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diff ratios and quarter miles....

7K views 14 replies 10 participants last post by  Rollin 
#1 ·
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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#2 ·
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.

Cheers
Russ
 
#3 ·
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.
 
#5 ·
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?
 
#6 ·
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...
 
#9 ·
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?
 
#10 ·
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.
 
#11 ·
Many people go really tall gears to use the engine's torque if you can save a gearchange. If getting into fourth is a foregone conclusion, then in theory you want to be around peak power in fourth as you cross the line. However, this leads little room up your sleeve for more mods later, as you may run out of revs as your terminal improves. Also, the engine has to be built to spin up quickly,(lighter flywheel, good cam, etc) or lower gears won't gain anything. Try it and see. If you have an EA T5 first gear should be 3.5 and fifth gear should be 0.78. Lower diff gears would probably suit a 3.28 first gear ratio XR box better, and you also then get a 0.73 fifth to compensate on the highway.
 
#12 ·
no i have a manual EB XR6, as per my website...

yeah there is so much to consider before changing diff ratios, i just wanted a few opinions.

i will never have forced induction unless i win the lotto, but i am very keen to get a little bit of N2O happening in the future, either a 50 or a 75 hp shot, depending on traction - i think it might be quicker with 50, due to the 225s at the back....

my next performance enhancement will be the biggest cam that will work with the factory computer, valvesprings, vernier cam gear etc, it will be set up to help the engine spin up high, i dont think i will run out of revs in 4th with a 3.7, unless my trap speed is more than 192km/h (by my calculations thats what it will do at 5500rpm in 4th gear with 3.7s in it)

so nearly a 120mph pass with N2O and a cam? i think not.

what i want to know is - will putting 3.7s in it help my quarter mile time? i think it will, cause she will rev up a bit harder at the end of the quarter, maybe only a little, but when you are racing commonwhores then every little bit matters...
 
#13 ·
Hey Rollin.

I just thought id mention this to you. With a set of 3.23 diff gears, you should pass the line at about 5000-5500 rpm in 3rd. With this ratio, I managed a low 15 sec pass, and another member managed a mid 15 sec pass in his almost stock EA. Both were running standard 15" rubber on the rear.

I would suggest to you that the 3.45's are high enough (that’s just my opinion). You might be better of investing in getting some better traction off the line.

Ciao
 
#15 ·
hey Bass, can you tell me a bit about your car? i have heard a lot about it but never anything specific - what engine mods do you have, and what stereo do you have?

my stereo is bloody heavy, and also EBs were a lot heavier than EAs, and EBII heavier than that, so i do think i will gain a tenth or two with the lower ratio, even if its to make up for the stereo in the boot...(thats if i dont mod the engine any further)

traction at willowbank is pathetic, at least in first gear, so i might be doing myself a favor by getting into second gear a bit earlier? i never get wheelspin in second unless it raining or im doing 'indy' doghies...

on the street 3.7s wont be a problem, as a normal road is way stickier than willowank IMHO...

but what you said about staying in third gear till nearly injector cutout is an interesting idea - my car kinda dies in the ass above 4700 so i was changing at 4600 in every gear - next time i go to the drags i might do a run death gripped to 3rd and see if its any quicker - once i get the cam it probably will be able to make power at those revs - i didnt take future engine mods into account when thinking about my current shift points.

all food for thought peoples, thanks a lot for your opinions, anything else you wish to tell me would be warmly recieved - i still cant make up my mind...
 
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