Rear end ratios

[mmagnus]

I just finally finished restoring my car. I put a R200 rear end with a 5 speed tranny in my 70z. The problem i am noticing that the shifting is very close in 1st, 2nd, 3rd and 4th and by the time I get it into 5th, it seems like has topped out as far as speed. I think the ratio on the rear end is 3.79. I took it up on the freeway and the top speed was very limiting and it seemed like there should of been another gear.

Any suggestion on how i can improve upon this. The motor is not stock. Its a fresh rebello rebuild 2.7litre, headers, tripple carbs, electronic ignition, nismo clutch, lightend flywheel.

Thanks for any help.

Mark

[xray]

From the sound of it, I bet you have a 77-79 five speed...I know of two options off the top of my head:

1) Change your diff final drive ratio to 3.54

2) Swap in a late (1981-3) 280zx non-turbo 5-speed. The fifth gear is supposedly taller and spins at a lower rate at highway speeds.

When you say "topped out," can you clarify--like a speed/rpm/gear point (5th gear @60 mph = 3400 rpm, for example)? What makes you sure it's a 3.7 ratio? That's pretty rare in 280zx (1979 only, I think) but popular in 300zx models. If it's from a 280zx, odds are it's more likely a 3.9 or 3.54 ratio

2 cents,

Steve

[jmortensen]

Get used to spinning the engine faster. My old setup with 23.5" tall tires, 3.70s, and the late ZX 5 speed theoretically topped out at 172 mph. Unless you're running 185/50/13s or something ridiculously short you should be in a similar range.

Here's a tool that will help you figure out your top speed: http://www.geocities.com/z_design_studio/transmission.html

If you just want to cruise at lower rpms on the hwy then get the 3.54 that the others suggest.

[zguitar71]

The motor you have is built to run at a higher rpm. If you are running 3500 or so at 75 I would not worry about it. I am assuming the motor is cammed up a little so the power band will be a little higher than stock too. Before I put in a ZX 5 speed I had a 4 speed with a 3.54 and short tires so I cruised the highway at 4500 rpm all the time and never had any problems and was allways in the power band so I never had to downshift (well I still do not have to down shift, but the acceleration over the passes around here is not nearly as brisk in 5th).

As far as topping out the speed in 4th, that could be the case. Depending on the tires size and gearing you might be able to go 135+ in 4th and going to 5th will actually lower the speed due to the rpm decrease thus falling out of the power band wile tring to slice through the air. Z are not very areo dynamic so it takes a lot of power to go real fast. 5th gear was designed to give better gas mileage/lower rpms not higher speeds.

BTW a restored with a Rebello 2.7 sound like a dream car to me, congratulations!

[mmagnus]

It seems as though when driving on the streets at normal speeds I am immediately shifting to 5th to get any speed out of the car. When i hit the feeway, i hit about 70 miles an hour and the RPM is about 3k. I am almost afraid to hit the gas as the car is running so high. There is a ton of torque but no real top end. Here is the breakdown of my car:

2.7 litre Rebello motor with Rebello cam.

Fidenza lightened flywheel

Nismo racing clutch

MSA headers and exhaust.

Mallory Unilite ignition. MSA Ignition module

5 speed tran. 3.9 R200 (I believe)

15X7 Konig Rewinds. 225 tires.

Tokico Alumina shocks and Tokico springs

Poly bushings.

It seems as though you are all suggesting swapping out the rear end to a 3.54. I am looking for top end cruising speed. Longer time between shifts...what do i need to change to?

[LanceM]

3K at 70 isn't bad, you're just going to have to get used to pulling higher rpm. Here is a spread sheet I made up showing rear gear ratios/trans ratios/tire size. This should help you get an idea what you have and what you might want to go to to get what you want. I got the tire size info from www.tirebarn.com and the trans ratio info from http://www.schlaver.com/zCarGo.html

Z gear ratios.xls

[xray]

70 mph at 3000 rpm is normal. I recall someone posting their dyno chart of a Rebello 2.7 and the HP curve was a steady inclined slope to 7500. Perceptions of maxing it out at 3000 rpm are likely inaccurate--you could also call up Rebello and talk to them about it.

Zs will not give you "top end cruising speed" because of inherent front end aero instability. Even with airdam and some rear downforce, most don't make it past 140 without generating sizeable lift.

...I suppose you could toss the stock R180 3.36 back in there. You'd have sluggish acceleration with the 5-speed, but you may have better top end.

Good luck!

[zguitar71]

Your motor should safely rev to 7K and run all day at 5K with out any problems. At 3K you are just getting into the power band. I would not change the rear at all. You will loose off the line acceleration. You probably have a zx 5 speed judging from the 3K at 70, the 3.9 rear is the standard most people use. Personally I would like to run a 4:11 rear with mine. If you want longer time between shifts then rev the motor to 7K before shifting. With the 5 speed and 3.9 I do not think you have the power to take it to 7K in 5th, that would be around 150+ MPH. Do you really want to go that fast in a `70 Z anyway? The motor was designed to make power up high no low like a V-8, do not be timid with it, the best thing you can do for the motor is run it like it was built to be run.

[jmortensen]

As the others are saying it sounds like you're missing out on about 3 or 4K rpm worth of fun. Rev the wizz out of it. That's what Rebello built it for.

[mmagnus]

Thanks for the feedback guys. I guess i am looking at it from a drivablitiy point of view. I am going to run with it for little while and then see if i want to change it over.

Thanks

Mark

[xray]

Mark:

Just remembered another option...I know some folks on hybridz.org who have mated a 240SX transmission with (I think) a 240-280 bellhousing. IIRC, still requires some creativity with the driveshaft, but a search on their website (and maybe a google search also) could help with some new ideas...

Steve

[mmagnus]

As i drive the car more, i am noticing a lot of vibration in the car above 2800 rpm. it seems to be coming from the trans tunnel but the whole car is violently shaking and is almost undrivable at higher RPM. Any idea what may be causing that?