Diff ratios

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Diff ratios

Post  griffinracing on 25/2/2013, 4:31 am

Im hoping to pick some brain (tough would we be racing if we had any).

I'm hoping to get to a few rounds this year and included will be a few new circuits. I now have a choice of 2 diff ratio's 4.2 and 3.7 for my Midget.

Now I dont have the horsepower of Greg or Damien or Simon (I've a lusty 103.8 RW) so need to get the ratio for each track right.

Which should I run at
Winton (long)... I was thinking 4.2
Phillip Island... I was thinking 3.7
Sandown... I was thinking 3.7
Wakefield Park is definately the 4.2

Let me know your thoughts.

Steve
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Re: Diff ratios

Post  16vmidget on 25/2/2013, 9:39 am

Hi Steve,

Sounds about right. Are you running an 50 or 60 aspect ratio tyre?

Cheers

Greg
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Re: Diff ratios

Post  griffinracing on 27/2/2013, 1:08 pm

Greg,

I'm running 205/60/13's. The 50 profiles are double the cost (so I thik I'll be sticking with 60's).

Steve
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Re: Diff ratios

Post  16vmidget on 28/2/2013, 9:39 am

Steve, a quick calculation shows that on your 60 series rubber, you are pulling 16 mph per 1000 revs with a 4.2 diff and 18 mph with the 3.7 in top gear, so thats a 14 mph difference at say 7,000 rpm in top. There are so many variables i.e engine torque, wind resistance, speed into and through corners, length of straights etc etc as to the minimum and maximum diff ratios you can pull, but I'd say what you propose will put you in the ball park. I run a 3.7 at PI and Sandown and 3.9 at Winton and Wakefield but with more like a 45 aspect ratio tyre so more revs per 1000. Its a compromise for me as I run out of revs at certain points at PI (taller diff would be better), but could probably be further up the rev range at Winton and Wakefield (lower in this case)....oh for a 6 speed box!
By the way, I don't have that much more power than you at the rear wheels, the car is pretty light though!

Cheers Greg
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Re: Diff ratios

Post  griffinracing on 4/3/2013, 6:19 am

Greg,

I hope you have a fair amount more HP than me or all the money spent on twincams and fuel injection is a waste.

I assume that even with the 6 sp gearbox the final drive is 1:1 same as with the cr & standard box. I have a 4.875 (CIG locked) diff I am going to try at the new Eastern Creek south circuit and have been wondering about giving it a run at Wakefield Pk too. I usuall change into 4th 20m before the start/ finish line so never use the full revs in top gear.

Winton and Sandown are the unknowns and running a 3.7 at PI will be new. I suppose I just have to make a guess and go with it. I might put an advert in for a 3.9 and a 4.5 standard s/h diff centres. Do a CIG conversion and have too much to choose from.

Steve
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