Blown Engine Considerations


High Output Ignition System Components


A more dense air/fuel mixture will require high voltage to ignite.  Although boost levels of roughly 4-10psi will require no more than stock components, any levels above 10psi may need aftermarket high output replacements. Thomson Superchargers offers these components.



Spark Plugs

At boost levels of 4-10psi stock spark plug heat ranges should be sufficient.  If during testing or operation you feel a cooler plug is required, a good direction to go is as follows.


  1. One heat range colder for boost pressures of 8-10psi
  2. Two heat ranges colder should be tried for boost pressures above 10psi


Note- If a colder plug tends to get deposits faster than stock, additional ignition voltage may be required to achieve plug life.


Ignition Timing


Ignition timing may benefit from a small retard function as boost rises and the fuel mixture becomes both more dense and more turbulent.  Contact Thomson Superchargers for more info on this subject.  Testing has shown that our supercharger actually decreases the Intake air temp substantially when making boost, the more the throttle is opened, the colder it gets. All of our R&D has shown this is due in part to a drastic increase in the thermal efficiency of the blower (due to the rotor design). The other major factors are the blower intake design (velocity/volume), and the way the fuel is delivered to the blower (atomization). The cooling effect is so pronounced that it actually requires a slight increase in ignition timing advance under boost, when earlier designs required a large amount of advance be taken away. This equates to……..MORE POWER.


Fuel System





The supercharger only adds one of the two variables needed to create more power,  MORE AIR.  Now you need a fuel system that can deliver the other, MORE FUEL.  This is the area in which large power gains are made or lost.  Because every engine configuration, and requirements are different, Thomson Superchargers has made every effort to baseline the included BEFI fuel system components.  For information on fine tuning these components to your configuration, please contact Thomson Superchargers.  


Air Filters


Only a high quality gauze filter should be used on your Thomson Supercharger.  Running your supercharger without an air cleaner or unfiltered velocity stack will result in internal damage to both the rotors and the blower case, voiding any stated warranty.  Proper air filter maintenance is vital to overall performance.


Exhaust System


All efforts should be made to reduce exhaust back pressure.  Blowers want to breath,  forcing more air in,  means more air out.  Contact Thomson Superchargers for the best head pipe sizes, and muffler designs for your application.


Crankcase Venting


Crankcase pressure is vented to the atmosphere through one or more vent lines.  Venting this air into the air cleaner as the stock system does, can change AFR readings because the air contains fuel/oil contaminated blow by.  This mixture reentering the intake tract reduces power and encourages detonation.  For the best performance,  the crankcase should be vented to open air.





Fuel Octane


The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites.  The lower the rating, the sooner this occurs.  When fuel ignites from compression,  rather than spark plug spark,  it produces a knock.  Nothing can be more damaging to any engine than spark knock (detonation).  The effective compression ratio of your engine determines what octane rating you will need to run.   




Clutch Components


There are many factors that can contribute to clutch slippage or failure when using a supercharger including, overall mileage, adjustment, operator abuse, ect. Testing has shown that stock Harley clutch components work very well for most street installs running low boost. It is highly recommended that a high pressure diaphragm spring, or lock up pressure plate be installed when running moderate boost. High boost levels will require more extensive modifications or a complete race type clutch upgrade.




Common stock, and performance cams used in the v-twin industry are designed for naturally aspirated engines. These cams can work when using a supercharger, but by no means will they optimize the performance available when using forced induction. Working closely with Craig Lockwood of Black Widow Cams, Thomson Superchargers can provide a custom blower grind cam that will let you realize the full potential of you blown application. Contact us for your needs.