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Clutch: Subaru WRX/Sti clutch FAQ

Clutch: Subaru WRX/Sti clutch FAQ

The first impulse when clutch shopping is to get “too much” clutch. This is often a very big mistake, as there will be compromises in the different types and compositions of clutches.

Clutches hold Torque, not Horsepower:
Most performance enthusiasts relate more to horsepower numbers rather than torque, but clutch capacity is measured in terms of torque. Think in terms of a high rpm 250 HP Honda Civic versus a 250 HP Ford Powerstroke turbo diesel. The truck will need about three times the clutch capacity because the engine produces about three times the torque.

Choosing what’s best for you:
It may be difficult to know what clutch is right for a particular application since there are so many different levels of personal tolerance and many variations in design. Some people can tolerate clutch chatter, or noise, or heavy pedal effort, or shorter clutch life, higher cost, or other trade-offs. But why tolerate unnecessary issues if you don’t have to? Get the clutch that suits your needs.

What are the various clutch materials? Other than unique or specialized compositions, clutches are generally comprised of:

1. Organic
2. Kevlar
3. Ceramic
4. Feramic
5. Carbon (initially invented in 1998 by Alcon Components for the Subaru World Rally team )
6. Sintered Iron

Depending on manufacturer specifications, this list also shows the general order of the amount of force the clutch materials can hold.

Organic: Metal-fiber woven into “organic” (actually CF aramid with other materials), original-equipment style. Known for smooth engagement, long life, broad operating temperature, minimal-to-no break in period. Will take hard use, somewhat intolerant of repeated abuse (will overheat). Will return to almost full operational condition if overheated. Material is dark brown or black with visible metal fibers.

Kevlar: High-durability material more resistant to hard use. Engagement is similar to organic, but may glaze slightly in stop and go traffic, resulting in slippage until worn clean when used hard again. Higher temp range in general, but can be ruined from overheating; will not return to original characteristics if “cooked”. Material is uniform yellow/green and may look slightly fuzzy when new.

Ceramic: Very high temperature material. Engagement is more abrupt. Will wear flywheel surface faster, especially in traffic situations. Due to it’s intrinsic properties, ceramic has a very high temperature range. Material is any of several light hues – gray, pink, brown.

Feramic: This unique clutch material is one that incorporates graphite and cindered iron. The result is a friction material that offers good friction coefficient, torque capacity, and smoothness of engagement.

Carbon: Very high temperature material. Engagement is more abrupt. Will wear flywheel surface faster, especially in traffic situations. Slightly more durable and flywheel-friendly compared to other aggressive clutch materials. Material is black.

Sintered Iron: Extremely high temperature material. Engagement is extremely harsh and is generally considered an “on/off switch” both due to it’s characteristics and the clutch types this material is generally associated with. It requires a special flywheel surface. Material is metallic gray in color.


FMIC (Front Mount Intercooler) STi/WRX FAQ

FMIC: The primary purpose of a FMIC (front mount intercooler) is to reduce post turbo air temperature prior to entering the combustion chamber via the throttle body.

FMIC install in a Subaru WRX STi.
FMIC install in a Subaru WRX STi.

HP gain is around 15HP. This figure can vary as results can be further enhanced with post installation tuning. This is one modification that is extremely difficult to put a traditional HP figure on as results truly vary from car to car based on tuning and turbo output in terms of CFM.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent “bad” FMICs on the market. Obviously, there may have been bad FMICs sold, but not enough to report as “bad” overall.

Which FMIC construction method is best? FMICs have two main construction methods:
1. tube and fin
2. bar and plate
There is much debate as to which construction method is best. There are many pros and cons with each design type, but no real hard data. Bar and plate designs are consistently reported as more damage resistant which gives them the edge with regard to appearance longevity. In the end, you are best advised to chose a FMIC based on other qualities rather than concentrate on construction design.

Spark Plug info for your Subaru WRX/STi:

Spark Plug info for your Subaru WRX/STi:

Refer to your owners manual for recommendations. Alternately, you can visit an auto parts store or online retailer for recommendations on suitable spark plugs designed for your vehicle. Major manufacturers are:
a. NGK
b. Bosch
c. Denso
d. Autolite
e. Champion

Sti spark plug location
STi spark plug location

Who are the specialty spark plug manufacturers? These manufacturers make specialty plugs that have unique compositions or designs that claim increases over traditional plugs. They are listed for advanced users or those with interest.
a. Torquemaster
b. Beru (specifically the Silverstones found here)
c. SplitFire
d. PREP spark plugs
e. E3 spark plugs
f. Pulstar plugs

What types are there? There are really three main types:
a. conventional nickel alloy (commonly referred to as “copper”)
b. platinum
c. iridium

Which type should I use? That depends on how often you are interested in changing the spark plugs. Conventional spark plugs generally last one year. Platinum or iridium can last, depending on manufacturer specifications, up to seven years.

What’s some good background spark plug information?

Materials: The three main types of spark plug materials are nickel alloy, iridium, and platinum. Copper can be used in the core all plugs.

All ground electrodes are made of nickel. The use of Platinum and Iridium, which are stronger, allow for much finer CENTER electrodes (the ground electrode is still Nickel). These finer electrodes do not quench the flame core as much as a conventional style plug. This increases ignitability, therefore increasing HP. It’s not a huge gain, but cylinder pressures are measurably higher.

Platinum or iridium can be used as a thin pad which is laser welded on the ground electrode (the “J” strap), this serves to increase the life of the plug.

Downpipe Turbo Subaru FAQ

Downpipe Turbo Subaru FAQ

Buying a aftermarket downpipe is a typical modification which removes or replaces the stock catalytic converter with a better flowing unit. It also increases the exhaust diameter for better flow.

HP gain is 15-20HP. This figure is highly debated as different manufacturers use different dynos with different cars with different levels of mods. It also varies because some downpipes use one high flow cat, while others are catless and actually extend far enough back to eliminate the 3rd cat as well.

I have an 06/07 WRX, is the downpipe the same? No. The 06/07 WRX has a unique exhaust in that it’s downpipe is both the downpipe and catpipe sections of the “older” exhaust. This means you must use a “long” downpipe to bolt up to the rest of your exhaust system. A full TBE will fit fine, but when replacing just the downpipe, you must use a long downpipe.

I have an 08/09 WRX, is the downpipe the same? No. The 08/09 WRX has a unique exhaust in that it’s downpipe is the same, fitment wise, as the Legacy GT. So if you have one of these models, you must ensure you state your model/year to your vendor or specifically request the “Legacy GT downpipe” for your car in order to assure proper fitment.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent “bad” downpipes on the market. Obviously, there may have been bad pipes sold, but not enough to report as “bad” overall.

What downpipe metal material is best? Downpipes are made from mild steel and stainless steel (304 & 321). There is no irrefutable evidence that one material is better than the other. Obviously, corrosion levels are higher with mild steel (coated or otherwise). Article on exhaust materials.

Which downpipe construction method is best? Downpipes have 4 main construction methods:
1. Blank plate: Identical to stock construction with the wastegate portion completely covered.
2. Bellmouth: Completely open design.
3. Split bellmouth: Similar to bellmouth only with a divider inserted to separate the wastegate.
4. Divorced or Twin Dump: Separate exhaust and wastegate piping that connect further downstream.

There is no irrefutable evidence that one design is better than the other. The thought process is that the greater the separation there is between the wastegate gases and exhaust gases, the smoother the overall exhaust flow.