After swapping the TII motor and drivetrain into my 'vert, i've noticed the fuel economy plummet, and a good deal of this has to be from the gear reduction. In 5th gear, i used to be at 75 mph while the motor spun at 3000 rpm on the NA 'vert hardware. Now in 5th, 3000 rpm is about 65 mph with the TII tranny and rear end. These speeds are whats indicated on the speedo, but it seemed pretty accurate when I went past some of those radar-speed-signs .


Are there other ring-and-pinion options out there for the TII rear end that would give me taller gearing for the highway? I know that the R-type transmission was available in other cars and trucks; what other 5th gear ratios are out there for this box?

Thanks!

 

There's a 4.3:1 and there may be a 3.9:1 may be from a Japanese turbo automatic.

 

Did the japanese turbo autos come with the same 8" diff as the turbos, or did they have the 7" like our NA autos?

 

iirc, the automatics from the Japanese turbo models were of the same strength as the Standard TII diffs.

 

Ok, great. I've got a torsen diff from an FD thats waiting to go in... now i gotta see if i can track down a turbo automatic rear end to go with it.

Would this rear end also have been on the cosmo?

 

I don't think the J-spec came with a 3.9?
All the J-specs either came with a 4.1 or a 4.3 - which doesn't help you.

Someone figured out how to stuff an MPV pinion into an FD rear, which gives you a 3.73 ratio, but I dunno if you can do this with an FC turbo rear.
Check out the 20B section for the details on this...

The S5 turbo 5th gear is taller than the S4 turbo 5th, so that might be an option?
I think it was 0.719 versus 0.762...


-Ted

 

If you're trying to put an FD gear in an FC then you'll probably need to create a hybrid pinion shaft with the FD gear on one end and the FC shaft on the other. Racers do this on the NA diffs in order to get high ratio gears in the long pinion FC case.

The mention of the 3.9 was just speculation.

 

Ok, would there be an option using 3.73 ring from MPV (which should be possible to get either used or new) and having just new (longer) pinion made, to match the ring and fit long FC differential?

 

Something is wrong. You shoul be going way faster than 65 in 5th at 3000 rpm asssuming any Turbo transmission and 4.1 diff.

You have an odd ball 5th gear or rear end. Where did you source your transmission and diff?

Maybe you got "lucky" and got the driveline out of an infini. (4.3 R&P)

 

A popular swap for the V8 guys is a mustang cobra rear end, super strong and almost endless gear choices, and it still has IRS. They're bolt in kits pretty much also.

 

I thought the T-bird rear was more popular for the FC, and the Cobra for the FC.

 

Yea, i thought it was spinning abnormally quickly.

I am not positive which generation transmission I got.. it was advertized as an S4, but when I got it, all of the plugs on it seemed to be S5-style and rounded instead of the more square S4 ones that were on the NA transmission that came in my car. However, working out the math, it would seem I have the higher-ratio S4 transmission after all. Any sure way of telling?

All the parts came from guys i met on this forum, and the diff was just advertized as a TII diff. No idea whether it came from an Infini or not.

The tires I have on it are on S5 TII wheels, and are 225/50/16's, which should be within 0.1% of the stock diameter, so i dont think that'll make a difference. (all my calcs were done assuming an effective radius 1/2" shorter than the unloaded radius)

Im almost positive that the speedometer is very close to correct not only from the radar-speed limit signs, but also from a few long-distance drives that google maps said would be 588 miles, the odometer read 590 at the end (only one pit stop, and it was def not more than a mile off of the highway). But based on this, wouldnt a rear end ratio of 4.3 make the actual distance travelled less than whats indicated, since the speedo is transmission-driven and is set up for a 4.1 rear end?

All of this is just confusing me, but mathematically the only way I get close to 3000 rpm at 65 mph is with an S4 transmission and the 4.3 rear end

 

I'll take that 4.3 Infini rear off your hands if thats what it is. I can trade my TII rear with you.

 

What size tires? What make and model of tire?

Best way to verify speed these days is with GPS. Do you know someone who has a garmin or something?

 

yea, i could borrow my roommates GPS and check it, that wouldnt be difficult.

The tires are Kumho Ecsta ASX 225/50ZR-16's, on stock S5 TII wheels. According to the chart on fc3spro, they are essentially the same size as stock (0.08% different diameter)

 

If you look at a j-spec transmission 5th gear ratio vs a spec transmission you will find the gear ratio is closer on the j spec. That is why your rpm is different. Also you have 4.10 instead of 3.90
Thanks Robert

 

If you post a picture of the tranny I can tell you

 

I know the ratios are different from S4 to S5, but i didnt know about jdm transmissions. Do you know what the 5th gear ratio is on those? Even with the switch from the vert's 3.9 rear end to the turbo, the math doesnt work out.

EDIT: its a lot closer if I use a tire circumfrence back-calculated from a known speed and gearing on the car when it was stock (all 'vert stuff). I'll need to check out the accuracy of the speedometer, and how far off my tach is (since i think there is a little discrepency between what it reads and what my megasquirt tells me)

Either way, a lower rear end ratio or 5th gear would be nice for fuel economy purposes.