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Diff Ratios


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#21 Zedman240®

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Posted 16 July 2008 - 07:46 PM

I was going to say as well the power you need to push a car through the air at high speeds is enormous! I've seen pics of cars that are equivalent aerodynamically to a chest of drawers after a high speed run, return with the roof caved in from just from the force of air against it. Unless this car is only going to be driven on a salt flat it will be useless on the public road. I Lycra clad cyclist will be leaving you at the lights! The secret to high speed is to have gearing low enough to help the engine multiply the torque at the wheels, and making it easier at high speed to push it through the air. Plus all engines make most power higher up the rev range. With a too low diff ratio, you are going to run out of road before you reach 4th! Why do drag cars have 4.11-5.8's?

I just think you are wasting time looking for a ratio like that when you can find a great 3.9 LSD for good all round performance! (plenty on epray)

#22 Brabham

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Posted 16 July 2008 - 10:29 PM

Have taken all this on board. I am looking to get to 300Kmh, so that is not all that fast. Car is not finished yet still deciding on ratio. I have the 3.7 and this is pretty good for our tracks here in Aus I agree, although 4.1s and shorter have better acceleration I just feel you would be giving away too much on the top end. However all our tracks are acceleration and hence why people like these ratios. I am thinking about getting a mental L28 ET, so power should be enough. I know that power and top speed is an exponential ratio. Given the 260 2 seaters came with a 3.364 I think my goals are achievable yet expensive :(. So my offer on a tall diff still stands if anyone has one in Sydney. Hopefully since no-one wants one, they might be cheap :) If I can not find one, I will settle for the 3.364 or 3.1. Anyway cheers for the advice but it is something I want to try. I can always go back to old faithful 3.7 if need be, and I will keep it as I may need it. I just wish more of our circuits were like the old Monza, flat chat in top gear with about one or two serious high speed corners. 

#23 Scando

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Posted 17 July 2008 - 05:14 PM

Makes a bit more sense now if you're going for an L28ET because they produce a lot of torque low in the rev range.  300km/h is still a very big target!
If you're going to one of the salt lakes mettings then maybe you could take a few different ratios and try different ratios each run.  I'd assume you have a fair bit of time to change the diff between each run?

Good luck with it  :)

#24 nat0_240_chevZ

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Posted 17 July 2008 - 05:36 PM

3.364 is still not really common, i am yet to see, hear, feel and or touch one in person and i have seen and touched a few odd things, no comments from the smart ass' please!
a 3.54 is easy and almost low enough for what you are after, otherwise 1100$ will get you a 3.3 or like from kameari, see other post benZ i think????
for the gearsets only.
or a g/box with very tall 5th

#25 Brabham

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Posted 17 July 2008 - 07:03 PM

Had a 3.545 in a car but sold it, wanted to keep it but alas it went with the car. Car was an auto - why do people put autos in sports cars aaarrggghhh. I have noticed that there are a lot of z people on here from Melb and Brisbane, but not many people here are from Sydney. Oh well at least there are three places here that do z parts so most are still obtainable- plus ebay and its damn paypal!

#26 620Z

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Posted 18 July 2008 - 08:55 AM

You are dead right Nato. My 3.36 LSD is the only one I have heard of or seen in a car. I wouldn't be selling it even if I was offered $1,500-. (I have plans) So I would be surprised if Brabham found one. Any R200 LSD is hard to find these days let alone the ratio you want. Took me a year to find my 3.9 LSD and that was through word of mouth.

#27 Scando

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Posted 18 July 2008 - 01:39 PM

Is an LSD really necessary for salt lake racing though?  You've got heaps of time to reach your top speed so the bit you'll loose taking off shouldn't make any difference.  I'm not saying it's going to make it any easier to find the ratios you're after but there's no point spending the extra on an LSD if you don't need it for the application you're using it for.  They're heavier and may actually cause a little bit of drag depending on what the pre-load is set at so it might marginally reduce your top speed compared to an open diff??? Just a thought.

#28 PZG302

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Posted 18 July 2008 - 02:01 PM

Why not chase up an R180?? For the high speed salt lake type work you won't be putting the torque loadings through the diff that you would with a shorter ratio, so shouldn't have to worry about breaking them. Plus I've found the difference in strength between the R180 and R200 for a circuit car to not warrant the extra weight of dragging around the R200.

#29 620Z

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Posted 18 July 2008 - 03:08 PM

I'm not really sure Brabham's priority was to do the Salt Lakes thing. That was more a plan for me. Since I have a diff for the proposed trip I don't see any need to look for another one. I think my 400klw motor might push an R180 to the brink. Something not worth testing at a real high speed!!

#30 Scando

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Posted 18 July 2008 - 05:03 PM

A mate of my dad's built a Falcon for the salt lakes.  He said there is no traction at all.  Sounds like you wouldn't even break an R160 from what he said no matter how much power you were putting through it!

#31 bluerat

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Posted 28 September 2008 - 05:13 PM

Howdy,

Brabham, try Nissan part no.38100-N3405 which is a 3.154 ring and pinion set listed for an R200 in the Datsun Competition parts list. I very much doubt they would be avalible new, but try US sites, as they were sold there new. Dont waste your time with Nissan Aust, the US is the answer, if you want one.

Also try www.locost7.info/gearcalc.php and dial in your numbers, It will tell how many RPM you will need to do what speed in what gear, It will be up to you to get the power to pull the numbers!!!

If you can get hold of a DVD called 'Big white Dyno', done at Lake Gardiner in '04 it will show you how hard it will be....

If you look at the DLRA results not mant cars do 200MPH (320kmh) goooood luck.

#32 Brabham

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Posted 28 September 2008 - 09:37 PM

Thanks for the info blue rat. It is something I will look at. At the moment I have the 3.7 and will just keep a casual eye out for anything that turns up on ebay or through word of mouth. I am still restoring the body on the car and have not fully decided on the drivetrain yet, but the diff will be an R200 with an L-series engine of some description, possibly turbo and methanol-water injection looks interesting, have been reading up a bit on that lately for a bit more power and extra cooling. :) :). Not sure what corrosive effects it would have on the engine though. Any thoughts? Singapore F1 tonight yay :)




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