Gearbox ratio issue

[playalistic]

In the paperwork for my GTO-3 I can see the gearbox was fully stripped and rebuilt by Quaife and they used the QKE28Z gearset and (for some bizarre reason) chose a final drive of 4.06. This means I will headbutt the limiter in 5th gear at approx 130MPH (tested on an airfield) and cruising at motorway speed puts me somewhere around 4100rpm in 5th gear.

Not good for obvious reasons and something the test drive didn't make too apparent unfortunately and thus I feel a complete dick but anyway my next step is to get it resolved.

It gets a bit nerdy here so please bear with me I got a tool called Gearcalc and plotted what it thinks are the figures for the standard 5-speed ratios and 3.82 final drive. The calculations are based on 265/35/18 wheels and max RPM of 6200RPM which I believe to be correct for the GTO-3?

Gear Mph per 1000 RPM Mph @6200 RPM
----------------------------------------
1 5.76 36
2 9.21 57
3 13.31 83
4 17.75 110
5 23.18 144


I'm not following this properly as my understanding was the GTO-3 had a top end of approx 165MPHish? Anyway, I then punched in my ratios and final drive (4.02) which produced the following:


Gear Mph per 1000 RPM Mph @6200 RPM
----------------------------------------
1 7.01 43
2 10.16 63
3 13.24 82
4 16.41 102
5 19.27 119


I was hoping I could get away with just changing my final drive down a notch to the nearest to stock that Quaife do: 3.84 but with my ratios that produced:

Gear Mph per 1000 RPM Mph @6200 RPM
----------------------------------------
1 7.41 46
2 10.74 67
3 13.99 87
4 17.34 107
5 20.36 126

And with their lowest final drive available which is 3.62 produced the following:

Gear Mph per 1000 RPM Mph @6200 RPM
----------------------------------------
1 7.86 49
2 11.38 71
3 14.83 92
4 18.37 114
5 21.58 134

If you're still with me here and haven't slipped into a coma, can someone toss me a log and lend some advice? I think the figures the tool has produced (Gearcalc for windows) are a little off so not sure I should be using them as a guide to start with

I'm starting to think I should just get another standard 5-speed and transplant the current one and be done with it.

[Gadgeroonie]

stick a 6 speed in it

who did the gearbox rebuild ? (ie asked quaife to do it)

your box would be good for a race track with lots of turns and few straights

there is a gearbox calculator program on www.gearboxman.co.uk

fitting larger rear wheels and higher profile tyres will also raise your final drive

try the calcls again using 7000 rpm for the max speed

Edited by Gadgeroonie on Thursday 3rd November 19:16

[playalistic]

Cheers for the advice. Doing the calcs at 7000rpm actually seems to properly tie in with what the top speed for each gear (standard) is claimed.

So with my current box, top speed in each gear (final drive 4.02):

1st - 49MPH
2nd - 71
3rd - 93
4th - 115
5th - 135

With the shortest final drive available (3.62):

1st - 55MPH
2nd - 79
3rd - 104
4th - 129
5th - 152

Which sacrifices some top-end but is closer to the standard. Is there any issue of going with a 3.62 final drive with the ratios I have currently? This is if I decide to stick with this box which is looking more unlikely as I might bite the bullet and go six speed.

[daddysumo]

If you are going six speed you should find out how many other parts have to be changed at the same time, you might be suprised....

[playalistic]

daddysumo said:

If you are going six speed you should find out how many other parts have to be changed at the same time, you might be suprised....

Yeah you're right there, had a little research and have figured out that could be pretty costly. I might just go with changing the final drive to 3.6 on the current box which will make a good difference and bring it almost back inline with stock 5-speed top end for each gear.

Being a total numpty in these matters, is there any issue with going down to a 3.6 final drive? i.e causing undue stress on components, premature wear, drivetrain issues

[andygtt]

so did Quaife make it clear what they were providing you i.e. a low ratio box? or did they simply now know it was for a noble?

If they knew and didn't make it clear its a major error on their part and makes it not fit for purpose in the eyes of trading standards.

You could change the final drive to bring it up, not sure how much work that is though... the engines rev to 7000rpm but std peak power is at around 6200rpm which is were you prob got that rpm from

I changed to the 6 speed from a 5 so drop me a note before you decide to do it so I can brief you on exactly what would be involved.

[playalistic]

They didn't do it under my guidance, it was the previous owner before the last owner. He apparently specified it that way to be an out and out track toy through Hexsham horseless carriages the garage that did the engine/box removal and refitting. He went against their advice despite them describing exactly what I've experienced. The previous owner from me was fully aware of the cars shortcomings and didn't mention them in the sale of the car although he did mention something about the gears being "slightly shorter" than stock but I'm not looking to blame anyone for this but myself to be fair.

Thanks for the advice so far, Jetstream have mentioned that it isn't a huge deal changing the final drive. Most of the cost is down to the engine/box needing to come out unfortunately!

[andygtt]

that was rather nice of the previous owner! think the best option is to change the final drive... you'll be in safe hands at Jetstream, my car is there being painted and having some tweeks

[D_G]

I'm thinking of changing mine to LSD, quaife list a MTX75 diff number QDF20Z. I assume this has the correct ratios?

[Gadgeroonie]

see if you can borrow some 19 inch rear rims and see what affect it has on top speed

might be the cheapest option and easiest

can you specify the wheel / tyre size in the program you are using ?

[andygtt]

IMHO upping the rolling radius of the wheel is a bad ie (ie going to 19's or fitting a larger 18in tyre)... it will change the ride height and unbalance the car.

fitting an lsd should have no effect on the gearbox ratios as the existing cwp bolts to the diff so you use your existing one

[playalistic]

andygtt said:

IMHO upping the rolling radius of the wheel is a bad ie (ie going to 19's or fitting a larger 18in tyre)... it will change the ride height and unbalance the car.

Yep, think I'll just get it done properly and suffer the costs this time round. I am very tempted to just bite the bullet and go 6-speed but I'll have a chat with Matt & Dave anyway and weigh up my options. Cheers chaps.

One more thing, changing the final drive on these boxes seeing as though they're FWD does that mean the housing needs to be opened or is it on the LSD?

[andygtt]

LSD is inside the housing as well so has to be opened regardless...

Biggest cost of going 6 speed is the lsd and gearchange kit... i have a spare 2nd hand starter clutch etc that I would let go and even a lightened flywheel if you so desired lol (5 speed flywheel is same as 6 speed so u dont have to change this btw). you also need all new engine mounts.

considering taking the engine out is quite a chunk of cost for you it might be worth biting the bullet as jetstream have done this conversion so know it inside out.... gearchange on an M400 kit and 6 speed box is leagues ahead of the 5 speed you have now.

Wonder how much your low ratio box is worth... to the right person it must be worth more than a conventional 5 speeder?

[playalistic]

andygtt said:

LSD is inside the housing as well so has to be opened regardless...

Biggest cost of going 6 speed is the lsd and gearchange kit... i have a spare 2nd hand starter clutch etc that I would let go and even a lightened flywheel if you so desired lol (5 speed flywheel is same as 6 speed so u dont have to change this btw). you also need all new engine mounts.

considering taking the engine out is quite a chunk of cost for you it might be worth biting the bullet as jetstream have done this conversion so know it inside out.... gearchange on an M400 kit and 6 speed box is leagues ahead of the 5 speed you have now.

Wonder how much your low ratio box is worth... to the right person it must be worth more than a conventional 5 speeder?

Yeah, must be worth a few quid as the reconditioning and gearset alone came to just under £3k and it has approx 5000 miles on it since done. I guess its just a case of finding someone who wants it... Thanks for the offers on your bits, I might well be in touch

[mgbond]

Am I right in thinking that the std 5 speed Box final drive is 3.89?

Will be worth getting the box to the gearbox man what ever you decide so he can strengthen it and put the cooling channels in.

Bondy

[playalistic]

Standard 5 is 3.82 FDR. God I could recite all the ratios of each gear at this stage I reckon

Looks like the 3.6 FDR is a 14 week (!!!) build to order from Quaife and comes in at a bargain £550. Ouch. Might as well get another MTX75 and have that to the gearbox man for strengthening at this rate.

[andygtt]

What we need is a 30mph per 1000rpm final gear... I have to admit that I have developed a silly complicated spreadsheets that compare all the tyre sizes and ratios at the same time so I could pick the ones for my GTT... I had a final drive made for the box in that giving me 36mph/per 1000rpm top and plan the engine to go to 8400rpm
I also wanted a 60mph plus first gear which i have comfortably lol

Wish I could do the same with the noble box.... I plugged my noble into this spreadsheet when i changed to the 6 speeder and played with tyre sizes to see it i could technically get the car to 200mph lol

[Gadgeroonie]

Andy - not sure that fitting bigger rear rims/tyres is going to be such a problem

it wil only raise the rear by half an inch and you can lower the springs by that amount to compensate

what top speed would a set of 19" rims give you ?

[andygtt]

Increasing the rr will change the geometry if you drop the chassis to compensate, wishbones are short so at a guess the roll centres would move considerably...... Also i dont think Half an inch will not make enough of a difference to the top speed... The 345/35/19s I have on the gtt would be ideal, but they are nearly 3 inches bigger in diameter and around 8kgs heavier lol

I will plug the figures into the spreadhsheet tomorrow to find out exact figures that tyres will make.

[andygtt]

plugged the figures in... so going to 19 rims and keeping the tyres the same profile ie 265/35/19... we go up from 134mph to 140mph at 7000rpm... wheel dia obviously goes up by 1inch meaning the car will sit half inch higher.

if you fit my 345/35/19's you will get a whoping 151mph but the wheel/tyre is then 4 inches larger and 8kgs heavier lol

BTW at 7000rpm my car with std gearing and wheels would be doing 177mph!