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Graham Bichard
Member
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Posted: Thursday January 24th, 2008 07:17 am |
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Can anyone tell me what head was used on the MPi min, and what the valve sizes were in imperial?
I suspect its 33.5mm inlet / 29mm exhaust (but what does that convert to?) and am guessing at a 12G940 head?
Anybody know for definite?
Graham
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DaveShreeve
Administrator
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Posted: Thursday January 24th, 2008 12:45 pm |
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| Can't quote numbers, but it is a one off. The Mpi is the only one that has the bosses for the alternator mounting bracket.
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Graham Bichard
Member
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Posted: Thursday January 24th, 2008 01:29 pm |
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Bugger!!!
Dave, as ever you are a font of knowledge! Many thanks.
My next question is - on the MPi head would there be anything in the way of modifications (ports/chambers etc) that would be different to the other 1275cc heads? I suspect the mods applicable to the head (and also applying to the choice of cam) would be limited by the ECU rev-limit of 6000rpm (I think from memory). Anybody care to comment?
I now know too, that the block is different - no machining for the dizzie, amongst other things!
Why aren't things ever simple!
Other than the ratios, is there anything I should know about the gearbox/casing?
Cheers
Graham
Last edited on Thursday January 24th, 2008 01:33 pm by Graham Bichard |
hawaiianblue
Member
| Joined: | Saturday June 2nd, 2007 |
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| Posts: | 214 |
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Posted: Thursday January 24th, 2008 02:51 pm |
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A 12G940 is of course just a bare Non MPi head with no machining, so a head with 12G940 cast into it doesnt really tell you much about the valve sizes, it's just an internal part number for a rough casting. All 1275 A+ engines will have a 12G940, just of varying specs.
But anyway, you can fit an MPi head on an earlier Spi/carb 'car', but not the other way around. Although you need to use a sandwich plate for the temp sender or drill and tap the head (not a hard job)
You could fit an earlier head on an MPi 'engine', however you would have to use the old side mount radiator arrangement to allow you to put the alternator back in its original position if you wanted to fit an earlier head to an MPi car.
The amount of modifications to the A series for the MPi shows just how much Rover (read BMW) spent on developing the engine, and considering it was at the very end of its life span, i mean the first one rolled off the production line in 1951! 803cc Beasts!
Other than the ratios, is there anything I should know about the gearbox/casing?
It splits open if you hit speed humps with it..... Seriously though, the box is the same as any other Rod change mini/metro box. The only actual difference is the final drive, the gear ratios them selves are the same as earlier Rover minis. The gearchange leak fix kit that most mini specialists try to sell you, was fitted by Rover as standard on all A series units from around 1988 onwards, so dont buy one.
The ECU limits the rev's to around 6000, i think it's a little over that. But most cars wont produce any more power over about 5000ish RPM as the fueling and ignition then limits it.
The ECU could actually cope with far more power, the same box is used on the MGf up to around 200bhp, but the firmware is different (hence the different part number). There is a supercharger kit availiable for the MPi, uses the BMW mini supercharger, it gives around 130bhp i think, with the standard ECU.
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Graham Bichard
Member
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Posted: Thursday January 24th, 2008 03:30 pm |
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hb, do you know if all the Rover 1275cc head (12G940) were of the 33.5/29mm inlet/exhaust sizes? (And what does that convert to in imperial - Vizards book only quotes imperial!).
Cheers
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hawaiianblue
Member
| Joined: | Saturday June 2nd, 2007 |
| Location: | |
| Posts: | 214 |
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Posted: Thursday January 24th, 2008 07:50 pm |
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basically it works like this.
They cast a basic cylinder head. It comes with the coolant ways and ports cast in, but without and other holes, just indentations to mark where they should be. There are no valve guides and only a small rough cast hole where each valve should be. This is a 12g940 casting.
The casting then goes off to the machine shop and has the various holes drilled and tapped. The valve guides are drilled out and pressed in. The valve seats are then cut and hardened seats are pressed in for unleaded models. Ports are also cleaned up and have the roughness polished out (well a bit)...
So a 12g940 can have various size valves pressed in, the face can also be milled down to the required compression ratio.
I 'think' all the Rover Cooper heads are the same internal spec, but if you get the head off a Mini Sprite or 1275 Mayfair the head will be a lower spec. The lower spec heads will still have a 12g940 casting number in them as they're finished from the same bare casting. The Same casting was also used to make Metro heads as well as Maestro 1.3's
But for conversions or imperial into Metric and vice versa http://www.onlineconversion.com/
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Itsarek
Member
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Posted: Thursday January 24th, 2008 09:19 pm |
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hb, do you know if all the Rover 1275cc head (12G940) were of the 33.5/29mm inlet/exhaust sizes? (And what does that convert to in imperial - Vizards book only quotes imperial!).
Cheers
 Divide By 25.4  = 1.319" / 1.142" 
Last edited on Thursday January 24th, 2008 09:35 pm by Itsarek |
Graham Bichard
Member
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Posted: Friday January 25th, 2008 07:25 am |
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hb, that clears it up quite nicely. Just one more question about the 940 head. Dave mentions the alternator mounting being part of the head - is the a machining job onto the 940 head (something like machining a flat on, tapping a couple of holes for a bracket to be mounted to) or is it an extra blob of metal as part of the casting which was then machined (as in a lug)?
itsarek, thanks for the info ref. the valve sizes - I wasn't convinced 25.4 would be accurate enough, and I'm damned if I can find my Zeus book! Thinking about it, and having just started reading Vizard's book (still a fair bit to go though), I've got the question in my head as to how much flow you could actually gain with an MPi, so would it be neccesary to go for the bigger (1.4") inlets.
He (Vizard) mentions that some of the 1275 heads were pretty good (but can be improved upon quite easily by cleaning up the valve stems, and the lips in the ports) but when you're limited to 6000rpm (give or take) by the ECU you aren't going to be able to produce a screamer (and engine speed produces power (along with torque I know) - 2 x pi x N x T (2 times pi times engine speed times torque)) so really I'm looking at getting the best torque, smoothest delivery and benefit from the resulting increase in power from this to aid acceleration. (Phew, I feel dizzy now!) I'm also thinking a 52mm alloy throttle body would benefit air flow, especially when matched to whatever inlet manifold the MPi has (I really think I'm going to have to have a look under the bonnet!).
I realise that good power results are available having read 'The Last Works Minis' but the Gp A minis used different electronics and as for the Gp N - well I think Bill Richards might be a bit better at this than me!
I think I'll have a lie down now!!!
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hawaiianblue
Member
| Joined: | Saturday June 2nd, 2007 |
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| Posts: | 214 |
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Posted: Friday January 25th, 2008 04:53 pm |
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A friend of mine had an unleaded head fitted at Bill Richards (he's Local), when he got it back the tappets rattled like hell. I told him to take it back to BR and ask if they did the tappets. He was told that adjusting the tappets would be extra, he'd already paid about £250 for the conversion! So apparently they fit replacement heads and dont adjust the tappets..... mmmmm Bill Richards might be better than you?
As for the alternator mounts there is extra metal cast onto the head which is then machined flat and tapped. So really the MPi head is a 12g940 (v.2) head. It might actually be possible to make an adaptor plate to bolt the existing alternator to the temperature sender hole on an older head.
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