Changing the diff ratio

[martinclan]

For various reasons I am comtemplating a change of diff ratio and there are a couple of questions I would like to pose the forum...

1. Am I correct in thinking that any of the small Triumph diffs will fit. (Herald, Spitfire etc.) There are a lot more of these knocking about than Marina ones.
2. As I understand it the main job in changing the diff is removing the halfshafts. I am sure I read somewhere that getting the halfshafts is a real pig of a job. Is that correct? Or am I confusing it with removing the hubs from the halfshafts which I guess I don't need to do.

Cheers (once again) Robin

[MarinaCoupe]

Robin, Assuming you have a Coupe, Saloon, Estate or 7cwt Van, then you are correct in saying that the light Triumph cars use the same differential. Early Herald diffs are weaker than the Marina ones. Compatible diffs can be found in early 4 speed TR7s, Dolomite (not Sprint), Spitfire Mk4s and 1500, late Heralds.

If you remove the axle hub, backplate and halfshaft as one peice then they come out quite easily, particularly if you use a slide hammer connected to the end of the halfshaft thread.

As a word of caution, if you are thinking of fitting a 3.63:1 (1800/1700) diff to a 1.3 engined car, you’ll find that the car will be very sluggish going uphill requiring you to change down to 3rd to build up speed again. The 3.29:1 diff fitted in the 2 litre Ital Auto is even worse.

The 1.3 engined cars and Commercials came with various ratios from 4.55:1 for the van, to 4.11:1 for the Saloons, Coupes, Estates and 3.89:1 for late 1.3 Itals.

[martinclan]

Thanks Chris that's very useful to know.
I currently have 3.89 diff as the doner was a 1.8sc. But I am changing wheel sizes to 15" and am currently negotiating for an alternate drive train (coventry climax based) which means a 4.11 diff would be more appropriate. Of course it might all come to nothing...

Cheers Robin

[balmy]

Pretty certain I have a brand new crown wheel and pinion in 4.11 if your interested.

[martinclan]

Thanks Balmy. But I am afraid my skills (and tools) aren't up to setting up a diff from scratch.

Cheers Robin

[balmy]

Apparently my skills weren’t good enough to buy the correct ratio I wanted


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[MarinaCoupe]

Thanks Chris that's very useful to know.
I currently have 3.89 diff as the doner was a 1.8sc. But I am changing wheel sizes to 15" and am currently negotiating for an alternate drive train (coventry climax based) which means a 4.11 diff would be more appropriate. Of course it might all come to nothing...

Cheers Robin

All the 1.8s and 1.7s came with the 3.63:1 diff ratio, including the 1.8 SC, so unless it’s been replaced at some point, you should currently have a 3.63:1 diff. The 3.89:1 was only ever available as standard in 1982-84 I the 1.3 Ital Saloon & Estate.

[martinclan]

Thanks Chris. I don't know where I got the 3.89 from. I thought I could blame the Haynes manual but it's correct in there.
Looks like it is academic anyway as the owner of the Coventry Climax engine I was looking wants far to much money. So back to plan A - the B series and a Peter Burgess head.
Cheers Robin

[MarinaCoupe]

Robin - David Balme has fitted a Peter Burgess on his TC Coupe and I ran locally modded head using data from the Peter Burgess B Series Tuning book. The different ‘recipes’ for combining certain parts, cams etc, makes it much easier to develop a car to the spec that you want.

[martinclan]

Thanks Chris. Yes I have a copy of the book. The plan is (now) to go for a Peter Burgess Econotune head with a rebored (+60 thou) block. I am looking to do this quite soon, even though the car is nowhere near completed, as Peter Burgess is putting his prices up in the new year!

Cheers, Robin

[PHUQ]

Sounds like a sensible spec Robin. Probably the one I should have gone for!