MPI to 73.5mm (1380)

[Extra 300 Driver]

Hi folks. My MPi engine is out and being stripped now. While its out I am going to have +20 (1293) pistons fitted but then I thought 'why not go all the way to 73.5mm'? I am fitting an SW5-10 cam and having it all balanced too, but I wonder if fitting 73.5 pistons will create any problems when it is all put back togther? I am running standard head and injection system.

Are there any issues in changing the cc of an MPi car or does the ECU take care of it all?

[Cooperman]

I'll give you one very good reason for not going to 1380. The MPI block is unique to the MPI and once you go to 1380 it may not take another bore should that become necessary. At 1380 the bores have to be offset and you cannot even sleeve back to standard. Result if you ever have a problem is one scrapped block. How would you find a new MPI block? It's the same with the original Cooper 'S' and blocks for those cost a small fortune, that's if you can even find one. They are never going to make any more A-series blocks. Even ordinary 1275 blocks are getting harder to find these days.

[Extra 300 Driver]

Cooperman said:

I'll give you one very good reason for not going to 1380. The MPI block is unique to the MPI and once you go to 1380 it may not take another bore should that become necessary. At 1380 the bores have to be offset and you cannot even sleeve back to standard. Result if you ever have a problem is one scrapped block. How would you find a new MPI block? It's the same with the original Cooper 'S' and blocks for those cost a small fortune, that's if you can even find one. They are never going to make any more A-series blocks. Even ordinary 1275 blocks are getting harder to find these days.

Good point (I have 3 of them but I dont want to write off block for the hell of it) So if I went to +20 would the ECU still take care of it?

[Cooperman]

The ECU will easily cope with +0.060" which is 1330 cc and which does not need offset boring. From that you can still sleeve back to 1275 should the need arise and pistons are a direct replacement with the same gudgeon-pin height.At 1330, assuming nothing else is changed you can expect to gain arond 4 bhp plus a bit more mid-range torque. With the MPI it is always worth cleaning up the combustion chambers and inlet/exhaust tracts a bit, especially smoothing out the 'short-side radius' from tract into chamber plus grinding away and smoothing-out part of the valve guide bosses. If you look closely at the stripped down head all will become obvious, especially the very poor profile around the valve seats where there will be a big step. This you remove, carefully, with a die-grinder. Then you need to skim the head about 0.020" to get the compression ratio back, and maybe increase it a small amount.

[skunk20]

1380 doesn't need offset boring, a decent head gasket is needed tho, not a cheapy off ebay.
I would agree with cooperman though 1330 gives you afew more choices should you have problems somewhere down the line. as far as ECUs go, I've no idea.

[Extra 300 Driver]

Cooperman said:

The ECU will easily cope with +0.060" which is 1330 cc and which does not need offset boring. From that you can still sleeve back to 1275 should the need arise and pistons are a direct replacement with the same gudgeon-pin height.At 1330, assuming nothing else is changed you can expect to gain arond 4 bhp plus a bit more mid-range torque. With the MPI it is always worth cleaning up the combustion chambers and inlet/exhaust tracts a bit, especially smoothing out the 'short-side radius' from tract into chamber plus grinding away and smoothing-out part of the valve guide bosses. If you look closely at the stripped down head all will become obvious, especially the very poor profile around the valve seats where there will be a big step. This you remove, carefully, with a die-grinder. Then you need to skim the head about 0.020" to get the compression ratio back, and maybe increase it a small amount.

Thaks Peter I will post up some pictures when she is going back together

[Cooperman]

I look forward to seeing them. Incidentally my grandson has an MPI which is being'got-ready' for when he is 17 in one years time. The young man next door is doing an Engineering Diploma instead of A-levels and he needed a project to do and write up. So he is going to re-build the MPI engine with my supervision and teaching. It is standards at 1275 and the bores are so good that we are just going to hone and re-ring, then fit all new bearings (crank is in superb condition), new oil pump, water pump, vernier timimg gears plus all the other wear parts. I am going to clean up the head a bit, as I suggested you do, just to make it run at its best for him.
Gearbox will have a centre oil pick-up and we will drop the diff ratio to 3.1:1.
Body, with just so many new panels now welded on, goes to the paint shop in a couple of weeks to be painted the same basic colour of dark green metallic, but with a silver roof. It will have Gp. 2 wheel arches and Mk 1 grill and surround with no stripes on bonnet of on either side. He has found a replacement boot-lid badge from a 1961 car which just says 'MINI MINOR' - very retro. Front badge will also be from an early Morris.
With a solid-mounted front sub-frame, new suspension bushes and re-built brake callipers plus KYB dampers it should handle well once the suspension is all set to the best geometry front and rear.
Once done it will, hopefully, be like a new car.

[Cooperman]

skunk20 said:

1380 doesn't need offset boring, a decent head gasket is needed tho, not a cheapy off ebay.
I would agree with cooperman though 1330 gives you afew more choices should you have problems somewhere down the line. as far as ECUs go, I've no idea.

At 1380 without offset boring it would be very susceptable to head gasket failure. I had to deal with such an engine last year which had been bored to 1380 without offset and with the bores too big by about 0.004". It threw oil out and the block was then scrap.
Personally I don't like these 'big' engines and find that the majority of such engines could give as much power at 1275 cc if more care was taken with head quality and build standard without, effectively, scrapping the block at next rebuild time.
My 1310 cc Cooper 'S' gives a measured 117 bhp at 6400 rpm with a 286 cam and a really good (and expensive) bench flowed head, correct exhaust system (1.75" int. dia all the way), Karl-Schmidt pistons, all lightened and balanced with 1.5:1 roller rockers, twin H4 carbs on a 'tweeked' inlet manifold, etc, etc.
No head gasket fears either.

[skunk20]

I won't argue, other than to say if the OP want 1380 without offsetting the bore, thats up to him, it's been done and there are some very reliable ones out there.
Thats an impressive engine spec, I'd be happy with a fraction of that power. Reading your profile im guessing thats a rally car? Also good luck with the mpi build sounds like it will be a cracker when its done.

[Cooperman]

Yes, that one is an historic rally Cooper 'S', 1964. I also have a 1973 Innocenti 1300 Export which is 1330 cc with a basically standard engine, that is to the original Cooper 'S' spec with twin HS2's. In fact, the Inno is unusual in that the later cars were really the last of the real Cooper 'S's as they had 7.5" brakes, 10" wheels, 11-stud head with the twin carbs, but actually used a solid-back 1275 block with 1,75" big-end journals. When/if that ever needs a re-build I shall probably just sleeve the block back to 1275.

[nick1275]

iirc i dont think the standard ecu is up to it, as an above post has mentioned i would look at getting the head up to a nice standard and change the f.d in the gearbox

[Cooperman]

The ECU is OK for 1330 cc and an improved head, although I've not tried a head with bigger valves. With the John Cooper 'S' works they used 1.5:1 rockers and had problems with emissions when the bores and valve guides got a bit worn, At 1330 with just an 'improved' head rather than full gas-flowing/big valves, etc, the result is quite good. A lower final drive does help to put a bit of 'bite' into the acceleration.