58mark

iTrader: (3)

I'm going to drop a 3.4 into my rs. I'm going to build it up to about 200HP

What I'd like to do is change from my 3:42 rear end to something that will drop my RPM's on the highway. I have been told that with my current 2.8 that is a bad idea because the engine will labor too much in getting it going. With a 200HP 3.4, I don't expect that to be an issue, but I still want your opinions

This is my goal... Better MPG, and I don't care about being more sluggish off the line. Speed isn't my goal at all, but I'm hoping the extra HP will offset the different gears.
I also would like the rear disc setup and the 4th gen offset, because I've got my eyes on a set of deep dish y2k wheels

How hard is it to find a 3:08 rear end from a 4th gen? wasn't that the standard rear end on all v6 cars? does include both automatics and 5 speeds?

1fas92

iTrader: (1)

Scribed ....

big gear head

A 3.23 is as far as you can go in your current rear end because of the 3 series differential.

58mark

iTrader: (3)

couldn't I replace the entire rear end, not just the gears?

drdave88

iTrader: (3)

You could get a rear out of a V8 car that has 2.73 gears in it, that a series 2 carrier.

built91Z28

What series his current axle is or what the new one would be doesn't matter if he's replacing the entire rear axle.

FWIW, my old 96 Firebird V6 was a 3.08 open rear end. Drum brakes though.

Base91

iTrader: (1)

For pure highway the 2.73 would be best but the 3.23/3.27 is a great compromise. Should be 2k rpm for 65 mph. Fairly relaxed.

big gear head

Correct, if you are replacing the complete rear end then it doesn't matter. Just don't go so far that you put too much of a load on the engine in overdrive. I put a 700R4 in a El Camino that had 2.73 gears in it and I could only use overdrive when I was at 70 MPH. Below that put too much load on the engine and I had to shift down to drive. You can actually cause the car to get worse fuel mileage by loading the engine too much.

Z28ricer

iTrader: (2)

Have you tested this with a V6 ?

Sure with a V8 possibly, with a V6 you'll likely be putting the engine down in an RPM range where its less efficient.

I wouldnt be doing anything other than a 3.42

Base91

iTrader: (1)

Yes I have the 3.23 and v6 and think it's a good all round ratio.

Z28ricer

iTrader: (2)

I was referring to the 2.7x comment, feel free to test it, then let us know how well it didnt work, before suggesting it.

58mark

iTrader: (3)

I'm sure he's right that a 2.73 would feel great on the highway, but it's not realistic for daily driving

jmd

iTrader: (4)

5th gear is pretty easy to change out on a T5. This is a good option for a highway driver if the gears aren't already set up for the tallest possible 5th already.
You'd want to get ahold of a Tremec supplier who can tell you what the input drive/countershaft driven gears up front and 5th drive / driven gears are to see if there's any room to make it taller.
Any idea what code your T5 is? (might have it on a metal tag at a tailhousing bolt)

58mark

iTrader: (3)

no idea... I looked into gears one time, but I was told it wouldn't help much

Z28ricer

iTrader: (2)

No, it would realistically be terrible, and result in worse fuel mileage.

jmd

iTrader: (4)

well, next time you dive under your car, see if the code is there.
sometimes it's up top on a decal.

58mark

iTrader: (3)

would you like to tell us how? right now I think I'm running about 2700 RPM at 70 MPH. there is no way the 2:73 is going to drop it so far that it's in the lugging area. I run in 5th all the time at 1500 rpm around the city and it's smooth, but I just have no acceleration

Z28ricer

iTrader: (2)

Because it doesnt have to be "lugging" to be far below the RPM where its most efficient.

58mark

iTrader: (3)

then what's the sweet spot? Driving song between 1500 and 2000 rpm's seems to be it for my car, and I can't imagine the 2:73 dropping it below that. are you saying it will?

I'm not considering putting the 2:73 in my car, but I am curious why you are so sure that it wouldn't produce better MPG if in theory, you were only driving on the highway

Z28ricer

iTrader: (2)

Because for actual use, and not theory, there is a trend between displacement, mpg, and cruise rpm on the highway, if the severe rpm drop were such a good idea we would have plenty of OEM's using it with factory vehicles.

yaj15

iTrader: (3)

If you really want to go after fuel economy why don't you look in to a T56 swap.

Some of those transmissions have a 5th gear ratio of 0.74 - 74% gear reduction off of the 1.00 4th gear ratio.

6th gear ratio of 0.50 - 50% gear reduction off of the 1.00 4th gear ratio.

I've got an L98 in my car with minor bolt-ons, a 700r4 and 2.77 gears. At 70mph I'm only turning 1,900rpm-2,000rpm. The gears that I have aren't the best in terms of performance but the V8 cars make a lot of low end torque that I really don't feel too much of a penalty with the high way gears that I have. Especially since I've got a higher stall converter as well. But I've got a bus load of low end torque with my car - as do most of the other factory powered V8 cars - compaired to the 2.8L & 3.1L motors that some thirdgen cars came with.

Even with the improved 3.4L motor that you are dropping in there is still less power and torque available at lower rpms (where you drive most of the time from idle to 4,000rpm) than a typical factory V8 car.

Keep the 3.42's that you already have. The V6's make less torque than the V8 cars and their torque and power bands occur at a higher rpm than with a V8 car.

If you are going after fuel economy, keep the 3.42's that you already have (so that you can keep good acceleration performance) and you have the deep double overdrive gears that will reduce your fuel usage.

You can keep the T-5 that you have and pair it with 3.23's although there isn't much of a difference between 3.42's and 3.23's.

The other poster is right a V6 car is going to be a real dog on the street with 3.08 or 2.73 gears. Yes in theory you will save fuel economy at cruising speeds because the engine will be turning at a lower rpm.

The question then becomes how much do you have to keep your foot down to maintain that speed with those free-way gears? Steeper gears also mean that you won't have to put your foot in the throttle as hard to get up to speed and maintain it.

This is really important in a basically stock V6 car like yours because your peak torque and power band occur at a higher rpm than a typical V8 car. This means that if you install highway gears (such as 2.73's) you will frequently downshifting to get your car back up in to the torque and power band. This gets old fast and you will decrease your fuel economy as well as create more wear and tear on your drivetrain due to all rapid changes in engine rpm all of the time.

jmd

iTrader: (4)

yaj, you may want to slow down the advice on combinations you've not experienced.

He won't be pulling a .50 sixth with 3.42 gears with a 3.4.

When trying to reinvent the wheel, look at all the combinations that were offered in the first place. You're not going to all of a sudden outthink engineers who intended to give the car driveability and economy with a T5. Yeah, if you take a V8 T5 G92 combination that worked well and want to prioritize mileage, a change to a .59 fifth gear might work. But you're not going to have awesome results by throwing in a 2.73 in place of a 3.42 with the trans. unchanged, or trying to pull a .50 sixth which would have miserable non-tractable results.

The other thing to consider is driveline speed. In the realm of the usual parts used in thirdgens, the slower you can turn the driveshaft and numerically lower rear gears are better balance-wise and stronger respectively. Across their typical RWD offerings in the third-gen era, GM used different diameter driveshafts and tossing a T56 with .50 sixth and spending $ on putting very numerically high gears and then finding you need to change driveshafts too wouldn't be a very happy result. It's the "change one part only to find you have to change another, which leads to a mis-match with others" problem that rears it's ugly head.

My neighbour asked about changing his Dana 60 from 3.73 back to the stock 4.10 gears. He thought it would help mileage. It didn't, because of drivability issues.

Base91

iTrader: (1)

Given your original set up idea (3.4 and highway) the 2.73 would definitely give better mpg. So would another taller gear. Of course there is loss for that gain.

yaj15

iTrader: (3)

Yeah I know. I haven't driven a V6 car with those kinds of gears before but from a mathematical and performance perspetive I know that those combination of parts (V6 paired with a 0.50 overdrive gear) wouldn't work well. I have driven thirdgen V6 cars (specifically the 3.1L V6 paired with the 700r4 and 3.23 gears in a couple of 1991 & 1992 RS models) so I have some first hand knowledge of the fuel economy for that drivetrain.

A big part of fuel economy is turing as few rpms as possible at any given road speed but there are a lot of other factors that come in to play as well that you mentioned.

I agree with you, if he had a V6 paired with the 0.50 overdrive gear he would have to keep his foot deep in to the throttle to keep the car going at a given road speed even though the engine would be turning at a low rpm. There would be a lot of load placed on the engine still because of the minimal torque multiplication factor that the 0.50 overdrive gear would provide.

I agree with the other posters, 3.23 and 3.42 gears would still give him good performance and good fuel economy at the same time with the new motor. The 3.4L motor is also more powerful than the 2.8L & 3.1L V6 motors that came in these cars so even if he left the rest of his drivetrain stock his fuel economy should improve becasue he won't have to get in to the gas that much to get the same kind of acceleration (or maintain a given road speed) as before.

jmd

iTrader: (4)

Until you see a thread indicating turbo, just stop.