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Beating The Synchro's? 5th Gear

SS8_Gohan
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SS8_Gohan Grand Master

SS8_Gohan

Posted
Posted

Hi all

i noticed something strange with my 5th gear changes at Sandown a month or so ago, all gear shifts would be fine, but when it came to 5th, it would make this crunching sorta noise. It would still shift into gear, but it would make a 'not so nice' noise on the way through. I could be quite forceful/quick with the changes of the other gears, but really had to watch 5th and shift quite slowly.

I then saw this on youtube the other day

http://youtube.com/watch?v=aIMHbMXi178

997 911 GT2 vs Z06, driven by Chris Harris from Autocar (he does great reviews IMO) and the noise you hear from when he changes from 1st to 2nd at 5:02 (time of the vid) is the same noise i was getting from 4th to 5th at sandown when i was going down the straights.

Chris mentions that the noise is "beating the synchro's", what exactly is this and is it doing a lot of damage to the gearbox when it happens?

Gearbox currently has Redline MT-90 in it, but was thinking of switching to Redline Lightweight Shockproof, would this make a difference?

Was also thinking of get a C's short-shifter, but would this just make the problem worse?

cheers,

daniel

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NYTSKY Grand Master

NYTSKY

Posted
Posted

Mine does that up into 5 and back into 4.I put it down to the syncros.No idea how to get rid of it.Changed oil 3 times.

A friend of mine bought a new box from JJ and that did it in a gear aswell.Cant remember which.I think its the nature of the beast.

If you change hard and fast from a high rpm the syncros cant catch up and thats where you get the crunch sound.

Id like to have a go against the GT2 though,see how close we are :P

Yawn Mentor

Yawn

Posted
Posted (edited)

I get it from 3rd to 4th and I have a C's short shifter too. I reckon its just a problem of being over-enthusiastic on my part. In the end a standard gearbox taking 350rwkw fairly reliably is still a feat in itself, so I just have to remember that the time I could save by shifting like its the end of the world is probably in the tenths of a second per lap. I dont know if this is common, but I assume it is, especially mine being a 19 year old box.

Its just a problem with the synchros not having enough time to basically synchronise itself with the next gear, leading to the crunching noise.

Edited by Yawn
jantolis Proficient

jantolis

Posted
Posted

I get that sound shifting into 4th and i thought it was that my synchro is rooted. It still changes perfectly but just makes a noise. If i change properly without a clutch it doesn't make any bad noise and i can shift it in quickly.

jeffboyracer Community Regular

jeffboyracer

Posted
Posted
Hi all

i noticed something strange with my 5th gear changes at Sandown a month or so ago, all gear shifts would be fine, but when it came to 5th, it would make this crunching sorta noise. It would still shift into gear, but it would make a 'not so nice' noise on the way through. I could be quite forceful/quick with the changes of the other gears, but really had to watch 5th and shift quite slowly.

I then saw this on youtube the other day

http://youtube.com/watch?v=aIMHbMXi178

997 911 GT2 vs Z06, driven by Chris Harris from Autocar (he does great reviews IMO) and the noise you hear from when he changes from 1st to 2nd at 5:02 (time of the vid) is the same noise i was getting from 4th to 5th at sandown when i was going down the straights.

Chris mentions that the noise is "beating the synchro's", what exactly is this and is it doing a lot of damage to the gearbox when it happens?

Gearbox currently has Redline MT-90 in it, but was thinking of switching to Redline Lightweight Shockproof, would this make a difference?

Was also thinking of get a C's short-shifter, but would this just make the problem worse?

cheers,

daniel

i have exactly the same thing happen to me to....at high speed going from 4th to 5th get a crunch noise

mine is complete stock bar pod and exhaust, had gear box oil changed too, anybody now for sure what is happening here?

Duncan Grand Master

Duncan

Posted
Posted

to get back to the start. for a given speed on the road, every gear has a different number of revs. so when you change gears you must get the engine to the right revs.

the synchros are soft brass rings that help get the gear spinning at the right revs, but being soft they do wear over time. Any time you try and put it in gear at the wrong revs the synchros are being worn. If you get the revs exactly right it will slip straight in.

synchros take time to work, the more difference in revs the more time it needs. You normally notice this with a quick up change in gears.

4th and 5th crunch most in these boxes because they have less synchro rings than 1-3.

So....if they are crunching they are worn and need to be replaced. In the meantime try redline oil in a worn gearbox it helps reduce the crunching or changing gear more slowly. But the only real solution is tear down the box and put new synchros in

some_cs_student Veteran

some_cs_student

Posted
Posted
Or if you don't want to do that just drive the car properly and change at the right revs.

You know he's talking about the track right?

And what do you mean change at the right revs?

I'm sure he doesn't have an issue changing at 4000 RPM, but its a track... :w00t:

godzl1975 Experienced

godzl1975

Posted
Posted
Never get it and i bascially had water for oil in the diff.. Dont know what your on about and ive double clutched 4-5th heaps of times.. a few times i poped a flat change

R34GTT box isnt as old as in most cases stated here R32 GTR ones

mine did that from 3rd to 4th and changed to redline shockproof and that helped.

jantolis Proficient

jantolis

Posted
Posted
You know he's talking about the track right?

And what do you mean change at the right revs?

I'm sure he doesn't have an issue changing at 4000 RPM, but its a track... :P

I would have thought that if you don't use the clutch and change at the right revs(track or street) it wouldn't make that crunch sound because it's going straight in.

when i had no clutch for a couple weeks my box never made that sound getting into 4th because i would have to change when the revs were matched and hence no grind or crunch.

jantolis Proficient

jantolis

Posted
Posted

not sure exactly most cars but this is how you can find out. whatever speed your doing look at the revs. then change to the next gear normally(with clutch so you can see0 and see the revs. next time you change from that speed to the next gear let the revs go down to what the next gear was at and whack it in.

Took me a couple days to get used to using all 5 gears without clutch.

for example i drive first gear to 3k and the get out of gear an put it in when it's at 2k if i remember correctly.

SS8_Gohan Grand Master

SS8_Gohan

Posted
  • Author
Posted

wow, big response, ok

as Gareth said, it was track related, not road related.

It was happening when i was going down the straights, shifting from 4th to 5th at say... 140'ish?? not sure exactly. That's the ONLY time i've ever noticed it happen in my car, at high speeds like that.

so...

1) The noise is indicating the synchro's are being damaged each times it happens?

2) Yes, how exactly do i "rev match"

3) Should i perhaps give Redline Lightweight Shockproof a go?

4) Would a short shifter exacerbate the problem?

Should i just shift slower and more carefully went at the track, moving into 5th?

cheers and thanks for all the replies guys

daniel

Duncan Grand Master

Duncan

Posted
Posted

working backwards

no it will make it worse

absolutely

when changing up - the only thing you can do is change more slowly. when changing down you blip the throttle to help "heel/toeing"

yes. buts its too late now so dont worry about that

seriously, all you have to do is put in redline and shift slightly slower. the only real fix is rebuilding the gearbox

scheris Collaborator

scheris

Posted
Posted

guys start sim ple and go fro there....

check for oil leeks.... seriously... if you start to get crap loads of heat and 4th starts aswell and 4tgh only gets worse...bingo.....

only at high revs..... for 5th.... lacking oil or syncros....

oil is cheaper and gets worse quicker...but not overly quick...depending on leek.

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