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Choosing gearbox ratios

Chris Perry

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Dear All,

Are there any gearbox ratio experts on Blatchat?

I am about to place the order for a new Ford Type 9 gearbox for my 1.6 140 bhp Sigma car.

Ideally when choosing the ratios do you want:

a) Rev drops between gear change to be the same.

b) Equal speed range change in each gear from shift to shift.

c) Equal ideal shift points.

Thanks in advance,

Chris Perry

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There's a vast amount on this in the archives, including the solutions (compromises) that other have gone for.

I think it boils down to three initial questions:

  1. What are you going to use it for?
  2. Have you already decided your final drive ratio and rear wheel size?
  3. Have you found a gear calculator that you like yet?


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what final drive (diff) ratio do you have as this will affect your choice of 5th ratio, either for cruising or track.

Don't be tempted to go for too high a 1st gear overall ratio (1st X diff ~ mph/1000 rpm).  Doing 60 mph in 1st sounds impressive in the pub but can make it hard driving in traffic and getting away from the lights, especially on a steep incline.  (Smelly clutch warning).

Have a look at SPC semi helical and recent posts regarding the recent addition of a slightly higher 1st ratio, which looks like a good compromise.


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Hi Chris

I used to find first gear far to short on the standard box - felt I was snatching for second far to soon! So I changed my first and second gear ratios (3.36 to 2.75 and 1.81 to 1.75) when I got my BGH box. I now find first a bit of a pain in stop start traffic and if I am trying to get off the mark quickly I often slip the clutch too much and do end up with that lovely burning smell! However once moving I much prefer the longer ratios - especially on the track - which I do not spend enough time on!

Have a Sigma with circa 200 bhp

Good luck.



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Firstly consider:

Road or track  (including touring)

Wheel and tyre size (rolling circumference)

Max RPM used.

Final drive ratio

Power / Torque characteristics - (the gearboxes sole purpose is that of a torque multiplier)


For a 140BHP road car I would be looking at a helical kit to keep the noise to acceptable levels.

A 2.75 is still too low for such a lightweight car, by using a 2.48 you'll get a first gear that can be used on the road for tight corners, you'll be able to select 2nd to 1st at 35-40 mph.

I've used 2.48 and 2.38 firsts, my current box (to be used behind a pokey Duratec) is being built with a 2.29 1st, as Steve (SPC) has made a small batch up after a couple of years of my nagging him.

Steve now offers straight cut and helical kits built to his own design, much attention has been paid to reducing rotating mass to speed up gears shifts and reduce the strain off critical driveline components, bearing design and size, tooth profile and pitch and surface finish al put his kits way ahead of anything else on the market.

Shift points on the ratios should get closer as the speed increases in order to offset of increasing wind resistance.

If you choose a new kit you'll often have the option of 5th gear ratios .97 .92 .87 .82 etc the .97 equates to circa 250 rpm drop so perfect for track use to keep the engine on peak power to pull top end speed whereas the .82 offers a more relaxed cruising with increased economy for zipping down France.

So based on 185/60/13 tyres with a max rpm of 7000rpm I'd be looking at:

A gearset with 2.48, 1.62, 1.22 1, .87 ratios which would offer the following: (3.92 final drive)

Theoretical Max (MPH) speed in gear  46.2, 71.2, 84, 114, 132,

RPM drop between shifts  2456,  1696,  1262,  910  (based on calcs to 2 decimal places)

Cruising RPM @ 70mph                     - 3714rpm

                            82mph (130Kph)    - 4351rpm

BM me if you need more details as I've spreadsheet with more comparisons.

Link to Steve's Tracsport gearsets at SP Components






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Has anyone purchased one of the kits from SPC and installed it themselves?  Is it easy so long as you take your time or one of those jobs best left to those who know what they are doing.

Do you still get the over run noise with the SPC kits?

Do you get over run noise with the Mazda box?

Thanks, Andrew

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I did my own a couple of years ago and it was reasonably straight forward. 

You'll need to specify whether you're converting a standard type 9, or the XR4 version which has a different (stronger) front countershaft bearing arrangement, and specify the correct input shaft length for your engine and whether the spigot end needs trimming.

A press is pretty essential too, although if you go and collect the kit from Steve he'll probably press the necessary bits for you.  This may only apply to the standard, non XR4 box.

Although they may be perfectly serviceable, consider replacing the bearings while you've got it apart, but they're not cheap.  Also the blocker bars and springs as the springs can wear and eventually snap.  Definitely replace the three seals and the top cover, tail casting and box-to-bellhousing gaskets, and don't forget to put sealant around the selector rod hole on both sides of the gasket, as it will leak if you don't.

Provided you're careful and take your time, it really isn't difficult and Steve will give you lots of advice, just be prepared for a very long chat!


ps my box is nice and quiet, but I'm comparing it to a Quaife straight cut *headache*

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Firstly Steve does not do 5 minute chats............... *biggrin* before calling him make sure you've had a pee and got a mug of tea to hand, Steve's a top bloke who is passionate about his products and will take the time to talk to you.

All Steves Tracsport kits now use the larger front layshaft bearing for both box options  ie Std type 9 which originally ran needle rollers or the 'stub shadf' design used on later V6 applications.

Fitting is reasonably straight forward though a press is required for std boxes as Steve uses an interference fit bush.

If your not sure for a very reasonably charge Steve will rebuild your box with a Tracsport kit.


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I think AndrewB's comments relate to the nosies generated by the stock Caterham supplied 5 speeder, in comparison to the SPC Tracsport kit.

By there very nature the boxes Caterham supply are reconditioned and non serviceable parts will have been replaced with parts from other boxes / sources which can affect running clearences, which in turn influences whine, shunt and laygear chatter. - in the same way that some final drives are musical and some being correctly built are silent

Steve's gearsets are custom made to his own spec and precise tolerances.

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Mine is indeed the Caterham standard 5 speed type 9.  Everything works a treat and so shouldn't complain it's just a tad frustrating to lift off in 4th and sometimes 3rd and get what sounds like a bag of bolts bouncing around in the gearbox.  It's not the end of the world but I do quite fancy the challenge of understanding a gearbox but wouldn't want to do so if I am likely to balls it up or make the over run worse.

Many thanks for the replies.

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I can't help wondering if the 'bag of bolts' on the overrun only happens with Duratec engines?  I have had several BGH modified 5 speed boxes in the past, bolted behind Vauxhall 16v engines, and had no noise whatsoever, I then put one onto a 2.3 Duratec in a CSR and it was a little noisy, I have done the same again and it is even noisier on the overrun, yes it is a bag of bolts alright and bloody annoying, although Chris at BGH Geartec says that there is nothing wrong, TADTS!  Maybe it is more common than I thought?

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Poor mapping will compound this and start the oscillations that will increase in magnitude and eventally induce driveline shunt.

Poor engine and gearbox mounts and sticky, poorly routed throttle cables and linkages with poorly engineered ratios will all contribute too.

I've run to straight cut boxes behind Duratec's and aside from the normal whine theres no shunt or chatter, but mine was mapped by a certain Mr Walker.

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  • 1 month later...
  • Area Representative

I have an SPC transport gearbox. Ratios are great for my car which is used for touring. The only downside is that it is much noisier than my original 5 speed box especially in 2nd and 3rd.

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Thanks gents.  SPC ratios look good but my existing standard type 9 is pretty quiet so I am now hesitant in taking the plunge. SPC website states 'the gear tooth profiles are much larger than ford original spec and are similar to a typical S/cut gear. The .82 5th gear pair have an increased helix angle and finer pitch teeth for quieter cruising'.   I guess that the larger gear teeth profiles will be stronger but the trade off is more noise (though nowhere near as bad as straight cut).  BGH don't seem to offer ratios quite as high as SPC but does anyone know if they (or any other suppliers) are likely to be any quieter/as quiet as a standard gearbox?





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