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MG MGB Technical - I think I have 10.5:1 compression!

The engine has a 60thou over bore so I think that is 467.6cc per pot.(Vs)
The chamber in the head has been skimmed down to 36cc as per PB's book.
The piston to deck clearance is 15thou giving 2cc PB's book say 4.5cc for the gasket and 6.5cc for the HC piston bowl. That makes a total chamber of 49cc. (Vc)
So (Vs+Vc)/Vc = 10.5

The cam is standard. It is a 2709 head. The carbs are SU HIF4. The filters are KN and I used the needles recommended for those. The plugs are the NGK ones recommended in PB's boof for a standard engine, can't remember the code.

The dizzy is a 20 degree advance one set at 10 degree static and I have put my plugs back to 25 thou gap. They were at 30 thou as my electronic ignition instructions recommended increasing the gap from standard. However I know that as compression rises it becomes harder for the spark to jump.

When I was swapping the heads over there were black and brown deposits on the pistons so I will run some redex or BG44K through it

At the moment I have a sticking exhaust valve so it does not run well. When my bank account has recovered I will take it for a rolling road tune but in the meanwhile other than putting high octane unleaded (97) in it are there any other adjustments I should make?
David Witham

David,

In the meanwhile, don't you need to fix the sticking exhaust valve?

Charley
C R Huff

Yes Charley. I was taking that for granted.

While I was at work yesterday my son got the head ready for removal. So when I got home we took it off and removed the sticking valve. It had picked up in the guide and some of the bronze has transferred to the stem. Although you can't see it with the naked eye I suspect we had managed to bend it a little at some point.
David Witham

Glad to hear that is being taken care of, David.

I always thought the decision for using high octane or not was based on the tendency for the engine to pink under load or to run on after shutting it down. I have not heard of high octane building up less carbon in the combustion chamber. Perhaps the carbon build up might indicate that it is running a bit rich?

Also, I would expect the ignition system should be able to handle a 0.030 inch plug gap with 10.5 : 1 compression ratio.

Charley
C R Huff

David you say that you are using the NGK plugs recommended for a standard engine but compression is probably the main reason to go to a colder range and with over 10:1 I would think a PBR7ES would be a better bet. Thats what we use in the supercharged engines and they last well and dont have any fowling problems. Also with that compression you should not want any more than 30* total advance or even a little less. A good road cam like a piper 270 would make the compression a lot easier to live with as the later closing of the inlet valve lowers the dynamic compression at lower revs.
Denis
Denis4

Once I stripped the head it became clear that there were issues with 3 exhaust valves. I took a trip north and called in at Peter Burgess. He identified that the problem was my effort to build my own head; not enough reeming of the guides, some issues with the exhaust seat fitting work I had done locally and my interpretation of taking the sharp edges off the chamber had been inadaquate. So thank you very much to Peter and his team.

The head was back on in time for drive it day and the only problem we had was a bit of pinking. So next I need to spend some time checking my ignition timing, carb adjustment and trying some cooler plugs. The aim is to have no known issues before having it rolling road tuned.

David Witham

David,

No doubt that you took the head to the right place.

Charley
C R Huff

This thread was discussed between 05/04/2011 and 19/04/2011

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