PorKen

A true luxury-GT, at last!

Easily one of the most rewarding things I've ever made/installed.

Driving, especially around town, is remarkably more pleasant. With an old 928 Specialists short shifter, shifting is snick-snick-snick-snick.


I used a 7" 1.9:1 ratio remote servo (brake booster). Still good clutch takeup feel. I had to fabricate a custom rear bracket to mount it but got lucky with the included front bracket which aligned perfectly with a spoiler screw. (There is a 8" 3:1 ratio version which would probably fit but it might have less feel.)



Compared to being able to barely depress the stock pedal one handed, with the booster installed and the helper spring removed(!), I can now push the clutch with my left thumb (about the same effort as the clutch in my BMW Z4 3.0i). Less travel is required and the pedal is closer to the seat so I was able to move the seat back (for more headroom).

Master pushrod adjusted to remove slack in the system. PKlutch (clutch stop) at max extension.



Simply replaced the flexible hose with longer ones. (FYI: clutch fittings are M12x1.0)



And one vacuum connection.

PorKen

Still some details I'd like to change, but good enuff to drive for a bit.

- Although there's no apparent restriction/delay, I'd like to have one piece hoses made to replace the many adapters and small lengths of hard line

- supposedly there's an alternate vent cap which covers the air filter to which a hose is routed higher up, but my box was missing this (and some hardware )



I guess I better get on these minor issues because I reckon Roger is going to be calling about a luxo-barge 3:1 booster for his SE.

Daniel5691

Sir, this is amazing.

Thanks for taking the time to document this.

Dan

Alan

Ken - wow you didn't waste much time - how long ago did we have that online discussion? and here it is done! Nice

Alan

PorKen

It took more time to research and agonize over whether it would work than it did to just stick it in there and see if it worked.

For those interested, the bore size is 11/16 (17.5mm) vs. the 928 master which is 19mm(?). I had read online that there might be a problem of the servo/booster slave piston bottoming out if the bore sizes were too mismatched, but it seems to have worked out fine.

Total hose length is 244cm (2 x 4'). I tried using -3 hoses at first, (even though I wanted -4 as the clutch lines are 6mm) because the local hose place had them pre-made in -3 (brake hose size). The smaller hoses created a small delay which allowed the clutch to slip on fast takeoff (kind of like a clutch-delay-valve some car makers put in to protect the driveline). Clutch lines are larger because they are moving something whereas brake lines are smaller because they only supply pressure.

The servo has enough reserve to operate the clutch just once with the engine off. Almost two but before it annoyingly sputters and goes back to the stock tractor effort. This is usually all that is needed to start the car but it would be nice to have an additional canister to supplement it. The servo would fit in the fender in place of the stock vacuum reservoir but it's tight. Obviously, there's lots of room under the left headlight on S4-up. The system weighs about 4kg (9lbs) with hoses, minus 125g (4.4oz) for the dorky stock helper spring assembly. If this booster had a been a factory option, I probably would have removed it to save weight.


Because the clutch pedal sits farther forward and takes much less effort to press, besides moving the seat back, I was also able to remove the factory clutch pedal spacer I had installed to compensate for my short-ish legs (for my 6' height).

I'm pretty sure I'm driving down to Sharktoberfest this year so folks can try it out, there.

Michael Benno

Looks great Ken. I need to get one of those PKlutch stops for my 5spd!

PorKen

But wait there's more! I was able to put the helper-spring back in* after loosening the adjustment nut almost all the way (a few threads showing). Not quite effortless, but so smooth. Seat back just a little more. Will probably have to make longer extension tubes under the PKlutch.

Because it's there, I'll still have to try the 3:1 at some point, though.



*With the helper-spring at stock compression the pedal would stay at the floor or take a long time to retract - the spring was stronger than the boost!

hlee96

Will you be putting a package for 5 spd owners looking to mod their pedals?

thanks-Hoi

Alan

Pretty soon you'll have the drivers seat out and be sitting in the back!

So how smooth & controllable is it for engagement?

Alan

PorKen

Ended up removing the helper spring again. It spoils the linear delivery and requires adjusting the preload such that there is a hitch at the very end of travel when releasing the pedal.

I drove around all afternoon, trying to get stuck in traffic. After many hours of driving my leg was not tired and my knee was fine. My foot was sore, though, so I am hot to try the 3:1 (already spoiled ).

I think this 1.9:1 might be fine for the dual-disk which was a bit easier to push as I recall. (Plus there is less room under the headlight pre-S4.)

I really like the new seating position. Both legs nearly straight! No more keeping the left partially bent for enough strength to control the clutch pedal. I felt much more connected to the car instead of feeling like I'm riding it. Dare I say, more like driving an automatic.

I think it's easier to finesse with the help of the boost since I'm not pushing so damn hard all the time. I'm hopeful the 3:1 will be even better.

It looks more and more like a do-able project. I will keep testing first to see if there is some major problem, though.

jpitman2

Boosted clutches are not that uncommon - I know Nissan 4x4 use them. That booster really looks familiar - the white plastic air valve looks like a Girling, and casing like a Repco VH44...takes me back a long way!
jp 83 Euro S AT 57k

Chris Lockhart

Great job Ken.

Weissach

How many pounds of pressure was it before the mod? It should be between 30lbs-33lbs stock. I've never had trouble with the 928's clutch after adjusting the helper nut to lighten. Clutch feel does go away the more you adjust the fly nut, but it's not that bad.

PorKen

I could not figure out a good way to measure the foot pressure with what I had around the house. It's too high for comfort. The 87-up single disk is especially rough in traffic. The dual disk was easier as I recall.

Mostly this has become a priority because I've been spoiled by the lighter weight action in my BMWs (only 3 liters, granted) and I would drive my S4 more often if it wasn't such a workout in town.

These are Lockheed and Girling copies (7" LE72696, 8" LE10117) used on MGB, Lotus, etc.

PorKen

3:1 is the ticket!



So, so nice, with none of the initial-push weirdness that I had with the 2:1. Had to adjust the pre-load to zero and reduce the clutch stop so I put the pedal-spacer back in. (No helper spring.) Kept the way-back seating position though because I can pretty much operate the clutch with my tippy-toes. Much easier to feather precisely - getting going on a hill, for example.

Unboosted feels like I'm trying to push out the firewall, now!

This one came with the alternate cap for the air valve with a hose connection to the engine compartment.



Only two issues presently. First, the short sections of smaller diameter steel line into and out of the servo means it takes more effort to push the clutch very quickly (which is easier to do with the higher boost). Second, it does not hold vacuum well at shutoff.