Higher 5th gear

[Dyaque]

I'm sure this has been covered but my search skills are failing me again...

I'm fairly convinced that my sticky 5th gear is due to wear on the cog (after 180something000 miles).  

Now pondering the replacement and wondering: has anyone here gone to a higher-ratio 5th gear cog to reduce the revs at motorway cruising speeds?  

If so, which ratio did you use and what were the resulting revs at 70mph / 80mph, etc?

[Ian Comerford]

I’ve seen an article on Pelican about this quite recently, can’t recall the heading but have a search if you can

This might help 

https://forums.pelicanparts.com/porsche-911-technical-forum/1178016-5th-gear-highway-cruise-gas-mileage.html

[Northy]

I have a longer fifth in my SC.  It’s from a mid year 3.2 that had the 915.  I can’t remember exactly, but it drops the revs about 500-750 revs at 70mph, something like that.  Worth doing whilst it’s apart.  I got the parts from Tuthills when the sage of the lakes rebuilt my box 

[mean in green]

I guess the starting point would be identifying what you currently have, but be sure to do your research as even modest gearing changes can have significant effects, all according to your car's particular variables e.g. wheel & tyre sizes, final drive, power and so on. 

[Mike911scrs]

The tallest' Porsche gear is from a 3.2 Carrera at 29:38 or ZD/T gear set,

your box being a 1983  915/62 box, should have a 28:22 SM gear set in there,, with a 8:31 CWP, the only other thing is Tyre/wheel size?? 

taller gears are available from GT in the states, and will need some mods to fit them, + there may be some noise/whine

to CORRECTLY change 5th gear, the box will need stripping and cleaning and re-assembling correctly with new pinion nut correctly Torqued up.

or you could just do it on the cheep, by taking the end cover off, gunning of the pinion nut, remove the input-shaft nut, change the gear set and put it back together with a tube of Silicon 😟

regards  mike

[Mike911scrs]

what is STICKY with the 5th gear??,

has the box been in bits??

if it has it sounds like the selectors are incorrectly set or possibly (most likely not) the syncro band/dog teeth are worn out, along with the 5th/Rev slider

regards  mike

[Dyaque]

Many thanks, all.  Pelican has been helpful but the posts I have found (including yours, Ian) have referred to earlier cars.

Mike, this is extremely helpful: thank you.  Sadly, I am already underwater on my rustwork (multiples of originally hoped-for budget) so this will have to be a cheap job despite my strong inclination to do it properly. 

"Sticky" means that, following smooth engagement, it is increasingly difficult to disengage 5th depending upon how hot the 'box is.  When on track or pushing on through the twisties, the force to pull the gearlever back feels like something is going to break.

The drive ratio info is an excellent guide for my research, though.  For completeness, I'm on 17 inch rims running 205/55 and 225/45 tyres (from memory). 

Design911 offer a number of ratios, and 2 options for each, being their in-house "OE Match" (which they say is the same spec as original including metal grade - goodness knows how to test this) or "upgraded" by NPR.  Any thoughts on these options? 

[Ian Comerford]

It’s not an answer to your last question but have you considered trying a different oil?  What is in there at the moment and when was it last changed?

[Type911]

Swepco is good..............................................

[PeterK]

Ooh, you little tease, you

[Dyaque]

Slow progress, but progress nonetheless.

I've been pointed to a couple of very helpful websites for gear info here:

https://www.cogscogs.com/Gear-Ratio-Charts-and-Graphs_ep_10.html

and here:

https://retro-sport.com/porsche-gears/porsche-915-gears/porsche-915-5th-gears

The latter has a wonderful interactive chart showing the expected revs v mph for the relevant selection of gears.  Using the data from the cogscogs website for "standard", you can play around with different options.

I'm still undecided, but am mulling over dropping the revs at 80mph from my current ~3,400 (final drive has been confirmed at 28:22) to just above the 3,000 mark.  One of my advisers is worried that the car will have no torque to accelerate from there, and the dyno chart from when it was done suggests there is a bit a flat spot in the torque curve there. 

I read the x-axis as being 300 rev increments and the number relating to the mark to its top right (see the 400).  If this is correct then I will be dropping the revs from the point that torque picks up again (3,400) back to where it flattens out (3,000-3,100).

Of course, longer ratios are available to drop the revs further at motorway speeds, and if I understand this chart correctly then maybe dropping down to ~2,100 at 70mph could work.  But that seems extreme and probably too much, given the car refuses to go above an indicated 125mph even on the long Nurburgring straight (which might be mildly uphill) despite the revs having plenty to go and the gearing chart saying it should.

Very curious to know what you all think of this.  Am I reading the chart correctly?  Is dropping to the start of the "flat" section a valid concern (I don't remember this being noticable when driving)?  Or is the concern negligible in the interests of more relaxed revs on the motorway passage (and you can always drop to 4th you need grunt)? 

This has turned into a much longer note than expected. 

If you're still reading, what do you think?

Edited February 5 by Dyaque
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[Ian Comerford]

It doesn’t mention torque but there’s a new Pelican discussion on the subject.  Maybe try contacting someone with the higher 5th, Petekz is always helpful

https://forums.pelicanparts.com/porsche-911-technical-forum/1188957-915-question.html

[Dyaque]

Great spot, Ian.  Thank you.  I have messaged PeterKz as he seems to have done exactly what I have in mind.