Light weight pressure plate driveability?

[wjdunham]

Hi guys, I've got a '76 mage case 915 and Motronic 3.4L motor that I am in the process of breaking in. The zero timed motor and and transmission are both new to the car, so I have no prior experience with either and I'm trying to get a sense for weather everything is adjusted and running properly. The car has a lightweight aluminum pressure plate, an otherwise stock 3.2 flywheel and clutch disk.

I revs very quick as one would expect, but it also drops back down really quick and I'm noticing that the shifts are bit jerky. The revs drop so fast, and the with the slight delay one needs to let the synchro do it's job before completing the shift the engine speed is well below where it wants to be when the clutch is disengaged and I get a bit of engine breaking before it revs back up. At least that's what I think is happening, is this a possibility?

I'm assuming I need some better footwork to blip the throttle before letting the clutch out that will take some time to develop? I have a stock 1985 3.2 and it's smooth as butter. Does a lightweight pressure plate make that much difference?

[Dpmulvan]

Lightweight aluminum pressure plates (Sachs powerclutch)

[famoroso]

Significantly lightened reciprocating mass combined with a balky / recalcitrant / or just slow shifting transmission is a bit of an issue as you're experiencing.

Hopefully, your transmission will shift more quickly as it breaks in / your technique improves.

I have a similar issue when driving my friend's 993 Carrera RS build with RS flywheel and RS gear ratios. The ratios are so close that any lazy / lackadaisical upshifts feel like downshifts given that the revs will go up when you let the clutch in. The vehicle needs to be driven "smartly." No slow upshifts.

[NY65912]

I had a lightened flywheel in my 3.8 IROC build. It revved instantaneously but I found it very difficult in the street. You really have to work at it.

[icarp]

When changing to a lighter clutch and flywheel most engines need to have the mixture go richer .
This can happen by more accelerator pump contribution , or wall wetter with injection .
Sometimes jet changes and map changes need to be implemented . Timing is also important .
Tuned well the lighter mass is only a benefit . Clutch compound can have alot to do with smooth engagement .
ie. tilton style compound is sintered and is very abrupt , on the other hand Kennedy makes a fantastic synthetic full dia. spring type disc and is smooth as silk
Ian

[jess p]

You mentioned your 915 mag case, I’m assuming you have the 7:3:1 pinion. Are you happy with it mated to the 3.4 from a gearing standpoint.

[wjdunham]

Originally Posted by jess p

You mentioned your 915 mag case, I’m assuming you have the 7:3:1 pinion. Are you happy with it mated to the 3.4 from a gearing standpoint.

It's a very late mag case unit from 1976, so it is an 8:31 unit, i had a chance to pick up an earlier 7:31 box but I was afraid it would be too short. I think I made the right call as the car is already fast enough through first, second and third gears.

Getting used the feel of the setup, definitely requires some more thought and coordination to drive smoothly. About the only issue I'm having now is a bit of bucking between 2000 and 2500 RPM's. Only seems to happen if I'm cruising along in that range and want to accelerate, I need to shift down a gear before adding throttle. I think I'm also hearing some detonation if I don't shift down. I only bring it up because my stock '85 setup (well, almost stock, it does have an SW chip and enlarged throttle body) does not exhibit any hesitation or balking. I never lug the engine but I don't ever really need to think about it. With this new 3.4 setup I definitely need to think about it. Would that be one of the side-effects of the 964 cams? Once the engine gets going, it will leave the 85 setup in the dust, but has to be kept a bit higher in the RPM range to take advantage of it.

Bill

[spuggy]

Originally Posted by wjdunham

I revs very quick as one would expect, but it also drops back down really quick and I'm noticing that the shifts are bit jerky. The revs drop so fast, and the with the slight delay one needs to let the synchro do it's job before completing the shift the engine speed is well below where it wants to be when the clutch is disengaged and I get a bit of engine breaking before it revs back up. At least that's what I think is happening, is this a possibility?

I'm assuming I need some better footwork to blip the throttle before letting the clutch out that will take some time to develop? I have a stock 1985 3.2 and it's smooth as butter. Does a lightweight pressure plate make that much difference?

Look into double-clutching; this matches the speeds of the transmission shafts to the engine & axle/road speed - and does most of the work of the synchros for them, so the transmission won't baulk at the shift. Which is the synchros slowing stuff down to match speeds.

That said, it's a 915, and it's kind of an art. Even the very best of them won't shift like a Toyota. Or a G50.

I've never driven a 915 with a cast iron pressure plate cage - all of mine were the lightweight alloy (saves about 10-15 lbs). I don't think that's your issue.

[spuggy]

Originally Posted by wjdunham

Getting used the feel of the setup, definitely requires some more thought and coordination to drive smoothly. About the only issue I'm having now is a bit of bucking between 2000 and 2500 RPM's. Only seems to happen if I'm cruising along in that range and want to accelerate, I need to shift down a gear before adding throttle. I think I'm also hearing some detonation if I don't shift down. I only bring it up because my stock '85 setup (well, almost stock, it does have an SW chip and enlarged throttle body) does not exhibit any hesitation or balking. I never lug the engine but I don't ever really need to think about it. With this new 3.4 setup I definitely need to think about it. Would that be one of the side-effects of the 964 cams? Once the engine gets going, it will leave the 85 setup in the dust, but has to be kept a bit higher in the RPM range to take advantage of it.

Bill

SC/3.2 cams do have more low- and mid-range punch/pickup than 964 cams (which is why I love/run them).

Sounds like either a tuning issue or you're lugging it around in too high a gear to me. I don't think I've ever driven 2000 RPM outside stoplight-to-stoplight traffic coming to a stop. Sounds like lugging it.

Well-configured turbo cars love tall gearing and cruising @ 2600-2800RPM - just open the throttle wide, instant load translates to instant boost, 80% peak torque @ 3000, peak boost very quickly..

An N/A car with 964 cams revs higher and makes a little more power at the top (10%?), but does this by giving up some mid-range torque against a power band that starts higher. Without a turbo to pack the charge in, it needs to be spinning to make power because 964 cams don't "breathe" as well at low RPM.

[autobonrun]

I have LWF in both my 993 and my 996 TT, but not in my 915 tranny SC. As you’ve noticed the revs not only rise quickly, but drop quickly as well. I actually love this as it allows me to blip the throttle and control the revs. I think you’ll get use to it but as others have stated, a combination of double clutching with brisk shifts will help. I’m not familiar with Motronic, but check if there is any decel adjustment. Slowing that rpm drop between shifts will help. And yes dropping down a gear before acceleration also helps.

[brighton911]

After I switched to a Sachs lightweight pressure plate I found more finesse was needed when pulling away from a stop. It's always a good idea to blip the throttle between shifts, makes life much easier on the syncros.

[Showdown]

Ditto on the Sachs lightweight PP; took me a minute to get used to is but now it’s second nature. Like most things with these cars, sometimes the remedy is just driving more.

[wjdunham]

Thanks for all the feedback guys, all makes sense. If I did not own the '85 I never would have even started this thread, just assumed that "this is the way it is" and I need to learn how to drive it. But I was not expecting such a difference from the more stock 3.2 915 combo in the lower RPM range.

[Tremelune]

It sounds like you're simply not matching the revs on upshifts. Practice going from 3rd to 4th and back, then 2nd to 3rd while maintaining the same speed as best you can. When you let the clutch out, the engine speed/pitch should not change and the car should not jerk in any direction. Don't worry about double-clutching—your issue does not sound related to synchros.

When you floor the throttle while cruising along at 2,000rpm, the car should accelerate with no great fanfare. If it's balking, bucking, or hesitating, it's likely an air/fuel tuning issue, rather than anything mechanical.

[wjdunham]

Originally Posted by Tremelune

It sounds like you're simply not matching the revs on upshifts. Practice going from 3rd to 4th and back, then 2nd to 3rd while maintaining the same speed as best you can. When you let the clutch out, the engine speed/pitch should not change and the car should not jerk in any direction. Don't worry about double-clutching—your issue does not sound related to synchros.

When you floor the throttle while cruising along at 2,000rpm, the car should accelerate with no great fanfare. If it's balking, bucking, or hesitating, it's likely an air/fuel tuning issue, rather than anything mechanical.

The issue I'm having is the latter, bit of balking adding throttle when cruising along at 2000RPM. I've talked to Steve Wong about this, but not much he can do without seeing the car and getting some data. It's a pretty standard configuration that he's done many times: 3.4L 9.5:1 compression, 964 cams timed per the stock 3.2 spec at 1.2mm, enlarged throttle body and SSI's. Maybe I need to get hold of a wideband O2 meter and see what's really going on.

[Tremelune]

Mm. Until a car has been on a dyno, I always think of them as running a get-me-there tune...I am far from an expert here, though!

[JackkTheMackk]

I have an rsr flywheel and sachs lightweight aluminum pressure plate. This set up weighs 1/3 of the stock SC flywheel and pressure plate. 0 issues with drivability. The throttle is very touchy i’ve grown to love it. If you blip the throttle the revs will shoot up and straight down.

I am running pmo 46 carbs on a 3.2SS. ITBs or motronic might be a little more difficult to tune and get perfect. YMMV.

[wjdunham]

Seeing where this thread has wound up, it looks like I really have two separate issues. On the drivability side, definitely the setup is doing what it is supposed to and I need to learn how it wants to be driven, no problem at all there.

But it also looks like I may have a bit of a tuning issue in the lower RPM range which I need to get to the bottom of. It's not a big issue, but I need to get some measurements to understand if there is more to be had. I also need to just drive the car more, get the rings fully seated and make sure it's not just more driver experience with the setup.

I did pull one of the plugs and it looks good to me.

[spuggy]

I think, if you're trying to add throttle at 2000 RPM, what you're hearing is distress. Pickup trucks do that pretty well. But this is a sports car - which you've added hotter cams to.

I've been running a knock mitigation system (J&S Safeguard) on the 930 since about 2010 or so - none of the knock detected (always below 3000 and always load - not boost - related) was ever audible to me. Clanking in protest at too high a load seems more likely; I selected 3rd once at 15 MPH with the G50 thinking I was getting 1st (used to a 915 gate/muscle memory). That was very audible...

What gear are you in, at what speed when you're doing this? And how fast are you jumping on the throttle?

It should be fine with low load due to a low gear - eg 1st or 2nd - to just open the throttle. With high load, shift down (preferred) or tip the throttle in gently - if you really insist on lugging it (both bad for the motor and pointless in terms of power output).

[wjdunham]

Originally Posted by spuggy

I think, if you're trying to add throttle at 2000 RPM, what you're hearing is distress. Pickup trucks do that pretty well. But this is a sports car - which you've added hotter cams to.

I've been running a knock mitigation system (J&S Safeguard) on the 930 since about 2010 or so - none of the knock detected (always below 3000 and always load - not boost - related) was ever audible to me. Clanking in protest at too high a load seems more likely; I selected 3rd once at 15 MPH with the G50 thinking I was getting 1st (used to a 915 gate/muscle memory). That was very audible...

What gear are you in, at what speed when you're doing this? And how fast are you jumping on the throttle?

It should be fine with low load due to a low gear - eg 1st or 2nd - to just open the throttle. With high load, shift down (preferred) or tip the throttle in gently - if you really insist on lugging it (both bad for the motor and pointless in terms of power output).

I am definitely not lugging the engine, I know that is neither necessary or desirable for the engine. I'm cruising along in third gear in the 2000-2500 RPM range nice and smoothly, and I feed on the throttle is when I feel the issue. I'm not going up a hill or otherwise loading the car, just on a nice flat road trying to accelerate. If I were in 4th gear and needed to accelerate I'd probably downshift first.