Best Gear ratio (rear-end ratio) for a turbocharged car ?

[fl327]

i guess the 3.54 is pretty good, i like the od ratio with short tires.

i would run a 3.7 in an instant if it was lsd, and not look back though..

[260DET]

A generalization...

Turbo motors like load. By having to shift earlier' date=' you have to get out of boost. The longer you can stay in a gear, the longer the turbo can spool. The 3.54 or better yet 3.36 allows that to happen. It is pretty much the opposite of the n/a set-ups where you want to run through the gears quickly. Running through the gears quickly with a turbo set up, does not allow you to take advantage of the turbo.[/quote']

 

Interesting point, which I would tend to agree with. The lower the diff ratio, the closer in effect the gearbox ratios are. Which may be good with a peaky NA engine but not so good with most turbo setups. And for a car used on the road consider the cruising revs in top gear.

[yo2001]

So does a turbo car like big heavy wheels? it'll put more load to the engine.. (sor of like heavy flywheel)

[Scottie-GNZ]

Something else to consider is a turbo engine (generally) has lower compression,

NOT QUITE! It all depends on what terminology you use and how you define compression. Remember that for every 1-bar of boost, a turbo car has the potential to ingest twice its displacement. Lets say you are able to ingest twice the displacement, that means at 14.7psi boost the 2.8L has 5.6L of compressed mass stuffed in the cylinders. From a physical standpoint, the c.r. is still say 8.0:1 because of the physical displacement differences of the cylinders between BDC and TDC. However, think about the mass that is actually being compressed (remember, that mass is already compressed) and believe me, you have the equivalent of more than 8.0:1. I would venture a guess it is more equivalent to 13+:1. Ever wonder how a hot L28ET with lots of boost can have 450lb/ft of torque? Probably because it is stuffing the equivalent the of almost 7.0L of compressed mass into its cylinders and then compressing that mass again.

 

As for the final drive ratio, Z-Gad is correct in that a turbo engine works best when under load for longer periods. That does not mean load as in dead weight . The actual ratio you need depends on the RPM limit of the engine, MPH thru the traps and the ability to spend a longer time in the meat of the torque range in each gear. A low-torque, high-revving turbo engine will demand different gearing than a high-torque, low-revving engine. The L28ET with the stock cam is not a high-revving engine despite many making it out to be one. It is pointless revving it pass 6000RPMs. The more boost you run, the more torque you make, the higher the MPH. With that you usually end up with less gears in the tranny or a lower numerical gear ratio or get the engine to rev higher. A perfect example of this is 240Z Turbo, when he ran 129+ with his 3.1L, he was pushing the engine upwards of 7200. I rarely push my engine pass 5700 (only 300 less that what a L28ET should run at) and at those speeds or higher, I run a 3.07. Granted, I have more torque, but if I was revving my engine to say 6200, I could and would run a higher numerical ratio.

[Lockjaw]

Hey lockjaw you're running the 3.9? Iis that the one out of the n/a car. If so what kind of tires are u running at the track. I just went to the track in my car and had one wheel spin and I ran a 15.8 with a bad valve. And I'm only running 215/70 on stock rim, although I do have a very bad diff mount. I would like to stay with 3.9 when I go turbo but I need lsd so i'm getting the 3.7lsd. Also how much does ur car weigh. Mine weighed 2750 without me 1/4 tank of gas, full radio, interior etc... TIA

 

I have a 3.90 diff that was a replacement for a customer at a nissan dealership, and they never installed it, so I tried it, and I liked it at the drag strip.

 

I run a 235/60r15 at the dragstrip, which is roughly a 26 inch tall tire. believe it or not, tire size has am impact on your overall gear ratio, although that gets way technical in nature, and I don't like all that mess.

 

I noticed when I was running some short autocross tires with my 3.5 diff that the car really moved out better, so I thought I would try 3.90's when I ran the DR's. Since they are 4 inches in diameter larger, you can imagine the difference with 3.5's was pretty noticable. Took longer to get into boost, did not spin up as easy, etc.

 

Again, there is more to this then just saying one gear is better then the other, there are a host of factors to consider, like vehicle weight, tire diameter, trans ratio's, engine mod's etc.

 

I am also running a non stock cam, although I wasn't at the time I ran my best pass on the old turbo, and I picked up a tenth of a second, on a hot day over my best pass with 3.5's on a cool evening. It I lost a mph or two, but attributed that to the heat more then anything else. Either way, its not the mph that counts, the the et, and my et's thus far have been lower with the 3.90.

 

Now when I get it back together, and run the JWT dry nos set-up, I will go back to 3.5's since I will be all over the rev limiter with 3.90's, although I will try it with 3.90's.

 

It just irritates me when someone says its a turbo and you have to stuff the tallest diff you can get in there, that is not always the case, if so, everyone should run a faster time starting their car in second gear from a dead stop.

 

Its also different from an Auto with a stall versus a stick and having to launch. Stall and auto means leaving with boost, doesn't always work that way with a stick.

 

of course all of this is just an opinion of one person, although all me friends here ran their best times in 280Z's with turbo engines running 3.90's, with the exception of one guy, and he ran a BW and 3.54's, but he was in a 240Z, so perhaps he is best excluded.

 

Either way, swapping a diff is not that big a deal, try and see.

[jeffp]

I ran the 3:54 gear stock in my car and it was good. When I setup the LSD unit it had 3:71 gears and that is what I kept. Keep in mind that was with the stock T5. Now I have the 3:71 gear set and a TKO2 tranny in the car. My topend with the gears, tires and the .82 overdrive instead of the .750 overdrive in the T5 my top end is about 165mph, fast enough for me.

I did not have nay spool problems with the 3:71 gear set. I could go from no boost to 20psi of boost when I took the car to 3K rpm, not bad at all.

If I was going to setup a full on race track acr what I would do is use the power glide like james is setting up, the 3:71 gear set with the taller tires to get me the topend.

The thing about the rear gear set that I dont think alot of people think about is the the lower ratio is a harder gear to turn then the higher ger set. My thinking is the even tho you say put out 400 foot pounds of torque say, the 3:71 gears will get better torque to the ground then the 3:54 or the 3:36 gears. see my point, get so that you get the mechanical advantage at the diff as well and you have a monster. More power will be put to the ground easier with the higher gear set, and for me the 3:71 seems to be the best ratio all around. Just my O2