I put a 3.45 in my ED Fairmont before I changed to a manual, and after quite a few engine mods. It works much better than the standard 3.08 ratio with both manual and auto, especially with a trick cam, with tends to sacrifice some low down torque. The initial acceleration is terrific with a manual, but the best part is that it's great on the freeway. I went to Canberra last weekend. At a true 110 (indicated 120), it's turning over at about 2500rpm,and is very quiet, but is sitting on the torque band, so it just sails up the steepest long climbs with the cruise on in 5th. With the auto it would kick down form 4th to 3rd. The overtaking acceleration is much better than with the taller diff. If there is a down side to the 3.45 diff, i havn't found it- fuel consumption seems the same- maybe because the throttle response is better, I don't nail it as hard. I think a 3.85 would be too low for normal use.
Well I have gone from 2.92:1 to 3.45:1 and Iam very happy.I have done some mods to my car as mentioned in pervious posts which is giving me 122kw at the wheels around 164hp in old terms.But its running lean but thats another story.The 3.45:1 makes the car more livlier due to the cam and now my power curve is higher it helps get off the line nicely.Its not so sluggish which is what you are probably experiencing at moment.We have discussed this in Mods and Audio section under diff ratios which goes into it alot better.Have a look at that.Any higher diff ratio then it will be revving out really quckly and you will end up with no top end at all.Its a comprimise.
Have you got 240hp at the wheels or at the flywheel??If its at the wheels then you must be using forced induction.122kw at the wheels I have been told is around 190-195kw at the flywheel which is 254 hp in old terms.
Lukey, its 240(ish)hp at the engine, not the wheels (i wish).
I think i have done about the same mods as you, thats why we are running our engines at around the same power.
I dont really care if i loose top end, as long as i can get to 180kph i'll be fine. My goal is to run the quarter with a top speed of 100mph+ i am at around 90-94 at the moment. Do you think that it is possible to pull 100mph+?
I just want to be able to get into drag racing (legal) an have a respectable time for a streed driven 6cyl sedan.
So what is with your engine running lean? I just put the crow cam in and mine is running the mixtures spot on, if not rich..... What model is your car? and is it CFI or MPEFI?
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.
Well Iam happy to say its a Series 1 12/88.One of the first cabs of the rank.The susposable shit boxes,maybe mines one in a million but I think there are more 88 models in the EA home page.I think BC's is a 87 and they all are still going strong.
ED and EA have the same computer ECC 4 but an ED has more learning capabilites then an EA.So maybe your computer has learnt the different cam or maybe I don't know exhaust and head might make it different in our cases.Its a good question but thats all I think of.The injectors are standard EA Bosch.
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