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Gearbox Mods


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#1 IronmanG

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Posted 03 December 2019 - 05:40 PM

So as a general rule of thumb what sort of mods are required for 100 hp, 120hp, 150hp.
Can helical gears handle that.
X pin diff seems to be a given

#2 imack

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Posted 03 December 2019 - 06:02 PM

N/A or boosted? Very different requirements

#3 IronmanG

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Posted 03 December 2019 - 06:22 PM

Could be either. Didn't realise it was that different

#4 nicklouse

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Posted 03 December 2019 - 07:51 PM

Depends on final drive as well.



#5 IronmanG

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Posted 03 December 2019 - 08:16 PM

Ok so supercharged would probably be 2.9
NA is 3.4

#6 DeadSquare

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Posted 03 December 2019 - 08:54 PM

BHP is torque X RPM, so if you can keep the torque near constant while doubling the RPM you get twice the BHP.

 

From this you can deduce that if you double the speed at which the power goes through the system, it will cope with twice the torque.



#7 Dusky

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Posted 03 December 2019 - 09:04 PM

From this you can deduce that if you double the speed at which the power goes through the system, it will cope with twice the torque.


No.

#8 mini13

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Posted 03 December 2019 - 10:02 PM

Indeed, it will cope with the same torque but higher power, they actually used to do this on the early focus wrx cars,

#9 IronmanG

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Posted 03 December 2019 - 10:09 PM

So what kind of mods do I need in order to cope with say 100 lbs ft and a 100 horse. And 125 lbs ft and 125 horse

#10 nicklouse

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Posted 03 December 2019 - 10:14 PM

Nothing. Just make sure it is in good order. And if you are running a FD for top speed then maybe a 4 pin diff. 3.1 to 2.9 or whatever)



#11 IronmanG

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Posted 03 December 2019 - 10:37 PM

So some people saying on here straight cut drops etc. That's overkill?. For total reliability on the road
Just a X pin diff

#12 grizzler73

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Posted 03 December 2019 - 10:59 PM

Not sure what you are asking here. You can put a fair bit of power (however you want to measure it) through a gearbox, just means it won't last that long the more work /stress you put it under. What are your aims?

Edited by grizzler73, 03 December 2019 - 10:59 PM.


#13 Anthony30

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Posted 03 December 2019 - 11:39 PM

I'm going for the minispares heavy duty helical gearset, mpi drop gears, and quaife atb on my 1293 turbo.  This is meant to be a good reliable setup. Straight cut gears are just a pain, and imagine they would be awful to live with. O_O



#14 nicklouse

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Posted 03 December 2019 - 11:50 PM

So some people saying on here straight cut drops etc. That's overkill?. For total reliability on the road
Just a X pin diff

My box is currently standard, and it had a 1380 286 etc on it as a road car. And currently has a 1293 full race engine on it both as FWD and RWD just with straight cut drops of a different ratio and a LSD. They are way stronger than people think.



#15 Cooperman

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Posted 04 December 2019 - 12:32 AM

The complications come with higher powered engines. To increase the bhp a 'hotter' cam is needed. With a hotter cam, the power band becomes narrower.

That, in turn means closer ratio gears are needed to keep the engine 'on the cam'. Then 1st gear becomes too high to pull away easily, especially uphill and a much lower final drive ratio is needed. A lower final drive, in turn, means higher revs in top gear for a given speed.

Thee is little point in having a powerful (say 120 bhp) engine, if the gearing means a lower powered car with a wider power/torque band will accelerate better and not keep on requiring new clutch plated due to the need for excessive clutch slipping to pull away.

Up to about a 276 cam (or equivalent), a standard gear box with a 3.44:1 FDR can work, but when you get to a 286 or similar, a SC CR gearbox is needed with around 3.76:1 or 3.9:1 FDR in order to get decent acceleration from rest or through the gears With a 296 or hotter race cam it is often necessary to run a 4.1:1 or 4.3:1 FDR.

It's all a bit complicated, but it easy to fit too high a FDR and then have poor acceleration.






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