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Diff ratio for Bathurst


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#1 RLY240

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Posted 22 December 2013 - 08:13 PM

We're off to the FOSC regularity event at Bathurst this Easter and I'm looking for some advice on what diff ratio to run.  Normally run a 4.3 or 4.11 with a max of 6500 rpm, I've also run 3.9 but not sure if I would be better off with a 3.7 or even a 3.5.
We see about 200 in rally trim but there's no Conrod straight in a rally so not sure what we need to prepare for.
I know some of you guys have run there before so any advice appreciated.
Roger


#2 PZG302

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Posted 22 December 2013 - 08:20 PM

3.9 will suit in my opinion, that's what I have used for a couple of speed weekends in my car, hillclimb up mountain straight and the run from the hump to the start finish line.

Whilst speed down Conrod is important you can't forget about the big climb up mountain straight and even worse the 1 in 8 climb out of the cutting which is 2nd gear low revs and pull from there up to Reid Park and then onto Macphillamy.

A big trap for young players is to worry about Conrod and forget about the climb up the hill.

I would take a 3.9 and a 4.1 and go with the 4.1 first and if to short drop to the 3.9.

#3 dat2kman

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Posted 23 December 2013 - 12:23 AM

I run a 4.1, but rpm a bit higher, 7700.
On GearCalc program thats 245 down Conrod, but the 4.1 gives pretty good climb up the hill.
3.9 or less gets a bit boggy

#4 RLY240

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Posted 23 December 2013 - 07:57 AM

Thanks guys, I don't think I have the horsepower for more than 210-215 so sounds like the 4.11 is the way to go.  I'm actually less concernered about the top speed than I am about running sustained high revs and of course pulling up the hill.  Nothing to win here, just need to be sure to get the car through the weekend in one piece and we're double entered for twice the fun.
Roger


#5 peterrx5

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Posted 23 December 2013 - 10:15 AM

Hi Roger, I don't think it is a FOSC event at Bathurst over Easter in 2014, several members of the ZCCQ have entered for this event through the NSW Road Racing Club and I am one of the hopefuls.

I am stuck with a 3.6 diff in my car so the climb up through the cutting is very interesting, should be better this time then at my last visit when I was running a 3 speed auto as I now have a manual box fitted.

Check out this video taken at this event in 2013, you will see that the driver actually selects first gear to pull up out of the cutting, and he averages lap times just on or under the 3 minutes.


May see you there.

Cheers
Peter

#6 RLY240

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Posted 23 December 2013 - 10:46 AM

Peter, you're right. It's the Bathurst Motor Festival not the FOSC.  My bad.
If I could crack a 3 min lap I'd be thrilled, happy to just lap up the experience and bring it home in one piece.
See you there.
Roger


#7 260DET

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Posted 23 December 2013 - 06:58 PM

Yeh it's being run by Ripplestrip and a few of us from the ZCCQ are going. My Zed will be there using a 3.9 diff, 7000rpm red line, not sure if I'm looking forward to it or not me being a scaredy cat with 420 horses at the wheels. There are a couple of Zeds coming from WA I believe, so now we have one from SA too. Excellent :)

#8 dat2kman

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Posted 23 December 2013 - 07:37 PM

Your target time is 2.45

That was in a Historic category Group S car, very unmodified and poor aero,  and 6" rims.
compared to the majority of sprints entered S30 Zeds, running triples, 8" rims, low profile sticky rubber etc

#9 Grandad

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Posted 02 January 2014 - 01:19 AM

Hi RLY240

Great to hear you're off to Bathurst at Easter, you're in for a great time.

My son and I have taken the Zed there twice (double entered), we usually run a 4.11 ratio but for our first Bathurst run we fitted a 3.5 to allow for the run down the hill which was fine but we lost out enormously going up and it didn't really work across the top so for our second attempt we fitted a 3.9 which was perfect, it left us with enough power up the hill, worked well across the top and didn't overwork the motor on the way down.

I was happy with my times but unfortunately for me my son was even happier with his best of 2.55.3, while the car is not group s spec but was on 6" rubber, for you to get an idea of how your car will rev with a 3.9 ratio diff I've attached a link to his hot lap.

Hope this helps in your decision making.




#10 luvemfast

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Posted 02 January 2014 - 09:42 AM

Great video.
Would love to drive it one day.

#11 260DET

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Posted 02 January 2014 - 06:52 PM

So what is stopping you, need more S130 goodness ruling the mountain :)




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