The good old 948cc 'A' Series

[philthehill]

A standard 1275cc engine should be limited to 6500rpm max.

Dependent upon the state of tune a 1275cc can reach in excess of 8500rpm and beyond.
A mildly tuned 1275cc should still be limited to 6500rpm unless balanced and component weight reduction undertaken.

The real benefit of the 1275cc is torque.

[smithskids]

I'll stick with the MG 1275 plus 270 Kent cam and 38mm rimflow inlet v/vs. 0.030 off head and a janspeed 3 branch into a modified minispares RC40 exhaust system plus HIF SU on water heated inlet. This lets me drive from 25 mph to70 plus in top gear. I gave up smoking starts when I passed 70th birthday.[img][img][img][/img][/img][/img]

[philthehill]

Torque is the way to go - makes for a more relaxed driving experiance.

[olderisbetter]

Phil the engine in my 58 traveller is a 948, I want to keep the car pretty much as it was but would love the extra power from my original engine, I think you have mentioned it can be done with parts from other engines, just wondering what parts i should start looking for, Thanks Rob.

[jagnut66]

Hi,
With a standard 948 engine would you improve performance and breathing by simply porting the inlet and exhaust holes where the manifold attaches and, if so, would things be then further improved with the fitment of a larger bore exhaust, (I believe 1 1/2" is the next size up from standard) with a corresponding larger SU fitted. I would want to keep things looking pretty standard, which is why I'm thinking about this.
Excuse my question if you think the answer is obvious but this is the first 948 engined Minor I've run, so I'm toying with a few 'minor' upgrades, though nothing along the lines of the 1275 route I took with Abby (been there done that, so out of my system), I would keep anything I did to Olwyn far simpler and closer to standard.
Many thanks,
Mike.

[philthehill]

Rob/Mike
I am in the process of building a budget 998cc based on a 948cc block.
Many of the non 948cc Minor spares I will be using I have on the shelf or obtained S/H off 'e' bay.
This is the process I am and will be taking.

Obtain a set of 1098cc Minor fully floating con rods.
Obtain a set of good set of late 998cc Mini three ring pistons (the piston rings will be replaced as a matter of course (new rings £74 a set)). Do not get the interference fit 998cc 'A' Plus rods/gudgen pin unless you have the special tool to pull out/in the gudgen pin.
Re-bore the 948cc block to 998cc. Important to bore the block to the pistons.
Obtain a good 1275cc camshaft (Marina/Midget) (whilst not as good as a Cooper 997cc camshaft Pt No 2A948) the extra valve opening/overlap period is of benefit). The 1275cc camshaft can be obtained quite cheaply (£15-£20). You will need a new oil pump/drive spider and oil pump spacer (dependent upon pump used).
Replace the cam followers as a matter of course.
Replace the head with a 12G202 head casting skimmed by at least 0.030" preferably 0.060". Better gas flow over the Minor one.
The 12G295 head whilst the best for the 948cc/1098cc engines is now so expensive (£300 plus) as to be of no benefit if cost/benefit analysis is undertaken.
Fit grooved valve guides to the inlet valves so as to be able to fit the top hat seals.
Replace the valve springs as a matter of course.
I have got a 12G185 Cooper S head to go on my 948cc/998cc block which does not require valve pocketing or recessing of the valves.
These heads were notorious for cracking between the valves when fitted to the 'S' but this one seems to be serviceable and in good condition.
Fit duplex timing chain assy and later neoprene seal timing cover (Mini).
There are several things you can do to the 1.25" H2/HS2 Minor SU carb to make it flow better (see CCC book on Mini Tuning) but the 1.5" H4/HS4 Marina is a better bet.
Gas flow the inlet manifold to suit the carb used.
Replace main and big end bearings.
Lighten the 948cc flywheel.
Fitting the 1098cc flywheel/clutch assy unless lightened will not benefit a 948cc/998cc engine as regards engine rev pick up.
Fit a 1098cc Midget standard exhaust system.
Getting the air in and the exhaust out is the best way of improving the 948cc.
A lot of the things mentioned above do not effect the look of the 948cc engine and at first glance it would look totally standard.
Whilst not a racing engine the modifications above will increase the power enough to make a difference and still keep the renowned 'A' Series reliability.
I hope that the above is of help. Please ask if further information required.
Phil

[don58van]

Phil: Thank you very much for this.

I have been thinking for a long time that I, and probably many others, needed an expertly prepared 'recipe' for moderate cost upgrading of the 948. You have provided exactly that.

Even with the help of two of Vizard's books and years of reading this message board and several others, I was still uncertain of how to go about this upgrade.

Just a few of questions:
1) When you say to fit a Midget exhaust system, which parts do you mean? Do you include the cast iron manifold in this? I thought it is the wrong shape to direct the pipe out of the engine bay.
2) Would a MG Metro cam be better than a standard 1275 in this application?
3) Would you recommend a good 12G185 over a 12g202 if one could be found at the right price (bearing in mind that machining of the 202 is required)?
4) If I recall correctly, 998 pistons are available as dished or flat-top. Do you have a preference?

Thanks
Don

[philthehill]

Don
Thank you for the comment.

Regarding the Midget exhaust:- Just the pipe and muffler. Mate those to the Minor front down pipe.
Regards the Metro camshaft:- A good camshaft but expensive (S/H averaging £80 on 'e' bay) and getting hard to find in good condition.
I have several 1275cc Marina/Ital/Midget/Sprite good S/H camshafts on the shelf and may as well put them to good use.
The 12G185 head is ideal as no machining or sinking valves required but finding one without any cracks is difficult. The combustion chamber is the right volume as well so no skimming of the head. My 12G185 head is so early it has only the 9 studs.
Regards the pistons:- I prefer the flat topped pistons. The later 998cc Mini 3 ring pistons are near comparable to the 998cc Powermax Piston specification but without having to spend lots of money. I paid £25 for a set of piston in very good condition but without the piston rings but they would be replaced as a matter of course anyway.

My original Minor 948cc engine was built to very nearly the same 998cc specification and the increase in power output was very noticeable. When I changed the engine to the larger 1275cc Midget lump the 998cc engine was sold ending up in a Sebring Sprite used in competitive events.

[jagnut66]

Thanks for the info Phil.
Best wishes,
Mike.

[olderisbetter]

Thanks Phil thats all good info and as my 948 is in need of work i am going to go with this method, The gain and cost make it the best way forward for me, Like most things in my life it is no big rush but i am on the look out for bits anyway, Thanks for all your help, Rob

[ampwhu]

this is good info, thanks