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3.9 Diff Ratio In A R33 Gtst With 390kw Atw ?

WARLORD
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WARLORD Veteran

WARLORD

Posted
Posted

I am wondering if anyone has any input on going to 3.9s i currently have 4.3 and love the acceleration of it but i thing its better suited with 4.11 or 3.9 which i have both of but cant decide what would be better hopefully someone has some facts that can help me decide.

the car will be used in circuit and 1/4 mile plus 1000 meter runs.

any input would be appreciated as i will be doing it this week.

KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

If you have the data for your existing 4.3 diff you can directly calculate the change points for your car with 4.11 or 3.9 and choose what's best for dragging. A taller ratio will reduce wheelspin but increase the distance between gears so it rather depends on the power band of your car and the nature of the circuits.

WARLORD Veteran

WARLORD

Posted
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Posted (edited)

Diff ------ 3rd ----- 4th ------ 5th km/h

4.35. ---- 147.13 ---- 191.57 ---- 254.74

4.11. ---- 155.72 ---- 202.75 ---- 269.62

4.08. ---- 156.87 ---- 204.24 ---- 271.60

3.9. ---- 164.11 ---- 213.67. ---- 284.14

3.69. ---- 173.45 ---- 225.85 ---- 300.31

At 7000rpm

This with wheel size 255/35/18

Im just not to sure what would work better as a over all package.

Wheel spin has stop thanks to the new federal 595 rs rr tyres

Edited by WARLORD
Duncan Grand Master

Duncan

Posted
Posted

well 400m should be easy enough to decide, but for circuit use it will depend which tracks you go to, and how you find those ratios at the moment.

Personally I prefer the shorter gears because in NSW we don't have tracks like Sandown or Phillip Island where you are in danger of hitting the rev limit in 5th

KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

If your main interest is drag and track the decision would seem simple - if 255 km/hr top speed is enough stick with what you have. If not pick the ratio that gives you the top speed that you require. The shorter diff gives you closer ratios (and I believe Duncan runs the 4.3s).

If you are a serious circuit racer and have some fast tracks as well as slower ones you could consider changing diffs to suit the track!

The main reason I like taller diffs is for comfortable road cruising.

Duncan Grand Master

Duncan

Posted
Posted

yeah sorry I've got no idea on 1000m either, although there are others on here who do that. In both 400 and 1000 I would guess that the shortest gearing that doesn't require an extra gear change is the quickest, which is how I read what XKALABA was saying

and yes I run 4.375 in the race car, its one of the few mods allowed. a bit like 400m there is a trade off between acceleration from shorter gears vs extra gear changes in corners.....I generally prefer a higher gear and midrange out of corners than screaming out sideways in a lower gear so shorter gears suit me. But that is only general, each track and lap time would be different

XKLABA Grand Master

XKLABA

Posted
Posted

That much ?

Wow with 11psi limit I would have pegged it around 210-220kw

There are a few reasons I say don't use 5th,

1) the time it takes to select 5th is time the car isn't accelerating

2) 5th gear isn't overly strong and shouldn't be used for racing unless necessary, like 1000m runs

3) 5th is an overdrive gear and defeats the purpose of using a shorter diff ratio

With Duncans car only having 240 or so kw on semis and 4wd he isn't really going to have many issues fighting traction but with 390rwkw and only RWD the shorter ratio will see alot of wheel spin through the first few gears

My R34 with about 420awkw and the factory 3.545 diff and the stock Getrag 6 speed it will spin all of 1st all of 2nd and most of 3rd with 295 semis :D

I have since changed that to a option 2 OS88 with a much taller 1st and moded transfer case and on 19psi (380kw) at Coota for Nulon Nationals it pulled away nicely, watch the Nulon Nationals round 4 2015 vid on YouTube I'm the only blue 34 and only did one pass

https://m.youtube.com/watch?v=rmxbttyMoJo&feature=youtu.be

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