4 Speed Rocket to 5 speed conversion

Need help from a fellow owner, something to air - discuss away!

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Joehale1987
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4 Speed Rocket to 5 speed conversion

Postby Joehale1987 » Thu Jul 04, 2019 10:57 am

Hi Guys,

I have a 4 speed rocket box on my Robin Hood at the moment. coupled to a 2.1 pinto on 45s.

It bugs me that I can't cruise down the motorway at 70mph without revving the hell out of the engine. Don't get me wrong you could do it but its far from 'ideal'. She is much more a brat around the forest roads kind of girl.

How complicated is a conversion to a 5 speed? I have swapped the box out on the car before so know about the engine removal and so forth but what are the issues I may come across when converting?

Different prop shaft? Different gear stick pos?

I am not 100% going to do this but its a good thing to know if / when I come to do it.

All help is much appreciated!

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Re: 4 Speed Rocket to 5 speed conversion

Postby greenwoo » Thu Jul 04, 2019 1:15 pm

others will no doubt know the technical answers, although i;m guessing that the flywheel may be interchangeable between a 4 and 5 speed? if not, then flywheel and clutch.

does it really rev the nuts off to do 70'ish?

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Fury1630
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Re: 4 Speed Rocket to 5 speed conversion

Postby Fury1630 » Sat Jul 06, 2019 3:17 pm

I've done it & you're right, the extra gear is in some spacers between the main case & the tail extension. So everything that lives in & around the main case is interchangeable, bell housing, clutch, speedo cable etc, everything that lives in & around the tail extension is about 70mm further aft, so that's gear stick, gearbox mount & prop shaft front UJ.

Is it worth trying a higher diff ratio & do the blatting in 2nd & 3rd instead of third & forth? The Rocket box ratios are better than the type 9 - first & second are quite low, so you may gain a cruising gear, but loose the close ratio selection - a diff is just a straight swap, so it my be worth a try before committing to all the work?

Jus' Sayin'
"Low Commotion" - Fury 2.0 Zetec. 160 - 170bhp, 600Kg & very very Yellow :D
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Joehale1987
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Re: 4 Speed Rocket to 5 speed conversion

Postby Joehale1987 » Tue Jul 09, 2019 8:54 am

Fury1630 wrote:I've done it & you're right, the extra gear is in some spacers between the main case & the tail extension. So everything that lives in & around the main case is interchangeable, bell housing, clutch, speedo cable etc, everything that lives in & around the tail extension is about 70mm further aft, so that's gear stick, gearbox mount & prop shaft front UJ.

Is it worth trying a higher diff ratio & do the blatting in 2nd & 3rd instead of third & forth? The Rocket box ratios are better than the type 9 - first & second are quite low, so you may gain a cruising gear, but loose the close ratio selection - a diff is just a straight swap, so it my be worth a try before committing to all the work?

Jus' Sayin'


Thats a good thought. I will have to check out what diff I have on her at the moment and see what other options are out there. I just don't feel that I could do any decent length run out without the revs being stupid high when on open fast roads. She's great for forest blatting and things like Zig Zag Hill but for motorways / dual carriage ways.... Not so great.

I will go and check my Diff now....Assuming it has a marking on the external case somewhere....

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Re: 4 Speed Rocket to 5 speed conversion

Postby Fury1630 » Tue Jul 09, 2019 11:46 am

On the type 9 & the Rocket, 4th is 1:1, fifth on the type 9 is 1:0.83, so 5k revs would be reduced to 4.1k - worth having, but it's good to know what you'll have at the end before setting out on an expensive mod program :D
"Low Commotion" - Fury 2.0 Zetec. 160 - 170bhp, 600Kg & very very Yellow :D
http://blatterbeast.blogspot.com/
http://www.pistonheads.com/regulars/ph-carpool/fisher-fury-ph-carpool/31848

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Joehale1987
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Re: 4 Speed Rocket to 5 speed conversion

Postby Joehale1987 » Tue Jul 09, 2019 11:56 am

Fury1630 wrote:On the type 9 & the Rocket, 4th is 1:1, fifth on the type 9 is 1:0.83, so 5k revs would be reduced to 4.1k - worth having, but it's good to know what you'll have at the end before setting out on an expensive mod program :D


Agreed, that's a great bit of knowledge to know. I mean a 900rpm drop is all that's really needed.

If I went from a 3.64 ratio diff to a 3.16.... What sort of difference would I see there? If you know....?

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Rob E
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Re: 4 Speed Rocket to 5 speed conversion

Postby Rob E » Tue Jul 09, 2019 2:42 pm

Joehale1987 wrote:
Fury1630 wrote:On the type 9 & the Rocket, 4th is 1:1, fifth on the type 9 is 1:0.83, so 5k revs would be reduced to 4.1k - worth having, but it's good to know what you'll have at the end before setting out on an expensive mod program :D


Agreed, that's a great bit of knowledge to know. I mean a 900rpm drop is all that's really needed.

If I went from a 3.64 ratio diff to a 3.16.... What sort of difference would I see there? If you know....?

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Re: 4 Speed Rocket to 5 speed conversion

Postby Fury1630 » Sat Jul 13, 2019 8:55 pm

Joehale1987 wrote:If I went from a 3.64 ratio diff to a 3.16.... What sort of difference would I see there? If you know....?


Hmm, this is a maths question.
Fortunately many years ago I made a spreadsheet to answer this question.

In this snapshot you see a Type 9 in 5th driving a 1:3.63 diff in the top box, with a rocket in 4th driving a 1:3.14 in the lower one - pretty much the same speed.
MPH-RPM.jpg
MPH-RPM.jpg (59.37 KiB) Viewed 121 times


In this snapshot you see a Type 9 (or Rocket) in 4th driving a 1:3.63 diff in the top box, with a Rocket in 4th driving a 1:3.14 in the lower one - so that's the difference in speed, all else being equal.
MPH-RPM 2.jpg
MPH-RPM 2.jpg (61.29 KiB) Viewed 121 times


The difference between the two boxes is that the type 9 has a ludicrously low 1st (for getting a laden Sierra moving while towing a caravan) & 2nd isn't much better, but the Rocket box's closer ratios will be a little wider spaced with a taller diff, but still closer than the type 9s.

I'm not advocating either - what's right for you isn't right for someone else, I'm just suggesting an alternative :geek:
"Low Commotion" - Fury 2.0 Zetec. 160 - 170bhp, 600Kg & very very Yellow :D
http://blatterbeast.blogspot.com/
http://www.pistonheads.com/regulars/ph-carpool/fisher-fury-ph-carpool/31848

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Joehale1987
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Re: 4 Speed Rocket to 5 speed conversion

Postby Joehale1987 » Wed Jul 17, 2019 7:39 pm

Now that is exactly what I was looking for, wow! You Sir are a legend, great work on the tables!

I will digest this and take a long think!


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