Changing 5th gear ratio

CVH and Zetec tuning, suspension and braking mods.

Changing 5th gear ratio

Postby ben0307 on Thu Jan 06, 2011 11:14 pm

I posted this on Fiesta-Si.com a while back http://www.fiesta-si.com/forums/viewtopic.php?f=13&t=20997 but still haven't found the solution...

I've seen a few people suggest changing the 5th gear in the 1.6 Si gearbox to a longer ratio (for better mpg / higher top speed / motorway use). Has anyone actually done it?

Standard Si ratio is 0.88, and it seems most other BC gearboxes have the longer 0.76 ratio. Apparently the 5th gear and the gearbox input shaft are a matched pair though, and should only be changed together. This is a problem because I need to keep the Si input shaft (want to keep the Si ratios on 1st - 4th). I have a 0.76 ratio 5th gear from an ERST S2 gearbox, and although the spline is basically the same, it doesn't seem to fit.

Image

Has anyone actually changed just the 5th gear, or have any experience of if it is possible?
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Re: Changing 5th gear ratio

Postby MB7201 on Thu Jan 06, 2011 11:51 pm

The splines might just be different for the older s2 box. I have an s2 diff and I had to change the crown gear, again because it was slightly different. Heres a list of common boxes, I would suggest trying a mk3 fiesta one: http://st2000.s5.com/ftransguide.html#g ... nformation
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Re: Changing 5th gear ratio

Postby ben0307 on Sat Jan 08, 2011 11:19 pm

Good point. I spotted the crownwheels had different part numbers stamped on them, but didn't think to look on Microcat to see if there are different part numbers for the 5th gear/shaft.
Have looked them up today, and come up with this:
Fiesta 1.6 Si :
* P92WT 7L226 CA
Escort RS Turbo S2 :
* P83WT 7L2226 AA (-1986)
* P92WT 7L2226 AB (1986-1995)
* P92WT 7L2226 AB (1986-1988)
Fiesta Mk3 1.8 Diesel :
* P91WT 7L2226 AB
* P92WT 7L2226 AB (1989-1995)
Fiesta Mk3 1.1 CFI :
* P91WT 7L2226 DA (1985-1995)
Fiesta Mk3 1.3 EFI :
* P91WT 7L2226 DA (1992-1995)
* P92WT 7L2226 AB (1986-1992)

Obviously I would expect different part numbers because they are different ratios - I think the last two letters denotes this.
It appears that the start of the part number generally represents the year that the part was designed or first manufactured. Given that the parts listed are nearly all 1991 or 1992, and the RS Turbo box must be between 1986 and 1990, I am thinking that this part has probably been revised for later gearboxes produced after 1991/1992. This may or may not have involved changes to the spline profile, but I figure it's probably worth trying to get a later 5th gear to try it.
Will get something posted in the wanted section...
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Re: Changing 5th gear ratio

Postby ben0307 on Sat Jan 15, 2011 10:46 pm

A closer look at the splines shows that they are slightly different - see photos below. Both gears fit their original shafts fine, but neither will fit when they are swapped over.
Image
The gear on the left is 0.76 ratio (45 teeth) taken from an ERST S2 gearbox.
The gear on the right is 0.88 ratio (41 teeth) taken from a Fiesta 1.6 Si gearbox.
Both gears have 25 splines, but the angle and depth of the splines looks different. The V shape on the Si gear is almost a right angle. On the ERST gear it's a sharper angle, and looks slightly deeper.
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Re: Changing 5th gear ratio

Postby ben0307 on Thu Mar 17, 2011 10:50 pm

I think I now have this sorted, after my dad spotted an advert for a company called GEARS in a magazine:
http://www.gears.tv/
They have an excellent article on their website that describes these gearboxes in detail:
http://www.gears.tv/features/rsturbo.pdf
This confirms that the later '92WT' gearboxes have a different design of spline to the earlier units (see page 4/5).
I had no luck trying to find a 92WT gearbox with the 0.76 ratio locally, but GEARS supplied me the 5th gear I needed for a very reasonable price. Thought I should post details here in case anyone else wants to do something similar in future.
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Re: Changing 5th gear ratio

Postby Mav Singh on Thu Jun 02, 2011 6:45 pm

howmuch did the gear cost?
wondering if they have a 0.70 ratio gear.
1.6 si
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Re: Changing 5th gear ratio

Postby ben0307 on Wed Jun 22, 2011 12:12 am

Think it was about £15 delivered for the input shaft gear on it's own (as I already had the main shaft gear from the RS Turbo box). Would be more if you need the pair - maybe about £30 or £40?
There are only 3 standard ratios based on the list I have:
0.83 only fitted to Mk3 Escort RS1600i (quite rare I guess).
0.88 fitted to Zetec Mk3 Fiestas (1.6 Si, XR2i 16V, RS1800). Not sure about Zetec Escorts.
0.76 fitted to everything else (so should be common).

The input shaft and gear are supposedly matched, so look for any coloured paint marks and try to get a gear with the same colour if possible, as it will fit the spline better.
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