C4 LT style manual diff control
Three Wheelin'
Joined: Feb 2003
Posts: 1,782
Likes: 109
From: Cambridge/Boston, MA
This is a fun topic. Very interesting stuff in the car in the video. I don't know the rally world, but that sure looks fun. It must help to have bodywork experience 
The *immense* response is probably the best reflection that few, if any, here have experience with this kind of conversion. The physical side of the pdas is pretty simple- just two levers that get pushed on by hydraulic pressure to alter the amount of locking. I don't think that the electronics would care, although swapping in a C2 abs ecu would probably be the proper way to deal with them. The real complication, in my opinion, is figuring out how to use it and how the trans parts would cope in a manual configuration. I believe that the Guard Transmission fellow has worked on a C4 lightweight box and mentioned that it had a number of different/stronger parts inside.
To implement all you really need are two master cylinders sized to provide enough hydraulic fluid displacement for 0-100% lock. The stock diff slave locks are pretty small. Likely that almost any small automotive clutch master cylinder would provide enough fluid movement. Not sure if motorcycle master cylinders would do it or not.
The car in the video appears to have a digital dual pressure gauge, the car in the photos has two analog bar gauges. Easy to tee a pressure sender into the circuits- something that I've wanted to do and data log in the stock system.
The big question is why? There isn't much weight savings to be had. When/why the factory pdas kicks in is still mysterious, there is an argument for doing away with that. I think you would need to be a professional chassis engineer to really know how to correctly set up an awd car with manual locks. Because everyone defaults to the C2 for track work, the body of knowledge on taking advantage of the pdas system is really lacking.
Not very helpful, but I would love to see some discussion develop.
The *immense* response is probably the best reflection that few, if any, here have experience with this kind of conversion. The physical side of the pdas is pretty simple- just two levers that get pushed on by hydraulic pressure to alter the amount of locking. I don't think that the electronics would care, although swapping in a C2 abs ecu would probably be the proper way to deal with them. The real complication, in my opinion, is figuring out how to use it and how the trans parts would cope in a manual configuration. I believe that the Guard Transmission fellow has worked on a C4 lightweight box and mentioned that it had a number of different/stronger parts inside.
To implement all you really need are two master cylinders sized to provide enough hydraulic fluid displacement for 0-100% lock. The stock diff slave locks are pretty small. Likely that almost any small automotive clutch master cylinder would provide enough fluid movement. Not sure if motorcycle master cylinders would do it or not.
The car in the video appears to have a digital dual pressure gauge, the car in the photos has two analog bar gauges. Easy to tee a pressure sender into the circuits- something that I've wanted to do and data log in the stock system.
The big question is why? There isn't much weight savings to be had. When/why the factory pdas kicks in is still mysterious, there is an argument for doing away with that. I think you would need to be a professional chassis engineer to really know how to correctly set up an awd car with manual locks. Because everyone defaults to the C2 for track work, the body of knowledge on taking advantage of the pdas system is really lacking.
Not very helpful, but I would love to see some discussion develop.
Thread Starter
Rennlist Member
Joined: Jul 2013
Posts: 1,080
Likes: 112
Thanks for the info. I drove a rally car on and off the road for years. It had a manual diff control too. I was never a good enough driver to use it to the full extent but I liked the car's handling predictability and consistency with manual settings. I have never done a hairpin like I could with manual diff. I know the C4 Lightweight is a different animal altogether and I see that the extent of the modifications are far beyond just doing some hydraulic plumbing but the thought of a manual diff is very exciting.
Thanks!
Andrew
Thanks!
Andrew
Rennlist Member
Joined: Jun 2010
Posts: 1,710
Likes: 4
From: Detroit (Rock City); 1990 C4
Rennlist Member
Joined: Jun 2010
Posts: 1,710
Likes: 4
From: Detroit (Rock City); 1990 C4
The ***** we find so fascinating would constantly apply pressure to the center and rear differential servos, guaranteeing some degree of clutch pack engagement (which is absent during 'normal' C4 operation). That engagement increases disc-to-disc friction and therefore wear, thereby increasing the frequency of required maintenance.
No big deal for a race team with a trailerful of spares, trained mechanics, and the factory parts support team on speed dial.
For a guy like me, I gotta go to Sunset Imports and order up 2x 928 332 726 05 ($15 each) and 4x 928 332 728 00 ($71 each) and then pay Fritz and Udo $120/hr to rip apart my trans and diff to install them.
I'm just guessing, but I think it would add up. Matt Monson at Guard Transmission would be the guy I'd ask about the whole thing.
No big deal for a race team with a trailerful of spares, trained mechanics, and the factory parts support team on speed dial.
For a guy like me, I gotta go to Sunset Imports and order up 2x 928 332 726 05 ($15 each) and 4x 928 332 728 00 ($71 each) and then pay Fritz and Udo $120/hr to rip apart my trans and diff to install them.
I'm just guessing, but I think it would add up. Matt Monson at Guard Transmission would be the guy I'd ask about the whole thing.
