4 speed swap - install over center spring or not?

[madmike]

Hi,

I'm in the middle of converting my '68 Charger to a 23 spline A833 four speed.  I just installed the pedal assembly the other day, no big deal.  Later, I was perusing this site, and I came across a few comments on the over center spring in the pedal assembly.  I think I got the general idea from other posts that if you run a diaphragm style pressure plate, you shouldn't use the over center spring, and if you run the three finger style pressure plate, it's OK to run the over center spring.  My car is correctly set up to run the larger 11" clutch, but I haven't purchased it yet.  I learned through experience (hard way) that it is easy to install the over center spring in the pedal assembly, when the assembly is out of the car vs installed.

From my understanding of clutches, I think the diaphragm style provides a little less leg effort, smoother engage/disengage, and more even clamping force, than the three finger style.  I also think the over center spring is there from the factory, just to help the clutch pedal return to the full 'up' position (not stuck against the floor).  I do not understand why it would make any difference which clutch style to use, the over center spring should still work fine.  My general feeling is if the factory engineered it that way, it ought to be in there.  Maybe, back in the day, they didn't use diaphragm clutches yet...

At this stage of my build, with no steering column or dash board installed, and no windshield in place, it would be relatively easy for me to remove the pedal assembly and take out the over center spring.  I am not concerned with pedal effort, so if your answer to my questions below only reflects more or less pedal effort, that won't affect my decision.  Probably less than 5% of all the cars/trucks I've owned in my life time were automatics, and I converted most of them to a manual transmission as well.

So, my questions:
1.  I don't know which style of clutch (diaphragm or three finger) I'll wind up with (price affects decision), should I install the over center spring or not?
2.  Why would the over center spring care which style of clutch is in the bell housing, it should work anyway, right?

Thanks for your thoughts and technical input!
Mike

[elacruze]

The difference is in the spring curves of a diaphragm spring and coil springs. Coil springs add pressure in a fixed rate to compression. Diaphragm springs have a plateau of pressure through the most of their travel. In a stock coil spring clutch, holding the pedal to the floor is much harder than pushing the first inch of travel, so the factory added a heavy overcenter spring to help hold the pedal at full travel while you're at a light or in traffic. It's nothing more than leg relief. Since a diaphragm spring has the same pedal effort through it's travel, it is much easier to hold to the floor, and the overcenter spring can actually overpower the return effort of the diaphragm and hold the pedal down at full travel.
<edit>
Note; you actually have to push *against* the effort of the overcenter spring during the first 1/2-2/3 of pedal travel, so it actually adds to pedal effort before it overcenters.



Here's a pretty awesome website I found giving some science to the coil/diaphragm clutch consideration;
http://www.the-crankshaft.info/2009/09/comparison-of-multi-coil-and-diaphragm.html

I just so happen to have an 11" 23 spline Centerforce under my toolbox, ordered in error which I planned to put in my pickup...but that's a long way off. If you're interested in it, PM me.

[Troy]

The factory used Borg & Beck style clutches so the overcenter spring was a necessity.

I currently have a standard clutch but no overcenter spring and it's really no fun in traffic.

Be extremely careful installing the pedal assembly with the spring attached. Always wire the pedal to the frame until you get everything hooked up. Without anything holding it in place it can cause a lot of bodily damage if/when it slams the pedal to the floor.

Troy

[471_Magnum]

If you go with a Centerforce clutch, you will be instructed to remove the OC spring.

[madmike]

Elacruze,

Thanks for the link, an interesting read.  I haven't yet shopped for a clutch, but maybe I should start looking, before I decide to remove the over center spring or not.  In all of the vehicles I have converted to manual, I have never used a Centerforce before, I generally just buy a heavy duty stock replacement type, which have always worked for me.  I did consider a Centerforce in my Chevy rock crawling truck, but I couldn't afford it at the time.  It just has a heavy duty stock type clutch in it as well, which has worked fine.

Thanks for your offer on your clutch, please let me do a little research before I commit to yours.  Thanks again for the link!

Mike