Preformance increase AND fuel economy?

linearaudio · Post by **linearaudio** » Wed Jul 16, 2008 8:26 pm

Hallo there. I would like to "pep" up my 1098 traveller a little, but with a view to keeping fuel economy up. The 12G940 head swap sounds good, maybe with a MG Metro cam and 38mm metro carb on the MG manifold, but is it going to drink the fuel? I don't want mega performance, just the ability to maybe pull a more relaxed diff ratio. ( I have got the Vizard Bible, but haven't tried anything yet) Any thoughts?

Post by **MoggyTech** » Wed Jul 16, 2008 9:29 pm

Increased performance doesn't mean bad fuel ecconomy unless you thrash it of course. If you drive a tuned car with ecconomy in mind, you can get better MPG than a standard car.

Post by **bmcecosse** » Wed Jul 16, 2008 9:33 pm

Yes - that's all great stuff. Pretty much like my engine - although I am still on standard cam - the 'spare' engine has that MG cam installed - but the engine that's in the car just keeps on going.
If you drive faster/ harsh acceleration - then yes you will use more fuel. But if you just cruise at a reasonable speed - then the higher compression ratio from the 940 head will make the engine more efficient, and so provided you have chosen a nice weak needle for the carb - it should give better economy. Also important to get the ignition timing spot-on (just short of pinking under load) - and make sure the vacuum advance unit is working too. If you can get a lower rear axle ratio - then for given speed the engine will be doing less revs - with the throttle plate further open (to get enough power at the low revs) and so the cylinders will be better filled with a higher 'barometric' (some call it 'dynamic') compression ratio - and hence better efficiency. Thus you should always try to drive in the highest gear the engine will handle - without it falling off the cam of course. This is where the standard AEA 630 cam may be a better bet than the MG cam - it peaks at lower revs and so may be a better 'slogger' if that's what you want. If you want a revver - then go for that MG cam - it is excellent. Remember to sink the exhaust valves into the 940 head by 40 thou - with either of these cams.

Post by **Kevin** » Wed Jul 16, 2008 9:38 pm

Sounds good to me apart from the Cam a Swiftune SW5 would be my choice (have a look at mini forums) also dont overlook the HS4 carb and matching inlet as the MG one is really for the Hif44.

Post by **bmcecosse** » Wed Jul 16, 2008 9:45 pm

SW5 has a higher lift - it would not be wise with the 940 head 'exhaust sinking' (too much sinking required !!) - instead you would need to make pockets (deep ones!) in the block - or both . Of course on a 1275 engine the valves can poke down the bores - so it's not a problem there. However my money is still with an MG cam - the higher lifting SW5 puts more strain on the valve gear and no power advantage over the MG. I believe it was designed as it is -in order to be able to be reground from a standard cam - hence they were limited on the timing that could be applied and could only go for extra lift.

linearaudio · Post by **linearaudio** » Wed Jul 16, 2008 9:54 pm

Thanks, I feel better for all that opinion! One more thing though-the exhaust- doesn't seem to be much around in the big-bore department, or would it be silly to adapt an RC40 silencer onto the existing peashooter? How much restriction is due to the pipe diameter, and how much due to the fartbox internals?

Post by **bmcecosse** » Wed Jul 16, 2008 9:57 pm

Quite a lot I think! The noise from my standard exhaust is a 'shriek' at high revs! Free flowing exhaust aids economy buy reducing engine back pressure (less pumping losses) - so yes it will be worthwhile fitting an LCB and a decent bore system - possibly adapted from a Mini. the MGB puts the exhaust under the rear axle - may be worth considering - although it does have straps to limit the 'droop' of the axle.

Post by **Kevin** » Wed Jul 16, 2008 9:59 pm

One more thing though-the exhaust- doesn't seem to be much around in the big-bore department,

The best is made by Maniflow but at a price or the alternatives from some of the Moggie specialists.

Post by **Peetee** » Wed Jul 16, 2008 10:01 pm

My choice would be a 12G295 head skimmed 80 thou to give a slightly higher compression ratio. With a water heated inlet designed for and using a HIF38 carb (1.5 inch) Good economy is down to good torque at medium revs. To achieve this you need high gas flow and the smothest inlet path possible - hence the matched inlet components.

linearaudio · Post by **linearaudio** » Wed Jul 16, 2008 10:04 pm

Thinking cheap and cheerful- could the Marina pipework be made to fit with better back box substituted?

Post by **bmcecosse** » Wed Jul 16, 2008 10:13 pm

Yes - i'm sure a Marina exhaust could be adapted to fit. You would need the bigger bore exhaust manifold to go with it. There is also a twin exit manifold - that system may take a bit more 'adapting' to make it fit into a Minor!