dual fire vs. single fire ign.
- Thread starter hdfozz
- Start date
Hoople
Account Removed
I would prefer single fire if I had a choice. Having dual fire means there will be a spark firing on valve overlap (wasted spark) as well as the compression stroke. Normally that is not a problem with most cylinder set-ups. But with Harley's there "can" be a problem if your using long duration cams. Since the Harley has 315° and 405° firing intervals, when the rear cylinder is fired on the compression stroke, the front cylinder is on the exhaust stroke - which is OK. But when the front cylinder is fired on the compression stroke, the rear cylinder is already on the intake stroke. A combustible mixture may exist in the rear cylinder at this point and the wasted spark causes a backfire through the carburetor. This is where the long duration cams can be a problem.
The only issue with converting a dual fire to single is that with single fire you need some way to phase the ignition to the crank. Normally a cam sensor is used for that. I have seen some aftermarket systems (Crane) advertised as phasing during cranking (or starting). During cranking, the ignition sees the engine slow down during compression via the crank sensor. From that slowing-down, the ignition now knows which cylinder is on it's compression stroke. I have never used a Crane myself but I feel that is a very risky way to do it due to the fact if "sync" is lost for some reason, you must restart the engine to re-phase the ignition. Now if you happen to loss sync by an "even" count, you would be OK. But an odd error count (1,3,5,7,etc) would be bad news. Using a cam sensor would be the most reliable way to phase a single fire ignition to a crank.
karlsbike
Active Member
Hoople is absolutely correct in his points about the stock dual fire (or wasted spark). Now he's got a TC, but at least on a carbed Evo the conversion to single fire is easy:
You've got your cam sensor in the nose cone. Under the nose-cone cover and the plate underneath it you've got your stock sensor (pick-up) sitting on the plate right in front of you. Your ignition knows the phase of the engine by sensing when the slot in your rotary cup open/close (hall effect, or magnetic field sensing).
The only practical difference between the two is that a single fire unit needs to know which coil to fire - front or rear, while a dual fire system just fires its single coil at an interval given by the pick-up (adjusted in your ignition unit for the 315/405 deg difference between front and rear).
Depending on the ignition system you're chosing, but with an advanced unit you set which advance curve you want (e.g. by dip switches), and then set static timing in relation to front cylinder TDC (comp stroke) and the cup positon (indicated by a light on the ignition unit). The rest is computed.
So, in practise, what it takes is a bit of simple re-wiring, and finding a place for the two coils required - or you can get a dual coil unit that most probably will fit in the stock poisition. The ignition and pick-up can be had as a fully nose-cone contained unit, meaning that you'll get rid of the separate ignition module as well...
You've got your cam sensor in the nose cone. Under the nose-cone cover and the plate underneath it you've got your stock sensor (pick-up) sitting on the plate right in front of you. Your ignition knows the phase of the engine by sensing when the slot in your rotary cup open/close (hall effect, or magnetic field sensing).
The only practical difference between the two is that a single fire unit needs to know which coil to fire - front or rear, while a dual fire system just fires its single coil at an interval given by the pick-up (adjusted in your ignition unit for the 315/405 deg difference between front and rear).
Depending on the ignition system you're chosing, but with an advanced unit you set which advance curve you want (e.g. by dip switches), and then set static timing in relation to front cylinder TDC (comp stroke) and the cup positon (indicated by a light on the ignition unit). The rest is computed.
So, in practise, what it takes is a bit of simple re-wiring, and finding a place for the two coils required - or you can get a dual coil unit that most probably will fit in the stock poisition. The ignition and pick-up can be had as a fully nose-cone contained unit, meaning that you'll get rid of the separate ignition module as well...
Breeze3at
Experienced Member
Great info in Hoople and Karl's responses. I put a Dynatec HD1 unit on my '92 Dyna Glide back about '94. I can (and have) go from single to dual fire with a flip of a dip switch. Honestly, the only difference I can tell, is the idle is a little more "lumpy" in single fire mode. I'm running a .530" lift, medium duration cam.
R_W_B
Senior Member
Hoop had an excellent post as usual, but I have read a lot in this area. As Hoop said if you install a longer duration cam the the intake is going to start opeing sooner.
But all things being OEM most experts and people who have tried it (like Breeze3at) say there is no noticable difference in the performance. One reason for this is that the physics of the ignitable mixture. A compressed mixture conducts a spark better than the uncompressed intake stoke mixture so a larger percentage of the coils input will go to the compressed cylinder. In other words at the upper end of the intake stroke you have a weak spark occurring in the wasted cyclinder.
I tend to like the old potato potato sound of the dual fire more. If you are going to create a performance bike then yes make single fire one of the options. Otherwise if you are just street riding enjoy the sound you are gonna feel the same power either way.
J
Jack Klarich
Guest
Another plus of single fire is you can adjust the rear cylinder timing independent from the front, most good units have built in diagnostics and lights for timing and selectable advance curves
karlsbike
Active Member
I have seen the same, but for the life of me I cannot see any advantages to removing the VOES - except, of course, if the VOES causes pinging. I will try out single fire with VOES
Same here, and I will try it out once the white stuff goes away (word avoided, as courtesy to Jack
. But, I am surprised that you experienced more lumpy idle in single-fire mode - I'd think it would be opposite...
Breeze3at
Experienced Member
I have seen the same, but for the life of me I cannot see any advantages to removing the VOES - except, of course, if the VOES causes pinging. I will try out single fire with VOES
Same here, and I will try it out once the white stuff goes away (word avoided, as courtesy to Jack
I might have that backwards... It's been so long since I changed it, and I changed the exhaust system at the same time (no balance tube now).
Of course I just took the manifold off yesterday to fix an air leak, so I can't go fire it up and switch it to hear the difference.