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MG Midget and Sprite Technical - Camshafts for a tweaked 948...

Hi to all,

Have a bit of a funny one here and please treat me like an idiot because my knowledge of A-series engines is a little limited...

Am in the process of rebuilding a Mk1 sprite that my grandfather bought 30 years ago and rotted away in a barn.

Over the last 10months have been stripping it down and have discovered it was a little more tweaked than expected....

The engine is a 948
Bored +0.060
Hepolite 18519 Flat top pistons
Fully balanced and lightened

It was bought with the original head and hs1's but we have since acquired some HS2s and a 12g295 for it..

However, until recently had absolutely no idea what the cam was. Have now got to the stage where can try and work out what it is:

Markings: Nil (apart from an A stamped on the timing driving end)
Oil Pump drive: Spider
Lobes: 1/2 inch
Rough profile has been worked out at :

Full inlet lift 115
in open btdc 35
in closes abdc 75
ex opens bbdc 70
ex closes atdc 40

in cam lift 0.3
ex cam lift 0.3

This seems to match closely to a factory 643/544 cam...

Our current working compression ratio without getting the head milling is 8.5:1...

Now the only advice I've managed to find so far is that frankly to get this to work effectively...gonna need a compression of 14:1..

Ultimately I want this car to be road drivable!

Options are limited by budget at the moment unfortunately
1) try and pick up a cheap 998 cam
2) Save up for a SW5-07

Other?

Basically does anyone have advice on for this predicament but also just for generally tuning 948's.

http://img.photobucket.com/albums/v252/talon123/P3307771-2-1.jpg
http://img.photobucket.com/albums/v252/talon123/P3307777.jpg

Kind regards

Build log at:
Sebringsprite.blogspot.com
D Paul

Why not look at a 266 or 276?
Not a big fan of the SW5...

And a 12G202 head might not be a bad idea.
When I built a hot 998 I was able to get very cheap 12G202 heads who with a bit of work flowed better than the 12G295 and could be milled more to get an acceptable CR.

Onno K

I run a 948 with a 567 cam and 12G202 head modded by Longman.
A Anstead

A Anstead,

Have you increased the compression ratio much for that setup? My calculations make it about 9.0:1 on a simular bore and flat top setup..

How do you find that set up for Heavy/light traffic?

The area of the world I'm based in will need some low RPM work for Traffic (read tractors) and single track roads.


Will have to make do with the 295 for now as a)haven't a clue where would find a 12g202 or b) afford it! right now (Will definitely have to think about getting it skimmed though :s)


Onno K,

Why the ambivilance towards the SW5? seems to have some rave reviews kicking around on the mini forums...


Thanks to everyone for the thoughts.

D Paul

Hi
I believe the CR to be in region 9.5 to 1
H2 Carbs with V3 needles. Stock dizzy with Aldon ignitor. +120 bore with mini 998 rods, lightened flywheel. balanced
The engine idles well and fulfils all my needs. It is good in traffic and long distance travel. I have run with this spec since 1992. It is what is suggested in the old BMC tuning manual. RR showed 62Bhp.
Alan
A Anstead

Bad experiences in a 998.
A lot of valve train noise (really a lot)
Have heard of several similar experiences (with the wear to match)
It produced around the 60BHP in that mini and was very tame at idle.

The engine could have done with a wilder cam and idle would still be survivable
Onno K

The only problem with running a 295 head on a 948 is that you need to skim quite a lot off to get a high compression ratio, due to the very large combustion chamber.

This can cause you to break through into the drilling for the oil feed to the rockers, causing more work to solve the problem.
Dave O'Neill2

This thread was discussed on 01/04/2012

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