'91 ZF 6-speed rpms?
Thread Starter
Instructor
Joined: Oct 2005
Posts: 143
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From: Lincolnshire,ENGLAND
'91 ZF 6-speed rpms?
Is 4th gear on the ZF direct drive? (im guessing it should be)
I have a '91 L98,standard wheels/tyre size and the console sticker is telling me i have a 3.45 rear end.
The reason i ask is that im pulling 1900rpm in forth @ 30mph,and about 4100rpm in fourth @ 70mph,Does this sound right to you ZF guys?
It doesnt to me,my tacho must be mis-behaving, or someones installed massive rear in the past?
Just installed a new 3.55 in my '77 vette wich has a borgw 4speed,its pulling about 1350 in 4th @ 30 mph and 3200 in 4th @ 70mph.
Doesnt pull as hard as my C4 in 1st but im about 60 horses down from the C4 to the C3.At first i was suspisious the supplier had sent me a 3.08 or something,but the '77 rpms look right with this rear.
What d'ya reckon?
I have a '91 L98,standard wheels/tyre size and the console sticker is telling me i have a 3.45 rear end.
The reason i ask is that im pulling 1900rpm in forth @ 30mph,and about 4100rpm in fourth @ 70mph,Does this sound right to you ZF guys?
It doesnt to me,my tacho must be mis-behaving, or someones installed massive rear in the past?
Just installed a new 3.55 in my '77 vette wich has a borgw 4speed,its pulling about 1350 in 4th @ 30 mph and 3200 in 4th @ 70mph.
Doesnt pull as hard as my C4 in 1st but im about 60 horses down from the C4 to the C3.At first i was suspisious the supplier had sent me a 3.08 or something,but the '77 rpms look right with this rear.
What d'ya reckon?
Team Owner
Joined: Sep 2002
Posts: 22,209
Likes: 12
From: OBAMA IS HITLER
the tach in the 90/91 (and it may be in later C4 years, i'm not sure) is NOTORIOUSLY bad. in fact, all the analog guages are off. try hooking up an engine enalyzer (such as TTS datamaster with a laptop) and you'll see the real deal. in my case, my tach is a couple hundred rpms off, on the high side.
yes, 4th gear is a 1:1 ratio with the ZF. here's the whole gamut:
1rst: 2.64
2nd: 1.78
3rd: 1.30
4th: 1.00
5th: 0.74
6th: 0.49
yes, 4th gear is a 1:1 ratio with the ZF. here's the whole gamut:
1rst: 2.64
2nd: 1.78
3rd: 1.30
4th: 1.00
5th: 0.74
6th: 0.49
Safety Car
Joined: Apr 2003
Posts: 3,793
Likes: 9
From: VA
With 275-40-17 tires your 30mph@1900rpm is a 4.8:1 rear, and your 70@4100 is a 4.5:1 rear
So not only are they unlikely numbers, but also not the same, so you have a problem with the tach and/or speedo.
So not only are they unlikely numbers, but also not the same, so you have a problem with the tach and/or speedo.
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
With standard tire size and 3.45 gears you should be running 42 mph at 1900 RPM and 91 @ 4100. If your tach was off this much you'd be idling at about 200 RPM's which I think you'd probably notice. More likely your tach is off a little bit AND somebody has installed 4.10 gears. With 4.10's you get 35mph and 76 mph respectively. with the RPM's you mentioned.
Play with my spreadsheet and see what you find
http://www.wku.edu/~nathan.plemons/calc.xls
The easy way to tell is to jack the car up in the rear and mark the tire and driveshaft. Rotate the tire around one full revolution and count how many times the driveshaft turns. Betcha anything it's closer to 4:1 than it is 3.5:1
Play with my spreadsheet and see what you find
http://www.wku.edu/~nathan.plemons/calc.xls
The easy way to tell is to jack the car up in the rear and mark the tire and driveshaft. Rotate the tire around one full revolution and count how many times the driveshaft turns. Betcha anything it's closer to 4:1 than it is 3.5:1
Thread Starter
Instructor
Joined: Oct 2005
Posts: 143
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From: Lincolnshire,ENGLAND
Originally Posted by Nathan Plemons
With standard tire size and 3.45 gears you should be running 42 mph at 1900 RPM and 91 @ 4100
Idles about 1100rpm during warm-up,looks about right?
Drive the car to work every day and get 17-18 mpu.s.g
I'll 'av a look underneath tomorrow & check out how many turns of the prop shaft i get.
Is there any codes stamped into the diff housing regarding the ratio it has i can look for while im at it?
Thread Starter
Instructor
Joined: Oct 2005
Posts: 143
Likes: 0
From: Lincolnshire,ENGLAND
Originally Posted by Nathan Plemons
Likely the diff itself would have 3.45 stamped on it and they just changed the gear set on the inside, won't be much help.
were is the code stamped?Also could have been replaced under warranty,could have been exchanged for a rebuilt unit?Just interested to know were it is and have a look while im under there.
My '77 was built with a 3.36 which i exchanged for a rebuilt unit that has 3.55 gears and a case stamped as a 1969 4.11
Old cars-numbers can lie! theirs got to be someone out there with a 2nd hand console lid from another C4 in there car!
(yes-i know the vin would'nt match up) I'll jack up the rear and give her a spin-count the prop.
