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 Post subject: EOI extra tall 5th gear
PostPosted: Sat Jun 14, 2008 6:44 pm 
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Just wondering if there would be any interest in an extra tall 5th gear to fit mk2/3/4/5.

The advantages of having a custom made extra tall 5th gear would be reduced rpm while in 5th gear similar to having a 3.52 final drive but would not require complete disassembly of gearbox and on a 3cyl could be installed by a competent DIY'er or mechanic without removing gearbox from the car .

Gears would be helical for quietness, and now the bad part, cost is generally around US$275-400 per gear as a standard ballpark figure and there are two gears.
Actual price is dependant on how many are made and complexity.

If there is interest then I will investigate the actual cost with my gearcutter.


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PostPosted: Sat Jun 14, 2008 9:02 pm 
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You know I'm already interested!

What are your thoughts / info on the new 5th ratio with the 4.4 final drive? I would actually like to go lower than the 5th with the 3.52 final as that's what I'm running now and I don't find it low enough for cruising (even with larger tires).


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PostPosted: Sat Jun 14, 2008 9:13 pm 
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stock is somewhere around .7:1 so if you could get a .5:1 gearset made I would be interested.

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PostPosted: Sat Jun 14, 2008 9:46 pm 
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Stock is 0.758, you don't want to go too low as the gap from 4th will be massive, I would think 0.6 would be plenty low enough, it depends which final drive ratio you have as to what your cruising rpm is, you guys tell me what cruising rpm is ideal for your travelling speed, ie 55mph? 65mph? and I'll work backwards to come up with a ratio and some graphs so you can determine rpm in each gear.


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PostPosted: Sat Jun 14, 2008 10:14 pm 
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my car is an xfi with 3.85:1 final drive and 175/70R14 tires so the rpms are not out of hand already but the weight and aerodynamics of my car a .5:1 would work fine for me. Since most cars came with a 4.10 or so a .5 would be good. I think 2000 at 60mph is what my ideal rpm would be :)

But then again I have no concern for performance or having to shift gears constantly. I just want the absolute best mileage possible and the gears are one of my main limiting factors.

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PostPosted: Sat Jun 14, 2008 10:20 pm 
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No worries, I'll crunch some numbers later tonight and report back, are you running those big tyres to reduce your crusing rpm? with a taller 5th gear would you change to a smaller tyre to lower the car and reduce drag??? can you get a 175/60/13 with low rolling resistance???


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PostPosted: Sat Jun 14, 2008 10:47 pm 
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Yep they are the tallest tire I could fit in the fender wells and still have a reasonable turning radius :) They only drop the rpms 250 at 60 but that is better than nothing. They are Sumitomo HTR 200 tires they are rated pretty good for traction and low rolling resistance so they seem like a good all around tire. They run at 51psi :shock:

Most people are trying to find the 3.52 gear set and I will probably swap to them as well if I can find some. I would be interested in a taller 5th anyway because it just doesn't have a big enough rpm drop when shifting to 5th from 4th. I like it to drop at least 1000rpms and right now I think it is more like 600.

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PostPosted: Sun Jun 15, 2008 1:31 am 
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I'd be interested, but I'd be hoping for the cheaper end of the scale since for me it would also require getting rid of my Mk1 box and finding a Mk2-4 one and making a new set of mounts. But if they were available... I'd definitely be interested.

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PostPosted: Sun Jun 15, 2008 2:33 am 
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Coyote X wrote:
Most people are trying to find the 3.52 gear set and I will probably swap to them as well if I can find some.

Bit slow aren't we, I just sold one this week, I can always get them, the trick is to get one at a good price, if you are willing to pay US$240 +US$44 for postage I can get one this week, otherwise it's a matter of waiting for one to come up at a good price ie less than US$150.

sbdeadelf I should be able to give you an answer this week, I may also be able to offer taller 5th gears for a mk1 if there is enough interest.

OK, I crunched some numbers, I've used 165/70/13 as a standard tyre size, I realise a lot of models have 155/70/13 but the difference is pretty small. I've settled on 0.611 as the lowest ratio that can be used properly.

image_id: 14269
Here is some standard figures, blue is a standard metro 5th gear and 4.1 FD, red is the same 5th gear but using a 3.52 FD, at 60mph the rpm drops from 2900 to 2490.
The green line is using a custom 0.611 fifth gear with a standard 4.1 FD, the result is 2338rpm at 60mph, so the new gear would give an even better cruising rpm than by changing the final drive to 3.52.
Coyote I'll crunch your numbers shortly.


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PostPosted: Sun Jun 15, 2008 5:30 am 
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Ok, here's where I explain why I won't go lower than 0.611.

image_id: 14272
This chart shows the effect when you change from 4th to 5th.
Follow the blue line, this is 4th gear with a final drive of 4.1 and tyre size of 165/70/13, at 3000 you change into a standard fifth gear, your rpm drops down to 2488, thats a 512rpm drop, now follow the red line, this is the custom 0.611, and changing from 4th to the 0.611 drops the rpm from 3000 to 2005rpm, thats a huge drop for a small engine.

Ok now I'll change the final drive, follow the green line and changing into a standard 5th gives the same rpm drop but the road speed at 3000rpm is much higher, the black line is the taller 0.611 which gives the same 995 rpm drop as the 4.1 final drive equipped box.
Hence the final drive ratio of the diff has no effect on the rpm drop between gears and too big a gap can mean worse fuel economy if you have to rev it quite high just to change into a higher gear.


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PostPosted: Sun Jun 15, 2008 6:20 am 
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Coyote X here is some data for you.
Your 'big' tyres effectively makes your 3.85 xfi final drive the same as a 3.52 with normal tyres.


image_id: 14271
The blue line is the standard 5th gear with 3.85 FD and normal tyres, this gives 2723rpm at 60mph.
The red line is the same gear and final drive but with your BIG tyres, = 2539 RPM at 60mph.

Ok so if we change the standard 5th gear to a custom 0.611 we get the green line which gives 2047 RPM at 60mph.
So that would give you a 995rpm drop between 4th and 5th and a cruising speed of 58.6mph at 2000rpm

If you want to get more radical you could use a 3.52 FD with your tyres and that would give you the black line on the chart which = 1871 RPM at 60mph which I believe would be too tall.
If you did use the 3.52 FD, custom 0.611 5th gear and changed to smaller wheels your speeds would be similar to the green line, the advantage is you would lower the car and also have the ability to lower the car further with lower springs without the wheels rubbing and I beleive you would significantly lower your cd.


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PostPosted: Sun Jun 15, 2008 9:39 am 
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Add me to the mk1 list. PM sent

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PostPosted: Sun Jun 15, 2008 12:08 pm 
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.611 sounds good to me :)

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PostPosted: Sun Jun 15, 2008 4:10 pm 
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With a 3.523 final and 23.4" OD tires:
2500rpm = 105kph (65mph)

With a 4.389 final:
2500rpm = 85kph (53mph)

With a 4.389 final and a 0.611 5th:
2500rpm = 104.6kph (65mph) ... which is back where we started.

With a 4.389 final and 0.550 5th:
2250rpm = 104.6kph (65mph) ... which is a drop of 250rpm.

This is the kind of 'upgrade' I'd be looking for in a new 5th, the rpm drop from 4th-5th is not important. You only use 5th for cruising (holding speed), and with the NA gearset, 4th will take you to 180kph (112mph) at 6500rpm.

1st-4th are acceleration, 5th is cruise. Make sense?


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PostPosted: Mon Jun 16, 2008 1:52 am 
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martinq wrote:
This is the kind of 'upgrade' I'd be looking for in a new 5th, the rpm drop from 4th-5th is not important.

That depends on what you want, if you have a 4.389 FD then you aren't after fuel economy, 4.4 is for performance.

The gap between 4th and 5th does matter for the fuel economy guys, there's not much point having a 5th gear so tall you have to rev to 4000 rpm just so you can get it into 5th without the revs falling below 2000rpm, as soon as you get to a small hill you would have to change down to forth and rev your engine hard and there goes your fuel economy.
I will figure out what ratio's will actually work between 0.5 and 0.63 and we can work from there.


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PostPosted: Mon Jun 16, 2008 6:11 am 
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It depends on terrain. The interstate freeway system is engineered with gradual slopes, typically 3% but i have seen 7%. Very low gearing might be a problem on roads in the mountains. how long will we be flat cruising vs how long will we be reving up to get uphill? Who keeps track? I guess mine is less than 10% hill -- for 1 minute. i am always in 5th except to make turns. 1800 rpm may be fine -- even without much torque.

and i wonder how many miles will it take to pay for the gear at say $5/gal ? :?: :?:

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PostPosted: Mon Jun 16, 2008 9:45 am 
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Dattman wrote:
That depends on what you want, if you have a 4.389 FD then you aren't after fuel economy, 4.4 is for performance.

I want performance and economy. :)


Dattman wrote:
The gap between 4th and 5th does matter for the fuel economy guys, there's not much point having a 5th gear so tall you have to rev to 4000 rpm just so you can get it into 5th without the revs falling below 2000rpm, as soon as you get to a small hill you would have to change down to forth and rev your engine hard and there goes your fuel economy.

We're only talking about a difference of 250rpm with the 4.4 FD. You wouldn't need to rev to 4000rpm (3-cyl maybe?), 3000 should be fine. Even shifting at 4000rpm would not significantly affect your economy. The big factors (aside from engine tune) is vehicle weight, aerodynamic drag and cruise speed. Once you are cruising (stable speed), you want your revs down and your throttle open to reduce internal losses.

The difference of a 0.550 5th is quite minimal (I would probably prefer a little more), but this may not be usable for the 1.0 litre guys. As it is (1.3 8v, 3.52 FD, 23.44" tire), I have more than enough torque from 65kph (40mph) on up, mind you, I don't live in a very hilly area. The current drop in rpm from 4th to 5th is so minimal it's almost worthless.


Dattman wrote:
I will figure out what ratio's will actually work between 0.5 and 0.63 and we can work from there.

Super!

Here's some numbers for the NA gearset with a 2.80 1st and a 0.50 5th.

shift point: 3000 RPM

1st 2.800 :1
2nd 1.895 :1
3rd 1.280 :1
4th 0.914 :1
5th 0.500 :1
Final: 4.389 :1

rev drop speed MPH
1st to 2nd 970 17
2nd to 3rd 974 25
3rd to 4th 858 37
4th to 5th 1359 52
5th @ 3000 95


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PostPosted: Mon Jun 16, 2008 2:18 pm 
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you can count me in as "on the fence", and I am willing to consider multiple ratios


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PostPosted: Mon Jun 16, 2008 2:31 pm 
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whoa back the truck up, Martin if you want to change 1st gear to 2.8 we can do that but it will cost you about US$1900 to do it as first and 2nd are on the input shaft so you have to make a new input shaft and 1st , 2nd gears, very expensive.
Your problem is your final drive, not the gears, the problem with making a 0.5 gears is it is fine for the absolute lowest geared combination which is what you have, fine for a 1.3 which has way more torque than the 1.0.
The majority of the cars are metros with a 1.0 and 4.1 final drive, it makes far more sense to optimise the target rpm for these guys.

I'm hoping to have some actual ratios today or tomorrow.


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PostPosted: Mon Jun 16, 2008 3:16 pm 
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here is some information that might help you that I have dug up.

* Swift GA and Metro Convertible had 165/65R13 tires, the same diameter as the 145/12 tires on the cars up to mid year 95
* Swift GT had 165/65R14 tires, the same diameter as the 155/13 tires mid year 95 and up.

and

The available transmission ratios and what they are out of:

* 3.52:1 1.3L SOHC Swift up to mid year ‘95
* 3.79:1 98-01 4 cylinder Metro
* 3.85:1 XFi Metro
* 3.95:1 mid year ‘95 up 1.3L SOHC Swift
* 4.10:1 Swift GT, GTi, 95 and earlier Metro
* 4.39:1 Metro Convertible, 96+ Metro

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PostPosted: Mon Jun 16, 2008 4:39 pm 
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Dattman wrote:
whoa back the truck up, Martin if you want to change 1st gear to 2.8 we can do that but it will cost you about US$1900 to do it as first and 2nd are on the input shaft so you have to make a new input shaft and 1st , 2nd gears, very expensive.

I know about all of that, I was just playing some numbers with the NA gearset. This truck ain't goin nowhere.


Dattman wrote:
Your problem is your final drive, not the gears, the problem with making a 0.5 gears is it is fine for the absolute lowest geared combination which is what you have, fine for a 1.3 which has way more torque than the 1.0.

The final drive isn't the problem, it's the solution. The only issue is getting the cruising revs down and that's what this thread is about. I'd like to get them down as low as possible.


Dattman wrote:
The majority of the cars are metros with a 1.0 and 4.1 final drive, it makes far more sense to optimise the target rpm for these guys.

Agreed, but maybe we could come up with an optimized gear for the 3-cyl and another for the 4-cyl.


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PostPosted: Tue Jun 17, 2008 2:33 am 
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martinq wrote:
Dattman wrote:
whoa back the truck up, Martin if you want to change 1st gear to 2.8 we can do that but it will cost you about US$1900 to do it as first and 2nd are on the input shaft so you have to make a new input shaft and 1st , 2nd gears, very expensive.

I know about all of that, I was just playing some numbers with the NA gearset. This truck ain't goin nowhere.

I know YOU know but I don't want any body else getting any funny idea's :mrgreen:

martinq wrote:
The final drive isn't the problem, it's the solution. The only issue is getting the cruising revs down and that's what this thread is about. I'd like to get them down as low as possible.

The way I see it, the 4.389 is optimised for acceleration and the 3.52 is optimised for fuel economy, they are opposite ends of the spectrum, if you want acceration AND economy then get something in the middle, ie 4.1 or 3.85, the final drive ratio has a much bigger effect on 4th and 5th gears than it does on 1st and 2nd.


OK here's the bad news, there's a MAJOR clearance issue which means I can't make 5th gear any taller than 0.657 and that still involves modifying the shifter fork slightly.
I can actually get a 0.568 ratio to work nicely but the gear is physically too big and would have to redesign a whole new shifter fork and thats a lot of work.

A standard metro with 4.1 would normally be doing 2900rpm@60mph and the 0.657 would drop that down to 2513rpm.

Let me know if any one is still interested, sorry there is no easy way to get 5th any taller.


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PostPosted: Tue Jun 17, 2008 5:52 am 
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Coyote X wrote:
here is some information that might help you that I have dug up.

* Swift GA and Metro Convertible had 165/65R13 tires, the same diameter as the 145/12 tires on the cars up to mid year 95
* Swift GT had 165/65R14 tires, the same diameter as the 155/13 tires mid year 95 and up.

and

The available transmission ratios and what they are out of:

* 3.52:1 1.3L SOHC Swift up to mid year ‘95
* 3.79:1 98-01 4 cylinder Metro
* 3.85:1 XFi Metro
* 3.95:1 mid year ‘95 up 1.3L SOHC Swift
* 4.10:1 Swift GT, GTi, 95 and earlier Metro
* 4.39:1 Metro Convertible, 96+ Metro


Thanks, the 3.79 and the 3.95 are the odd ones out, they don't work out mathematically, if it is the same gearbox design then they have changed to a different sized tooth, if anyone has either of those ratio's I'm really keen to know how many teeth are on the crownwheel and pinion gear.
Coyote I crunched the numbers for you, the 0.657 would lower your current setup by 338rpm down to 2201 @60mph, changing to a 3.52 would bring it down to 2012rpm @ 60mph, if you want it any lower then the only option is to make a custom final drive of say 3.03:1 that would lower your rpm down to 1732@60mph.
Without the custom 5th gear the 3.03:1 would give 2201rpm @60mph so neither option works out very well by itself but together would work very well.


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PostPosted: Tue Jun 17, 2008 8:16 am 
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I'm interested depending of course on the cost and ease compared with swaping the whole tranny to one with a higher final drive .

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PostPosted: Tue Jun 17, 2008 8:47 am 
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re..I believe the teeth count on the 3.79 gears is 19 & 72.. :D


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