The Triumph Dolomite Club - Discussion Forum

Don't forget MX5's have bigger diameter wheels than Dolomites. So there is possibly a difference in circumference of tyre that will affect the overall ratio. It also depends which engine you put in, the 1.8i has slightly more bhp than a Sprint, not sure about the torque curve though.

I've got no plans on putting mine on a track. It's just a car I use every day. I want a higher ratio coz at the moment the larger engine drains the dolomite tank a bit quicker than the original engine. The Mazda tank would take up most of the dolomites boot! It's also lacking in top end speed so still reving high on duel carraige ways and motorways.

Karlos wrote:Don't forget MX5's have bigger diameter wheels than Dolomites. So there is possibly a difference in circumference of tyre that will affect the overall ratio. It also depends which engine you put in, the 1.8i has slightly more bhp than a Sprint, not sure about the torque curve though.

So what I mean is I think the Sprint axle will probably work well. (Was on train and didn't quite finish what I was saying)

If the overall diameter is larger then yes, it will help a little.
These webpages can help
http://www.mintylamb.co.uk/

cliftyhanger wrote:If the overall diameter is larger then yes, it will help a little.
These webpages can help
http://www.mintylamb.co.uk/

Don't really want to put larger diameter wheels on as I'm trying to keep it looking original until you open the bonnet.

philgroom wrote:

cliftyhanger wrote:If the overall diameter is larger then yes, it will help a little.
These webpages can help
http://www.mintylamb.co.uk/

Don't really want to put larger diameter wheels on as I'm trying to keep it looking original until you open the bonnet.

It is mean't the other way around: Imagine if you put smaller wheels on the MX5, the engine RPM would be higher at the same road speed than with the larger wheels. Need to consider all aspects of the drivetrain:

• gear box ratio: 5th gear of MX5 vs 4th + Overdrive of Sprint
  final drive ratio: MX5 Diff vs Sprint Diff
  Rear wheel diameter: MX5 14" or 15" vs Sprint 13"

Mazda specs.
Diff ratio.....4.3:1
5thgr ratio......0.814
Circumference of 14" wheel......1.814m

Sprint specs
Diff ratio.....3.45:1
4th+od ratio.....0.82
Circumference of 13" wheel......1.816m

Karlos wrote:

philgroom wrote:

cliftyhanger wrote:If the overall diameter is larger then yes, it will help a little.
These webpages can help
http://www.mintylamb.co.uk/

Don't really want to put larger diameter wheels on as I'm trying to keep it looking original until you open the bonnet.

It is mean't the other way around: Imagine if you put smaller wheels on the MX5, the engine RPM would be higher at the same road speed than with the larger wheels. Need to consider all aspects of the drivetrain:

• gear box ratio: 5th gear of MX5 vs 4th + Overdrive of Sprint
  final drive ratio: MX5 Diff vs Sprint Diff
  Rear wheel diameter: MX5 14" or 15" vs Sprint 13"

Can't believe the Mazda diff is 4.3! The Japs do favour short final drive ratios, on their small and crowded islands (sound familiar?) acceleration seems to top a good top speed in the desireability stakes and for that reason, they always seem to suffer from heavy fuel consumption. Especially when you consider that a lot of Jap engines don't get properly "on the cam" till 4k RPM or higher.

What is interesting is the negligible difference between wheel circumferences between the Sprint and Mazda wheels (though you don't quote tyre sizes, i'm guessing the Sprints will be the traditional 175/70/13 and the MX5s about 185/60/14) and the fairly insignificant difference between the upper gear ratios. Then a truly massive variation in final drive! The diff you have now (1500HL?) will be either 3.63 or 3.89 depending on year and either of those is still a big step up from 4.3. The Sprint diff MAY be a bridge too far, spacing the intermediate ratios too far apart to keep the engine on the boil. It might be an idea to find out which diff you have and if its the earlier 3.89, consider trying the 3.63 which is the strongest of the "small" diffs and will bolt into your existing axle case and not need a propshaft mod. Fitchetts even sell an "uprated"3.63 with stronger bearings, mostly for Spits and other small chassis cars, but it will still bolt into your Dolomite case. I used one of these in my GT6 for several years after destroying 2 x 3.27s in very short order! That combo gave me just short of 23mph/1000rpm in overdrive (0.75) top on 185/60/13 tyres.

Steve

Carledo wrote:Can't believe the Mazda diff is 4.3! The Japs do favour short final drive ratios, on their small and crowded islands (sound familiar?) acceleration seems to top a good top speed in the desireability stakes and for that reason, they always seem to suffer from heavy fuel consumption. Especially when you consider that a lot of Jap engines don't get properly "on the cam" till 4k RPM or higher.

What is interesting is the negligible difference between wheel circumferences between the Sprint and Mazda wheels (though you don't quote tyre sizes, i'm guessing the Sprints will be the traditional 175/70/13 and the MX5s about 185/60/14) and the fairly insignificant difference between the upper gear ratios. Then a truly massive variation in final drive! The diff you have now (1500HL?) will be either 3.63 or 3.89 depending on year and either of those is still a big step up from 4.3. The Sprint diff MAY be a bridge too far, spacing the intermediate ratios too far apart to keep the engine on the boil. It might be an idea to find out which diff you have and if its the earlier 3.89, consider trying the 3.63 which is the strongest of the "small" diffs and will bolt into your existing axle case and not need a propshaft mod. Fitchetts even sell an "uprated"3.63 with stronger bearings, mostly for Spits and other small chassis cars, but it will still bolt into your Dolomite case. I used one of these in my GT6 for several years after destroying 2 x 3.27s in very short order! That combo gave me just short of 23mph/1000rpm in overdrive (0.75) top on 185/60/13 tyres.

Steve

Those are the numbers that I found out about the donar car I used. Earlier I got my chassey number and it's wk2392dl so it's an early one and found that they changed the diff ratio at chassey number wk13577dl. So I suspect that I have got the 3.89 diff. How much difference to the cruising speed does the 3.63 diff have? I'm not very good at calculating these sort of things! Experience of these diffs is better than doing the maths and hope it's alright.

This stuff is calculated by the factory and takes many factors into consideration. My Vauxhall powered car has a 3.45 diff, a very similar 5th gear ratio and 195/50/15 tyres. this gives me 100mph (true, not clock) at almost exactly 4500 RPM. This suits my car perfectly, but my engine is a larger torqueyer plodder than your Mazda lump, which is probably gonna fall off the cam at a more sensible motorway cruising speed. Which would mean having to change down at 70-80mph to get any acceleration going if you needed it.

I can't give you any definitive answers because I haven't done what you have. You could say I just have a hunch! But it's a hunch with a lot of experience behind it! You may just have to suck it and see, but a 3.63 is cheaper to buy and much easier to fit than a Sprints 3.45 (you also won't have to worry about the oversize rear brakes on the Sprint axle, which come as part of the package) So I'd try that first, get a 1500 Spitfire diff, unbolt the rear case and stub shafts and it's ready to fit. They are cheap enough that if you break it in a week, you haven't lost much! But my money says it will last ok, at least til you turbo it!

Steve

Carledo wrote:This stuff is calculated by the factory and takes many factors into consideration. My Vauxhall powered car has a 3.45 diff, a very similar 5th gear ratio and 195/50/15 tyres. this gives me 100mph (true, not clock) at almost exactly 4500 RPM. This suits my car perfectly, but my engine is a larger torqueyer plodder than your Mazda lump, which is probably gonna fall off the cam at a more sensible motorway cruising speed. Which would mean having to change down at 70-80mph to get any acceleration going if you needed it.

I can't give you any definitive answers because I haven't done what you have. You could say I just have a hunch! But it's a hunch with a lot of experience behind it! You may just have to suck it and see, but a 3.63 is cheaper to buy and much easier to fit than a Sprints 3.45 (you also won't have to worry about the oversize rear brakes on the Sprint axle, which come as part of the package) So I'd try that first, get a 1500 Spitfire diff, unbolt the rear case and stub shafts and it's ready to fit. They are cheap enough that if you break it in a week, you haven't lost much! But my money says it will last ok, at least til you turbo it!

Steve

An experienced hunch is usually the best thing. Thanks for taking the time to answer and help. Knowledge comes with experience and I'm agricultural and plant machinery trained so most thing I work on are big and strong from the word go. I havnt broken this diff yet and it has had a bit of stick down country roads! Do the stub axles go straight into the spitfire diff as that has independent rear suspension? Might start finding out some prices tomorrow. Need to replace all seals on the axle as they have started to weap a little. And again thanks for all your help.

If you start with a Spit diff, first unbolt and remove the 2 stub axles (4 bolts each, they just pull out then) then unbolt and remove the rear case section. The diff is then ready to fit in your car, your 1/2 shafts should be the same spline and diameter as the Spit stub shafts and the propshaft flange is the same. To be sure it is a 3.63, not something else, look for the code FH on the diff number.

Steve

Called up a few places about diffs today and fitchetts came out on top. They were the most knowledgeable and were able to come to an answer straight away. Just mentioned I wanted a 3.63 diff from a spitfire to go in my dolomite 1500 axle and they said you won't need the spitfire mounting points or the rear or intermediate cover with the stub axles so they said I could have a discount as I'm only wanting a bare diff. All the other people I called up just said they couldn't help or I would have to get the whole assembly.

DOH, i'd forgotten about the front mounting plate! Not that it makes any difference, but you may have to put bolts with copper washers in the 4 mounting holes for where the plate was fitted as sometimes the holes are not as blind as they are supposed to be!

I've always found Fitchetts helpful and they are very close to me too so an obvious first choice for bits.

I just assumed that, since this is a bit of a tryout, you'd be looking for a pre-owned one.

Steve

Carledo wrote:DOH, i'd forgotten about the front mounting plate! Not that it makes any difference, but you may have to put bolts with copper washers in the 4 mounting holes for where the plate was fitted as sometimes the holes are not as blind as they are supposed to be!

I've always found Fitchetts helpful and they are very close to me too so an obvious first choice for bits.

I just assumed that, since this is a bit of a tryout, you'd be looking for a pre-owned one.

Steve

It's trying to find a pretty owned one that hasn't been abused and thats the right ratio is the problem. At least with a recon one it's been set up properly by people that do it all the time. I'm pretty sure that the ratio will give me the extra top speed that I'm after. I'm not planning on going racing just want to be able to go on motor ways without reving the engine to the max. Fitchetts arnt close to me but their customer service and knowledge out did the one near me.