How does a lightweight flywheel improve performance?
A transmission can be thought of as a fulcrum and lever in a car. First gear has a really long lever; second gear has a shorter lever, etc. The lever represents the mechanical advantage that gears give your vehicle. When your car is moving, you have two factors that are present during acceleration, one is driveline losses, which are constant and the variable, which is vehicle weight and the mechanical advantage supplied by each gear. While changing to a lighter flywheel will give the user little to no changes on a dyno, the apparent changes are quite dramatic due to the greater mechanical advantage. Consider these made up figures for consideration: Drive line losses, 45 pounds and vehicle mass (weight) at the driveline (remember your gear's mechanical advantage reduces your actual car weight). We know that within reason, vehicle mass is a constant. Now imagine if you reduced the driveline loss from 45 to 35 with the use of a lightweight flywheel. Since the engine has less drivetrain losses to compensate for, this means the "gained" horsepower can be applied to moving the vehicle mass. Using mathematics, one can realize that the higher you go up in gears, the less effect that a lightened flywheel will have to the overall equation.
How much will a lightweight flywheel affect my car's performance? This Excel document
will allow you to find out for your WRX or STi application.
Are there any downsides to a lightweight flywheel?
While the performance characteristics of a lightweight flywheel seem to be the perfect solution, there are compromises:
a. Low end performance is affected. This usually means that higher revs are necessary for smooth starts due to the reduced rotational mass. For drag racers, this can be a BIG issue.
b. Possible missfire check engine light.
c. Possible chatter, like missfire this affects some users and not others.
What causes a missfire CEL with lightweight flywheels?
No one is really sure. There are theories though. One theory is that the Crank Position Sensor senses the rotational speed of your crankshaft. Since a lightweight flywheel reduces the rotation mass of your engine, your crankshaft accelerates and decelerates quicker than OEM specifications. Another theory is that there is a missmatch of information from your Crank Position Sensor and the Cam Position Sensor during acceleration and deceleration. This may or may not be caused by the slow reaction of the belt tensioner which will cause enough belt slop to give erroneous readings to the ECU. Since missfire CELs (due to lightweight flywheels) do not occur as often on the STi and the STi has two Cam Position Sensors, this adds further evidence that the missfire CEL has something to do with the Cam Position Sensor since there is only one on the WRX.
Which lightweight flywheel will not throw a missfire CEL?
In truth, there is no lightweight flywheel that will not throw a missfire CEL. The odds
of you throwing a missfire CEL are higher the lower in flywheel weight you go. Even with a “heavier” lightweight flywheel, you may throw a missfire CEL.
Will the use of a lightweight pulley increase my chances of a missfire CEL with a lightweight flywheel?
Yes. To a smaller degree, lightweight pulleys also decrease rotation mass, adding their quicker acceleration and deceleration into the equation.
How do I fix a missfire CEL with a lightweight flywheel?
The safest course of action is to use a portable OBDII code instrument and clear the codes frequently. Once you know the frequency of the missfire CELs, it will aid you in determining if this is an actual code or perhaps something to investigate further. While the missfire CEL code is a "safe code" (just an indicator and won't throw your car into safe or limp mode), it is never a good idea to drive around with the CEL on for extended periods of time as another, more serious code can be present without your knowledge. Additionally, it is not a wise idea to remove the missfire CELs via engine management software. Missfire codes can be an indication of a problem and removing their presence will remove possible symptoms of an actual problem.
Who is a good or a bad candidate for a lightweight flywheel?
Arguably, the users whose driving technique is most highlighted by the benefits of a lightweight flywheel are people who autocross frequently. Many people are also genuinely happy with the performance increase in their daily driven vehicles as well. The only group that is generally dissatisfied are people who competitively drag race as the reduced rotational mass does not lend itself to their severe launch techniques.
My special thanks to the following personnel for their assistance in the formulation and research on this FAQ:
* NASIOC member DrDRum who came to my rescue and made the WONDERFUL Excel chart for lightweight flywheels. I wanted something ghetto, and he created something highly configurable and very pro looking.
* Tom @ Kartboy
for giving me hands on experience with a complete clutch assembly and explaining the basics.
* David Koyle
at ACT for humoring my questions after I met him at the Rim of the World.
* Andrew Yates
for verifying a lot of my information and going over advanced clutch information.
This post was created because I wasn't able to find a good clutch/flywheel FAQ. I came up with the text based on LOTS of searching here. It was also created to be intentionally brand neutral so that it serves as a stepping stone for further research. Upon reading this you should have an idea of what type of clutch and/or flywheel best suits your needs. The manufacturer is up to you.
If you find an error in this FAQ, please PM me with factual details and I will update this post. Responses such as, "I have XXX's clutch and it's great!" or "XXX's flywheel cracked after 1 month" are not appreciated here, that is what the Car Parts Review Forum