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S30 rear end change - 3.545 to 4.36


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My car is a daily driver that I like to auto X. Turbo 2.8, the power band starts at 3500 RPM. It has an 81 to 83 close ratio 5 speed, so 5th gear is 0.745. I've calculated that my engine RPM at 75 miles per hour would go from 2880 to 3540. That is on my current 23.1 inch tires, if I go back to bigger tires, the engine RPM would go down a little bit maybe 3350@75mph.

 

Again, this is a daily driver. In my head this sounds doable. That is quite a few RPM at cruise, but it will put me closer to my power band so I don't think it will be in issue. I don't think it will bother me. But I don't know. What do you guys think?

 

Q1 Is 4.36 too short for a daily driver? how annoying will it be?

Q2 How much faster would my car be?

Q3 How much would it help me at the autocross?

Q4 Would you do it? Why or why not?

 

Thanks, Brazle.

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Go autocross with the local club. Try to drive whole course in second gear. Adjust gear ratio as needed to be able to avoid shifting during your race without blowing motor. That is the gear you need. If you go to 4:10 or lower you will hate it for a daily driver and find that you need to shift more during autocross. Been there and done that. 3.70 can be a nice compromise.

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Q1 Is 4.36 too short for a daily driver? how annoying will it be?

 

No and you're the only one that can judge how annoying it might be.

 

Q2 How much faster would my car be?

 

Probably won't be any faster.

 

Q3 How much would it help me at the autocross?

 

You typically gear for a max of 70mph in second gear for an autocross. So if the 4:38 gear does that, it will help.

 

Q4 Would you do it? Why or why not?

 

No. I would spend money on a seat time first, LSD second, and shocks third. Gearing is a distant 5th on 6th on my list when it comes to going fast on an autocross course.

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Thanks guys, good thoughts.

 

Sorry, I should have specified - my old diff is making noise, so I'm going to replace it. And my friend has a 4.36 that's in good shape. So that's why I'm considering this, since I have to swap it out anyway.

 

I've been auto crossing for 5 or 6 years. The track I go to is pretty fast, it's more like a small road course (it's a sheriff's training track). I usually get well into 3rd gear.

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Wow, sounds like fun. If the price is right, get it and try it. What have you got to lose other than a day of work? Take it easy exiting turns on the street until you get used to the new gearing. I spun my car on an on-ramp on the maiden test drive the day I switched from 3:36 to 4:11. Destroyed A hub,strut, air dam, control arm and two wheels. Dam n lucky I didn't kill myself. Took me a whole week to fix it (I was in my 20s-would take a month now). Wish I was in your club, our tracks are super tight due to small facility.

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Personally I had a HELL of a time keeping traction in my turbo car unless I had really good tires with a 3.7 on stock height.

 

One thing you can do to 'try it out' is go look for someone with a set of 13X7's and low profile tires. If you have a 20" tall tire, you can pretty much simulate the gears with a little calculation.

 

We put 13X7's with sticky slicks on them, with the stock gearing. Made it act like a 4.11. You can do the same it's easy to do on the back, you don't have to clear any calipers!

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I don't have any issues with traction right now, at least not in a straight line. And I'm only running 205s at the moment, stretched on 15x8s (not my choice, I just got a good deal on wheels and tires when I needed new tires).

I don't launch hard and I don't shift hard, and even when I roll hard into the throttle, I don't have any trouble keeping traction.

 

Tony, I'm not sure i know anyone with wheels and tires that small.

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I'm leaning heavily towards trying it out.

 

Here's a video of the track, (only 1 of many configurations)

 

That is an awesome auto-x location! We only have a couple of event locations that I could get into 3rd gear and that was with a 4.40 FD in my old honda ~75mph max.

 

I don't think the shorter gears would really help you too much but if you do plan on taller tires it might be streetable depending on what annoys you and the type of driving you do. Better tires would be high on my list but I understand you are trying to repair what you have.

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I ran the same setup as you with a 4.11 and turned 3100 rpm at 75 mph. There is so much other noise in my 73 that the extra revs don't make a big difference. I recently swapped to a T5 with 3.54 that keeps the same ratio in 1 and 2, but lowers in 3-5 and now turn 2800 rpm at 75 mph. There's no doubt that the car is a little more "relaxed" cruising, but the difference is small.

 

I think the bigger issue is going to be running out of revs and having to shift more with a 4.36. On the road course I frequent, I was constantly bumping up against the rev limit in third before, but the extra 10 mph in that gear now let me focus more on driving and less on shifting.

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I run a 4,44 ratio with '75 4 speed. I was amazed by the crisp acceleration when starting from 0, for some reason however my performance was worse than when using 3,90. I think that traction while accelerating might be an issue - especially when wet. In my case - when combined with vlsd traction was also an issue when decelarating in a corner.

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