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MG MG Y Type - 8.6 compression ratio

Why not raise the compression ratio to utilise the potential of modern fuels.
The YA was produced shortley after WW2 and tuned to the available pool petrol which was little better than parafin.
I had the simple mod carried out in the 1970's which entailed no more than 3/32inch machined off the cylinder head and four 3/32 spacers under the rocker pillers which I made myself.
The pro's and con's are
A very useful increase in power.
A reduction in fuel consumption [mine went from 28 to 33 mpg ]
And an improved acceleration.
The only debit if you could call it that, was a slightly reduce power range.
However the engine has even with the mod a flexability far better than any modern product. I often drove over the mountains of Switzerland and France, and without fuss could pull away up steep hair pin bends with a 4 birth caravan in tow. This would be far beyond the ability of my modern car.
My advice is that its worth considering.
Bryan Mellem

Bryan, you are absolutly right. How much did you advance your ignition and did you have to fit a richer needle in the carburettor?

Without knowing a lot of owners already have higher compression ratio's because of earlier rebuilds.
I almost exchanged my worn but standard height (=low compression) head for a rebuild head from a TD who's owner found out that he had to high a compression for the compressor he fitted... Alas, before the deal was struck he found a better one.
Willem van der Veer

Thanks Willem for your comments.
I stand to be corrected but the ignition is slightly retarded, by how much was very much dependant on fuel available and wear in the distributor. Accelerate in top gear and listen for knocking is my meathod. W E Blower advises TDC.
As for the carb. he advises E.S. needle Jet.090inch but this was to run twin carbs. on a home made fuel, so this is unhelpful!
I just used the carb. as it is, adjusted in the normal way. One hopes to use less fuel not more.
Tappets I left at 19 thou but 20 may be better.

I must add that thicker washers need to be fitted under the cylinder head nuts.
Bryan Mellem

Blower, and the factory, recommended tuning procedures, one of which was to take around 1/8 in. off the head to give over 9:1 cr. and many people,myself included (TC), have done this over the years. The factory originally recommended putting equivalent thickness spacers under the pillars as Bryan has done but this has been rejected by many (as well as the factory later) as it spoils the rocker geometry. If you must compensate shorten the pushrods but normally this is not necessary.

Ian Thomson
ian thomson

Ian is totally correct, under no circumstances should packing pieces be put under the rocker shaft posts, as this will result in the rockers hitting the valve stems past the horizontal. As the rockers continue on their arcs to open the valves, the valve stems will be pushed sideways against the valve guides. This results in the guides becoming worn earlier than they should. In fact if the geometry is checked carefully, there is some argument to suggest that the posts should have a small amount machined off, rather than added, irrespective of cylinder head thickness. The pushrods may then need shortening.
Roger Wilson.
Roger Wilson

Thanks Ian and Roger for your comments. Blower's advice refers to mod-ing the TC which I think has an 8mm lift cam where as the YA is only 6.5mm. Anyway so long as the push rods are clear of the tunnel walls there is no problem, I can only comment on 8.6 comp ratio and well over 150 thousand miles since uprating the engine. I still think the result is very worth while. While still on the subject I cannot understand why the YA was so de-tuned not only with a single 1.25 carb but a special low lift cam. One can only conclude that MG's thought that the public were not ready for a pocket rocket.
Bryan Mellem

Sorry to keep on about the 8.6 compression mod. but after a restles night the penny suddenly dropped. If 3/32 spacers are fitted under the rocker pillers this will be correct for the push rods, but this is the error - the valve stems will be 3/32 below the other side of the rocker arm. In fact the spacers need to be only 3/64! I have sometimes wondered why the adjusters were almost at the end of their travel. In practice it has'nt made an atom differance since the engine has run for a considerable time without any problem. So there we have it - a happy set of rockers.
Bryan Mellem

This thread was discussed between 24/03/2007 and 06/05/2007

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