The Triumph Dolomite Club - Discussion Forum

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PostPosted: Thu May 16, 2019 8:42 am 
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As some of you may know, I sprint and Hillclimb my ’75 Sprint which I have kind of race prepared for the discipline; even though it is road registered with an MoT and is taxed etc. it is not generally a road car.

To help with the faster venues like Goodwood and Blyton where gearchanges cost time, I have compiled a list of different gearboxes noting their ratios and predicted output speeds when applying the standard 3.45:1 final drive.

Clearly, these are hypothetical as they don’t take into consideration that car is shaped like a big unaerodynamic brick etc. but they are useful for giving a feel for the differences in ratios.

I’ve looked at the Ford type 9, 4 and 5 speed (V6 and straight 4) TR7 4 and 5 speed and RX7 boxes as the latter seem cheap and plentiful.

Looking at the figures, the best box by far for a better speed spread is the TR7 4 speed.

What can anyone tell me about these ‘boxes? Are they similar to an 1850 ‘box? It has been suggested that it might be out of a 1.8 Marina….hmmm…

Has anyone done the conversion…what might I need to do to make it fit…..will it be strong enough? I’m aiming for something like 150 BHP (at the wheels).

Looking forward to receiving your wisdom chaps! 😊

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PostPosted: Thu May 16, 2019 9:51 am 
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Joined: Thu Sep 28, 2006 12:13 pm
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You describe the 4 speed TR7 'box fairly accurately; I suspect it would be short lived with 150 bhp behind it.

I have the 5 speed '7 'box in the Quicksprint, it is OK for road use, but on a relatively tight circuit such as Castle Combe I found the ratios just wrong. Mad Mart was going round simultaneously and I could jealously hear him exploiting his overdrive whilst I was having to waste time with clutch and stick! It might be better on a more 'long legged' circuit such as Silverstone, but I haven't had opportunity to find out...yet...

Other disadvantages of the LT77 5 speed 'box are it is heavier than a Sprint o/d 'box....even after you've hacked away at the bellhousing to get it to fit in a Dolomite engine bay!


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PostPosted: Thu May 16, 2019 1:12 pm 
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Location: Ayrshire, Scotland
The Land Rover box, both LT77 and R380, are not only heavy but clunky old things. I have an R380 in my 3.9 litre MGB V8. It can handle the torque OK but is not a pleasant thing to use. Fortunately with a low revving V8 gear changing is not necessary too frequently. I once read a magazine report of the Morgan V8 when it first came out. The tester said that to win the traffic light grandprix, you just had to select a gear - any gear would do. My B is a bit like that!

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Mike
(MGB GTV8, BMW Z3 2.2, and Dolomite 1850HL)


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PostPosted: Thu May 16, 2019 5:05 pm 
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The Mazda RX8 5 speed boxes are meant to be stronger than the 6 speed.
6 speed are electronic speedo.

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PostPosted: Fri May 17, 2019 5:49 am 
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Joined: Fri Oct 06, 2006 8:57 am
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Location: Auckland, New Zealand
I've inherited Toyota 'boxes converted to fit a Dolomite. The T50 (Sprinter) was not strong enough and soon wouldn't reliably go into third. The W5* series (Celica / Supra) tend to be heavy and ratios aren't necessarily what you need for racing so you need to know what ratios you want before you buy.

I've also tried standard Sprint non overdrive, ST close ratio non overdrive and a standard Sprint overdrive gearbox. Depending on the circuit, the ST close ratio and standard overdrive produced the best lap times. However, the ST close ratio was difficult to get off the start line with such a high first and consistently lost grid places before reaching the first corner.

As a result I ended up sticking with the standard overdrive 'box. I modified it to use overdrive on second, obviously you can't change from second into overdrive with full power, BUT changing into second overdrive while braking into a sharp corner (hairpin) produced a very useful additional ratio to keep on the power band when exiting a corner when second was too low but third just too high. Might only be for a few seconds but acceleration out of a tight corner can mean picking up a place. With seven potential ratios it also meant a narrow powerband could be used effectively. However 4OD was reserved only for the end of long straights. This was using a 4.1:1 diff ratio and most importantly an LSD. I would start with the diff ratio and LSD rather than gearbox as this, coupled with good tyres will improve race times the most.

Also I used to 'crash' the gears ALL the time. You just can't wait for the synchro, but I found the 'box lasted a very long time (many years) even subject to that level of abuse. I actually only broke one over 17 years and 105 race meetings.

I also found a large yellow warning lamp on the dash that showed when OD was engaged was extremely useful in the heat of a race to remind me what was going on!

All this with a lightened flywheel, 7.25in clutch and centre release bearing from Rob at Sprintspeed

Hope this helps

Geoff


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PostPosted: Fri May 17, 2019 7:48 am 
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You need another diff.

Jeroen.

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