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Apologies - Another Straight Cut / Helical Question


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#1 Chris Jones

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Posted 22 April 2013 - 01:00 PM

I know this has been discussed a lot on here and I've sifted through many posts, but I can't get a definitive answer on this:

Basically I'm looking to be running near enough 100bhp in my MPI for now, but may look into an Emerald ECU in the future and possible supercharging, so any work carried out now will have that in mind.

My problem is that I got my mini roaded at 94bhp at the fly and under 50 at the wheels (I know the fly isn't totally accurate as it's not been dyno'd but it's a decent benchmark). Also I don't like the sluggish 2nd to 3rd gear.

I'm wanting to get a bit more power to the wheels so invested in a x pin and thought about straight cut gearsets.

My question is that I'd be fitting a straight cut close ratio box to solve the sluggish 3rd gear, but would this also help in getting power down, or would I need to fit sc drops for that too? Is there any point running helical drops with a straight cut box? Has anybody done this and can tell me how loud the whine would be as I know most of that does come from the drops? I love the whining sound but imagine it can get a bit annoying on a South Cumbria to Leeds run for example....

Thanks in advance, Chris

p.s If anyone knows of any SC rod change boxes going please let me know.

#2 jaydee

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Posted 22 April 2013 - 01:07 PM

Out of curiosity, which wheels are you using and at what pressure they were set?
Thats a bit too much power sapping to be the gearbox only.

#3 Chris Jones

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Posted 22 April 2013 - 01:11 PM

Ultralite 6 x 12's @ ~30psi I'd imagine, it was a few years back.

#4 Chris Jones

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Posted 22 April 2013 - 01:16 PM

http://www.theminifo...95#entry1826595

This was my original thread a few years back. When I wrote this I wasn't too clued up on anything 'bottom end' and just got a bit baffled by everyone arguing :lol:
I spoke to a few people and they said with that fly figure I should be seeing around 70 at the wheels anyhow.

#5 jaydee

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Posted 22 April 2013 - 01:24 PM

A drag of about 43 bhp is double the drag i'd expect from transmission indeed, 70-75 ATW, the print out is really strange, the RR should read power at the wheels and calculate the power loss to give you an 'ideal' flywheel figure.
Can you do a simple power run on a dyno to see what you got now? The shouldnt charge you a lot for 10 mins on the rollers.

#6 Cooperman

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Posted 22 April 2013 - 01:26 PM

The SC Cr gears close up the ratios between gears so that with a cam with a narrow top-end power band it is possible to keep 'on the cam'. For example, a 286 cam will give peak power at about 6400 rpm and the best power is between 5500 and 7000 rpm. A SC CR box is then ideal, but a lower ratio final drive becomes necessary as 1st gear is then very high so cruising speeds are lower (or cruising revs are much higher).
The SC drop gears are really intended to reduce the end loading on the transfer gears whilst enabling quicker changes of overall ratio for different circuits without having to pull the engine out to change the diff. On a circuit you obviously need to be pulling maximum revs in top just before you brake at the end of the longest straight. So for different circuits a slightly different overall final drive is needed.
SC drop gears are very noisy and it is expected that a crash helmet will be being worn. With my Cooper 'S' rally car which has SC drop gears I use ear plugs or ear defenders on the road as they really are uncomfortable on any journey over about 5 miles. I intend to replace them with helicals next time I have the engine out.
Getting the power down is always a problem and it's just clutch control and power application really. With high output engines a X-pin diff is ideal, but that just reduces diff pin wear. A limited slip differential is good for racing, but simply horrible on the road or on a rally stage as the car leaps about and torque steers.

#7 Chris Jones

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Posted 22 April 2013 - 01:28 PM

Can't do much yet as the cars half stripped ready for some bodywork and engine out for cam, clutch diff (maybe gearbox).

#8 Chris Jones

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Posted 22 April 2013 - 01:32 PM

Thanks Cooperman. so you think that a setup with sc drops and sc helicals will see an equal transmission loss? I was planning on running an sw5 cam, sc box and 3.1 final drive.

Edited by Chris Jones, 22 April 2013 - 01:33 PM.


#9 Cooperman

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Posted 22 April 2013 - 07:32 PM

The difference in power loss between helical and straight-cut gears is minimal and certainly won't be noticeable.
If the car is for road use then SC drops are highly not recommended as they are horrible to listen to and will cause ear damage in the long term.
May I ask what the car is being used for? If it is a road car then you would be best served by using the standard gearbox & transfer gears, but fitting a lower FDR to effectively close up the gear ratios and make the car stay on cam a bit better.
Remember, fitting a 'hotter' cam will always mean a narrower power band and thus the need for closer gear ratios, but not so much closer as the competition SC gear ratios give.

#10 Chris Jones

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Posted 22 April 2013 - 08:18 PM

I didn't fancy s/c drops due to the noise, that's why I was planning on running helical drops and a straight cut set in the box.
It's a road car, but a 2nd "weekend car", therefore it doesn't do any motorway miles or long A road stints. I live in the Lake District so mainly B roads etc.
Yeah I was given the advice of switching from 2.7 fd to 3.1 and have been offered a straight swap from my local mini shop when the engines out and my diff arrives so was going for that too.

#11 Cooperman

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Posted 22 April 2013 - 09:52 PM

With straight cut close ratio gears you will find 1st is very high. I once tried a 1293 'S' with about 100 bhp with a 3.44:1 FDR and even then 1st was very high and needed a lot of clutch slipping to pull away, especially up a slope. 0 - 60 was slow because of that and I changed it to a 3.9:1 which gave fantastic 0 - 60, but cruising was only 14.5 mph/1000 rpm in top.
Basically you get 'owt for nowt' and if you gain in one area you lose in another.
With your engine sped you would find a standard helical box and drop gears fine but a 3.44:1 FDR much more useable. That is virtually the original 1275 Cooper 'S' gearing although the early 'S' had slightly closer ratios which may well still be available. I'll take a look on the web to see.

#12 Cooperman

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Posted 22 April 2013 - 10:08 PM

With straight cut close ratio gears you will find 1st is very high. I once tried a 1293 'S' with about 100 bhp with a 3.44:1 FDR and even then 1st was very high and needed a lot of clutch slipping to pull away, especially up a slope. 0 - 60 was slow because of that and I changed it to a 3.9:1 which gave fantastic 0 - 60, but cruising was only 14.5 mph/1000 rpm in top.
Basically you get 'owt for nowt' and if you gain in one area you lose in another.
With your engine sped you would find a standard helical box and drop gears fine but a 3.44:1 FDR much more useable. That is virtually the original 1275 Cooper 'S' gearing although the early 'S' had slightly closer ratios which may well still be available. I'll take a look on the web to see.


It appears that Mini Sport do a re-con rod-type gearbox with the original Cooper 'S'/1275GT gear ratios which are super and slightly closer ratio than the standard ratios. These ratios, with a 3.44 FDR, were very popular and gearing for twisty roads was just about right. Take a look at their web site.

#13 Ethel

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Posted 23 April 2013 - 01:39 AM

Metro wheels 'n tyres are a bit taller, you could use Guessworks calculator to make comparisons. A Mini with a 3.1 would be about the closest to the MG, but a 3.44 in a Mini works well. It's probably what the majority left the factory with, especially when the primary concern was making them drive well, more than noise or economy.

#14 Chris Jones

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Posted 23 April 2013 - 10:13 AM

Cheers guys. Been doing a bit more reading up on the advice you've given and it all sounds spot on. 3.44 FD probably will suit my driving style more than 3.1, as that seems to be a kind of compromise between performance and economy, the latter I'm not fussed about.

Just a quick question though... Could I not do the conversion myself instead of having minisport work on my box for me? Pretty sure it's in good shape at the moment so won't need a complete overhaul. Minisport don't do any helical kits as such for the conversion you mentioned, just SC, but I've read that it's just 2 gears you really need to change?

#15 Gr4h4m

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Posted 23 April 2013 - 04:13 PM

It's doable you need to have a good read up, once you've done it it doesn't seem that hard. I did my first box at the end of last year wasn't too bad. I replaced all the brearings, lay shaft, bulk rings, and added a 4 pin diff and central oil pickup.






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