Basic Guide to a Hydraulic Clutch Set Up
Peer pressure can be tough, everywhere you turn someone is telling you how you should build your project. You need to go with this engine, those tires suck, chrome is out, polished is in, heck just paint it all black. Same goes with a transmission conversion, some of you out there have done a new 5 or 6 speed conversion and some of you are thinking about it. Then the question comes up, do I go with a mechanical linkage or hydraulic clutch engagement? The majority of conversions end up going with a hydraulic conversion for one reason or another, but if you're not sure or want some more info....read on!
Hydraulic clutch engagement has been around for quite some time now and became standard equipment in manual transmission cars/trucks throughout the 1980's to present. In the most simplified form a hydraulic system works similar to your braking system only on a much smaller scale. The clutch pedal pushes a rod connected to a piston inside of a master cylinder. That piston pushes the fluid through a line down to a slave cylinder or hydraulic release bearing which then applies force against the pressure plate to disengage the clutch. When you release the clutch pedal the fluid looses its pressure and the spring force of the pressure plate moves the bearing back allowing the disc to re-engage. Easy enough, but there are some physics to all of this that can make having a hydraulic clutch a dream or a nightmare depending on the set up.
Sometimes both are called "slave cylinders" but to the left you see a hydraulic release bearing and to the right is an external slave cylinder.
If you've had any experience driving an older manual transmission equipped car or truck that was set up with a mechanical linkage, chances are you've driven one that had a clutch pedal that only a body builder would feel comfortable pushing down. A couple of things happen there. 1. At some point a "high performance" clutch was installed. One way to get a clutch to hold more power is to give the pressure plate more spring rate enabling it to clamp harder onto the disc to keep it from slipping. This in turn makes it harder to push in and that translates all the way back up to the pedal. 2. The mechanical linkage isn't set up properly and you aren't gaining enough leverage against the pressure plate springs to get a nice easy push. 3. You need to quit skipping leg day at the gym.
Hydraulic clutch systems can also suffer from the same hard pedal issues that the mechanical clutches did, but only because the master cylinder bore size and pedal ratio aren't optimized for your situation. Hydraulic pressure can be utilized to move more load with less effort when set up correctly. This is why it's critical to always seek advice when putting together a hydraulic clutch system. Most of the vehicle specific kits on the market today are optimized for the vehicle they're built for, and you will typically end up with a great pedal feel and an overall cleaner installation. Of course the one variable to keep in mind is that pedal feel is subjective, especially for those of you who keep skipping leg day at the gym. What feels great to one person may feel too hard or too soft to another person and if you find yourself in that situation you can usually swap to a different master cylinder bore to correct the problem.
Master cylinders come in all sizes and styles to allow for a wide variety of applications.
The basic rule is the smaller the bore size the easier the pedal feel, as long as you're keeping the motion ratio of the pedal in the same spot. Most manufacturers of master cylinders will recommend around a 6:1 pedal ratio for the best feel. You may not always achieve the optimum pedal ratio due to constraints of your set up and it may be necessary to adjust master cylinder bore size to make up the difference. This is also an area that you need to be cautious in, as you decrease the bore size you also have to increase the piston stroke to get the equivalent amount of fluid movement. Each hydraulic bearing or slave cylinder will require a certain amount of fluid movement to get full travel. Be sure to match the master cylinder fluid movement to the requirements of the bearing or slave cylinder you want to use.
Once you have the parts picked out that you're going to use it will be equally as important to install them correctly. If you are custom building a system from scratch it is critical to ensure that when the clutch pedal is pushing in the piston of the master cylinder, it is doing so in a straight line. We've seen many master cylinders get ruined from a side load condition. Most master cylinders have a little wiggle room here, but don't take a foot when you're only given an inch. We also recommend using high quality fittings and lines to make your connections, don't go cheap here when the best can be had for only a few dollars more. One of the best parts of a hydraulic clutch kit is the lack of a bunch of moving parts that get in the way of your headers and exhaust system. This doesn't give you free reign to wire tie the hydraulic line to the collector pipe though. Keep all hydraulic lines as far away from heat and abrasion as possible to ensure a trouble free operation. The same goes for inside the bell housing. Some bearings will require a flexible hose connected to the base of the bearing inside the bell housing. Be sure to secure all your lines so that they can't move around. We've seen more line failures due to contact with the pressure plate, than any other failure.
There isn't much space inside the bell housing once you cram in the flywheel, clutch, and bearing so it's critical to keep those flexible lines secured.
Although there are still some critics out there who will tell you that hydraulic clutch systems are risky and have the potential for failure so you should stick with a mechanical linkage, but there are enough high quality options available today that will give you great results when put together correctly. The OE manufacturers have been using hydraulic systems for years without any major issues so there's no reason to believe that just because your car didn't come that way that it can't be made to work that way. Like anything else you modify on your project, with careful planning and execution, a hydraulic clutch system can be a wonderful upgrade.
Stay tuned as we discuss the specific set up and bleeding of a hydraulic clutch system in an upcoming newsletter. If you like what you're reading be sure to share it with your friends.
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