mark kibort

where is this S4 5th gear located?

Cheburator

I think one of Erkka's magician friends managed to repair a broken one by welding the piece, but I am not a 100% certain it worked. I hope he chimes in. I think you would be amazed about something else - a while ago I managed to source a fresh factory rebuild GTS 5-spd box with about EUR7k spend on it on parts alone. Well, the diff preload was set-up incorrectly... 'Nuff said... We tweaked it and the whine disappeared...

mark kibort

ive broken them before...common failure i believe, BUT, i dont think there is anyway to weld or fix them. maybe if you took the entire gear set out and took it to someone that can weld shafts together, then maybe but i have no idea if that kind of welding could be possible . are cranks ever welded when they break off the rear nose?

i wonder if this is possible

if 5th is 1:1, how does it change the layshaft ratio, or is it because the layshaft gear is different , the 5th gear has to be different. i thought the ring and pinion was the main diff, no? (i dont have a clue other than the specs i see in the little book. )

doesnt sound like something that could handle 300 to 400ftlbs of torque after a weld job. i have much less of a clue than you do, but it sounds iff'y

Ducman82

an experienced welder would be needed, and some serious heat treating would need to done. Carbon would creep in to the weld zone and make it peanut brittle if not done correctly.

GregBBRD

At this point in my life, whenever someone says one of these transmissions has had work done in the differential section (like having a new limited slip installed), all it does is frighten me.

Very few people have the knowledge or the tools need to do this job.

I've seen just about everything....from people assuming that all the differentials are exactly the same dimensionally (and just throwing one in without checking the preload and backlash) to someone installing an complete differential with the ring gear attached from an entirely different transmission.

Frightening!

James Bailey

The gears you see on 5 th gear are the layshaft drive gears...there are different ratios of layshaft gears used with the different ring and pinions....When in 5th gear power goes straight to the ring and pinion but 1-4 it transfers over to the layshaft then gets back to the pinion shaft via those 1-4 gear sets.
So actual gear ratios combine the lay shaft ratio, each gear ratio, and the final drive ratio.......the USA 2:20 5th gear layshaft ratio effectively makes a 2:75 ring and pinion act like a 3:09 ring and pinion in gears 1-4 .....
We have discussed this at length before......simply search layshaft ratios 5th gear.
Also why 5 th is more efficient since the layshaft is under no load, power is not forced to jump over to the layshaft then jump back to the pinion shaft. Anytime you change directions you suffer a loss of power mostly into heat.

mark kibort

Thanks.. i know lots of information on a search, but wanted the cliff notes..... that was good. i think.

I know that the layshaft ratios are different with most all the models, and understand that if you put a 2.75 on a 2.2 box, all the gears are reduced by that amount and get the 3.09 equifilant reduction of those gears (of course the 5th will still be 2.75 in the change) but my question was on the 5th gear set. if they are different but all 1:1, how does it change the other ratios (i.e. using a GT5th gear on a S4 box (2.2) and still using the 2.2 rear end)

funny story about my failure as i shifted into 3rd at thundrehill way back when, and ended up with no drive. spun the 5th gear set and nothing else was seemingly damaged. i didnt think about the 5th gear NOT using the layshaft, and being direct drive. i could have driven my car home in 5th gear, instead of panicing about using a tow truck.. fortunately , the Petty's helped me with their trailer and brought the beast home for me. However, i might have run all those metal bits into the gears if i did, so it all ended well.

SO, does the holbert transmission neeed only a 5th gear set? i think so. and the question is again, can i use a GT 5th gear without changing any of the gear ratios?

James Bailey

the layshaft gear is part of the 5 th "gear" set so you can use any s-4 or GT 5 th for it BUT it moves 1-4 effective ratios around.....but as you always maintain it makes little difference for You ! So answer is unless it had a GT 5th gear made for use with the 2:75 before it will change the gearing..... for 1-4

mark kibort

so, for example, how would a GT5th gear change the layshaft ratios (i assume it would change all of them) .
and yes, "little" difference is a correct term,because of the trade offs over multple speed ranges. what works at laguna, might not work at sears point as optimallly and as you have witnessed first hand, anderson didnt better his time going from the 2.2 to the 2.72 GTS box. why is that? uh oh... man,..... i just cant help myself.. NO i dont want a gearing argument. Peace brother!!
so, tell me the effects of that 5th gear gt change? maybe its very small, no?
thx

Rob Edwards

All 1st thru 4th gears are identical worldwide, 87-95.

G28.12 and G28.55 5th gear has 22 teeth on the mainshaft gear, and 32 teeth on the layshaft gear. So the layshaft gear is turning 68.7% as fast as the input shaft

G28.13 is 21 teeth main, 33 teeth layshaft. So the layshaft speed is 64% of the input shaft.

G28.57 is 24 main, 35 layshaft. Layshaft speed is 68.6% of the input shaft.

mark kibort

so contrary to what jim was saying, using a GT with the 1.45:1 layshaft ratio vs 1.57:1 layshaft ratio of the S4, you might end up with a S4 with 8% taller gears overall by using the GT 5th gear which determines the Layshaft ratio?

James Bailey

Yes the GT layshaft ratio moves the 1-4 gears higher but the more common use of the 2:72 ring and pinion with the 2:20 layshafts lowers the 1-4 gears . Porsche used the layshaft ratios to try to keep the actual 1-4 gears total gearing pretty close regardless of the ring and pinion ratios.

mark kibort

yep, the ole trade off. most of the gears , as you say, are not that different (10-12% or so).... but I certainly, with how I use the car, wouldn't want to go any taller. however, at laguna I could then use 1st out of the turn 11 and get to redline before the hairpin...

Vilhuer

That was maybe 15 years ago!!! Pictures should still be up in here somewhere. It still works but early '78 where that '85 gearbox is in has seen only very little and light use in recent years.

GTS 5th gear insides and pinion shaft front end are different than any earlier box. I think GTS 5th can be paired with '87-91 pinion shaft if suitable sized roller bearings can be found to take take up larger space in between shafts. In any case it creates previously mentioned changes due to different 5th+layshaft gear ratio pair.

Porker928

Hi Greg
I'm a bit confused with what you said above? According to my edition of PET the two roll pins that secure the 1st and reverse shift fork (928 303 115 41) are referenced (N 013 317 1).

900 309 013 00 is also a roll pin but it's shown securing the preselector lever (A 928 303 032 06) and there's only one off them. Are you saying that these should be replaced as well? Please explain.

Also - What happens when the shift location spring (928 303 034 09) fails? What is the future for this part? Are Porsche going to start making them agian or will someone else step in. Would it be easy to get one manufactured?

Many thanks.

Ben