Diff / engine queries

Post by **sowden** » Thu Nov 13, 2008 9:48 pm

Hi all,

I have recently spoiled my diff by running it short of oil (I knew I should have persevered with that stuck filler plug!!). I'll be fitting a replacement, but I wondered about fitting a 3.9 - which I understand will make motorway cruising a little less revvy (is this true?).

What are the downsides of a 3.9 over a 4.2, anyone?

Also, am contemplating a 1300 marina engine, but are these a straight swap, other than the flywheel mods (Its the exhaust and cables / hoses that bother me)?

Also - if you ran a 1300 through a smaller bore 1000 exhaust, what would be the loss in bhp?

Sorry to ask what millions will have asked before but I searched the site and of my 86 pages of hits, I managed the first 7 pages only...forgive my weakness!

All the best to you all

Russ

1968 2 door
Trafalgar blue
daily driver

Post by **mike.perry** » Thu Nov 13, 2008 9:53 pm

Dealing with the Diff. A 3.9 will give you a loss in acceleration and hill climbing ability over a 4.22 but will be ideal for a 1275 engine.
I'll leave the engine comments to others that know more about it than I do.

Post by **bmcecosse** » Thu Nov 13, 2008 10:00 pm

Welcome to the Forum.
I have just fitted a 3.7 diff to my Traveller - which at present has a 'warmed up' 1098 engine. It pulls that diff fine. And certainly if going to a 1275 then it will easily cope. 3.9 diff may be less expensive than a 3.7 - the prices seem to fluctuate quite a bit!
1275 engine is straight enough swap (you seem to know about flywheel etc) - you can certainly use the Minor exhaust complete - it will only lose some power due to strangulation at high revs. Obviously a larger bore exhaust using the larger bore manifold from a Marina will be better - but you can add that later. You may need to move the battery slightly to clear the heater tap on the 1275 engine - or just fit either a straight pipe or one of the early screw-top heater valves. Also may need to use a 'bendy' top hose to the radiator - or a thermostat cover from (I think) a B series engine puts the hose at a better angle to make that connection. Wiring and accelerator/choke etc are all straight swap.

Post by **bigginger** » Thu Nov 13, 2008 10:03 pm

OK - 3.9 diff would mean fewer revs @ M way speed, but slower acceleration and less 'power' for any given revs. I also don't think the 1098 engine is really up to using one, but I could (well) be wrong. 1275 is a straight swap, you may find you need a longer throttle cable if you use a different carb, and some thought over the choke arrangement as well. Power loss with the smaller exhaust - never seen a figure for it, but all reports are that it's a fair bit. Certainly made a lot of difference when I fitted a bigger one to mine. HTH

*Oops - cross post*

Post by **sowden** » Thu Nov 13, 2008 10:27 pm

Guys,

thanks for your super-quick and really useful responses. Now my dilema starts....I've always maintained that moving away from a standard car is a Pandora's box where you are always chasing the next must-have mod., and I've resisted up to now. But I would like that extra 10mph on the motorway when I'm late for choir practice (background sound of Pandora's box lid creaking upon being opened...)

Enjoy your Minors!

Post by **bmcecosse** » Thu Nov 13, 2008 10:32 pm

The diff may not give you extra 10 mph - it will give you less noise! The 1275 should provide the extra speed - although 70 is the limit anyway!
Remember also that any diff change upsets the speedo calibration.

Post by **mike.perry** » Fri Nov 14, 2008 9:16 am

Despite changing to a 3.9 diff and fitting slightly greater rolling radius tyres my speedo was spot on when checked with the sat nav. It was just miles out before. See my topic on speedo calibration.

Post by **Peetee** » Fri Nov 14, 2008 9:41 am

although 70 is the limit anyway!

Which a healthy 1098 should be able to manage anyway.

Post by **bmcecosse** » Fri Nov 14, 2008 10:22 am

Indeed -and more, off road of course!
m.p - you say your speedo is now accurate - but is the mileage recorder accurate ? That's the part which is gear driven - the speedo relies on a weak spring that I suspect loses strength over the years and so is likely to progressively read higher and higher!
The trick is to find a speedo with accurate mileage, and then tweak the spring to get a good speed reading.

Post by **rayofleamington** » Fri Nov 14, 2008 11:27 am

What are the downsides of a 3.9 over a 4.2, anyone?

A 3.9 diff with a standard 1098 engine will be fine as the standard gearing is a little low by modern standards anyway.
it may actually bring your gearing closer to the original! Crossply tyres had a bigger rolling radius than 145 radials do.

I've run a 3.7 diff with a rather tired 1098 and it's great apart from hills.

Post by **mike.perry** » Fri Nov 14, 2008 11:32 am

The mileometer calibration will have to wait until I do a run up the motorway with a passenger, my eyes aren't up to reading the display without glasses. Sunday I will be checking the Series MM speedo on the way to NEC, again with a passenger.

Post by **bmcecosse** » Fri Nov 14, 2008 12:00 pm

Will be interested to hear the results - from both cars - and can you get your sharp-eyed passenger to note the little 'tpm' number from the speedo on the later car ?

Post by **Kevin** » Fri Nov 14, 2008 3:40 pm

1275 engine is straight enough swap

Hi Russ the 1275cc Midget engine is easier than the Marina one as less mods are needed.
If you just want a bit more oomph that can be achieved by a bit of gas flowing of the cylinder head a better cam and a slightly bigger carb.
To start of in a simple way use an 1 1/2" su & combined manifold from a mini it will go straight onto the Minor exhaust.
Next would be a seperate inlet manifold and the inlet removed from the combined one.
You could then go for an improved cylinder head and better cam, so you do have options apart from the larger engine, if unsure which way to go just try the carb & manifold first. Also if you do go down the 1275cc engine route your insurance company may expect upgraded brakes as well.

Post by **mike.perry** » Fri Nov 14, 2008 7:27 pm

1408 TPM on the Traveller and 1600 on the MM, although the MM has been calibrated by Speedy Cables and has the correct TPM on the speedo case. Incidentally the MM speedo has a trip meter, I think it came from an MO Oxford.

Post by **bmcecosse** » Fri Nov 14, 2008 7:36 pm

Thanks for that - 1408 is normal for a 4.22 diff car.

Post by **sowden** » Tue Nov 18, 2008 11:23 pm

Guys,

many thanks indeed for your comments.

Kevin, I like the sound of a warmed up 1098 as I like the car to be as standard as poss (although the racket I get from my rear end going over the appalling roads in West Yorks has made me seriously think about a suspension mod). Will examine the Mini bolt-ons + gas-flowing. If the 3.9 diff would go OK with that, that would be peachy for me!

Thanks once more y'all

Russ