was so I could calculate the dwell angle vs rpm

[68+86auto]

Using an oscilloscope would be the best option, do you have one you can use?

I can guess what the video would've said and I have documents with scope patterns to show what you are looking for.

 

 

Your guess of dwell looks reasonable, could you just increase the dwell between 0 and say 400rpm?

I think option 3 would be a bad idea.

[ACDodd]

Option 3 with the lowest ohm coil you can fire, limit the dwell at high rpm to 55 if using a low ohm coil. This option is what is used in modern electronic ignition conversions, it’s also fitted into factory ignitions since 1984 in the form of the 65d dizzy

Ac

Edited by ACDodd, 05 December 2023 - 07:26 AM.

[timmy850]

Using an oscilloscope would be the best option, do you have one you can use?

I can guess what the video would've said and I have documents with scope patterns to show what you are looking for.

 

 

Your guess of dwell looks reasonable, could you just increase the dwell between 0 and say 400rpm?

I think option 3 would be a bad idea.

I don't have an oscilloscope unfortunately as this would have made it a bit more straighforward

 

The basic concept of the video was:

20 degrees at idle gives peak coil power without saturation

20 degrees at 4000rpm gives misfires as the coil isn't at full power

60 degrees at idle saturates the coil, and basically adds heat

60 degrees at 4000rpm gives peak coil power without saturation

 

Option 3 with the lowest ohm coil you can fire, limit the dwell at high rpm to 55 if using a low ohm coil. This option is what is used in modern electronic ignition conversions, it’s also fitted into factory ignitions since 1984 in the form of the 65d dizzy

Ac

Thanks AC. Would something like this be suitable:

1000rpm = 20 degrees

4000rpm and up = 55 degrees

 

And I could open the spark plug gaps up to 0.030" too?

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Edited by timmy850, 05 December 2023 - 09:36 AM.

[ACDodd]

I would make 5000rpm 55degrees, and use a 0.8ohm coil if your module will fire it. If it’s a 1.5 ohm coil use 25degrees starting dwell.

That’s a very strange advance curve you have programmed in there?

Ac

Edited by ACDodd, 05 December 2023 - 12:28 PM.

[timmy850]

I would make 5000rpm 55degrees, and use a 0.8ohm coil if your module will fire it. If it’s a 1.5 ohm coil use 25degrees starting dwell.

That’s a very strange advance curve you have programmed in there?

Ac

Thanks I will update the dwell accordingly. I have fitted a 1.5 ohm coil and the unit is rated down to 1.5 ohm

 

The advance curve is still a work in progress...

The distributor is set to 15 degrees base advance so all the programmed numbers are additional to the base 15. If the unit ever fails I can bypass it and drive it a short distance, and it might help with rotor phasing (compared to having 30 degrees of programmed advance)

With the Dellorto carb it seemed to like to ide at 1500rpm, so I added a timing ramp under this to increase the idle speed back to 1500. This needs to be updated now I have twin HS2 fitted and it idles a bit lower

 

The general shape of the curve was basically my attempt to avoid the irregular combustion I was getting at 3000rpm. The fundamental cause of this was the compression ratio being much too high (11.4:1). I had previously checked and changed nearly everything I could think of, and the only thing that really helped was to add octane booster in addition to the 98 octane fuel. The curve also needs some work since all the changes I have made

 

The engine is an 1100

Bored out +080 with flat top pistons, zero deck height

Heavily modified 12G202 head with 26.4cc chambers, 27.2mm / 1.07" exhaust valves, 31.75mm / 1.25" inlet valves

RE266SS cam

3.44 diff ratio

[ACDodd]

Ok with the 1.5ohm coil us 0.025” plug gaps and have 27degress dwell at idle and then up to 60 degrees at 5000rpm.

Ac

[Spider]

Tim, for not a lot of money, you can pick up an Oscilloscope through the likes of ebay, something that is 20 MHz or more would be well enough for just about any automotive application. You may need to get a better quality probe to go with it though. There are also a few PC based ones available, some better than others, but I would suggest steering away from one of these for this application.

To 'connect' it to the HT system, you only need put a few turns of wire around what ever lead it is you'd like to 'look' at and connect the probe to one end of that, you wouldn't even likely need to connect the Earth of the 'Scope to anything (and that maybe desirable. Be sure of course yr HT lead is in good condition though !  You can alternative use a pick up off an old Timing Light or if your current one plugs in, there you go.

 

Using the 'Scope may take a little time to learn, but I know you'd pick it up in short time, don't be put off by that. By using a 'Scope here, you can see what you have and how well it's working, adjusting to suit the Coil and HT set up you have as all parts of the HT system will have an effect on Dwell.

[timmy850]

Tim, for not a lot of money, you can pick up an Oscilloscope through the likes of ebay, something that is 20 MHz or more would be well enough for just about any automotive application. You may need to get a better quality probe to go with it though. There are also a few PC based ones available, some better than others, but I would suggest steering away from one of these for this application.

To 'connect' it to the HT system, you only need put a few turns of wire around what ever lead it is you'd like to 'look' at and connect the probe to one end of that, you wouldn't even likely need to connect the Earth of the 'Scope to anything (and that maybe desirable. Be sure of course yr HT lead is in good condition though !  You can alternative use a pick up off an old Timing Light or if your current one plugs in, there you go.

 

Using the 'Scope may take a little time to learn, but I know you'd pick it up in short time, don't be put off by that. By using a 'Scope here, you can see what you have and how well it's working, adjusting to suit the Coil and HT set up you have as all parts of the HT system will have an effect on Dwell.

Thanks Chris, I think an oscilloscope would be the best option. I only have one use for it at the moment, so a little hard to justify the cost 

 

I have updated the dwell curve as per AC's guide. I did a few laps around the block and it's running really nicely so far

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