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Old 01-24-2003, 22:55   #1 (permalink)
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diff ratios and quarter miles....

hey peoples, i have a question about diff ratios and their effect on quarter mile times.

i have a 3.45:1 LSD at the moment, but a guy at work says he can get me a set of 3.7:1 diff gears for about $80, plus fitting and lapping into my housing (havent got a quote yet)

my current revs @ 60 in 4th are 1600, (1:1 ratio) and i figure that a set of 3.7s will see it turning at 1715 (or thereabouts ) at the same speed/gear, so maybe these extra revs will help stop me creeping over the speed limit cause it is reving a little harder.

what is the 5th gear ratio on a T5? i want to work out what revs @100km/h

i have done a 15.7, there is a 15.6 in there definately and maybe a 15.5 if i get it all perfect, 15.7 was @ 88.5MPH - how/will this be affected if i get the shorter ratio diff?

working off the terminal speed i was doing about 3800rpm in 4th as i crossed the line, and max power is at around 4500 (seat of the pants dyno lol)

do you think it will be worth it and where can i get a speedo gear to keep my speedo accurate?

sorry for so many questions, thanks for your help in advance!
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Old 01-25-2003, 00:18   #2 (permalink)
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Ok one answer at a time.

The T5 5th gear ratio is 0.73 or 0.72 dependant on which vehicle model and when.

Revs @ 100 km/h will depend on your tyre size but if we work on 235/45/17 as an example then your overall tyre diameter is 25.326 inches (I know thats archaic but that's how I learnt to do these).

With your present 3.45 and an overdrive ratio of 0.72 for 5th gear your revs @ 100 km/h will be around 2060 rpm or about 48.5 km/100 revs.

A change to the 3.7 will change your 100 km/h revs to 2209. or 45.2 km/1000 revs.

As to the change this will make to your 1/4 mile time this requires a bit more info and some fancy maths but I will try and work it back from the numbers provided.

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Old 01-25-2003, 00:47   #3 (permalink)
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I've been wondering this same thing myself (which diff ratio is optimal for the 1/4 mile). It depends on so many factors. A taller ratio means less gear changes, and it potentially also means better traction. One person I spoke to thought that a 2.77 or 2.92 was the optimal ratio for a manual EA. An ED XR6 revs higher so a shorter ratio would definitely be better, but I'm not sure a 3.7 would actually be an improvement.

Would be interested in everyone's thoughts.
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Old 01-25-2003, 04:50   #4 (permalink)
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Hey rollin, if you buy the 3.7's, will you sell your 3.45's to me?
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Old 01-25-2003, 14:07   #5 (permalink)
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Rollin

Note that as the the rear gears get shorter that you are more liable to get wheelspin off the line on a hard launch. Consider using a slick type tyre if you seriously want to lower your 1/4 et.

Also what model Falcon are you driving and do you know your race weight?
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Old 01-25-2003, 19:17   #6 (permalink)
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my car and all the info i can give you is on my website: see signature...

i have heard that for an optimal quarter mile time it is best to have your engine spinning at 10% past peak power as it crosses the line, and since it is spinning at 3800rpm already, a slightly shorter diff ratio might get it up a bit closer to peak power as i cross the line.

russelw: thanks for that, you are nearly spot on with your calculations, it spins at about 2k @100km/h at the moment, though its got 215/65/15s (stock EB XR wheels) so 2200 is acceptable i reckon.

Jack thats and interesting theory and it would be interesting to go to the drags on the same day with a 2.92 and a 3.45 and see what is faster, but i dont think the 2.92 would be quicker, at least in my car.

crocodarian: maybe, but the gears are only the crownwheel and pinion, not the planetarys or the LSD mechanism, so you will have to get a diff shop to fit them to your diff and lap them in - i havent got prices for that yet - do you already have a LSD or open centre? a 3.45:1 open centre diff would be interesting...

anakha: wheelspin is not that much of a problem, i have a VERY heavy stereo in the back, best 60ft with current tyres comes from letting it out at about 3000-3500rpm, it doesnt even really chirp into second, even if i pretty much flat-change it.

its only a stock EB XR6 with extractors and a pod filter, so there isnt a great deal of power to deal with. i think it weighs about 1550kg, probably more if you include me inside.

a am seriously investigating a slight N2O addition to the boot though so the traction issue may change...
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Old 01-25-2003, 19:21   #7 (permalink)
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Hmm... i have a single wheeler lol.
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Old 01-25-2003, 19:34   #8 (permalink)
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Rollin - Get me 3.7s for $80 (3.9s would be even better) lol !!!!

I have 3.45s atm and will be reaedy to sell them as soon as i source my 3.9s... They are immaculate.
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Old 01-25-2003, 19:43   #9 (permalink)
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the 3.7s will be second hand though, so no gaurantee of their durability, and also i dont know how many sets he can get.

your are on performance forums too arent you? how is that tailshaft hitting thing going?
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Old 01-26-2003, 01:24   #10 (permalink)
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Hey rollin if you can get your hands on those 3.7 gears for that price they are a bargain, well all depends on the drive line from the engine also, manual or auto, power of engine N/A or forced induction, well I have a EB its supercharged, I have 3.45 LSD in there at the moment, 4 speed auto, these gears are to low for my setup as the blower produces the power, so I need a higher ratio like 3.9s, as I found out over the quarter, I didnt even get into 4th gear I ran a 14.5@95mph.
Im guessing yours is a manual, go the 3.7s you wont be dissapointed, but make sure you have the running gear to suit.
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