As a daily drive fuel efficiency is my number one priority above power and comfort.
I have two engines, a solid block 1275 with a 1.5" carby and a 1098 with a 1.25" carby.

First thought is generally smaller motor uses less fuel but as demonstrated on top gear an underpowered engine can use more fuel than a well suited larger motor, so I dont know:(

The motor is going into a morris 1100, not a mini (sorry:p)

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The fleet:
1949 Fiat Topolino
1965 Morris 1100
1966 Mini Van (project)
19?? Morris 1500 (for sale)
1979 Chrysler Regal
1990 FSM Niki

I would say the 1089 with smaller carb. Slower, less sting and so more fuel efficient. But those two motors I would go with the 1275 for traffic driving.

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In a Morris 1100 body I'd go for the 1275. The 1098 box was short geared (4.26:1) to cope with the body weight; IMO the 1275 will use less fuel when cruising and be much nicer to drive overall.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

i think the chrysler regal would definately get 10mph.... go for that one out of your stable....

if fuel efficiency is your motivator go modern,,,,,

a daily mini is possible , but just needs a bit more care and attention,, but the good thing is you can still do stuff on them without a diagnostic thingo

now that i have those thoughts off my chest, stick with the 1275

In my previous Moke, I went from a moderately modified 1100 (Cooper 997 cam, single HS2, 12G295 head, extractors) to a pretty much bog standard Morris 1100S engine. I got similar performance from both of them, but the 1275 used significantly less fuel than the 1100.

Tim

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1951 Morris Commercial J Type Van
1955 BSA C11G
1961 Morris Mini Traveller
1969 Triumph TR6R
1977 Leyland Moke Californian

If you lightly modify and engine with a good exhaust, air filter and generally improve the flow, you will see an increase in fuel economy, provided you drive the same. But many like using the extra performance - a better flowing engine can save fuel, but you can use more fuel if you drive it heavily.

As soon as you add a performance cam, your fuel economy will go out the window. Too much overlap means too much fuel goes straight through the engine. You can probably get a good cam with decent fuel economy, but most cam upgrades are focused on performance.

That's a slightly different kettle of fish there. That was on a race track, and the bigger engined car was only keeping up with the smaller one. If your sole objective is the most fuel economy then the smaller engine is what will do it. If you want good economy with respect to performance, a larger detuned engine is what you want.

My 2c.

I've got a rebuilt 1275 with 1 3/4 carby and just sold my other mini which had 998 with 1 1/2.
I got better fuel economy out of the 1275 then the 998 both are Clubbies.

What happened to the supercharged 1275?

Ash.

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----------1984 Mini Mayfair with not much 1984 Mayfair left of it!-----------
----------1968 fake Cooper S that messed with a tree -----------------------------

Haha yeah the chrysler only uses 20l per hundred km. I basically drive lead footed to the speed limit.. Thats what made me think maybe the 1100. Gearing is irrelevant, I only have one diff.
Ash, it's the 1275 I'm talking about, I'm going to return it to standard specs and sell the fast bits, it drinks fuel and I cant afford it

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The fleet:
1949 Fiat Topolino
1965 Morris 1100
1966 Mini Van (project)
19?? Morris 1500 (for sale)
1979 Chrysler Regal
1990 FSM Niki

I thought our AP5 valiant was bad with 14l/100kms!

It might pay to think about maintenance issues too. Surely that would eat into any potential fuel saving?

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"Beware the lollipop of mediocrity: lick it once and you'll suck forever" Brian Wilson

Maintenance is no drama, I'm a mechanic:) plus all my cars are old and I've been banned from buying more unless I sell some of the ones I have, which I don't want to do:) I want to drive my 1100 again and one way or another the engine needs to come out for gearbox repair, so I thought I'd take the opportunity to put in whichever engine will be most economical

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The fleet:
1949 Fiat Topolino
1965 Morris 1100
1966 Mini Van (project)
19?? Morris 1500 (for sale)
1979 Chrysler Regal
1990 FSM Niki

Sounds like you need the tennis ball mod.

take one tennis ball, insert under accelerator pedal, guaranteed to improve economy

I'd just stick the 1275 in anyway. Even if it does use more fuel, would it really be that much more to give up the extra grunt/driveability? (Is that even a word??)