General Product Information


Why do I need a Twin Tec ignition?

The Twin Tec ignition gives you several important benefits:

Adjustable RPM limiter. Depending on the camshaft and other valvetrain modifications you make, peak power will likely be around 6000 RPM, well above the RPM limit of the stock ignition.

Adjustable advance. You can fine tune the advance to meet your requirements. The Twin Tec systems comes with enough adjustment range for most applications. You can also program a custom advance curve using our optional PC link and free software.

Single fire (further explained below). Note that all Twin Cam 88 engines are already factory equipped with single fire ignition. Other Harley-Davidson engines will benefit from conversion to single fire. If you make certain camshaft and carburetor jetting changes, conversion to single fire may be required to avoid backfiring problems.

Multiple spark (further explained below). Enhances starting and idle quality. Reduces plug fouling - especially if the carburetor has been re-jetted. 


How do I set a safe RPM limit?

Twin Tec ignitions have an adjustable RPM limiter. You can set the RPM limit from 3,000 to 9,900 RPM in 100 RPM steps by means of two rotary switches. The only exception is our Model 1005-OE that has a fixed RPM limit. 

Harley-Davidson stock ignition modules generally have the RPM limit programmed at 5,600 RPM. If you modify the valvetrain, peak horsepower will probably occur above 6,000 RPM and you will need to increase the RPM limit. Check with the manufacturer. They can recommend a safe RPM limit. We do not recommend increasing the RPM limit of a stock engine above 5,800  RPM.


How can I optimize the timing advance?

Twin Tec ignitions allow you to easily tailor the ignition timing advance to your application. The advantage of a Twin Tec unit is that you will always be able to optimize the timing to match your exact requirements. 

Models 1005, 1006, and 1007 come with two families of advance curves: street and race. The actual curves are listed in the installation instructions that you can download. Use the street advance curves for stock or mildly modified engines. The mode switch is used to select the advance curve family. Once you have selected an advance curve family, you can use the advance slope switch to change the shape of the curve. Higher switch settings result in a more aggressive curve. You can experiment with progressively higher switch settings. As a general rule of thumb, you will obtain maximum power by sing the highest setting possible without audible spark knock. Additional details are given in the instructions.

The TC88 unit has a 3D advance surface based on RPM and manifold pressure. Since Twin Cam 88 engines have a non-adjustable crankshaft position sensor, there is no mechanical means for setting the initial timing. The TC88 unit solves the problem. Initial timing can be adjusted over a 5 range by means of a rotary switch. A second switch allows you to change the slope of the advance surface. Higher switch setting result in more aggressive advance. The use of  a 3D surface and the wide adjustment range afforded by the two switches allows you to accommodate most applications. 

If you are doing dyno tuning or have specialized requirements, you can program your own advance curve or surface by using our optional PC link and free software. The software allows you to graphically edit the advance. You can also offset the rear cylinder timing over a 5 range. Harley-Davidson engine tuners have found that the rear cylinder often runs somewhat hotter and thus reaches the knock limit first. Slightly retarding the rear cylinder allows more overall advance and may generate more power. 

The table below summarizes the effect of various engine parameters on the ignition advance requirement.  

Engine Parameter Volumetric Efficiency Flame Front Velocity Combustion  Time Ignition Advance Requirement
Engine RPM VE peaks near torque peak Increased at VE peak Reduced at VE peak Less relative advance at VE peak. However, predominant effect is that more advance is required as RPM increases due to less time for crank to sweep through a given angle - thus requiring spark initiation at a greater angle BTDC.
Increased compression ratio Minimal effect Increased Reduced Less advance
More radical camshaft (increased duration and overlap) Less at low RPM; greater at high RPM Less at low RPM; greater at high RPM Less at low RPM; greater at high RPM More advance at low RPM; less advance at high RPM
Improved exhaust  scavenging or less  back pressure Varies throughout RPM range Lower levels of  exhaust gas residuals in cylinder increases velocity Reduced Less advance within the RPM range where exhaust is most efficient
Improved intake system efficiency (bigger throttle body or low restriction air cleaner) Generally greater at high RPM for H-D engines Increased Reduced Less advance
Increased fuel octane  No direct effect Reduced; less likely to reach knock limit Reduced More advance; increased knock limit
Air/fuel ratio No direct effect Optimum near stochiometric 14.7 A/F ratio Optimum near stochiometric 14.7 A/F ratio More advance required for rich mixtures
Improved fuel atomization  Minimal effect Small fuel droplets burn faster Reduced Less advance 
Increased intake air temperature Lower Increased; may reach knock limit where end gases ignite Reduced Less advance; lower knock limit as temperature increases
Increased humidity Slight reduction as water displaces air Reduced Increased More advance. Extreme example is water injection used to increase knock limit.
Increased cylinder head temperature   Minimal effect Increased; may reach knock limit where end gases ignite Reduced.  Less advance; lower knock limit as temperature increases
Spark plug position in head; number of spark plugs No direct effect Minimal effect Affected by distance from plug to farthest cylinder wall. Ideal location for single plug is center of squish area Less advance for centered spark plug or dual spark plug designs
Greater bore/stroke ratio Minimal effect unless valve shrouding occurs in large bore designs Short stroke increases rate of compression and results in higher  velocity Large bore requires more time to burn from spark plug to cylinder walls.   Very long stroke or large bore (over-square) engines may require up to 10 more  advance than an equivalent CID engine with optimum bore/stroke ratio 
Combustion chamber design with high squish and swirl  Minimal effect High swirl increases velocity High squish  designs take less time to burn to farthest reaches Less advance for efficient combustion chamber designs



What is the difference between single fire and dual fire?

Single fire and dual fire refers to the number of times the spark plug fires during each four stroke cycle. The terminology is somewhat unique to Harley-Davidson engines and is by no means consistently applied. For example, Custom Chrome Industries, one of the largest distributors of Harley-Davidson aftermarket parts, uses the opposite terminology. Their single fire systems correspond to what most other companies refer to as dual fire.    

With the exception of the new Twin-Cam 88 and late model Sportster 1200 engines, all carbureted Harley-Davidson engines have been dual fire. A single coil winding with two high voltage output terminals fires the spark plugs on both cylinders simultaneously. Each plug is fired twice during each four stroke cycle. This approach was used to cut costs as it eliminates the need for a distributor or a second coil and additional electronics. Dual fire results in a number of potential problems.

Most late model automotive engines are distributorless. Many of these engines use coil packs where a single coil winding fires two spark plugs. When one spark plug fires on the compression stroke the other spark plug is firing on the exhaust stroke. This approach is termed "wasted spark"  and is widely used. The wasted spark always occurs on the exhaust stroke because the engines have even firing intervals (i.e. 90 for a V8) and cylinders are always paired so that the pistons are 360 out of phase (i.e. one on the compression stroke when the other is on the exhaust stroke). The wasted spark causes little energy loss and no harmful effect on the exhaust stroke.

The situation is quite different with a dual fire ignition on a Harley-Davidson V-twin engine with 315 and 405 firing intervals.  The graphic below shows what occurs. 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! Under some conditions, a combustible mixture may exist in the rear cylinder at this point and the wasted spark causes a backfire through the carburetor. Long duration camshafts and improper carburetor jetting can contribute to the problem. Additional information on this subject may be found on the Mikuni web site.

The Problem with Dual Fire Ignition  

A single fire ignition eliminates the backfire problem and enhances idle quality. The single fire ignition uses separate coil windings and electronics to fire each spark plug independently. Spark firing occurs only on the compression stroke. Conversion of older carbureted Harley-Davidson engines to single fire is highly recommended. There is no downside to single fire, other than the cost of the conversion.

Twin Tec Models 1005, 1006, and 1007 have switch selectable single and dual fire operating modes. You can initially install one of our units and run in dual fire mode with your original equipment coil. You can then easily upgrade to single fire by adding an appropriate coil. If you have a tach, it will continue to operate properly when connected to the tach output from the Twin Tec ignition (some competitive systems require that you purchase a tach adapter).  


How does multiple spark work?

Many readers may be familiar with automotive multiple spark ignitions, such as the MSD 6 series. These are capacitive discharge systems that fire a series of short sparks. The Twin Tec multiple spark technology works on a somewhat different principle but the end result is similar. Coil primary current is repeatedly switched on and off by the processor.  When the current is switched on, energy is stored in the coil's magnetic field. When the current is switched off, the stored energy is discharged as a spark. All Twin Tec units can fire a continuous series of sparks from the advanced timing point to top dead center. 

Multiple sparks are of no benefit if conditions of homogeneous mixture distribution and good swirl turbulence exist in the combustion chamber. Under these ideal conditions, a single spark (even of relatively short duration) can generate a flame kernel that will rapidly propagate into a well established flame front.  Multiple sparks can provide significant benefits under less than ideal combustion conditions. If a flammable mixture is not present at the spark plug when the first spark is fired, better conditions may exist during subsequent sparks. Instead of a complete misfire, it may still be possible to initiate combustion and generate some power.

Multiple spark will improve starting  and idle quality. It will also reduce lean surge during part throttle cruise.  


What spark plugs wires are recommended for optimum performance?

Three types of spark plug wires are commonly available: original equipment style carbon core suppression, low resistance spiral core, and solid core. Carbon core suppression wires cause some energy loss due to their high resistance (about 5,000 ohms/foot). Replacing carbon core suppression wires with low resistance spiral core wires only increases spark energy by about 10%. Contrary to any claims, you will not see  a performance improvement by changing spark plug wires. On the other hand, carbon core wires can deteriorate over time and any wires more than a few years old are candidates for replacement. If you are going to install new spark plug wires, buy a set of low resistance spiral core wires. Most are about 500 ohms/foot. At that point, almost no energy is lost in the wire.  

Do not use solid core spark plug wires. These radiate excessive electromagnetic noise that will cause radio interference and may even cause the processor in the ignition module to glitch. We do not recommend Nology spark plug wires. These have a partially shielded jacket that forms a energy storage capacitor. When the spark plug fires, energy stored in this capacitor is rapidly discharged causing a short but intense arc discharge. There may be some advantage if the spark plugs are fouled, but the Nology wires tend to radiate more electromagnetic noise.


How about spark plugs?

Contrary to much of the marketing hype, novel spark plug designs generally fail to show any measurable performance improvements. Don't waste your money! The biggest problem with spark plugs for Harley-Davidson engines is fouling. The trick is to find a heat range that is a good compromise between fouling and pre-ignition. A platinum tipped plug would also be a waste of money, since it would likely foul long before it eroded. Stay with the recommended plug gap.

Use only resistor type spark plugs. The non-resistor type radiate excessive electromagnetic noise that will cause radio interference and may even cause the processor in the ignition module to glitch.


to find a heat range that is a good compromise between fouling and pre-ignition. A platinum tipped plug would also be a waste of money, since it would likely foul long before it eroded. Stay with the recommended plug gap.

Use only resistor type spark plugs. The non-resistor type radiate excessive electromagnetic noise that will cause radio interference and may even cause the processor in the ignition module to glitch.