rpms/transmission question

[go z racer, go]

Hey Gang--

Can anyone assist my very good friend from Texas with this particular issue (see bellow). I myself, am at a loss. Your efforts would be greatly appreciated, thank you.

--Jerry

jbrockm@webtv.net rpms/transmission question

Date:___May 5, 12:55pm

"here is my rpms/speed with a 3.54 diff. on my 76z

with a 79 trans@2500 rpms i was about the same as it shows on the transmission calculator..15,25,42,55,and then 5th is 65(my readings)---i put in a 81-83 trans and i got 15,25,38,50,55----according to the calculator there is no z transmission that would give me these rpms.---i should be at 72 in 5th.---------it sounds as if i have an intake leak now and i added headers.----what am i missing.any ideas is very welcomed.---i really need help as i am not sure if i should take this trany back to the yard or what?"

[Royce]

I have tried a couple calculations and don't get what he lists at all, what size tires does he have!?

[wolf76]

hi all-

that is my car above with the problem.its a 76 that i took a 79 5spd from and replaced it with what is suppose to be an 81-83 na 5spd....the rpms that is listed is what i am getting from the car itself-------checking with the transmission calculator it doesnt make sense...that is my problem...even if i was given the wrong transmission it still dooesnt make sense...but yet that is what it shows.btw the tires are stock sized 195-70-14.thanks in advance.

[Royce]

Okay, it sounds to me like your speedo is not correct! Which speedo gear are you using in the 280ZX trans that is currently in your 280Z car?

According to http://www.geocities.com/z_design_studio/ a '76 280Z coupe with auto or 4-spd would have a 3.54 diff ratio. Then a '79 5-spd 280ZX coupe would have had a 3.36 diff ratio and the '81 - '83 280ZX 5-spd coupe would have had 3.9 diff ratio. So depending on the tire size for those give years you may have three different speedo gears to choose from. Sounds like your best bet would be to use your original '76 280Z speedo gear.

Also, if all your gears match up except 5th than you might have the rare '80 5-spd that has the same 1-4 ratios as the '81 - '83 but 5th is a little shorter!

Hope it makes sense! Let me know if I need to try to explain myself better.

[2ManyZs]

Are you sure you have the correct speedo gear in the transmission for the rear end ratio you have?

If you are not sure, there's no way you can go by the speedo as it might not be correct.

Depending on the model of ZX it came from it could be geared for either a 3.54 or a 3.70 rear if I remember right.

That will make a larger difference in the rpm reading vs the speedo reading if the improper speedo gear isn't used.

[wolf76]

royce you have a great car---2manyzs thats a weird/cool looking dog.

well if my memory is correct the 76 came with the 3.54 and the only other time the 3.36 was used after the 240 was in a 4spd zx...reagardless i am still using the same dif. now as i did when i had the 79 tranny in.i didnt change anything out but the trans(added headers2)---i am still using the cable that came with the 76 for both trans.if it was the 80 transmission i should still have a higher speed/lower rpms than i did with the 79 trans.------with the addition of headers i dont even have an opinion if it sounds like the rpms matches.i will take it out on the major roads today and see how everyone else is driving.

[Royce]

The speedometer gear is in the trans at the point where the speedometer cable attaches. There is one 10mm bolt holding a lock tab in place. If I remember correctly, once this is removed you can pull the speedometer gear assembly out of the car. Swap this unit with the unit from your '76 280Z trans and you should be in good shape. Or better yet swap the diff with a 3.9 from a 280ZX and you should be pretty close. Your speedo would probably be off a little because of the difference in tire size, don't know what came stock on a 280ZX.

Have fun! Thanks for the compliments on my car! Unfortunately, it looks better on film than in person! But I still love it!

[wolf76]

well i changed out the trans. gear thingy from a blue one to a black one from the original 4spd....it brought 5th up to 60 but that was it....i will investigate and make sure the 81-83zx came with the blue wheel and if so i will just leave it alone for now..unless you guys have any other suggestions.maybe i will blow by a cop and see what he tickets me at.

[Royce]

Hmm... 60MPH sounds right for a '77 280Z - '79 280ZX trans. Sounds like your new trans has the same ratios as your old one, just the speedo gear was different! Maybe because someone installed an early 5-spd into a later '81 - '83 ZX body. Then put in the correct speedo gear to go with the 3.9. It is the problem getting used parts from 20 to 30 year old cars, the source vehicle may have been altered from stock.

Does that make sense???

[wolf76]

yes i am sorry to say that it does make sense.i was hoping for some reason not to pull it out again and exchange it for another one...i have about a week left on the warranty.---5th gear is also not right but i had convinced mself it woul d ge better with use.----thanks for all yalls help.