Subaru hydraulic clutch time..., Need help from the gurus! Options

[Chris H.]

Hey all,

I'm at the point where the hydraulic clutch setup needs to be mapped out for my Subie conversion. The donor trans is from a 2006 WRX with a 13/16 slave cylinder. Initial reaction is to buy a 13/16 generic (wildwood or CNC) master cylinder so they match and give a “factory” clutch feel but after some internet searches it seems that pedal travel is also a factor and might mean a different size would be better. I’ve seen some GREAT hydraulic clutch threads but could not find one for the Subie trans setup.

I’d assume the pedal throw is probably similar but could be wrong. BIGKAT is using a 3/4 and mentioned he might rather have a 5/8. His trans likely has a ¾ slave.

Any insight or thoughts on this? You don't have to be right! Brainstorming is great too. (IMG:style_emoticons/default/biggrin.gif)

[PRS914-6]

There is more to it that just matching the cylinders. If you are doing a direct cable to hydraulic swap on a 914 system (not Subaru) and rotate the push pin on the pedal all things remain the same and 1:1 sizing is what you should use. However, that is not the case when switching from another car. The Subaru pedals are longer/shorter, the pivot point to push rod is different and overall leverage is different. Bottom line, it's not that simple and some calculations are in order.

You can achieve the same goals by changing hydraulic cylinder size as well as mechanics leverage. If it were me, I would measure the full stroke of the slave, calculate the volume based on diameter\stroke

Next I would measure the full stroke DISTANCE of the 914 clutch pedal PUSH ROD rod and using that stroke distance, calculate what diameter equates to the volume needed to fully actuate the slave. This takes into account both mechanical and hydraulic motion and leverage.

Hope I worded this simple enough....

[Chris H.]

There is more to it that just matching the cylinders. If you are doing a direct cable to hydraulic swap on a 914 system (not Subaru) and rotate the push pin on the pedal all things remain the same and 1:1 sizing is what you should use. However, that is not the case when switching from another car. The Subaru pedals are longer/shorter, the pivot point to push rod is different and overall leverage is different. Bottom line, it's not that simple and some calculations are in order.

You can achieve the same goals by changing hydraulic cylinder size as well as mechanics leverage. If it were me, I would measure the full stroke of the slave, calculate the volume based on diameter\stroke

Next I would measure the full stroke DISTANCE of the 914 clutch pedal PUSH ROD rod and using that stroke distance, calculate what diameter equates to the volume needed to fully actuate the slave. This takes into account both mechanical and hydraulic motion and leverage.

Hope I worded this simple enough....

Whoa... I think I GET IT. And if I get it...everyone else will probably get it... (I'm slow (IMG:style_emoticons/default/biggrin.gif) ) Great work...

First, thank you Paul for the very thoughtful response...should help everyone that is in the same boat. So the question I have is do we want the "stroke"of the pedal (volume of fluid pushed) to equal the stroke of the slave cylinder ideally so they both move at exactly the same rate and the pedal bottoms at the same time the slave cylinder does or does the master just need to push AT LEAST enough to get the slave fully depressed? I'm thinking if the master was too strong the clutch would engage at an awkward time...too low maybe.