Compression Ratio with 86mm liners

[Norman D]

After deciding to buy 86mm liners and pistons for my 3a (see previous thread " Vanguard Pistons") I now read in the latest issue of Traction that this will cause me compression ratio problems and result in an intractible engine for road use! Surely this is a very common modification, does everyone who does this have thir combustion chambers enlarged and by how much? Or am I worrying unecessarily? Help.

[ianc]

I looked up my article which appeared in Newsletter 13 (long before the days of TRaction - must have been about 1973), which is reproduced in Section P2 of the Technicalities CD. The article explains in detail how to make the calculations, but there is one sentence relevant to this: if you go from 83 to 86mm liners on a TR3 and do nothing whatsoever to the cylinder head, you will increase the compression ratio from 8.5 to 9.0 (approximately). However, most people would do some re-shaping of the head to suit the larger bore, so that's where you might need the full article because you need to measure the volume in the head itself, in the gasket and between the top of the piston and the deck (top of the block).

A CR is not going to be intractable for road use, but you can't beat a TR2 with its low port head and smaller carburettors if you want really tractability!

If you change the CR you'll need to think about the carburettors and the timing (distributor), but these are not big issues as it has been done so many times by others.

If you don't have access to the article or the CD, e-mail me (ifcornish@btinternet.com) and I will scan it for you.

Ian Cornish

[Kiwifrog]

Norman look out for a good second hand TR4A head and your problems are solved

 

Alan