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Thread: Diff Ratios - Tarmac Rally

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Diff Ratios - Tarmac Rally

    I currently have a Hilux diff in my Mk1 Escort that has a 4.88:1 ratio. At 8000rpm that gives a theoretical top speed of around 215km/h (from memory). During tarmac events the most i have seen in reality is 196km/h. The car usually struggles at high speeds especially if it is even slightly uphill.

    I am considering whether i need to increase the diff ratio to 5.1 or even 5.3 for more punch out of corners, better pull up hills but at the expense of a little top speed.

    What ratios are others using in their tarmac rally cars?

    Is there any mechanical reason not to do this? Does the diff get weaker at higher ratios?
    1970 Mk1 Escort Tarmac Rally Car

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    Re: Diff Ratios - Tarmac Rally

    what you call a higher ratio, most of us over here refer to as being a lower ratio, it might be numerically higher but gives less speed so we call it lower! anyways, yes your correct as you get lower (uk) the pinion gear gets smaller and has less teeth so the load isnt spread as far so they do become weaker

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Re: Diff Ratios - Tarmac Rally

    Quote Originally Posted by Graham View Post
    ...as you get lower (uk) the pinion gear gets smaller and has less teeth so the load isnt spread as far so they do become weaker
    do ppl use as LOW as 5.x in tarmac rally? Soft tyres, aggresive LSD etc
    1970 Mk1 Escort Tarmac Rally Car

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    Re: Diff Ratios - Tarmac Rally

    I use a 5.1.
    Tarmac Rally car, 13" wheels, sticky tyres, 1600cc 4age engine, type-9 straight-cut clubman gearbox.

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    Re: Diff Ratios - Tarmac Rally

    Quote Originally Posted by muckerbarry View Post
    I use a 5.1.
    Tarmac Rally car, 13" wheels...
    I have 15" wheels so if my brain is in gear correctly then I would need greater than 5.1 to have the same overall ratio as you (assuming the same gearbox ratios). Hmm I wonder if a 5.3 is in the relm of Ok then...
    1970 Mk1 Escort Tarmac Rally Car

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    Re: Diff Ratios - Tarmac Rally

    Quote Originally Posted by MK1_Oz View Post
    I have 15" wheels so if my brain is in gear correctly then I would need greater than 5.1 to have the same overall ratio as you (assuming the same gearbox ratios). Hmm I wonder if a 5.3 is in the relm of Ok then...
    In theory yes, but it depends on a lot of factors....

    For example my engine has a redline of almost 8000rpm, if however i had a pinto with a redline of 7000rpm then my diff would be too low.
    Gear box ratios also comes into it, do you have a 5 speed with an overdrive 5th? or a 4 speed with a 1:1 4th gear?
    How often do you actually get to drive it flat out in top gear? I can count on one hand the amount of times my car has hit the limiter in top gear.

    Its not an easy question to answer unfortunately!

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    Diff Ratios - Tarmac Rally

    A look on eBay and there are atlas ratios of 5.3 and 5.8 are available from quaife. So that would suggest there is a demand for these. I assume they are out there for a Hilux.


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    Re: Diff Ratios - Tarmac Rally

    I run the same diff as you (Hilux with Kaaz LSD) and I had a 5.3 in it but have gone up to a 4.315 as I do circuit racing not rally

    So there def are these ratios available for the hilux. If you were in melb I'd swap you for mine

    Cheers

    Matt

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Re: Diff Ratios - Tarmac Rally

    Quote Originally Posted by Matt75 View Post
    I run the same diff as you (Hilux with Kaaz LSD) and I had a 5.3 in it but have gone up to a 4.315...
    Is the 5.3 still kicking around? If I change ratio I will build a separate unit so that I have both 5.1/3 and 4.8 available to me.
    1970 Mk1 Escort Tarmac Rally Car

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    Re: Diff Ratios - Tarmac Rally

    Yes, im looking to get another one too to replace it but unsure whether to go 4.8 or 4.1

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    Re: Diff Ratios - Tarmac Rally

    swap me the 4.88 for a 5.36

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