Got my '71 Riviera out of storage this past weekend. I had converted to a hydraulically-actuated 4spd last summer. BW ST-10 with Centerforce II 11" single disc clutch pressure plate, disc and TO brg. New TA Perf flywheel. Due to exhaust manifold clearance issue, a pull-type slave cylinder is used to actuate shift fork. I was gentle on the car last summer to adhere to Centerforce's 500 mile break-in suggestion. Pedal pressure was/is fine, but last summer I wished I had more pedal travel between fully enagaged to fully released clutch, but figured that was just the nature of the hydraulic actuation. Saturday I tried putting the posi to work by dumping the clutch. Rev to 4 grand and dump the clutch and no tire spin, but clutch slip instead. Massive slip generating smoke from clutch. Tried it twice with same results. Pedal feel of clutch actuation during normal driving has now actually improved. No doubt due to some loss of clutch material (!?). Tires still bark between shifts under hard throttle. Tires also bark if downshifting and popping clutch. I know clutch is fully engaged when pedal is released; shift fork is not holding clutch back. Flywheel is clean and free of any oil leaks. I thought the Centerforce technology is especially meant to prevent this slippage from happening? I haven't caled their Tech Line yet. Just wondering if anyone has any ideas of what may cause this and what I should look for as a remedy.
A few questions: 1. What's the 1st gear ratio of the tranny? 2. What's your rearend ratio? 3. Weight of the car? These are all contributing factors.
1. 2.64 1st gear (3 circumferential grooves around input shaft) (Trans date stamp of 2.24.78) 2. 2.93 posi rear 3. approximate weight of 4500 lbs with me onboard So, what do these numbers tell you?
With the weight of the car and the 2.93 gears, it takes a lot to get the car rolling. Especially a car that weighs 4500 lbs. Something has to give and in most cases it's the clutch. The clutch can't handle the friction and subsequently slips. I really don't think the clutch is bad or hurt too bad. Once you let it cool down, I assume it grabbed the same. I'd just say stay away from 4000 rpm dumps. Try this calculator and change gear sizes to see what would work for you: http://www.5speedtransmissions.com/startlineratio.html But you'd be giving up the highway cruiser ability if you start shortening them up. Try this, get the car rolling to about 10 mph and floor it. Does it break the tires free? This might be a way to save your clutch and do decent burnouts at the same time.
My experiance with Centerforce last year in the Regal was poor at best. I believe it to be highly over rated. You may have "greased" the surface with this slipage. Me and my brother had a BBB powered Chevy 4X4. We ran a Ram clutch and PP on an 11" billet steel flywheel. We could drop the clutch in 2nd and smoke off the big ole mudders. We had 4.11 rears, but the big tires reduced the ratio quite a bit. Like Paul says, you are trying to launch alot of weight with a poor first gear and rear ratio (Poor for launching anyway). You may want to investigate the kind of clutch truck pullers use, like the Ram we used, to do what you want.
Thanks to all for the info. I run a roughly stock height 28" tire on 17" wheels. I can't readily change rear gears because the only better set available for this 9-3/8 rear end is the 3.42 ratio, which is super hard to find. Everybody wants 'em. I'll probably just run the current clutch setup for awhile and change to the RAM setup as finances allow. I'm not a big clutch-dumping fiend. I know it puts a tremendous shock load on the driveline and uses up tires. I just wanted to try it a couple times for fun is all. I haven't read your calculator/gearchange link yet but, what would you recommend as a 1st gear ratio? The trans came out of a '78 Z28. I imagine the Z28 probably had a 3.73-ish rear gear? Would a 1st gear change really make a great change, or will I always be hobbled by the 2.93 rear set? I don't think I "greased" the clutch disc with the clutch slip episodes. Clutch grips fine under all other driving conditions. Like I mentioned, it actually has noticeably better engagement/disengagement feel in the pedal travel.
After reading the gear change/calculator info at the website you linked, I made an inquiry to Kajun Jon Bergeron of KAJUN ENTERPRISES, a 4spd guru. (www.KajunJon.com) Here's what he had to say: "Steve, the 3.42 Super T-10's were built for cars with less than 250 horse power. It really is a weak transmission, which is because of the 17-tooth input and 31-tooth cluster gear. To get the 3.42 1st gear, you must change the Input, Cluster, 3rd and 1st gears." ------------------------------------------------------------ Also, here's the response I just received from Roger Warner in the Technical Dept. at Centerforce: "Hello Steve, In reply to your email, you may be correct in your statement that the clutch is inadequate for your vehicle. The holding capacity of our 11" centerforce II assembly is 500 foot lb's of torque at the crank. However, with your gearing combination, tire height and weight of your application, this clutch might not be suitable, especially if the clutch is not fully seated-in or has any throwout bearing preload. I would not recommend our dual friction 11 clutch either being the gearing combination will result in poor service life of the clutch. If you have a 26 spline transmission input shaft, you might be able to use our 12" clutch, part number DF612909. This clutch has been used behind the 572 crate engines from General Motors with good success. Feel free to contact us at (928) 771-8422 if you have any questions." ----------------------------------------------------------- Well, that makes me feel better. I thought I had some other unknown issue I was going to have to chase down.:beers2:
Steve, some lateral thinking here. With your pull slave cylinder, have you always had sufficient travel to fully disengage the clutch without any preload on the thrust / throwout bearing? Bear in mind that your clutch may now have bedded in / worn a bit. Your pedal pivot ratio / clutch fork ratio and master cylinder bore size all have a direct relationship to the required throwout bearing stroke. I believe McLeod recommend 0.5" throwout bearing movement from 'just touching' the pressure plate to a full depressed (disengaged) clutch. The clearance quoted in my McLeod data between the pressure plate and clutch disc is 0.050". With the stock GM clutch fork ratio of 7"/3" and a (Wilwood) pull slave maximum stroke of 34mm this means you need a corresponding stroke length at your master cylinder. For a typical setup, based on cylinder piston surface areas, my calculations show for the following master cylinder bores (being aware that the nominal 7/8" Wilwood pull slave has a 3/8" rod that needs to be deducted from the total surface area) - 7/8" bore master requires 23mm stroke 13/16" bore master requires 26mm stroke 3/4" bore master requires 31mm stroke. Example - you have a 3/4" bore master cylinder that requires 31mm stroke to fully disengage the clutch. However, the actual 3/4" master cylinder fitted happens to only move 28mm. Therefore you are 3mm short on stroke and your clutch won't fully disengage. Solution - fit a 13/16" master cylinder (although this will result in a heavier pedal force required). Do you know what bore size and stroke length master cylinder you have?
Steve, Jon's right. I have heard of T-10's living for a long time but only if it was an early one. I knew the B body rearend gears are hard to find. So I guess you're kind of stuck in a tough spot. I do think if you rebuilt the tranny and made it beefier internally (essentially only using the case) you could customize the gears to get you moving better. The only problem would be finding a set that would not have excessive RPM drop between gears. If it's too much drop, you'll be out of the power range and the engine will lug in 4th if you slow down to around 35 MPH. My car did this with 3.08's and was detonating because of lugging. There's a large T-10 following out there and I think you can really build one up to suit your car if you can. Centerforce is right about the clutch going away prematurely. This is another by product of improper gearing.
I don't plan to run 35mph in 4th gear much. I can run comfortably in 2nd at 35. I'll most likely run my existing clutch for the rest of this year. I drove the car to work today and it shifts smoother and has better pedal feel than ever before, go figure. As for master/slave cyl sizes, I'm using a 3/4" master (Wilwood 260-6089) and 7/8" slave (Wilwood 260-1333). I orginally ran a 3/4" Wilwood slave, but found pedal pressure rather stiff. On the Wilwood Tech Dept.'s recommendation I switched to the 7/8" slave. I'm now good to go for now. I'd still like to hear your calculations/recommendations though. Thanks all!
The advantage of the Centerforce clutch is that the weights act to help with clamping pressure as the rpms increase. But, guess what, our BBB engines create lots of torque at low rpms. So the Centerforce is not best suited for our engines, but instead for engines that need lots of rpm to create torque. McLeod also makes a 12" diaphragm type clutch that will bolt to the stock 11" pattern. But it requires slightly more pedal travel than stock.
Some have gotten away with just trimming the rubber bumper to get more travel, while others had to weld an extension on to the lower part of the z-bar.
