Going by the Equation in Street Machine Mag and from what some people have told me you want the car to be in top gear or shifting into top before the 400 metre mark.This is the equation::
old diff ration divided by the top gear ratio equals new ratio.
2.92 / 0.70 = 4.4:1 diff gears. which is around 4.11:1 in borg warner gears.These will give you very quick accerlation but you might loose at the half way mark once the car you are racing has is getting into its top end where as you will have been in your top end for ages then you can't change up nor change down.The old Falcons don't like hitting 6000rpm to many times even though I have been told they can handle it under racing conditions of course. But then they say don't go more then 15% as it will be a pain to drive if you want to pack up and go down coast.
So going from the above info I would not go past 3.7 but 15% of 3.08 would mean the 3.45 would fit straight in the middle.Its up to you. You might have to talk to someone who races and daily drives there streeter to and from the track.You don't really want to trailer your pride enjoy as the fuel bill driving it there might put you off, if you sit on around 3000rpm at 100km/h.
With regards to the car well,I had 107kw with just bolting Hm's on then I rebuilt the engine fitting Crow 825 cam,variable cam gear,top to bottom rebuild and Street Force head.Injectors replaced the list goes on and on.Once run in I had the variable cam gear dialed in.I gained around 2-3kw at the wheels but gained a better power curve.To my amazment I only achieved 110-111 kw at the wheels.I was not happy and wanted answers.Dyno bloke said exhaust is to small.I had 2 1/4 from cat back."you'l gain power from fitting 2 1/2"Rang the bloke that did my head,My Q was why spend all this money when its not that much more powerful then before,mind you in his favour it did pull better.He said what size is your exhaust.I told him he said remove 2 1/4 and fit 2 1/2.In his words"your motor my not be making the power which you thought but it has the potential to do so,all the ingredients are there bar one thats is your exhaust, its restricting the flow of gas and due to your cam overlap burnt gases are re-entering the combustion chamber making your engine not breath to full capacity or what I have set the head up for.
So I had a 2 1/2 inch mandrel bent exhaust system fitted.My cat as it turns out went from 2 1/4 to a cat that was around the wrong way then to 2 1/4 then to 2inch and back to 2 1/4.Once modified to 2 1/2 all the way through and correct direction plus the rest of the exhaust I gained around 12kw at the wheels.Now I have gianed the mentioned "potential power" but now its flowing that much air its running around 14.5:1 which is not good for power nor engine.Before the exhaust mods it was 13.5:1.So fitting the exhaust has gained power but made it run lean.So next step is the chip.Sorry to make it along post.Hope this explains it and helps you with the diff gears.I knocked just over a second when I fitted my diff gears and that was before the exhaust.
What did you get at the wheels??O and before I forget changing diff gears can change kw reading at the wheels depending on how high tec the dyno is,what infomation has been put in the screen,what gear its done in.Late model dynos might be accurate but sometimes take way to much into account such as slippage e.t.c.Make sure you go to the same dyno as I have found out they all lie and depending on the day can change by 10kw at the wheels.Its only a comparitor comparing what you had before and what you have now once mods have been done.
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