Good trans/diff combos for L28ET 240Z?

[Guest DaneL24]

Obviously lower gears will allow the car to accelerate faster, but I have heard that if the gearing is too low the turbo won't spool as quickly because there is less load on the engine. Whats a good compromise between lower gears for fast acceleration with decent turbo spool? If you have a turbo set-up, please give your trans/diff combo and the RPM that the boost kicks in in 1st gear.

 

Thanks for the input.

[Mudge]

I'm shooting for a 3.54 and ZX 5 speed, T5 is another choice but I prefer to stick to the NA close ratio.

[BayAreaZT]

I'm running a 3.54 with a ZX 5-speed. Much better for my daily driver than the 4.11 I had in there. I can build more boost between gears and better on the freeway.

[Lockjaw]

IMHO, 3.56's with the ZX 5 speed are a little tall, 3.70's are nice, and 3.90's work well too.

 

The T5 and 3.56's work well, but requires more work to get the trans in, and you have to shorten the driveshaft I believe, and you have to mod the crossmember.

 

I liked the ZX trans with 3.70's. It kind of split the difference between 3.56 and 3.90's. I liked the 3.56's on the highway though, I had one hell of an over drive, but I felt the car was a little to sluggish in first and second. Perhaps with what I have now, my opinion would be different. I had a stock turbo, and stock actuator at the time.

 

3.70's are harder to come by though. If you can find a set, I would grab them, they will work well with any trans, and I would like to have a set right now, as I am running 3.90's with the T5 in my ZX. I like that combo too, and expect to be shifting into 5th gear before the 1/4 mile is up now.

[Mudge]

I myself can't stand the 3.90, 3400-3800 RPM on the freeway with the early 5 speed (.864 5th).

[Lockjaw]

That transmission sucks. The gear spacing is terrible, especially between second and third gear.

 

The ZX trans with its .745 overdrive and 3.90's is a much better choice. I don't ever recommend a Z 5 speed.

 

Here is a handy link to check out.

http://www.geocities.com/z_design_studio/

 

It has lots of transmissions for the Z car, and rear gears, and even has a 6 speed, don't remember if it was a t56 or what.

[Mudge]

Yep, thats why I can't wait to get rid of it. When dumping the fluid a good number of metal peices came out with it, and boy that fluid looks like it was never changed in its entire lifespan! The oil was more silver than oil colored (NO LIE). NA ZX 5 speed for me.

[Guest DaneL24]

Here is what I’m trying to figure out:

 

With an N/A engine, you can put in as low of gears as you want and there will be no tradeoffs as far as acceleration is concerned...the tradeoff will be highway drivability and gas mileage. But if I undertstand it correctly, putting lower gears in a turbo motor not only has the tradeoff of drivability and gas mileage, but also the tradeoff of slower turbo spool. It just doesn't make sense to increase the torque from the engine so much if you have to reduce it through the gearing anyways...you might as well just make a stout N/A motor and use some really low gears to get just as fast acceleration as a turbo. (Well, not exactly...but you see my point)

 

Lets say I replace a 3.36 diff with a 3.7 diff. In theory, that will increase force to the wheels by about 10%, as well as taking 10% of the load (force required) off the engine. Will that 10% decrease in engine load really slow down the turbo spool? Do you see what I'm trying to figure out here? At what point is it not worth it to get lower gears for more acceleration because it slows down the turbo spool...and therefore gives you less acceleration?

 

I know that highway drivability can always be retained by getting a 5 speed with an overdrive, but I’m concerned with 1st gear acceleration and turbo spool.

[Lockjaw]

I disagree with the concept that a turbo needs a numerically lower gear to work properly. Think of it this way. What exactly is a turbo engine? Is it not a very high compression engine when under boost?

 

If your turbo boost is chasing the tach, you don't have something dialed in right on your set-up, or you have to big a turbo.

 

I run a T5 with 3.90's in my ZXT. I dropped a solid tenth of a second in the quartermile over the stock 3.56 diff, and did this on a hot day, versus the cooler evening with the 3.56 times. In addition, my 60 foot times were better and so was my 1/8th mile. I run Drag TA's so it is not a traction issue. Even though I was spinning the hell out of the DR's on the hot day. Same boost levels, so the gearing helped.

 

Also something to consider is this. Turbo engines are generally lower in compression, and the numerically higher gear helps with off boost drivability.

 

Nissan put the T5 in turbo cars, and it has decidedly different gearing in first and second gear then it did in other applications. They must have had a reason for doing that.

 

Also a friend of mine has a 280 turbo, and he ran the T5 and 3.90's, but swapped it for a ZX trans, but kept the 3.90's. His warped logic for swapping? He had to shift into 5th gear before he finished the 1/4 mile with the T5, but did not with the ZX trans. Since they both have a 1:1 4th gear, which transmission was faster in his application? The T5.

 

T5's can also use a hurst short throw shifter for a Mustang T5, lets see you get one for a ZX trans.

 

I am sure I will have some naysayers say I am full of crap, but my times have proven that numerically higher gears do not hurt my performance, and with this bigger turbo I have now, I expect even faster times.

 

Thats the LJ stand on the matter.

[Scottie-GNZ]

Well, just go ahead and call me a naysayer. :D. I suppose this could be argued until kingdom come, but trust me, from my experience, the more boost you run and the more HP you make, the better the car will run in the 1/4-mile with a lower numerical ratio, especially in a light car like a 240Z.

 

I started out with a 3.90 and "progressed" to a 3.70, then a 3.54, then a 3.36 and now a 3.07. Know what, If I went to a 2.90 or thereabouts, the car would be even quicker. Instead I plan to go from a 26" tire to a 27". It is difficult to accept especially when you are so familiar with the NA concepts, gear multiplication, etc. I certainly do not intend to debate the point any further but let me throw this out. Look at included URL and see what happened to this person. Note the timeslip with the note on it and compare that to the one with the lower numerical ratio and you will see the car is quicker in every segment. This is exactly my expreience.

 

http://www.toohighpsi.com/BudgetTT/capriTS.htm

 

I know some of you guys are not at that stage yet, but if you plan to increase HP but not do any mods to move the torque curve higher, IMHO a higher numerical ratio is the wrong direction. You do not gear a turbo engine to run it quickly up to redline.

[SleeperZ]

I enjoy this arguement. I am going to prove to myself which is the correct one, at least for my car. I am running the T5 with 3.90 gears, and my first run at the track is tomorrow. I will be swapping a 3.54 back into the car soon - it is prepped and lubed, ready to install. Hopefully there will not be too many variables, and I will get a definitive answer.

 

Personally I don't like how the 3.90 with the T5 works on the street anymore. I jump on it and I'm out of gear immediately. It's just too low.

[yo2001]

One thing that I've notice is that stickshift car is hard to get a boost off line without slipping the clitch with E-brake. My low 1st gear helps me get out of hole. It does run out fast but I was getting 8psi in first, 10psi in second and on so I didn't really complain. May be If I was running more boost it would have made a difference.

 

Now with 60-1/stage III combo, I still getting 6 psi before the tires boil over.

[Lockjaw]

Well for starters, my car is not light, nor is it as rpm limited as a GN V6. Perhaps in your case Scottie, you are making enough power but are limited by RPM's. So in that case, you probably could go faster by having a numerically lower gear.

 

I know when I break mine down, I was 2 tenths faster in the 1/8th, 1 good tenth faster in the 1/4 mile, on a hot day. My 60 foot was good considering I was spinning, and the only thing off was mph, but only by 1. And my turbo was on its last leg while I made these improvements.

 

You also run an auto with a stall which helps with torque multiplication, which those of us with a manual don't have.

 

I am not saying you are wrong and I am right either, I am just saying perhaps there is not a universal standard, or there is more to it then just the fact that the car is turbocharged.

 

Sleeper I would like to know what you find out. I like the 3.90's in my car, but it does hamper me on the interstate a little. I know one thing, it goes thru the gears in a hurry. I can swap back and forth as well, but I have taller tires when I have the drag TA's on my car, so I was feeling a little sluggish, and I noticed I did not have that hard of a boost hit in first as I did with my regular tires on.

 

I will update with my times when I get back too.

[Mudge]

With a 3.90 rear and early 5 speed I'm in 4th by 35 minimum, if I'm cruising I'm in 5th gear 35+ MPH, I think this rear end sucks with my tire setup, trans setup, and just in general for a street car. It really makes me feel like its the 60s all over again, 3400-3800 RPM on the freeway is ridiculous WITH OVERDRIVE.

[Lockjaw]

If you have a Z transmission, you don't have overdrive.

[Scottie-GNZ]

Just a point of clarification. Not much point in pushing a stock L28ET pass 5800-6000 (I know most are facsinated with high RPMs) and I am pushing mine to 5600. Not much difference. My stall speed is 3000-3200, not what I would call "high".

[Guest Anonymous]

Lockjaw, why do you say no Z trannies have overdrive? I thought overdrive was anything numerically less than 1:1. 4th gears are 1:1 in Z 5-spds, and the 5th gear, whether the early .8xx or late .7xx are less than 1. Isn't that overdrive?

[Lockjaw]

Is your stall high enough to leave on boost? Stick cars don't leave on boost without clutch slip. Also auto's don't drop boost between shifts, sticks do. You also have 1 liter more displacement, which hardly makes a comparison between your car and an L6 fair and valid.

 

I cannot shift mine before 6k either, at least not at full throttle. Maybe in 3rd or 4th, but not in 1st and 2nd. And to follow the numerically lower gear content, my car would speed up if I put 3.56's or 3.36's in it, but how would it do that when it takes longer to get into boost? Its driveability will suffer since most of the time it would be operating below the boost threshold, and it would take longer to get there.

 

As I said earlier, I really think there is more to it then just some general rule. There are alot of factors that determine whether it is beneficial to go higher or lower, and just because a car is turbocharged does not mean numerically lower is the best way to go.

 

In my own experiences, I have not gotten slower by running a numerically higher gear, and while some people may have experienced different results, idicates that there is another variable involved that ultimately determines the outcome.

[Lockjaw]

Lockjaw, why do you say no Z trannies have overdrive? I thought overdrive was anything numerically less than 1:1. 4th gears are 1:1 in Z 5-spds, and the 5th gear, whether the early .8xx or late .7xx are less than 1. Isn't that overdrive?

 

I was kind of being a smart alec. Mudge is basing his comment on the fact he has the Z tranny with the lamest overdrive other then a 4 speed (with none) . His complaint is RPMS at freeway speeds, and he blames it on the 3.90's which is crazy when he has a .86 OD, and should be running a trans with a .745 OD. A simple trans swap would drop him down into the 3k range at 75mph, which is plenty sweet if you ask me.

 

To me it seems unfair to blame the gearing in the diff when it was clearly intended for that kind of gear to be run with a .745 OD tranny. (Nissans intentions anyway)

 

I guess it is like looking at a glass and arguing over whether it is half full or half empty. Neither is technically wrong.

 

I hate that Z transmission, it is more like having 2 4th gears then having a trans with an overdrive. I have always been more pleased with the ZX tranny versus the Z tranny, and the ZX tranny is very balanced with 3.90's, and 3.70's are ok, but 3.56's are a little to tall for my tastes.

 

I guess in the end it all boils down to personal preferences.

[Mudge]

Crazy? Do the math and I'd get a 200 RPM drop with a new trans. Your right the early 5 speed SUCKS, HOWEVER I still think the 3.90 is overkill for a car that sees alot of street time. I was out of first gear so damn fast that I stopped using it after awhile, the 3.90 would be great on a car with a HUGE cam that needs the gearing, otherwise I do not consider it appropriate for my own cars.