Cortina

I am going to rebuild my box soon (it's for competition use) and have the choice of a) older gearchange mechanism (wound spring) or b) later one (leaf spring) .............. yes I have different cases etc !

Apart from the fact that leaf springs tend to break , and are no longer available ? - which type of system should one opt for and WHY. (considering intended use)

One would like to think Porsche changed for the better .... but I'm assuming nothing !!

cheers

Note - have been using later system for about 20 rallies now with essentially no issues.

Cortina

PS. I am on the hunt for a set of 1.65 ratio lay gears (I can buy new but they are expensive) ... anyone got anything ... std in a G28.08 box I think ?

... or even a set of 1.5714 (std in a G28 11/13)

Please let me know quickly before I place an order !!

Hilton

Have you contacted Paul Anderson of 928spares in the UK? He's the most likely place to find trans bits.. plus he races his early one too

Cortina

Yes , I spoke with Paul today - nothing available in terms of parts for the later boxes BUT ....
he was able to answer the question posed initially

Ans ... The later boxes change MUCH better and quicker .

So now that I have woken up to the fact that I can't mix gears between the older (.07/.08) boxes , and the BW type (.10./.11) ,,, (tell me if I am wrong please !! ) due to the different syncro arrangement ... at least I have a plan

There is a higher layshat ratio available (1.5714) which will give me better acceleration than the current 1.4545 that I have ... so at least that will be a step in the right direction (1.65 out of the earlier box would have been better still .. but for reasons mentioned above I don't think it's a starter !!)

James Bailey

Using the higher layshaft ratio gives you a bigger RPM drop from 4 th to 5 th. Does give you a slight torque advantage in first gear does nothing really after you shift to second. Pushes mph down for redline in 1-4 . Such a box would FEEL faster as you would need to shift gears sooner might use a different gear in a specific corner.
You are correct that you cannot mix early and later gears due to the changes for the different style syncros.

Cortina

Not sure that I just totally agree with you.
Yes , 4th to 5th - much bigger drop
the rest - it's simple maths for the numbers (the tyres I use do 806 revolutions per mile , and I am assuming reving to 6000 , diff is 2.72)

speeds in gears are as follows (before - 1.45 , after 1.57 lay ratio)

before 43.5 / 65.2 / 91.5 / 120.9 / 163.8
after 40.2 / 60.3 / 84.6 / 111.9 / 163.8mph

so I would expect the biggest change in performance to be real and also "felt" in 3rd and 4th gears. Average speeds on stages are circa 55mph - the amount of time on a ten mile stage spent over 100mph is very small.

Depending on exactly how you do the sums , the above represents a change of about 7.5% ... we have experience of changing diff ratios in a previous rally car from 4.4 to 4.7 .... or 6.8% , and the difference in performance was VERY noticeable !

food for thought ..............

James Bailey

Not wanting to go Kibort on you but.....in the example above the 1.45 would be accelerating faster from 60-65mph,85-91mph,111-121mph as it is in the lower gears with higher torque multipication of the engine runnng at near full HP. The lack of alternative individual gear ratios means you end up with the same RPM drops between gears 1-4 so all that changes is the timing of when you shift and what gear you are in at a specific MPH.
I totally agree that it FEELS quicker and is in first gear. The other gears not much but you have to up shift sooner which seems faster

Cortina

I can see we are on different wavelengths .. If I follow your argument (logic) , then are you saying that lowering the ratio even further would give even better acceleration .... ??

we can't both be correct !

simple question - Is the 0 to 100 time (on the stopwatch) quicker or slower with the higher ratio ?

How could it be faster in first , but not in 2nd etc ??

PS I have no idea what a Kibort is

Mike Simard

You soon will.....


Jim has valuable advise. The 4-5 gears would be ideal to use if you are going speeds to take advantage of 5th.
It's an efficient direct drive in 5th and the 4-5 shifter movement is simple.
It would be ashame to change the layshaft ratio and mess that 4-5 up.

IMO the early boxes and GTs with the 2.7x final have perfect ratios and cannot be improved.

While you're at I'd look at the internal shift rails. I used an early box that was prone to getting stuck from 2-3 when shifted enthusiastically. The internal linkage was allowing both gears to be 'chosen' at the same time under certain circumstances.

It might be helpful to spend time contemplating the linkage while you have a couple of them apart.

Cortina

Let me start again ....... this is for a competition car ........ it never sees 5th gear (try watching some of my Youtube in car clips) in competition

4th to 5th change is irrelevant to me ... I need to get to approx 100mph as quickly as possible.

I'll keep quiet now.

James Bailey

0 to 100 means the time you spend in first gear is important however when you start talking about time spent from 40 mph to 100 MPH it is far different. Kibort( the plague of rennlist when it comes to torque horsepower and acceleration) can go on forever about horsepower seconds and tradeoffs.
If you could get close ratio gears for 1-4 and a high ratio ring and pinion like the ultra rare after market 3:09 you can have a trans where the RPM drop between gears is smaller so the higher horsepower combined with lower gears makes more torque overall. And I have watched your videos and very much enjoyed them.