15 replies to this topic

[Artful Dodger]

hey guys,

i have a 1275 GT with an engine that will be putting out around 95 - 110 bhp at the fly wheel

i will be using this car as a weekend fun toy and track day car so it will not be an every day car.


what is the benefit of straight cut gears??? i know that with straight cut gears there is less power lost at the wheels through the transfer, but what else?? and what is the power output difference in BHP?


cheers guys:)

[Cooperman]

Straight-cut (SC) gears ae normally part of the close-ratio gear set which is used with 'hot' engines giving high power outputs. With an engine with a high-overlap cam, such as a 286 or similar, the power band is narrow and close-ratio gears are necessary to keep the revs in the optimum power band. However, the SC gear sets have a high 1st gear, so a lower final drive is needed to enable the car to pull away from rest without excessive clutch slipping. The SC gears also, it is claimed, take less bhp to run, so for competition another benefit.
The SC drop gears reduce the end loads on the transfer gear and primary gears so reducing wear on the thrust bearings. The downside of SC drops is that they are quite noisy which is not an issue when wearing a crash helmet, but wearing on the ears when a helmet is not worn (I wear ear plugs on the road on my rally car due to the SC drops). I now wish I had stuck with helical drops in my rally Cooper 'S' and just fitted the SC box and low final drive (3.9:1).
It has to be said that a Mini with lots of power and a SC gearbox is an absolute joy to drive quickly on twisty roads, but less pleasant in traffic.

[icklemini]

as above and

- Less tooth contact with SC - some say this makes them 'weaker'
- SC = vastly reduced side loads on the bearings. good for layshaft life and of course the bearings...

[Artful Dodger]

ok thanks guys:)

but is it a big enough benifit to be shelling out 700 quid for straight cut gears and drops?? is it just a habit that people with hot cars go through or do 120 bhp minis ruin helical cut gear quickly?


and also does the reduction in side loading increase the bhp at the wheels??

Edited by MiniclubmanGT, 28 August 2011 - 05:47 PM.

[AndyMiniMad.]

Straight cut gears are not really needed as the normal helical gears can handle the power just fine. But as cooperman says, for competition use they are prefered as they sap less of the all important power....And can you tell us the spec of your 110bhp 1275 engine please....Im curious to know how you have managed this.....Is it turbo charged?

[Cooperman]

IMHO straight cut drops are not worth the cost and the noise rteally is not good.
The reason for SC gears is because with a high-powered Mini the useable power band is narrow, so closer ratio gears are needed or it will 'fall off the cam' and won't go as well as it coulkd. There is little point in having, say, 115 bhp if when you change up the revs are too low to give the power which could be available.
If close ratio helical gears were available with the same ratios as the SC ones then that would be ideal for 100+ bhp engines.
The lower final drive is needed due to the high first gear with a SC gear set.
A 100+ bhp Mini really is for competition use.

[jakejakejake1]

I was looking into a turbo charged a-series engine and was wondering what it is about the gearbox that means it can only be used for up to about 140bhp (average figure from turbominis), they say after this straight cut gears are needed.
I'm assuming this is due to the excessive side load on the gears, are there any other options rather than SC gears, as I have heard they are quite loud!

[Artful Dodger]

Straight cut gears are not really needed as the normal helical gears can handle the power just fine. But as cooperman says, for competition use they are prefered as they sap less of the all important power....And can you tell us the spec of your 110bhp 1275 engine please....Im curious to know how you have managed this.....Is it turbo charged?

sorry,, i didnt describe it properly, its a 1360cc engine weber 45, 285 cam crank wedged balanced ect stage 4 head and all of the other shiny

I was looking into a turbo charged a-series engine and was wondering what it is about the gearbox that means it can only be used for up to about 140bhp (average figure from turbominis), they say after this straight cut gears are needed.
I'm assuming this is due to the excessive side load on the gears, are there any other options rather than SC gears, as I have heard they are quite loud!

and bits that make it go:)


and jake. i dint think so, apart from sequential boxes but that is just crazy!

[Cooperman]

With high powered Minis is the transmission is always the weak point. Most of the retirements of Minis on rallies is transmission related.
In fact, the drive shafts are particularly vulnerable if not of the EN24 competition type (very expensive) and CV joints also fail. The diff is weak unless of the x-pin type as well. In fact, the actual gears are not too bad, but close ratio ones are needed for normally aspirated engines with 100+ bhp for reasons stated above.
For forced induction engines the helical box can be fine so long as everything is in top condition, but the final drive and drive shafts are particularly vulnerable.
There is no easy answer except to fit the comp shafts, x-pin diff and, maybe, do the Maxi CV joint mod which used to be popular.

[Wil_h]

As far as I am aware driveshafts rarely fail on turbo cars. I have bust one pot joint in 10 years, and that happened after I landed from a two wheel moment with my foot still planted!

For a road car I would always stay hellical, my 998 turbo with 120bhp seems to be ok.

For my hillclimber the straightcut box was a revelation compared the the helical, no bogging down going from 2nd to third.

[MJWarren]

As far as im aware no one makes brand new helical gears. Cost to manufacture has a big part in that. If they did You could have helical gears with closer ratios?

[Cooperman]

Personally, for a Mini for road use, I would rather have a bit less top-end power by the use of a softer cam, such as an MG Metro or Kent 266, than have a very hot engine with, say, a 286 or 296 giving high power with a peak at over 6400 rpm. Such hot engines need lots of revs all the time to extract the power and thus need close ratio gears and a low final drive. Then there is the added gearbox noise on any sort of journey and the low cruising speed on any sort of main road plus the higher fuel consumption and the need to re-build the engine more often.
For competition use a 110+ bhp engine is ideal with all that implies in terms of gearing, noise, transmission strength, etc, but it's acceptable as you do it all to try to win. These really powerful engines are built for a specific application and whilst they may perform very well for that applikcation, for other use they may be horrible to drive.
The exception may be forced-induction engines which have a wider power band and pull well in mid-range. The wider ratio gear ratios can be used and the entire packege can be road-use friendly.
Thgis may be a bit of a simplification, but engine and gearbox specifications for modified engines is always a bit complex, and is certainly not as simple as the 'comics' (Mini-MagaWorld) might have you believe.

[Ethel]

Also, if your gearbox is genuine 1275GT it will have closer ratios than other helicals.

It tends to be torque that breaks gears, if you're getting your power from spinning the engine faster, that's less of a problem than when using a Turbo to get more power at the same revs.

[Cooperman]

Also, if your gearbox is genuine 1275GT it will have closer ratios than other helicals.

It tends to be torque that breaks gears, if you're getting your power from spinning the engine faster, that's less of a problem than when using a Turbo to get more power at the same revs.

Yes, the 997/998 Cooper, Mk.1, 2 & 3, Cooper 'S' and 1275 GT all had closer ratio gears as standard, but for a really hot cam like the old 649 or the Kent 286 or 296, they were still too wide despite having a lower final drive ratio of 3.76 in the 997/998 Cooper and early 'S' and 3.44 in all the others

[Artful Dodger]

thanks guys:) ima gonner have a lok at the budget and see if i can afford said gears,

any 2nd hand ones lying around???