L28 is toasting my 240z clutch

[AlexS13]

Well, I just finished installing my freshly built L28, P79 shaved 0.080, aftermarket cam, header, intake, running round top SU's. I also swapped over to a zx 5 speed and a 3.90 diff. My stock 240z clutch had a few thousand miles on it and was in excellent shape so I thought using it with the new motor would have been just fine. Well, I've been driving the car for about 2 weeks being easy on it, breaking it in and everything has been fine. Tonight I went out for a little drive and started pushing the car a little harder and loading it up and I noticed my clutch will start to slip pretty bad once i get up to about 4000rpm under load. 1st gear just likes to spin the tires so I dont notice it, and 2nd gear isnt too bad but in 3rd and 4th it slips pretty bad! Anything under 4000rpm it holds just fine but I guess when the power band of the cam comes on its just too much for it? I must say though, the power and torque of the L28 along with the 5spd and 3.90 is just amazing! It completely transforms the car!

Has anyone run into this with the stock 240 clutch? And what would you guys recommend, getting a stock 240mm turbo clutch or upgrading to a aftermarket 225mm unit?

[Lazeum]

I've got a setup very close to yours: same trans/diff, L28 with flat tops just rebuilt, 225mm clutch but with 3xDCOE, Rebello cams, ported P79 head. The clutch for now holds power fine, I've never experienced slippage (but I was concerned as well during the rebuild). Not sure though if it is a stock clutch, there's nothing written on it letting me know if it would be a aftermarket unit. I got the clutch with the car.

[Carl Beck]

Just a thought - under the "been there - done that" heading

Recently rebuilt L28 - you might want to watch for any signs of the rear seal on the engine seeping a bit of oil. Be sure your PCV valve isn't clogged or stuck as well, sometimes they are overlooked on an engine rebuild.

Other than that - I've never had a problem with a stock 240Z Clutch. For that matter all the 240Z clutch pressure plates you get today are what were originally 280Z Pressure Plates. So they should hold a mildly modified L28 with no problem.

If you swap to a 240mm 2+2 Clutch - I'm pretty sure you also need the matching flywheel.

FWIW,

Carl B.

[Diseazd]

......not only do you need a 240mm flywheel, but also a 2+2 clutch collar. I've never had any problems what so ever with a 240z clutch in either of my modified L28 engines.......I would suspect a used clutch or as Carl mentioned, oil on the disc. IMO

[madkaw]

It seems odd to me that it doesn't slip during the "burn out's" or whatever below 4000. You would think the ample torque at those rpms would also be enoug to slip it. But I agree with above statements, I would think the L24 stock clutch would be enough to hold your motor.

Having the larger flywheel and clutch, I have noticed my clutch is much stiffer than a stock clutch.

[AlexS13]

Thanks for the input guys, I was extra careful when putting on the rear main seal and the rear crankshaft bearing making sure I sealed up the corner's like it said to do in the "How to Rebuild Your Datsun" book so I would like to think thats not my problem. Like I said, anything under 4000rpm it holds just fine, taking off from a stop it engages smooth. It doesn't seem to feel as if the clutch disk has oil on it just because of how normal everything feels until the 4K mark.

I was looking at the 240z centerforce 1 clutch kit on the MSA site and they say you must use a 280z clutch collar with that kit. I always thought the collar must match the clutch you are using?

[Zedyone_kenobi]

Physically it makes no sense. The torque required to get you moving is exponentially more than when you are already moving and the motor spins from 3000-5000 rpm. NO offense, but your motor does not 'come on' that strong at 4000 rpm to cause slippage. Unless something is really messed up with the pressure plate...

Again something is weird here.

[Carl Beck]

I was looking at the 240z centerforce 1 clutch kit on the MSA site and they say you must use a 280z clutch collar with that kit. I always thought the collar must match the clutch you are using?

Exactly - as I said above: "For that matter all the 240Z clutch pressure plates you get today are what were originally 280Z Pressure Plates".

The original 240z Pressure Plates were "thick" - and the matching collars were short. The 280Z Pressure Plates are thinner than the original 240Z Pressure Plates - - so you have to use the longer 280Z collar. For many years - ZOOM and ZOOM though NAPA had kits that came with the complete clutch assy - and a new matching throw-out bearing collar. Sadly they no longer come with the collar..

FWIW,

Carl B.

[carrott]

I would check that you have correct amount of free play at the slave cylinder and master cylinder , also if the ports are blocked in the master cylinder it can be slow returning and prevent it from returning and holding pressure against slave . Also make sure that the slave has the return spring fitted.

[AlexS13]

I adjusted the free play a little tonight so I will see what that does next time I drive it. I'am just as stumped as you guys are believe me! I think the torque the clutch must hold taking off in 1st gear is far less than what it is in 3rd or 4th at 4000 + rpm. I could be wrong here but that would explain why I don't feel anything in 1st but I do notice it in 3rd and 4th.

[Lazeum]

The torque resistance for your drivetrain is much greater on higher gears. You always start to feel clutch slippage on 4th or 5th gear on highway while you go WOT because it is where the torque coming from the motor becomes much smaller than the torque to move the car fast (it's a so-so explanation but I hope you get the picture )

[LeonV]

Physically it makes no sense. The torque required to get you moving is exponentially more than when you are already moving and the motor spins from 3000-5000 rpm. NO offense, but your motor does not 'come on' that strong at 4000 rpm to cause slippage. Unless something is really messed up with the pressure plate...

Again something is weird here.

You have that backwards. In no way is torque at take-off in 1st gear "exponentially more" than in high gear at torque peak. Refer to Lazeum's post.

The torque resistance for your drivetrain is much greater on higher gears. You always start to feel clutch slippage on 4th or 5th gear on highway while you go WOT because it is where the torque coming from the motor becomes much smaller than the torque to move the car fast (it's a so-so explanation but I hope you get the picture )

In higher gear, the road has an increasing mechanical advantage over the engine. In low gear, the engine has a dramatic mechanical advantage over the road through gearing. The proper test for clutch slippage is flooring it in high gear, on an incline. This is where you'll see the highest load on the clutch.