20 replies to this topic

[Chris1992]

 

So I was in the same boat as you before I had my 1380 twin cam built. I was so torn on what setup to go for considering I do a LOT of motorway miles getting to shows and never trailer my car. In the end I went for a full straight cut close ratio setup and a 3.1 fd. At first I regretted it, as with the 3.1 and CR box combo I was no longer able to burn away from traffic lights, as you have to really pedal it to get it off the line smoothly now. The straight cut gears, while good fun at first, were also piercingly loud.

 

However!! I have now put nearly 2k miles on this setup since I dropped the engine in a few months back, and I have to say that on the whole I am very happy. Now that I have got used to the feel of the clutch, being able to rip it up past 30mph in first gear is hilarious fun (I do miss fast traffic light launches though). I also find that the CR box puts 2nd and 3rd in the absolute perfect place for country road blasts, and the 3.1 fd means I can cruise at 70 in relative comfort. Regarding the noise, as Spider mentions above, once you get into into 4th the gearbox quietens right down, and something else I have noticed is that as I've put more miles on it, the wine from the box has become much quieter. Definitely still there, but I can now hear my exhaust and stereo over it lol. I've fallen in love with the sound it makes when pushing it through the country lanes, and every person that gets in it always says "it sounds like a race car" 

 

One last thing to note, I am running around 125 bhp with 90 lb ft of torque from 3k rpm. 

The fact that you're getting wine from your gearbox is a compelling argument for close ratio gears! ;)

 

 

Helps numb the pain in my wallet! 

[nicklouse]

I've taken the plunge and have given a gearbox rebuild a go. Stripped it down with no real issues and everything looks ok, to me, except for maybe excessive end float on the laygear but that's a problem for another day. So I'll be fitting a X pin diff and a rebuild kit with centre oil pick up pipe, nothing out of the ordinary. Seeing as it's all apart, it's an A+ box, I'm toying with the idea of a close ratio gear set but I'm not sure I'll be able to put up with straight cut gears. I've never been inn a mini with a straight cut gearbox so have no experience of what kind of noise they make. So I guess my only real option is the Minispares kit C-AJJ4032A? My Mini is naturally aspirated and I usually go 

on runs of 50 miles or more, and for the most part drive in a sensible fashion! I live up in the Shetland isles and our usual mainland port of call is Aberdeen, I'm hopeful of heading down to a few shows or events with her next year so any journey I make will be from there which could mean I'll be driving for a fair while. So is the extra cost of a close ratio set, money I could use on the next stage which is a rebuilt short block, really worth it for the driving experience seeing as it is all road use and possibly longish drives? I'd like to hear others experiences of these gears and do you ever think you should've spent the money elsewhere? .

    

That gear set will be fine. Don’t go to the 3.1 with it though.

[Tommyboy12]

Quick question.

 

What's the point of having straight cut gears in the box if you stick to helical drop gears ?

 

It was a compromise for me between strength and comfort. Straight cuts reduce side loading which is particularly important in high torque engines and are stronger on paper than helical (although thats not quite as clear cut). However I didnt want to be deafened by gearbox whine on the motorway, where I spend a lot of time travelling to shows. Ultimately you dont want to grenade your gearbox for a road application and limiting the twisting forces in the gearbox will help that.

 

Its worth remembering that the Metro turbo/ERA turbo were only 90hp because any higher and the gearbox warranty bills were too high for Austin Rover to stomach. Straight cut would have obviously been unacceptable to the average road user. They also had an early form of torque limiting to reduce loads on the gearbox/drop gears. Nowadays every single diesel and turbo car on the road has torque limiting to minimise gearbox damage in low gears.

[Spider]

Quick question.

 

What's the point of having straight cut gears in the box if you stick to helical drop gears ?

 

The only technical advantage is less thrust loss and opens up the opportunity of introducing alternative ratios in this gear set to help tweak overall drive ratios.

 

 

It was a compromise for me between strength and comfort. Straight cuts reduce side loading which is particularly important in high torque engines and are stronger on paper than helical (although thats not quite as clear cut).

 

If the material the gears are made from is the same, the tooth profiles are correct and they go through the same heat treatment etc, the helical gear will always be much stronger than any straight cut. With Straight Cut Gears, there's only ever one tooth from each gear in contact. With Helical Gears, it's anything from 1.2 to the sky being the limit, though realistically, 4 is not out of the question. This not only spreads loads across more teeth but also spreads the loads around more of the core of the Gear too. This is what is known as 'Tooth Contact Ratio'. To be fair with very cleaver design and on gears of the same tooth count, you can achieve a contact ratio of 1.1 with straight cuts but that is the absolute limit for them. The only other ways of getting more strength in to them is to go wider and / or move to better materials.

 

All the factory made gears, both the Helical Cuts and when ST had them, the Straight Cuts have a ground finish on the tooth. This too goes a long way to the noise (or lack there of) factor. All the currently available Straight Cut Sets I've looked at have a Machined Finish on the Tooth of varying degrees. This is borne out and as mentioned in this thread that they quieten down with some use, essentially lapping in to each other. It's also been said that Ground finished teeth are stronger, though that maybe debatable.

Certainly the service life many are getting from their straight cut gear sets (again of most / all currently available) suggests that they are made from the same or lesser materials and / or post machining treatment. For reference, the Factory Gears, Helical and Straights, were made from EN361 grade steel. While this isn't the ultimate material for gears, it is classed very highly for it.

[Cooperman]

One of the best set-ups for road use I have used is a 1330cc engine, with an MG Metro cam, a 3.44:1 FDR and an original 'Cooper ratio' gearbox. Ratios are ideal and it is possible to pull away from rest without excessive clutch slipping.

[skoughi]

One of the best set-ups for road use I have used is a 1330cc engine, with an MG Metro cam, a 3.44:1 FDR and an original 'Cooper ratio' gearbox. Ratios are ideal and it is possible to pull away from rest without excessive clutch slipping.

Pretty much what I have in my Clubman at the moment, well I have a Kent 266. I actually got my hands on a 1300GT engine and remote box a couple of years ago but gave away the box to a mate who has An Austin 1100 or 1300, can't remember which. That would've had the close ratio gear set in it but I didn't want the remote box. I could always source an early rod casing, the three boxes I have are all DAM5626 cases, or just try out a SC close ratio set from Guessworks. To be honest though I think I'll end up sticking with the standard gear set but fit a cross pin diff with a 3,44 and rebuild the gearbox to the best of my ability. At least that way I should then have a reliable box that'll do me for many miles. I'm going to finally get round to sorting a rebuilt short motor to set on top to replace what I have in the Clubman at the moment so will need all the pennies I have spare for that. Good to read all the comments from experienced users of straight cuts though, some day I'll get the opportunity to hear what a SC box is like and only then will I know if it's something I can live with.

Edited by skoughi, 03 September 2021 - 08:01 AM.