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Clutch hydraulics and pedal feel for V8/T56 swaps

Clutch hydraulics and pedal feel for V8/T56 swaps

Old 06-03-18, 03:11 PM
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Clutch hydraulics and pedal feel for V8/T56 swaps

I did the requisite searching and came up short. This has been sticking in the back of my mind for awhile now. So I have a LT1 T-56 transmission waiting to be assembled and retrofitted into Project OldTree behind the 20B-REW. After doing some careful measuring and a few talks with QuickTime bellhousings a few years ago, they offered to make a conversion bellhousing to whatever specs I need, which is awesome. As it turns out, most of the detailed measurements between a 13B/20B and LT1 are pretty close.

What is the regularly accepted clutch master/slave setup for V8 swapped FCs, and how does it feel compared to a stock TII or FD? Original Mazda MC is 5/8" bore, SC is 3/4" while a LT1 T56 (F-body version) MC is 3/4" and SC is 1".

Punching in the numbers here: Pressure in the Hydraulic Press Calculation resulted in the following

Based on the fluid displacement aspects, FC/FDs have a displacement ratio of 0.693:1, meaning that for every 1" inch that the MC travels in its stroke, the SC moves 0.693". F-Body is 0.563:1, so 1" of MC stroke equates to 0.563" of stroke at the SC. Coincidentally, the hydraulics boost the user's force by the inverse of these ratios in the same manner as a floor jack. Hydraulic Assist Ratio for FC/FD is 1.44:1, F-body is 1.777:1. With a set input force of 100psi at the MC, the FC/FD's hydraulics generate 144psi on the clutch fork, while the F-body's clutch fork sees 177.778psi

As it turns out, the clutch is a series of 3 levers. The clutch pedal (pivot to pedal pad vs pivot to MC rod), the hydraulics (MC vs SC) and the clutch fork (SC to pivot vs pivot to throwout bearing center). See how it got complicated?

The original clutches' pressure plate springs are rated at 1540 lbs (S5 TII), 1619 lbs (FD), and 2400 lbs (F-body LT1). Result is that the LT1 needs a considerably greater amount of force to operate its clutch. As its clutch needs to hold more torque, this is to be expected.

Could we get some specifications on the pedal length/MC offset, clutch fork/SC offset, to calculate the correct amount of force needed to engage both? This would be especially useful in making sure swapped cars have sufficient travel at the throwout bearing to engage properly. I have some of the specs from various sources (GM forums), but it seems that the sources may be lacking in accuracy.
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