Taller 5th Gear - Is It Time?
Discussion
Dear all,
Due to the rather unfortunate recent failure of my 5th gear synchro that I believe was the direct result of running the nasty dragging Helix clutch for almost 20,000 miles, I am now involved in another round of gearbox work
As this work will involve replacing my now worn 5th gear synchro hub and ring which I now realise I should have replaced when the box was apart last, I find myself returning to the idea of a taller overdrive.
My calculations show with my 3.45 diff if I switch from my current 0.80 5th gear set to the taller 0.72 set I will knock a useful 350rpm off my engine speed at 80mph which is the speed I generally like to cruise at.
Engine speed would fall from it's current 3,000rpm at 80mph to a more relaxed 2,650rpm which also seems to be the engine's fuel economy sweet spot on gas. If I cruise at 2,650rpm with my current 0.80 5th my road speed is around 70mph, which feels painfully slow TBH
At lower speeds with the 0.72 gear set and the engine turning at 1600rpm my road speed will be bang on 50mph in 5th, given 'Ol Gasbag' rumbles about quite happily at 1500rpm and pulls cleanly from there I'm now starting to think the taller overdrive idea could work.
When I last looked at this I was concerned about too bigger drop in engine speed from 4th to 5th, I didn't want to fall in a hole or lose the car's current 5th gear flexibility. But it now seems this may not be as much of an issue as I'd first feared, for example:
With the taller overdrive changing up from 4th to 5th at a healthy 3,500rpm would see a drop of 980rpm, the engine is now spinning at 2,520rpm and the car obviously remains at 75mph ready to slide easily up to my 80mph cruise speed. If I do exactly the same with my current 5th gear ratio I go from 75mph at 3,500rpm, this time only dropping 700rpm to 2,800rpm but she is starting to come on cam so will easily surge on to 120 pretty rapidly which is nice
Will I miss this surge, will the car feel a bit flat, or is what amounts to being just 280 revs behind the game be more or less unnoticeable in the real world
The way I see it is the real challenge will come with those early lazy up changes from 4th into 5th at say 2,500rpm (54mph), with such lazy shifts into the new taller 5th the engine will now be turning at lowly 1,800rpm, very much like if I change from 4th to 5th at 40mph with the current 0.80 overdrive ratio. Now I do drive the car like this from time to time and it'll certainly take such treatment in its stride, but the big question is will pulling out of 40mph at 1,800rpm feel too sluggish and start annoy me?
Perhaps the answer is just to re-calibrate my 4th to 5th up shifts by executing them at engine speeds 500rpm higher than I do now?
Anyway sorry for all the blah blah blah, I'm really just thinking aloud and trying to play Devil's Advocate on myself to test the taller 5th gear idea. While I thrash it all out in my own mind what I can say with complete confidence is the car will obviously remain exactly the same in the first four gears, it will accelerate just as hard and be just as flexible. And because the back of the box is all apart once more I will never have a better opportunity to fit the taller 5th gear set!
So should I turn this latest frustration to my advantage for not too much additional outlay.... or should I leave it exactly as it is?
Let me have your thoughts chaps
Due to the rather unfortunate recent failure of my 5th gear synchro that I believe was the direct result of running the nasty dragging Helix clutch for almost 20,000 miles, I am now involved in another round of gearbox work
As this work will involve replacing my now worn 5th gear synchro hub and ring which I now realise I should have replaced when the box was apart last, I find myself returning to the idea of a taller overdrive.
My calculations show with my 3.45 diff if I switch from my current 0.80 5th gear set to the taller 0.72 set I will knock a useful 350rpm off my engine speed at 80mph which is the speed I generally like to cruise at.
Engine speed would fall from it's current 3,000rpm at 80mph to a more relaxed 2,650rpm which also seems to be the engine's fuel economy sweet spot on gas. If I cruise at 2,650rpm with my current 0.80 5th my road speed is around 70mph, which feels painfully slow TBH
At lower speeds with the 0.72 gear set and the engine turning at 1600rpm my road speed will be bang on 50mph in 5th, given 'Ol Gasbag' rumbles about quite happily at 1500rpm and pulls cleanly from there I'm now starting to think the taller overdrive idea could work.
When I last looked at this I was concerned about too bigger drop in engine speed from 4th to 5th, I didn't want to fall in a hole or lose the car's current 5th gear flexibility. But it now seems this may not be as much of an issue as I'd first feared, for example:
With the taller overdrive changing up from 4th to 5th at a healthy 3,500rpm would see a drop of 980rpm, the engine is now spinning at 2,520rpm and the car obviously remains at 75mph ready to slide easily up to my 80mph cruise speed. If I do exactly the same with my current 5th gear ratio I go from 75mph at 3,500rpm, this time only dropping 700rpm to 2,800rpm but she is starting to come on cam so will easily surge on to 120 pretty rapidly which is nice
Will I miss this surge, will the car feel a bit flat, or is what amounts to being just 280 revs behind the game be more or less unnoticeable in the real world
The way I see it is the real challenge will come with those early lazy up changes from 4th into 5th at say 2,500rpm (54mph), with such lazy shifts into the new taller 5th the engine will now be turning at lowly 1,800rpm, very much like if I change from 4th to 5th at 40mph with the current 0.80 overdrive ratio. Now I do drive the car like this from time to time and it'll certainly take such treatment in its stride, but the big question is will pulling out of 40mph at 1,800rpm feel too sluggish and start annoy me?
Perhaps the answer is just to re-calibrate my 4th to 5th up shifts by executing them at engine speeds 500rpm higher than I do now?
Anyway sorry for all the blah blah blah, I'm really just thinking aloud and trying to play Devil's Advocate on myself to test the taller 5th gear idea. While I thrash it all out in my own mind what I can say with complete confidence is the car will obviously remain exactly the same in the first four gears, it will accelerate just as hard and be just as flexible. And because the back of the box is all apart once more I will never have a better opportunity to fit the taller 5th gear set!
So should I turn this latest frustration to my advantage for not too much additional outlay.... or should I leave it exactly as it is?
Let me have your thoughts chaps
Seems like a plan to me.
It'll only be the lazy driving that it might annoy,,, having to raise the revs higher in 4th before changing up to 5th to cruise without labouring the engine slightly,,
My car is roughly doing 70mph at 2400 revs in 5th and
80mph at 2800 revs which I think is still slightly to high
How much more work would it be to change 4th gearing so to take out some of the difference,
Would that add cost considerably,,
I always consider the car to be like a 250 cc motor bike,,, low geared!
Even first and second could go up a tad considering the power we have,,
It'll only be the lazy driving that it might annoy,,, having to raise the revs higher in 4th before changing up to 5th to cruise without labouring the engine slightly,,
My car is roughly doing 70mph at 2400 revs in 5th and
80mph at 2800 revs which I think is still slightly to high
How much more work would it be to change 4th gearing so to take out some of the difference,
Would that add cost considerably,,
I always consider the car to be like a 250 cc motor bike,,, low geared!
Even first and second could go up a tad considering the power we have,,
ClassiChimi said:
Seems like a plan to me.
It'll only be the lazy driving that it might annoy,,, having to raise the revs higher in 4th before changing up to 5th to cruise without labouring the engine slightly,,
My car is roughly doing 70mph at 2400 revs in 5th and
80mph at 2800 revs which I think is still slightly to high
How much more work would it be to change 4th gearing so to take out some of the difference,
Would that add cost considerably,,
I always consider the car to be like a 250 cc motor bike,,, low geared!
Even first and second could go up a tad considering the power we have,,
Alun, it sounds to me like you need the 3.08 diff, Daz has just fitted one and claims it's working well for him in his rather more powerful than 'Ol Gasbag' Chimaera. Changing the diff ratio obviously changes all five ratios, it's also an expensive exercise so it may not be the the best choice for my pocket right now and I'm not 100% convinced 'Ol Gasbag' will find it compatible with her 250 torques anyway?It'll only be the lazy driving that it might annoy,,, having to raise the revs higher in 4th before changing up to 5th to cruise without labouring the engine slightly,,
My car is roughly doing 70mph at 2400 revs in 5th and
80mph at 2800 revs which I think is still slightly to high
How much more work would it be to change 4th gearing so to take out some of the difference,
Would that add cost considerably,,
I always consider the car to be like a 250 cc motor bike,,, low geared!
Even first and second could go up a tad considering the power we have,,
Just swapping to a taller 5th on the other hand only really increases the step from 4th to 5th and with what's going on with my box right now it could prove a cost effective way to knock a few rpm of my 80mph cruising gate without spoiling the car's flexibility?
It will also have zero impact on the car's current rate of acceleration in its first four gears which is something I'd hate to blunt!
The only thing that really needs careful consideration is how the car will feel with the new wider step up between the 4th gear ratio that stays and the new 0.72 taller 5th?
Spacing wise the current 4th to 5th step is actually closer than on all the other gears, the gap between 4th and the new 0.72 5th will actually end up the same percentage step up as 2nd to 3rd & 3rd to 4th are right now and they all work great. My guess is the narrower gap between 4th to 5th was chosen for a reason which is why I'm being cautious, perhaps you need that closer gearing due to the greater wind resistance encountered at the higher speeds at which you commonly upshift into 5th?
Has anyone actually fitted a 0.72 overdrive gear set to their TVR Chimaera, and if so I'd be interested to hear how it panned out for you?
I don't know how you drive, Dave, but on the road I tend to use 5th only as a cruising/motorway gear, an "overdrive".
If out for a spirited Sunday B/A road drive, then I am in 2nd to 4th all the time, making the best of the lively engine.
On a track day I only use 5th if 4th has completely run out of legs, ie over 125 mph, and generally I am changing down again within a few seconds.
So while I admire your detailed analysis before proceeding (I am usually a measure once, cut twice kinda guy myself), I think your conclusion that it will be just fine is spot on. If you are in 5th on an A road in traffic at 48 mph (the favourite speed of the queue-leading driver on a single carriageway national speed limit road, with 25 irritated drivers stuck behind) the car will still cruise just fine, saving fuel. And if you spot a 26 car length gap in the 48 mph hordes coming the other way you would drop down to third or fourth for the glorious overtake anyway, regardless of the gearing of fifth. I really cannot see when it would be a problem, as you have so much torque, even in a tiny 4 litre with so much rear ballast (the car, not you.
). Shirley your car must run quite happily at 30-40 in 5th if you asked it?
If out for a spirited Sunday B/A road drive, then I am in 2nd to 4th all the time, making the best of the lively engine.
On a track day I only use 5th if 4th has completely run out of legs, ie over 125 mph, and generally I am changing down again within a few seconds.
So while I admire your detailed analysis before proceeding (I am usually a measure once, cut twice kinda guy myself), I think your conclusion that it will be just fine is spot on. If you are in 5th on an A road in traffic at 48 mph (the favourite speed of the queue-leading driver on a single carriageway national speed limit road, with 25 irritated drivers stuck behind) the car will still cruise just fine, saving fuel. And if you spot a 26 car length gap in the 48 mph hordes coming the other way you would drop down to third or fourth for the glorious overtake anyway, regardless of the gearing of fifth. I really cannot see when it would be a problem, as you have so much torque, even in a tiny 4 litre with so much rear ballast (the car, not you.
). Shirley your car must run quite happily at 30-40 in 5th if you asked it?QBee said:
I don't know how you drive, Dave, but on the road I tend to use 5th only as a cruising/motorway gear, an "overdrive".
If out for a spirited Sunday B/A road drive, then I am in 2nd to 4th all the time, making the best of the lively engine.
On a track day I only use 5th if 4th has completely run out of legs, ie over 125 mph, and generally I am changing down again within a few seconds.
So while I admire your detailed analysis before proceeding (I am usually a measure once, cut twice kinda guy myself), I think your conclusion that it will be just fine is spot on. If you are in 5th on an A road in traffic at 48 mph (the favourite speed of the queue-leading driver on a single carriageway national speed limit road, with 25 irritated drivers stuck behind) the car will still cruise just fine, saving fuel. And if you spot a 26 car length gap in the 48 mph hordes coming the other way you would drop down to third or fourth for the glorious overtake anyway, regardless of the gearing of fifth. I really cannot see when it would be a problem, as you have so much torque, even in a tiny 4 litre with so much rear ballast (the car, not you.). Shirley your car must run quite happily at 30-40 in 5th if you asked it?
I agree, so that's three that say "do it" and no one saying don't.If out for a spirited Sunday B/A road drive, then I am in 2nd to 4th all the time, making the best of the lively engine.
On a track day I only use 5th if 4th has completely run out of legs, ie over 125 mph, and generally I am changing down again within a few seconds.
So while I admire your detailed analysis before proceeding (I am usually a measure once, cut twice kinda guy myself), I think your conclusion that it will be just fine is spot on. If you are in 5th on an A road in traffic at 48 mph (the favourite speed of the queue-leading driver on a single carriageway national speed limit road, with 25 irritated drivers stuck behind) the car will still cruise just fine, saving fuel. And if you spot a 26 car length gap in the 48 mph hordes coming the other way you would drop down to third or fourth for the glorious overtake anyway, regardless of the gearing of fifth. I really cannot see when it would be a problem, as you have so much torque, even in a tiny 4 litre with so much rear ballast (the car, not you.
). Shirley your car must run quite happily at 30-40 in 5th if you asked it?Like you say most if not all of my spirited driving happens to the complete exclusion of 5th, it's really just a motorway fast clear A road & touring gear.
Typically I up-shift into 5th no lower than 2,000rpm and that's in lazy mode, currently 2,000rpm equates to 50mph in 5th but when in super lazy mode I do sometimes choose modulate my speed between 40-50mph without bothering to drop to 4th which equates to 1,500 to 2,000rpm. Modulating road speed between 40-50mph with the new taller 0.72 gear set drops the engine speed picture to between 1,350 to 1,700rpm which 'Ol Gasbag' will tolerate no problem but it is in the part of the fuel map we made a bit richer to counter shunting so the truth is for economy it would be better to drop to fourth even though the car can be drive like this in 5th no problem at all.
What I've just discovered is the if I compare fitting the expensive 3.08 diff and installing the taller 0.72 overdrive but keeping my 3.45 diff, the results are more or less exactly the same in 5th gear. So fitting the 0.72 gear set gives me all the cruising economy advantages of the 3.08 while retaining my current brisk acceleration in the lower four gears.
All the advantages where you need them, and none of the blunted acceleration disadvantages where you don't, all this for a lot less money than a 3.08 diff. With the tail housing off and the 5th gear synchro hub already removed this one is starting to look like a no-brainer
Damn it, I'm selling myself yet another modification here aren't I
It's a good call Dave - the noise at motorway cruising speeds has long annoyed me. Lower revs would be great.
I will be doing just what you suggest next time my gearbox is out.
My Saab Aero manual does 25000 motorway miles a year, similar gearing/revs to the TVR, and I always drop down 5th to 4th to overtake when the trucks get out of the BMW lane at last.
By the way, I was told on a (cough) speed awareness course that you shouldn't drive everywhere in the highest gear possible, labouring the engine. The best mpg is obtained by being in the correct gear for the speed so that you don't labour the engine. I have proved this by driving so as to keep my Saab's turbo gauge at as low a boost as possible, and this means being in lower gears than I am used to. The mpg read out is an eye-opener.
I will be doing just what you suggest next time my gearbox is out.
My Saab Aero manual does 25000 motorway miles a year, similar gearing/revs to the TVR, and I always drop down 5th to 4th to overtake when the trucks get out of the BMW lane at last.
By the way, I was told on a (cough) speed awareness course that you shouldn't drive everywhere in the highest gear possible, labouring the engine. The best mpg is obtained by being in the correct gear for the speed so that you don't labour the engine. I have proved this by driving so as to keep my Saab's turbo gauge at as low a boost as possible, and this means being in lower gears than I am used to. The mpg read out is an eye-opener.
When I was building the gearbox for my car I specified the 6th gear ratio to replace the 5th. So I have done this.
My logic was that the car was to be used for cruising to Italy at the time and it was before the more stringent French speeding laws so I wanted the car to not be cruising for hours at 4000 odd rpm.
Since then I have sold the place in Italy and noticed that UK motorway speeds are a lot slower than they used to be. I have slightly taller gearing anyway due to larger wheels and probably less torque at low RPMs than you but it means that at 70 I am teetering on the brink of needing to change down and in A roads I am usually in 4th. I suspect that if I had the slightly smaller wheels of a normal Tiv this wouldn't be the slight inconvenience that it is and on the whole the car is much quieter when cruising.
Would I do the same again? I'm not entirely sure but I think that I probably wouldn't but at the same time it is very borderline and I think it is only the bigger wheels and the lower torque of the speed6 at low rpm that make it a 50:50 call. With the torque of a V8 at lower rpm and the slightly smaller wheels then I would still be 100% happy with the decision.
My logic was that the car was to be used for cruising to Italy at the time and it was before the more stringent French speeding laws so I wanted the car to not be cruising for hours at 4000 odd rpm.
Since then I have sold the place in Italy and noticed that UK motorway speeds are a lot slower than they used to be. I have slightly taller gearing anyway due to larger wheels and probably less torque at low RPMs than you but it means that at 70 I am teetering on the brink of needing to change down and in A roads I am usually in 4th. I suspect that if I had the slightly smaller wheels of a normal Tiv this wouldn't be the slight inconvenience that it is and on the whole the car is much quieter when cruising.
Would I do the same again? I'm not entirely sure but I think that I probably wouldn't but at the same time it is very borderline and I think it is only the bigger wheels and the lower torque of the speed6 at low rpm that make it a 50:50 call. With the torque of a V8 at lower rpm and the slightly smaller wheels then I would still be 100% happy with the decision.
I've just been looking at your number crunching Dave,,
According to my Rev counter and speed tests
I'm doing 1600 revs at 50 mph in 5th so similar to what you'd achieve with the higher 5th gear,,
I'm my view the engines sucking plenty of air in at 1600 revs so it responds well and isn't an issue at all.
I have 17 in rims and 45 profile tyres so the gearing is probably out slightly but I think this is why I get good fuel figures,
The 308 is out of my price bkt as is any other mods right now but if mines higher geared than yours what freaking dif have I actually got?
So if I put in the higher 5 th gear and using your calc id be doing 2050 revs at 70 mph and 2400 at 80 mph,,, give or take a hundred revs ,, that sounds toasty to me.
The more you think about it the more you think our light cars will benefit from this mod,,,
Unless I want to boot it the engine power is always way more than you need in normal driving conditions so I can't see it being anything but beneficial.
Dave your engines purrs, it's more than powerful enough I'd imagine.
Can you estimate what the costs are yet Dave,, I have a mate who loves taking gearboxes apart,,, is this a difficult task to undertake for a competent person!
I'll bell him in the morning and ask him to get reading
My simple observation whilst driving through France was I could have done with a higher 5th just to keep it off cam enough to have good power but far quieter at 80 mph.
The real question as you mention is wether that off cam driving range will actually be detrimental but I did many miles at about 2500-2650 just to give my headache a rest and the engine performs lovely at that range anyway.
This is very exciting and I hope you go for it and all works out, you ain't got much choice but to pull the box apart so why not give it a try,,
Some people would like it to happen, others make it happen
According to my Rev counter and speed tests
I'm doing 1600 revs at 50 mph in 5th so similar to what you'd achieve with the higher 5th gear,,
I'm my view the engines sucking plenty of air in at 1600 revs so it responds well and isn't an issue at all.
I have 17 in rims and 45 profile tyres so the gearing is probably out slightly but I think this is why I get good fuel figures,
The 308 is out of my price bkt as is any other mods right now but if mines higher geared than yours what freaking dif have I actually got?
So if I put in the higher 5 th gear and using your calc id be doing 2050 revs at 70 mph and 2400 at 80 mph,,, give or take a hundred revs ,, that sounds toasty to me.
The more you think about it the more you think our light cars will benefit from this mod,,,
Unless I want to boot it the engine power is always way more than you need in normal driving conditions so I can't see it being anything but beneficial.
Dave your engines purrs, it's more than powerful enough I'd imagine.
Can you estimate what the costs are yet Dave,, I have a mate who loves taking gearboxes apart,,, is this a difficult task to undertake for a competent person!
I'll bell him in the morning and ask him to get reading
My simple observation whilst driving through France was I could have done with a higher 5th just to keep it off cam enough to have good power but far quieter at 80 mph.
The real question as you mention is wether that off cam driving range will actually be detrimental but I did many miles at about 2500-2650 just to give my headache a rest and the engine performs lovely at that range anyway.
This is very exciting and I hope you go for it and all works out, you ain't got much choice but to pull the box apart so why not give it a try,,
Some people would like it to happen, others make it happen
Ok so this is what I've learnt, firstly the final ratio for fifth depends on the number of teeth on the input shaft and counter gear and there were basically two main gear sets put in the T5.
The first set has a 3.35 1st gear and the second is the type fitted to TVRs with a 2.95.
For a 3.35 T5 a 25/51 5th gear pair will yield 0.73 (0.724638) which is standard for a 94-98 V6 Mustang, the standard set for the rest of the 3.35 Mustang T5s is the 27/59 pair which yields 0.68 (0.676492)
For our 2.95 T5 we got a 33/53 pair that yielded our 0.80 (0.804245) 5th, if I put the 25/51 pair in my box that would give me a 0.63 (0.63317) 5th. The only other set available is the 27/59 pair that would produce a super tall 0.59 (0.591102) 5th in my box.
So given I'm starting with a 2.95 box, to change my 5th gear ratio it looks like I only have three overdrive gear set options open to me:
Option 1: Fit the 27/59 set to achieve a 0.59 5th gear ratio
Option 2: Fit the 25/51 set to achieve a 0.63 5th gear ratio
Option 3: Keep my current 33/53 and stick with my 0.80 5th gear ratio
It seems there's nothing available to help me create my perfect 0.73 5th, and you can only create such an overdrive ratio if you start with a 3.35 T5.
The 0.59 5th option is way too tall so that's immediately out of the question, and while 'Ol Gasbag' would probably happily pull the 0.63 5th, the step up from fourth looks to be too much of a jump.
Lets see how that 0.63 looks on a gear calculator:
Just look at that big jump from 4th to 5th, I'll play around with the figures a bit more and run some further scenarios but that's already looking like a step too far. To stop the engine bogging down or at least feeling very flat it looks to me like I'd need to hang onto 4th a lot longer than I'd like before the upshift to 5th became practical. I fear with the 0.63 overdrive set the engine will be screaming away in 4th then dropping in a low RPM hole when I selected 5th.
If creating the ideal 0.72 5th gear ratio is not possible due to the lack of gear set availability to make it happen on a 2.95 T5, then I'm inclined to think I'd be better off sticking with my 0.80 set.
Its been fun thrashing all this out, but it's very much starting to look like a dead duck now
The first set has a 3.35 1st gear and the second is the type fitted to TVRs with a 2.95.
For a 3.35 T5 a 25/51 5th gear pair will yield 0.73 (0.724638) which is standard for a 94-98 V6 Mustang, the standard set for the rest of the 3.35 Mustang T5s is the 27/59 pair which yields 0.68 (0.676492)
For our 2.95 T5 we got a 33/53 pair that yielded our 0.80 (0.804245) 5th, if I put the 25/51 pair in my box that would give me a 0.63 (0.63317) 5th. The only other set available is the 27/59 pair that would produce a super tall 0.59 (0.591102) 5th in my box.
So given I'm starting with a 2.95 box, to change my 5th gear ratio it looks like I only have three overdrive gear set options open to me:
Option 1: Fit the 27/59 set to achieve a 0.59 5th gear ratio
Option 2: Fit the 25/51 set to achieve a 0.63 5th gear ratio
Option 3: Keep my current 33/53 and stick with my 0.80 5th gear ratio
It seems there's nothing available to help me create my perfect 0.73 5th, and you can only create such an overdrive ratio if you start with a 3.35 T5.
The 0.59 5th option is way too tall so that's immediately out of the question, and while 'Ol Gasbag' would probably happily pull the 0.63 5th, the step up from fourth looks to be too much of a jump.
Lets see how that 0.63 looks on a gear calculator:
Just look at that big jump from 4th to 5th, I'll play around with the figures a bit more and run some further scenarios but that's already looking like a step too far. To stop the engine bogging down or at least feeling very flat it looks to me like I'd need to hang onto 4th a lot longer than I'd like before the upshift to 5th became practical. I fear with the 0.63 overdrive set the engine will be screaming away in 4th then dropping in a low RPM hole when I selected 5th.
If creating the ideal 0.72 5th gear ratio is not possible due to the lack of gear set availability to make it happen on a 2.95 T5, then I'm inclined to think I'd be better off sticking with my 0.80 set.
Its been fun thrashing all this out, but it's very much starting to look like a dead duck now
Edited by ChimpOnGas on Thursday 2nd June 07:11
gacksen said:
have a 0.68 gearset on my side that i dont need anymore so in case of interest drop me a pm.
PM sent, can you confirm the number of teeth on both gears please?How are you getting to 0.68 in our TVR 2.95 T5?
My understanding is sets are only available to create the following 5th gear ratio options in the TVR 2.95 T5 box?:
Option 1: 27/59 set to achieve a 0.59
Option 2: 25/51 set to achieve a 0.63
Option 3: 33/53 to achieve a 0.80
Perhaps I've misunderstood and am more than happy to be educated on how I can get the 0.68 you're offering me as it would be more or less perfect for my needs
This is my understanding of how the available gear sets create the different 5th gear ratios:
Sadly it's looking increasingly likely the gear set I need to create my perfect 0.72 fifth gear ratio is not available and may never have been.
If you slot a 25/51 set in a T5 box with a 3.35 first gear you do indeed create a 0.72 fifth gear, but fit the same 25/51 set in my box with it's 2.95 ratio first gear what it looks like I'll end up with is a 0.63 ratio fifth, which apart from being a bit taller than I'd like would more worryingly create a rather big jump between fourth and fifth.
Lets say I'm bowling along in fourth at 60mph the engine would be turning over at 2,800rpm at it always has, the road ahead then opens up so I decide to change up to my new 0.63 fifth. Suddenly I've lost a whopping 900rpm and the engine is now spinning at a lowly 1,900rpm at the same road speed. In practice the act of changing gear, the pause in power and wind resistance all mean the engine will probably be turning at more like 1,850rpm by the time I get back on the throttle.
In my above real world scenario the act of suddenly dropping from 2,880rpm to near 1,800rpm could well feel like I've falling in a hole, and I'd worry climbing out of that hole back up to my fifth gear cruising speed could prove tiresome.
I feel a 0.63 fifth may just give too bigger gap and leave the car feeling flat and lifeless until I pile on enough engine speed needed to reach my 80mph fifth gear cruising road speed. In conclusion I need to be very careful I don't just spend a bunch of money here only to find out I've spoiled the car's current excellent 0.80 fifth gear flexibility.
Edited by ChimpOnGas on Thursday 2nd June 10:28
Really good and informative thread as always Dave.
My personal opinion here is leave the gearbox alone, I think you may be playing with fire here. I think you risk possibly losing the flexibility of 5th gear and needing to keep changing down into 4th. I can leave mine in 5th and still accelerate faster than most things at motorway speeds and I like that. Another thing to bear in mind as well as you pointed out with 250 torques I am not sure you can afford to go taller and keep it useable. Of course this is all down to personal preference but in your case I would be tempted with possibly the same 3.23 diff as Alun and I have as a half way house between the 3.45 and the 3.08.
I am considering the 3.08 as I can probably get away with it with around 360 torques, but then on the flip side it is probably costing me 3 times as much as you in fuel.
My personal opinion here is leave the gearbox alone, I think you may be playing with fire here. I think you risk possibly losing the flexibility of 5th gear and needing to keep changing down into 4th. I can leave mine in 5th and still accelerate faster than most things at motorway speeds and I like that. Another thing to bear in mind as well as you pointed out with 250 torques I am not sure you can afford to go taller and keep it useable. Of course this is all down to personal preference but in your case I would be tempted with possibly the same 3.23 diff as Alun and I have as a half way house between the 3.45 and the 3.08.
I am considering the 3.08 as I can probably get away with it with around 360 torques, but then on the flip side it is probably costing me 3 times as much as you in fuel.
For what it's worth (and that's probably not a lot), I tend to drive like this... 30mph limits I stay in third gear, 40mph speed limits I drop it into 4th gear (seems very happy buzzing away). 50mph limits... yep you guessed it However, the 60 and 70 NSL's I usually accelerate up to from 2nd gear to 3rd gear, then change straight into 5th, missing 4th gear altogether... Given that the old girl is doing 65mph at 6k rpm in second, there are occasions where she gets the 2nd gear bootfull (Oh my Lord the noise...) then straight into 5th for the leisurely cruise..
Dave apologies if this information serves absolutely no meaningful purpose whatsoever with regard to your 5th gear considerations...
Chilli
However, the 60 and 70 NSL's I usually accelerate up to from 2nd gear to 3rd gear, then change straight into 5th, missing 4th gear altogether... Given that the old girl is doing 65mph at 6k rpm in second, there are occasions where she gets the 2nd gear bootfull (Oh my Lord the noise...) then straight into 5th for the leisurely cruise..Dave apologies if this information serves absolutely no meaningful purpose whatsoever with regard to your 5th gear considerations...
Chilli
Matthew Poxon said:
Really good and informative thread as always Dave.
My personal opinion here is leave the gearbox alone, I think you may be playing with fire here. I think you risk possibly losing the flexibility of 5th gear and needing to keep changing down into 4th.
I'm inclined to agree Matthew, I've always said while the 5th gear may be a bit low giving 3,000rpm at 80mph I do also enjoy the flexibility and brisk 80-120mph acceleration it gives, I certainly wouldn't want to lose these positive character traits.My personal opinion here is leave the gearbox alone, I think you may be playing with fire here. I think you risk possibly losing the flexibility of 5th gear and needing to keep changing down into 4th.
As you quite rightly point out I am running the very real risk of spoiling the car by fitting a 5th gear that's too tall, and for what? a couple of MPG and some slightly more relaxed cruising?
My personal conclusion is.... The risks clearly far out way the benefits.
Then there's the question mark over what ratios I can even create with the parts available, the 0.72 looks to be the best ratio but my research says that only exists as an option if you slot a 25/51 set in a T5 box with a 3.35 first. If I fit the same 25/51 set in my box with it's 2.95 ratio first gear what I'll actually end up with is a too tall 0.63 ratio fifth.
Until someone can conclusively point me in the direction of a gear set that will definitely give me a 0.72 (or 0.68 at a push) 5th gear then I'm parking the whole idea. A taller diff remains an option but is an expensive one, in an ideal world I'd have a six speed box, a TKO-6060 with exactly the same first five ratios I have now just with the addition of a sixth ratio somewhere in the region of 0.70.
But a TKO-6060 would require a custom bell housing and a new clutch so this idea would probably be three times the cost of a taller diff. This all brings me full circle back to the sensible conclusion that sticking with my 0.80 fifth and 3.45 diff combo is the best, safest and clearly way cheaper option.
Thanks for listening to my ramblings chaps, it would have been cool if I'd found the perfect gear set but that's not looking likely, the purpose of thrashing it all out on here was to help me explore the viability of a taller fifth gear and to make sure I didn't end up doing something that stood to make the car worse not better and our discussions have certainly achieved this.
Using gear calculators and debating an idea on here is a damn sight cheaper than blindly slapping a set of 25/51 cogs on the back of my box only to find I had a huge step from fourth to fifth and the car fell in a huge hole unless I proper spanked it in fourth before upshifting.
Never mind hey, "nothing ventured - nothing gained" and all that
Dave
Matthew Poxon said:
Really good and informative thread as always Dave.
My personal opinion here is leave the gearbox alone, I think you may be playing with fire here. I think you risk possibly losing the flexibility of 5th gear and needing to keep changing down into 4th. I can leave mine in 5th and still accelerate faster than most things at motorway speeds and I like that. Another thing to bear in mind as well as you pointed out with 250 torques I am not sure you can afford to go taller and keep it useable. Of course this is all down to personal preference but in your case I would be tempted with possibly the same 3.23 diff as Alun and I have as a half way house between the 3.45 and the 3.08.
I am considering the 3.08 as I can probably get away with it with around 360 torques, but then on the flip side it is probably costing me 3 times as much as you in fuel.
I'm with Mathew on this Dave. that leap from 4th to 5th may get irritating. I recall 'brummie' had a taller 5th with one of his TKO boxes [very similar ratios] and he said it was not pleasurable even with his 500+ hp. The reason I kept to my ratios when changing boxes. My personal opinion here is leave the gearbox alone, I think you may be playing with fire here. I think you risk possibly losing the flexibility of 5th gear and needing to keep changing down into 4th. I can leave mine in 5th and still accelerate faster than most things at motorway speeds and I like that. Another thing to bear in mind as well as you pointed out with 250 torques I am not sure you can afford to go taller and keep it useable. Of course this is all down to personal preference but in your case I would be tempted with possibly the same 3.23 diff as Alun and I have as a half way house between the 3.45 and the 3.08.
I am considering the 3.08 as I can probably get away with it with around 360 torques, but then on the flip side it is probably costing me 3 times as much as you in fuel.
Edited by macdeb on Thursday 2nd June 18:39
macdeb said:
I'm with Mathew on this Dave. that leap from 4th to 5th may get irritating. I recall 'brummie' had a taller 5th with one of his TKO boxes [very similar ratios] and he said it was not pleasurable even with his 500+ hp. The reason I kept to my ratios when changing boxes.
Cheers Mac, I think its all pretty conclusive now, I'll be keeping her stock 
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