I dont understand all this diff info and I dont know which diff does what

my basic understanding is the lower the ratio the faster the acceleration but lower top end speed.

but this obvioulsy varies depending on what engine I install and how much power it runs

I plan on building a Hayabusa powered MK Indy R but which Diff is best suited for this application?

I'm not too bothered about top speed as 120-130mph is fast enough for most tracks but at the same time I dont want a car that feels like its trying to hard when driving on motorways.

any help would or pointing in the right direction would be much appreciated

Thanks

tyre size can make a big difference too....

but most people seem to want a 3.38 for a busa or a 3.14 for a turbo busa
( 3.38 diff for sale linky )

try using a gear calc program, and look at the rpm for a given road speed
( http://locost7.info/gearcalc.php )

(just remember: it won't tell you about acceleration and top speed will be limited by power / airodynamics not engine speed)

[Edited on 4/8/09 by mcerd1]

Have a search for any post by Chris Gamlin. He has a link to an excel spreadsheet in his signature, which is gear ratio / bec diff ratios.

Save a copy to your 'puter, then modify it to suit.

It doesn't have a busa engine option, but I'd imagine that a bit of internet research / Haynes manual would allow you to find the correct information to replace one of the engines that are featured.

For example this is the speed chart for an R1 engine with a 3.62 diff:-


R1 3 62 Diff

As you can see, it'll hit 50 mph in 1st, before you have to change up, and top out at just shy of 120 mph in top.

By comparison, this is the speed chart for an R1 with a 3.14 diff:-


R1 3 14 Diff

As you can see it'll hit 60 mph in first (but will need more revs to pull away cleanly) and top out at just over 130 mph.

* All results with the all important 13" wheels / tyres .

You can then monkey around further with the spreadsheet to reduce the rev limit. So rather than see the maximum speed in each gear, you can see what speed you are doing at certain revs. For example, this is the speed chart for an R1 engine with a 3.62 diff at 7,000 rpm:-


R1 3 62 Diff 7000 rpm

So you can see at 7,000 rpm in 6th you'll be doing 70 mph. It'll also give you an idea of the mid range speeds in the gears.

I'd be asking the owners / builders of other Busa powered cars which diffs they are using too.

Hope that helps.

Mike

It's confusing, but my understanding is the LOWER the ratio, the HIGHER the top speed. If you had a 1:1 ratio the wheels would turn at the same rate as the prop shaft , ie bloody fast.
If you had a 10:1 ratio you could go up the side of a mountain, but rather slowly.

Thanks for the pointers guys, I will find that excel doc and have a play around with it

Good shout on asking the busa guys what they are running (I hadnt even thought about asking that)

Thanks

what people talk about as a high gear/ratio (ie you go faster) = a lower number

eg. 3.38:1 means 3.38 turns of the input for 1 turn of the output

so 3.92:1 is a lower gear than 3.62:1 and so on

[Edited on 4/8/09 by mcerd1]

quote:

---

Originally posted by Macbeast
It's confusing, but my understanding is the LOWER the ratio, the HIGHER the top speed. If you had a 1:1 ratio the wheels would turn at the same rate as the prop shaft , ie bloody fast.
If you had a 10:1 ratio you could go up the side of a mountain, but rather slowly.

---

quote:

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Originally posted by MikeFellows
Good shout on asking the busa guys what they are running (I hadn't even thought about asking that)

---

type 9 ratios (BGH E8)

Both Danny K and Chris Maries run 3.14 diffs in the busa powered Indy RR race cars,with 13" wheels.

Jenko - Sorry but re: the diff ratios you are completely wrong.

The lower the diff ratio the higher the top speed - provided of course the engine has enough power to pull the revs.

The reason some (E.G CBR1000F & Busa) BECs tend to use 3.38 and 3.14 diffs is to do with the primary gearing used in the box.

a 3.14 diff will accelerate faster than a 3.90 but will have a lower top speed is my understanding of it

by the sounds of it i need to find a 3.14 diff for reasonable money. cheapest ive found so far is £300 + then would like to find a LSD to transplant into it

PPC this month has quite an interesting article on diffs.
Cheers, Pewe

quote:

---

Originally posted by MikeFellows
a 3.14 diff will accelerate faster than a 3.90 but will have a lower top speed is my understanding of it

by the sounds of it i need to find a 3.14 diff for reasonable money. cheapest ive found so far is £300 + then would like to find a LSD to transplant into it

---

3.90 will give better 0-60, 3.14 will give better top end, in theory,

if you car will do 120 at 6000rpm with a 3.90, then because the 3.14 needs less rotations to get to 120, you may be doing 1000rpm less, so you still have a 1000 rpm left to go to a high speed.

the higher the number is lower the ratio, so a 3.90 is lower gear then a 3.14.

but to really confuse things, in theory the 3.90 will give a better 0-60 than the 3.14, BUT, it can come down to the torque of the engine and the ratios in the gearbox, if you can got to 60 in first by putting a 3.14 in the axle, then because you don't have a gearchange then you could be as quick or even quicker to 60.

i'm running a 2.73:1 axle as this gives me 61mph in first, i have nearly 600lb/ft torque and a car weighing about 800kg, so to me i want long gears because i don't have the rev range to run a 3.90, i'd be changing gear to often!!!!! but i also have a .50 6th gear giving me the 70 speed limit at below 2000rpm !!!!

[Edited on 4/8/09 by bj928]

Guys thank you very much thats cleared that up

Mike

If i can give my 10pence worth...
i've got a 1400zzr in me Indy, this engine has very similar gearbox ratios as the Hayabusa.
Originally i had a 3.62 LSD which gave me 118mph and ballistic acceleration, but, i was always running out of revs and loads of wheelspin...it needed to be 'tamed'.
Went to 3.14 (hybrid LSD) this now gives me 0-60 about 3.5secs but gives me a topend of 137mph, which was much better for track and gave more relaxed cruising (5300rpm at 70mph)
Dave

quote:

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Originally posted by Shandylegs
Jenko - Sorry but re: the diff ratios you are completely wrong.

The lower the diff ratio the higher the top speed - provided of course the engine has enough power to pull the revs.

The reason some (E.G CBR1000F & Busa) BECs tend to use 3.38 and 3.14 diffs is to do with the primary gearing used in the box.

---

I said it was confusing

but its not too bad if you think about it like this:


3.92 to 1 means:

3.92 turns of the input = 1 turn of the output

[Edited on 4/8/09 by mcerd1]

Or to put it in basic terms - the higher the number , the lower the ratio .
My V6 is a total animal with a 4.1 in

No, Jenko - you were right!

Something like a 3.14, 2.xx etc etc, although numerically low numbers, are referred to as high gear ratios in car speak. Conversely, your 3.92, 4.27 etc etc are referred to as low ratio diffs! Just as 4th, 5th, 6th are referred to as higher gears than 1st, 2nd, 3rd although they are numerically lower. And just as 4x4s have a low range for off-road use which is actually numerically higher.

Not sure whether the confusing use of terminology would have originated simply from the numbering of car gears - i.e. 1 is lower than 2,3,etc, or perhaps because we're talking about reduction, i.e. lower number (closer to 1:1) = less reduction = higher speed??

Liam

[Edited on 4/8/09 by Liam]

mcerd1,

Not sure if it matters but should you not put the engines max power e.g. '160'(bhp) in the 'max power' cell on that gear calc spreadsheet?

Mark.

no thats for revs @ max power

it dosen't calculate your theoretical top speed - it'd need aerodynamics and rolling resistance for that

(if I put 160 in, it says I can only do 3mph in 5th )

quote:

---

Originally posted by Liam
No, Jenko - you were right!

Something like a 3.14, 2.xx etc etc, although numerically low numbers, are referred to as high gear ratios in car speak. Conversely, your 3.92, 4.27 etc etc are referred to as low ratio diffs! Just as 4th, 5th, 6th are referred to as higher gears than 1st, 2nd, 3rd although they are numerically lower. And just as 4x4s have a low range for off-road use which is actually numerically higher.

Not sure whether the confusing use of terminology would have originated simply from the numbering of car gears - i.e. 1 is lower than 2,3,etc, or perhaps because we're talking about reduction, i.e. lower number (closer to 1:1) = less reduction = higher speed??

Liam

[Edited on 4/8/09 by Liam]

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I think the confusion comes about because the ratio is written as "number of input turns per output turns", which I see as somewhat counter-intuitive.

It makes more sense if you use "number of output turns for every input turn" i.e. swap the ratio calculation around.

So, our 3.14:1 final drive becomes a 1:3.14 diff (or 0.318:1 if you will)

Our 3.90:1 final drive becomes 1:3.90 (0.256:1)

3.14:1 -> 0.318:1 (higher ratio drive)
3.90:1 -> 0.256:1 (lower ratio drive)

Hope that helps...

Chris

[Edited on 5/8/2009 by iti_uk]