ZCar Forum banner
Status
Not open for further replies.
1 - 9 of 9 Posts

·
Registered
Joined
·
1,770 Posts
Discussion Starter · #1 ·
Does anyone have actual test data displaying
the performance gained by switching from a 3.54 to a 3.90 rear end? ( e.g., 0-60 times, 1/4 mile etc.)
 

·
Registered
Joined
·
625 Posts
GEAR IT AND HAVE SOME FUN!!

I never dragged with my old 3.36 rear end ,but when i installed my 3.9 it felt like I had added another 30 HP to the engine.The torque multiplication in the lower gears was INCREDIBLE.Also the nicest benefit is that you dont have to spin your tires to get good off the line performance! This much I can tell you.When I got my nitto drag tires I was not impressed with my 1/8 mile times with the stock 82 ZX 5 speed and 3.9 rear end in my 71 240Z.I had run BETTER times with my normal street tires,the lack of wheel spin was not allowing the engine to get in its powerband before the tires hooked up. So ,I installed a 79 transmission input shaft and front counter shaft gear on my 82 zx 5 speed.This modification essentially changed my 3.9 reargear ratio on the first 3 gears to an effective 4.356. This may be a little confusing but changing the shaft on the tranny increases the 1st,2nd,3rd,and 5th gear ratios 11.7%.4th is not affected because it is 1:1. The math 3.9 x 1.117=4.356 4.356-3.9=0.456 increase in gear ratio Anyway after doing this my 1/8 mile time went from an 8.85 to a best of 8.55! So you can see that by changing from a 3.54 to 3.9 gear should pick you up nearly 2-2.5 tenths in the 1/8 mile!!! Going from 3.54 to 3.9 will give you about a 10% increase in torque in all 5 gears because you will actually be changing the rear gear ratio!! You should change your 3.54 to 3.9, it is the cheapest thing you can do to any Z car for a REAL noticable improvement in off the line acceleration and is mandatory when going to bigger cams which hurt off the line performance!! Later,norm
 

·
Registered
Joined
·
1,770 Posts
Discussion Starter · #3 ·
Re: GEAR IT AND HAVE SOME FUN!!

> I never dragged with my old 3.36 rear end
> ,but when i installed my 3.9 it felt like I
> had added another 30 HP to the engine.The
> torque multiplication in the lower gears was
> INCREDIBLE.Also the nicest benefit is that
> you dont have to spin your tires to get good
> off the line performance! This much I can
> tell you.When I got my nitto drag tires I
> was not impressed with my 1/8 mile times
> with the stock 82 ZX 5 speed and 3.9 rear
> end in my 71 240Z.I had run BETTER times
> with my normal street tires,the lack of
> wheel spin was not allowing the engine to
> get in its powerband before the tires hooked
> up. So ,I installed a 79 transmission input
> shaft and front counter shaft gear on my 82
> zx 5 speed.This modification essentially
> changed my 3.9 reargear ratio on the first 3
> gears to an effective 4.356. This may be a
> little confusing but changing the shaft on
> the tranny increases the 1st,2nd,3rd,and 5th
> gear ratios 11.7%.4th is not affected
> because it is 1:1. The math 3.9 x
> 1.117=4.356 4.356-3.9=0.456 increase in gear
> ratio Anyway after doing this my 1/8 mile
> time went from an 8.85 to a best of 8.55! So
> you can see that by changing from a 3.54 to
> 3.9 gear should pick you up nearly 2-2.5
> tenths in the 1/8 mile!!! Going from 3.54 to
> 3.9 will give you about a 10% increase in
> torque in all 5 gears because you will
> actually be changing the rear gear ratio!!
> You should change your 3.54 to 3.9, it is
> the cheapest thing you can do to any Z car
> for a REAL noticable improvement in off the
> line acceleration and is mandatory when
> going to bigger cams which hurt off the line
> performance!! Later,norm

Thanks Norm, what about RPM's @ 60 MPH and the effect on top speed?

Rob
 

·
Registered
Joined
·
625 Posts
Re: GEAR IT AND HAVE SOME FUN!!

Rob, if you go from 3.54 to 3.9 you will turn about 10 % higher RPM. As far as top speed goes ,my car ran faster with the 3.9 than 3.36 rear gear(the appropriate speedo gear for the 3.9 was installed).Twisting the engine at a higher rpm raised my top speed about 10 miles an hour by putting engine in more favorable part of the powerband. Later,norm P.S. if you go to 3.9 rear gear you WILL need a 5 speed if you don't already have one.
 

·
Registered
Joined
·
1,770 Posts
Discussion Starter · #5 ·
Re: GEAR IT AND HAVE SOME FUN!!

> Rob, if you go from 3.54 to 3.9 you will
> turn about 10 % higher RPM. As far as top
> speed goes ,my car ran faster with the 3.9
> than 3.36 rear gear(the appropriate speedo
> gear for the 3.9 was installed).Twisting the
> engine at a higher rpm raised my top speed
> about 10 miles an hour by putting engine in
> more favorable part of the powerband.
> Later,norm P.S. if you go to 3.9 rear gear
> you WILL need a 5 speed if you don't already
> have one.

Thanks Norm, I appreciate the response. I do have a 5 speed on my '78 and will go find a 3.9!
 

·
Registered
Joined
·
477 Posts
I have Excel file with ratios

> Thanks Norm, I appreciate the response. I do
> have a 5 speed on my '78 and will go find a
> 3.9!

Rob, I created an Excel file with the different transmissions and ratios. I also have calculated the gear spacing ratios for each transmission and have a [email protected] tablet to indicate new rpm with diff changes. Let me know if you could use this.
 

·
Registered
Joined
·
290 Posts
Re: I have Excel file with ratios - Yes Please !

Yes...please include me in the mailing or post here for all. This is commonly requested info, and your knowledge could help everyone.

> Rob, I created an Excel file with the
> different transmissions and ratios. I also
> have calculated the gear spacing ratios for
> each transmission and have a [email protected]
> tablet to indicate new rpm with diff
> changes. Let me know if you could use this.
 

·
Registered
Joined
·
1,770 Posts
Discussion Starter · #9 ·
Re: I have Excel file with ratios

> Rob, I created an Excel file with the
> different transmissions and ratios. I also
> have calculated the gear spacing ratios for
> each transmission and have a [email protected]
> tablet to indicate new rpm with diff
> changes. Let me know if you could use this.

Dave,
Yes I would.
Thanks for the response.
Rob
 
1 - 9 of 9 Posts
Status
Not open for further replies.
Top