Sports Sedan Forum

Can anyone advice me on what diff ratio to run at Phillip Island?
It is a 13B running 9800 rpm with 570 x 13 Rear tyres.
Thanks.

Stefan

I am pretty sure your Needham box is a 1:1 5th yeah? I would suggest a max of 250kph is achievable for a fast 13B. So with your 570 tyres maths would suggest a 4.3:1 diff will do 250kph at 10000rpm. So I would run a 4.1:1 to be safe if you do have a 1:1 5th gear.

Thanks for that. It is 1:1 and I have a spare diff that is 4.11
How do you do the calculations?

Stefan

Bit of maths for you -

before we start some symbols - / (means divided by), x (is multiplied by)

9800 (engine RPM) / 4.1 (final drive ratio) / 1 (top gear ratio) = 2390 rounds per min ( how many times the wheels will go round per minute)

circumference of a circle = 2 pie r so 2 x 3.14159 x 28.5cm ( 57cm / 2 = 28.5cm) = 179.070 cm or 1.79m traveled every time the wheels turns a full revolution once.

2390 rounds per min x 1.79m = 4278.1m or 4.2781KM travelled per min so 4.2781KM per min x 60 (mins in an hour)

= 256.686km/h

So all you have to do is change around your rear end ratio around to find out what speed you will do in top gear.

How fast the car will actually go is determined by frontal area, horse power, drag coefficient , pressure density assuming sea level 1013.25mb) etc

nerd cap off :)

Thanks 2JZSS that was really informative and I actually understood it. :D

Stefan

Sorry, these questions are a bit basic but how then do I work out
what ratio is better from track to track when going there for the first
time?

Stefan

Well i'm assuming you don't have any gear sets to change like a transaxle or holinger etc?

All you can really do is set up your rear end ratio to give you your top speed then drive the car to its gearing.

Ideally some real world testing before you get there would be the way to go but if you cant do that i'd be looking at how quick you think you would be getting up to (top speed) and work the rear end ratio to that.

IF you had the ability to change your ratios you could tailor your gearing to the track alot better. Ie certain gears for certain corners and be right in the rev range where you need to be.

clear as mud?

The Corolla has a normal box so I will need to do it via the Diff ratio.
It was a lot easier with the Sprintcar. Maybe a Quick Change diff
might be the go. :rolleyes:

Or perhaps a jerico box or something? then you would have a box you would never break with the ability to get whatever ratios you were after.

bit more expensive than a quick change rear end though!