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2003 R1: Which diff ratio?
JoaoCaldeira - 16/6/10 at 01:00 PM

Hi all.

I currently have a 2003 R1 engined MNR, with a 3.62 open diff, push in half shafts.
I have 205/50x15 at the rears.

In the longest straight at Estoril GP, in the last few hundred meters (200 - 300 meters = 4 - 6 seconds)) the rev limiter hits.

I was thinking about having a longer diff, but I'm not sure which one (3.38 or 3.14), considering that I want to later upgrade to LSD (not Sierra, as I don't want to change the whole arrangement (diff, halves, etc...)

Diff - Theorical speed limit - real (GPS)
3.62 - 124 - 120
3.38 - 133
3.14 - 143

I was thinking about going right to 3.14, but a friend of mine argued that it would be way too much.

What do you think?

Best regards,
Joao

coozer - 16/6/10 at 01:01 PM

Well, I wouldnt fancy 143mph in mine.. 110 is fast enough ta.

jimgiblett - 16/6/10 at 01:27 PM

In a 7 bodied car a 3.62 is probably fine as you probably won't top out. 3.38 would be a bit better for cruising but are like hens teeth. 3.14 will be to long for 1st gear IMHO.

- Jim

CRAIGR - 16/6/10 at 01:39 PM

Would say go to a 3.38 as mine with 205/60/13 is hard work on a run and runs out of revs on the straight at Brands Hatch.

spdpug98 - 16/6/10 at 01:41 PM

I have a 3.38 diff in my 2003 R1 MNR, in 6th gear the shift light comes on at an indicated 118mph with another 1200 revs or so to go

adithorp - 16/6/10 at 02:07 PM

I've got a freelander 3.21 in my 5VY. To be honest it's a bit tall in first (70+ to the limiter).

In a seven I'd go with a 3.38:1

adrian

Ivan - 16/6/10 at 02:33 PM

I would say leave it as it is unless you are racing and willing to test one against the other.

My thinking is that in a 7 you are close to your aerodynamic top speed anyway as it is so the 7% power loss that the 3.38 will give you compared to your current diff might mean that you will not reach the same speed on that straight and could be slower round the rest of the track.

If my maths is right you might be losing something like an average of 5 km/hr for 5 seconds on that straight which means something like 7 m in position which should be easily made up by the extra acceleration your 3.62 gives you on the other 90% of the track.

Johneturbo - 16/6/10 at 02:36 PM

3.38 would be a good compromise

i've tried them all in my 5VY R1 and now i run a 3.62 lsd but i think the 5VY run a taller primary

JoaoCaldeira - 16/6/10 at 03:28 PM

Hi all.

First of all, thanks for all your replies.

Just for clearing up, with the 3.62 I was running out of revs at the end of the main straight for 4 to 6 seconds. It was the first time it happened, and curiously enough, there was like a 30 - 40 km/h wind on my back - could that have helped?


quote:

so the 7% power loss that the 3.38 will give you

Ivan: I didn't understand how will I loose power exchanging the diff. Could you please explain further? Thanks.


Best regards,
Joao

Ivan - 16/6/10 at 04:04 PM

quote:
Originally posted by JoaoCaldeira
Hi all.

First of all, thanks for all your replies.

Just for clearing up, with the 3.62 I was running out of revs at the end of the main straight for 4 to 6 seconds. It was the first time it happened, and curiously enough, there was like a 30 - 40 km/h wind on my back - could that have helped?


quote:

so the 7% power loss that the 3.38 will give you

Ivan: I didn't understand how will I loose power exchanging the diff. Could you please explain further? Thanks.


Best regards,
Joao


Joao - Yes the tail wind would definitely have made the difference as your car thought that aerodynamically it was doing 30 -40 kp less which is a huge difference so you would have gone a lot faster down the main straight.

Re the loss of power -all things being equal the wheels see the torque at the propshaft times the diff ratio

Assuming the torque at the prop shaft at say 8000 rpm in any one gear is 150 ft.lbs then the torgue at the wheel discounting wheel size as a constant is:

For 3.62 diff = 3.62 X 150 = 543 ft lbs
For 3.38 diff = 3.38 X 150 = 507 ft.lbs
For 3.14 diff = 3.14 X 150 = 471 ft.lbs

Clearly for a lower diff ratio the wheel experiences less torque so accelerates less.

What the above means that excluding wheel spin your car will accelerate differently by direct proportion of the difference in diff ratio

mangogrooveworkshop - 16/6/10 at 04:54 PM

3.38 for sale search my post

progers - 17/6/10 at 07:11 AM

3.38 is ideal for a 5PW based track car. I had a 3.21 in my RGB race car and it was a little too tall for all but the fastest tracks. You do NOT want to install a 3.14.

- Paul

[Edited on 17/6/10 by progers]

JoaoCaldeira - 17/6/10 at 08:22 AM

Many thanks for all your replies.

Joao

Rocket_Rabbit - 17/7/10 at 02:36 AM

quote:

Re the loss of power -all things being equal the wheels see the torque at the propshaft times the diff ratio

Assuming the torque at the prop shaft at say 8000 rpm in any one gear is 150 ft.lbs then the torgue at the wheel discounting wheel size as a constant is:

For 3.62 diff = 3.62 X 150 = 543 ft lbs
For 3.38 diff = 3.38 X 150 = 507 ft.lbs
For 3.14 diff = 3.14 X 150 = 471 ft.lbs

Clearly for a lower diff ratio the wheel experiences less torque so accelerates less.

What the above means that excluding wheel spin your car will accelerate differently by direct proportion of the difference in diff ratio


Sorry, was reading through some posts and came accross this gem

A diff will NOT lose you power.

Yes, the torque ratings differ which means in gear acceleration will also differ.

However....

The Final drive is a multiplier of what your primary drive gear ratio is

What does that actually mean?

Well, lets say you are accelerating from 30 - 100.

Now with the 3.62 diff, 2nd gear takes you from 50-70, 3rd from 70-90 and 4th past 100.

With the 3.14 Diff, 2nd takes you from 60-85, and 3rd takes you past 100.

So because we are dealing with multiples, this is what we end up with:

From 30 - 50 the 3.62 wins (1st gear * 3.62)

From 50 - 60 the 3.14 wins (1st gear * 3.14 offers more torque than 2nd gear * 3.62)

From 60-70 the 3.62 wins (2nd gear * 3.62)

From 70 - 85 the 3.14 wins (2nd gear * 3.14 offer more torque than 3rd gear * 3.62)

Etc etc

So ultimately, what are the benefits of changing final drive?

Well, it's all about 1st gear and matching the track.

From standing/low speed starts, the higher final drive will offer you that extra acceleration.

From then on it's about matching the gearing to corners.

However, under NO cirumstances does a diff lose you power (well, yes it does, but that would be to do with design and mechanical losses, nothing to do with ratios).

Remember elementary physics:

Power is the rate of which work is done.

Work is what is required to accelerate.

The engine produces the work.

So unless you change the engine (or its operating rev band) the same work is produced.

afj - 18/7/10 at 01:20 PM

Regarding the above which i agree with, what would be faster 0-60 with the r1 5pw engine
0-60 in first gear using a 3.14
or
0-50 in first and the 50-60 in second including the gear change the r1 5pw with the 3.62 diff on 195-50-15 does 55mph in first meaning the gear change for the sprint to 60mph
also same for 0-100 3.62 using 1st 2nd 3rd 4th
or
0-100 1st 2nd 3rd using a 3.14 diff one less gearchange??

Rocket_Rabbit - 22/7/10 at 09:31 PM

In the real world, I'd say the 3.62 diff - especially with the speed of which these things change gear.

The main factor to consider is launching which will be easier to do with the 3.62