Please Release Me
A fitting definition of a modern performance car might just be one that has superior control over every system. EFI offers outstanding digital mastery over fuel and spark and it could be said that hydraulics offer similar and more specific control over clutch actuation. You could think of it like this: back in the ’30s, vehicle control made a giant leap forward when hydraulic brakes made their appearance on production cars. But like EFI, hydraulic clutch release systems have been slow to catch on with enthusiasts. The systems appear simple on the surface, but executing these systems properly can be sometimes problematic. But there are suitable solutions.
This becomes important with today’s Pro Touring and LS engine retrofit schemes where expectations are that a properly built muscle car can integrate 21st century components into ’60s sheetmetal seamlessly and perform much like a brand-new Camaro or Corvette.
Manual transmissions are far from dead on the street, but late-model, overdrive six-speeds often demand a late-model hydraulic clutch release system as opposed to a mechanical system. This story will look at some of the basics for hydraulic clutch actuation. The system—similar to a car’s hydraulic braking system—uses a hydraulic master cylinder to create line pressure plumbed (usually) to an internal hydraulic release bearing (HRB) positioned on the transmission’s input shaft.
Why would you want to convert to a hydraulic clutch assembly? Mechanical clutch systems can be a troublesome conversion in non-stock engine swap applications like slipping an LS engine into a Chevelle, Camaro, or early Nova. LS engines were never fitted with a provision for a mechanical Z-bar linkage mount so a bracket must be fitted or fabricated and the linkage modified to compensate for the difference created by the LS engine’s repositioned flywheel mounting surface. Other headaches that hydraulic actuation solves include header clearance issues and a reduction in pedal effort, making the driving experience more pleasant.
We spoke with McLeod’s Fred Taylor who has decades of experience with all aspects of clutch design. He emphasized that many enthusiasts think that a hydraulic system will reduce the pedal effort. If the original mechanical linkage is worn where it creates excessive friction, then a hydraulic system will be better. But generally, Taylor says, if you want a lighter clutch, you will need to change to a lighter pressure plate because the overall ratios between a hydraulic and a mechanical clutch are very similar.
Taylor describes it this way. If your pressure plate requires 500 pounds of force to release the clutch and you have a 10:1 overall ratio in the release system then it will require 50 pounds of effort on the pedal to release the clutch. It’s that simple.
Factory hydraulic release systems like those on late-model manual transmission-equipped cars and trucks are virtually bulletproof because of their OE engineering. Aftermarket conversion kits offered for specific model cars are also very efficient. Problems arise when attempting to convert an older car, like a ’54 Chevy or maybe a unique application like a mid-engine Corvair, using parts assembled from multiple sources. Knowing how to integrate the systems and how some of the more arcane aspects of the installation affects the system’s overall performance can make the difference between a pleasant, factory-like pedal feel versus a system that requires both legs to push in the clutch pedal.
There is a wealth of information on setting up the integral hydraulic release bearing with the proper clearance over many miles of street driving so we will expend minimal energy on that subject. Instead, we will focus on the issues of positioning and installing the master cylinder and the clutch pedal geometry.
There are several issues that can plague hydraulic clutch release systems. The smart move is to employ a master cylinder, pedal assembly, and hydraulic release bearing (HRB) that are intended to work together. But for applications where a specific kit does not exist, examining how these systems are designed can be helpful.
There are several issues that potential conversions need to consider. The first is simple hydraulics. We’ll limit this discussion to the integrated hydraulic release bearings since these are the most popular.
A properly matched system will produce the correct pressure without undue pedal effort, which means the force created by the hydraulic master cylinder is correct. Where enthusiasts get confused is with the relative bore size of the master and the slave cylinder/HRB.
The other side of this equation is volume. It’s possible to have more than sufficient pressure to actuate the release fingers on the pressure plate yet have insufficient volume. Overall, a small diameter master will create more pressure than a larger piston but may suffer from insufficient volume, which means the HRB will not move far enough to release the clutch. Conversely, a master with an oversized piston diameter offers more than sufficient volume but will suffer from lower pressure, resulting in a very stiff clutch pedal. This is where the complete kits work well by integrating all the components.
Among the most common problems with hydraulic clutch conversions is a master cylinder seal leak. This results from an incorrectly located master cylinder actuation arm on the clutch pedal in instances where the system has been sourced from multiple suppliers. Clutch master cylinders are extremely sensitive to excessive angles between the master and the pedal assembly. The key to success is a properly located arm that maintains a minimal angle between the clutch pedal and the master cylinder. Taylor says the minimal vertical travel of the master cylinder arm is achieved when at half-pedal travel the clutch pedal arm is 90-degrees to the pedal. This is more difficult to achieve than it might appear because as the clutch pedal is depressed, the arm travels in an arc.
One of the innovative solutions to this is American Powertrain’s HydraMax master cylinder mount. This mount sandwiches the firewall between two large stainless steel plates with a fully angle-adjustable mount.
The clutch pedal rod position has also been addressed by other companies that are now building specifically engineered kits that offer bolt-on systems to allow the builder to take advantage of the control and ease of a hydraulic clutch on a mid-’60s Chevelle, Chevy II, and even now some C10 trucks with a package that offers a similar feel to a 2019 Camaro.
Not all that long ago, enthusiasts were forced to mix and match parts from other applications to achieve their goals. While the die-hards can still go the long route, the rest of the world can enjoy the benefits of conversion kits from companies like American Powertrain, Driveline Components, McLeod Racing, Modern Driveline, and others. Often, this includes a modified clutch pedal that ensures proper actuator rod angle.
The setup of any system is still critical, but most of the important details are dealt with in the instructions. Taylor says one aspect that many enthusiasts fail to take into consideration is that as the clutch wears, the pressure plate’s release finger height increases. This wear moves the pressure plate fingers closer to the release bearing and requires re-adjusting a manual linkage to maintain the proper bearing free play. Hydraulic release systems build in this additional free-play so no further adjustments are necessary.
Let’s use the setup for a McLeod hydraulic release bearing as an example. In the case of a 1300 series release bearing, it has a total travel potential of 0.800-inch. If the pressure plate requires 0.400-inch of travel to fully release the clutch and we establish an initial clearance of 0.200-inch, this allows a total travel of the release bearing of 0.600-inch, which is more than enough to release the clutch.
The reason for the 0.200-inch of initial clearance is to account for future clutch wear. This initial “free-play” is actually taken up by the master cylinder when the pedal is operated the first time. But this clearance is important because as the clutch wears, the pressure plate fingers will become taller and this 0.200-inch clearance will be reduced, but clearance will still be present. As the piston is pushed back inside the HRB, the hydraulic circuit compensates by pushing fluid from the HRB back into the master cylinder. This also means that when the clutch master cylinder is installed, a very minor amount of clearance is required between the clutch pedal actuation rod and the back of the piston.
This clearance ensures that the master cylinder piston fully retracts when the clutch pedal is released. With the piston fully retracted in the master cylinder bore, this uncovers a transfer port so that hydraulic fluid can return from the circuit back into the reservoir. This allows the HRB to compensate for clutch wear. While simple in operation, you can see how important it is to set the proper piston actuator rod clearance when installing the system. Once the master rod is set, there is no reason to adjust it. Never use this rod to adjust the clutch release point.
There are quite a few more details involved with hydraulic clutch systems but this overview should offer a better understanding and appreciation for a hydraulic clutch system and the benefits it provides. CHP
Mechanical Leverage 101
Clutch linkage—either mechanical or hydraulic—uses leverage to do its job. Let’s say that a performance pressure plate requires 500 pounds of force to compress the fingers that clamp down on the clutch. With mechanical linkage, the leverage starts with pedal ratio. On an early Camaro, we measured a 3:1 ratio. The Z-bar adds another 2.1:1 along with a 2:1 ratio from the release arm that actuates the throwout bearing. Multiply these together and we get a 12.6:1 ratio.
When we divide pressure plate force of 500 pounds by the total linkage ratio of 12.6:1, the result is 39.7 pounds of pedal effort required to disengage the clutch.
According to Taylor, the McLeod system generates a hydraulic ratio of 2:1 between the master and the hydraulic release bearing (HRB). So when the clutch master piston moves one inch, this moves the HRB 1/2-inch. This requires the pedal ratio to become 6:1 in order to make the numbers come out similar to the desired mechanical linkage ratio of 12:1.
If we apply a 42 pound force on the pedal multiplied by a pedal ratio of 6:1, and multiply that by a hydraulic ratio of 2:1 we get this: 42 x 6 x 2 = 504 pounds of force on the pressure plate to release the clutch. This reinforces the idea that with similar ratios, a hydraulic system generates a similar pedal effort to a properly performing mechanical system.
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