SouthernSon

First let me say I have been into corvettes since my first car in 1969, a 327/365 coupe. I now have a growing collection I guess, four. My recent purchase is an '18 GS M7. I also have a C5Z, straight shift of course. I find the C7 GS to be a little notchy/crunch when shifting. It has only less than 9K miles. The C7 forum has a lot of information on this. Seems after it warms up it becomes smoother. I have had the '96 LT4 since '99. I just took it for a drive after sitting for a few weeks. The ZF6 has 141+K miles on it but shifts smoothly and positively. I have entertained the idea of selling it since purchasing the '18 but the drive today makes me realize what a great car it is, especially that ZF6 tranny. I know it is heavy but it is smooth and reliable. I know I will learn to like the '18 GS but the later transmissions in C5 and C7 just can't match it for smooth operation. Not to disparage any later models but to simply give the ZF6 its credit where credit is due.

yakmastermax

I think I need to service the clutch hydraulics on my 1989 ZF6 with 57k miles. When the car is cold sometimes 1st and reverse are pretty hard to get into. Once the car is warmed up, other than the "linkage detents", the shifting on my ZF6 really is smooth like a hot knife through butter. There is that tiny bit of positive, tactile, and predictable resistance from the detent, but once it is overcome it just glides right it. Really unlike anything I've ever shifted before. I hopped into my 2007 Nissan Xterra 4x4 with its 6 speed manual the other week when it was snowing. Man that thing shifts something awful compared to my ZF6. Not to mention things like the 5 speed in my old 2006 Subaru WRX, or the 4+3 in my old 1985. Did I mention the SM465 in my 1983 K5 Blazer lol.

I'm excited to see what it feels like with a fluid change in the tranny itself, plus replacement hydraulic system. I'm thinking about just doing the entire thing at once, master, hydraulic line, and slave plus new fluid obviously and bleed.

SouthernSon

I just put amsoil fluid in it a while back and I like it. I did have to replace the slave cylinder but that was all I have done to it except for the fork ball that came loose and started interferring with the clutch (awful sound when clutch depressed). But that was a long time ago. When I replaced the slave cylinder I used the phoenix system that pushes the fluid up from the slave cylinder to the M/C. It works great now.
amsoil - Product Code: MTFQT-EA

I have used the recommended BMW motor oil for it in the past but I believe I like the amsoil better.

ZWILDZR1

Okay I will play the devils advocate . It has weaknesses. One is the oiling system . Yep poor oiling . If you are going to drive at sustained high speeds you want to have the ZF blue printed. It can even happen if you don't drive for many miles at high speed. My 91 ZR1 saw frequent trips up to 150 + mph for a few miles here and there . Plenty of trips to the drag strip also . Once after doing a holeshot at Bowling Green it developed a high pitch whine . Letting it cool off made no difference. The whine did go away but the damage was done . Under light loads in first gear it would sporadically jump out of first gear . Never did this happen leaving hard .

I had talked to Bill Boudreau about it and he thought due to the ZF poor oiling that the reverse gear welded itself to the main shaft . I still have the transmission in my garage . GM ran a special to blow out existing stock of the later 93-95 ZF ZR1 transmissions for right around $2000.00 in 2004 and I bought one.

Marc Haibeck also spoke on this subjet at a seminar at the Gathering in BG. It's actually available on YouTube. He spoke of a ZR1 that was sent to his shop by the new owner prior to be shipped to Sweden. He went over the car making sure it was freshly serviced and no issues. The owner called to tell him the ZF locked up on him while driving over 140 mph for an extended period of time .

Some ZF's have the oiling issue as Bill Boudreau finds the issue when working on some ZF's . My car the previous owner switched over to Red Line Heavy Skock Proof full synthetic. It could be that the thicker Red Line contributed to the poor oiling. If I ever do anything with the old ZF I will know what damage was done and why . In my case it didn't lock up .

Marc did touch on that GM talked about possible issues driving above 130 mph for extended periods . You can find info on Bills website ZF Doc or watch the video recorded in Bowling Green by Haibeck.

SouthernSon

Sorry you have had that problem. Mine never has developed a problem. I am running 372hp/353torque to the rear. I let everything come up to temperature before I romp and stomp on it. I don't baby it. I change fluids regularly. I bought it in '99 with 10K miles on it. The straight cut gears do whine some. The pilot bearing will give a little whine too when clutch is out in idle. When clutch is pushed in, it stops. I have had the transmission in the C5Z rebuilt twice. Yes, the ZF6 is a joy to me and I don't have much doubt it will serve me well for many more years. I don't power shift and don't stress it beyond its capabilities but I do love to feel the torquey acceleration when driving.

I did find this interesting:

https://www.corvsport.com/1989-corvette-zr1/

AgentEran

Had a zf lock up on me as well

ZWILDZR1

I think the ZF in my first 91 had the oiling issues that Bill Boudreau has seen in some. This combined with the heavier / thicker Red Line Heavy Shock Proof created a perfect scenario for it to happen. I always let things warm up both engine and transmission before pushing it. My current 91 is making 448 at the rear tires. I use the Amsoil Syncromesh full synthetic. The car does have a Fidanza lightweight flywheel . Many owners with the lightweight flywheel use the Red Line Shock proof to help gear noise . Mine is pretty good . Haibeck does a great job on the install and it's quieter than most .

MatthewMiller

I think your cold shifting reluctance is a function of the transmission fluid needing to be changed. Many good transmissions are balky at shifting when cold, especially into 1st. My 2020 Camaro (T6060) does the same thing, right off the showroom floor. If it shifts well when warmed up, your hydraulics are fine. It makes sense to flush the hydraulic fluid, but I would not suggest replacing either cylinder unless there's a clear malfunction: the replacements available now are not as good as the OEM parts. I do recommend replacing your transmission fluid.

*89x2*

I do love the ones I have owned - and still own today. And I have driven more miles on a manual Corvette than probably anyone I've ever known. The 1989 ZF6 was a wonderful introduction.

84 4+3

My 4+3 doesn't like the first few shifts when cold. I wouldn't call it good though.

The oem parts part is spot on. My original master and slave are a substantially heavier casting than any replacement I've had. The replacements seem to last 3-4 years at best.

Tom400CFI

I've tried to convey this many, many times. Especially in threads where people are debating swapping a ZF-6 for a T56. NO F'N WAY! I've had 4 T56's in 4 cars and while they're "OK"....there is no comparison. The TR6060 in the later C6 and up, and the TR6070 are supposed to shift better than the T56, but I HAVE FOUND LITTLE EVIDENCE that it does. Your post backs that up.

The ZF-6 is a pretty special trans, and we're lucky that it ended up in the later C4's. The thing is hard to beat. Thanks for posting!



























.

SJW

Count me among those who have a very high opinion of the ZF6.

Live well,

SJW

Blue Bucket List

The only vehicles that I would compare for shift feel are the 5 speeds in the NA Miatas and the 6 speed in the S2000 Honda. I love the feel of the ZF in my 92 and it is one of the main reasons for choosing the C4 over the C5 that I drove prior to buying my 92. The 2001 C5 that I tried out just didn't have as positive feel in shifting that the ZF provides. Both cars had similar miles and condition but the things that the C5 brought to the table didn't override that one thing that I find very important for driving pleasure. Having spent a goodly part of my life driving class 8 trucks with a variety of gearboxes 9/10/13/18/20 speed including twin sticks for millions of miles has given me an appreciation for a good shifting manual trans and the ZF has it in spades.

yakmastermax

My Vette was off the road for the past 4 days as I got an irreparable flat on one of the literally brand new NT 555 G2 315/35R17 tires I had put on maybe 3 weeka ago. Was waiting for Discount to get the replacement tire in from the warehouse

Tire came in, got it on, went and got groceries in the vette.

First thought that came to mind as I was driving it around was DAMM this thing shifts nicely.


I read a post a in a different thread a while back where someone stated that the ZF6 starts to get noisy id you put more power through them? Any opinions on this statement? It was a bit odd because the poster didn't refer to the transmission as a ZF6 but referred to it by some other part of its code name S640 I think, and also considering the variety of engines and power numbers that ZF6 transmissions found themselves behind, L98s, LT5s, LT1s, LT4s, I find it hard to believe that a small change in power would all of a sudden turn the ZF6 into a chatterbox...

Tom400CFI

It's hard to believe. Either that poster was off the mark, or some other issue caused his symptom.

MatthewMiller

It gets noisy if you ditch the dual-mass flywheel for a solid flywheel. And the noise gets louder the lighter the flywheel, the choppier the idle (more overlap, IOW), and the lower the idle. It only happens in neutral with the clutch engaged, though. It's rollover noise on the countershaft. Supposedly you can shim the countershaft to reduce the noise, but it doesn't hurt anything.

tpi 421 vette

The ZF6 speed especially the black tag is probably one of the strongest transmissions ever put in a car. My 91 probably makes around 1000hp and tq, a 468sbc+300 shot of nitrous. I dump the clutch between 5000 to 5500 rpm with 29" slicks. And once the clutch is engaged it get a 300 shot of nitrous. The car runs 9.70s@144 to 146 with a DA of 7500ft. I have run 1.3 60ft times with it. I have abused it for 20 years and never broke it. I have destroyed about everything in the stock IRS before going to a Ford 9" and 4 link. But the trans just keeps taking it. Definitely one of the strongest trannies made.

topfuel67

It would be nice if there were different gear sets available for the zf6. I'd like to change that 2.66 ratio first and even the second gear. Make the fist in the 3.0 range and the second a little higher etc.

ajp01

Yes that's right. The ZF internal model number for our gearbox is S6-40, only the hyphen was missing.

GREGGPENN

If fluid weight is really a problem (maybe it is?), you'd have to wonder why Bill included BMW fluid as an alternative. Haven't looked at his site for years, but you probably know what I mean. Big debate, over the years, whether Amsoil or BMW is the "right" alternative fluid. I've always used the Amsoil -- since it's the same weight.

Of course, no one has said whether the issue is global or within certain years/runs? I'd be in the camp of those who really like the ZF6...assuming I never have another DMF fail! LOL

BTW...Redline makes another fluid that I've heard recommended. I think it's lighter weight too...and is the reason for it's occasional recommendation. Quick check of their site....It might be MTL? Just putting this out there for Redline fans. I'll continue using Amsoil.