Valve train

Ok, so disassembling the engine showed some considerable wear. Replaced the main and big end bearings with trimetal bearings. Didn't take any pictures as it all went well.. However as always the adage of not "forcing" anything was important as the front main bearing was tight until I realised that that timing chain was preventing movement of the crank.

Next to think about the valve train. I already know that I am going to need to replace the rockers, camshaft and followers as these are all very worn (see previous post). I also need to think carefully about choosing components that will work with my plan to supercharge the engine. This means:

1) Choosing a camshaft that complements the supercharger. This has quite different requirements compared to a normally aspirated (NA), non-supercharged engine. After a lot of discussion on the TR register website two key aspects seemed to become clear. Firstly the camshaft should have minimum overlap. In NA cars, overlap, having exhaust and inlet valves open simultaneously helps flow thought the engine at high speed. In supercharged cars having both valves open can lead to the supercharger "blowing" unburnt air/fuel mixture straight through the engine. This reduces efficiency and increases emissions. So minimal overlap is required which can be equated to maximal separation between the lobes. Ideally above 112 degrees compared to the more usual 105-6 degrees of a high performance camshaft. In addition it also seems to be important to increase the lift on the exhaust valve to give time for the exhaust gasses to leave the combustion chamber..

 So... what to choose... Well looking at various sites (including this really useful one) I chose a triumph original for a Triumph 2.5 PI MkI which has a lobe separation of 110 degrees and a lift of 0.34" (better than the Triumph Tune Road 89 I had before. Interestingly there is supercharger camshaft available in from APT  (113 degrees with lift of  0.28") the US, but it is a bit too expensive for my purse

2) Ensuring that the compression ratio is below the detonation limit for the supercharged engine. Supercharging has the effect of increasing the effective compression ratio of an engine. If this gets too high detonation can occur in the engine damaging pistons etc. One way round this to increase the octane rating of the fuel, but in the UK this is not really possible. Another way is to reduce the compression ratio of the car so detonation is not reached. As standard my CP series TR6 has a 9.5:1 compression head (Casting number: 308351 Stamping: 516816). This is thought to be OK with the Moss supercharger kit but I think could be borderline. Closer examination of my head also showed it had been skimmed in the past and was closer to 10:1 which isn't going to work.

So... Back to the drawing board... Triumph offer 2 choices from the factory in addition to the 9.5:1, in the form of 8.5:1 (used in the US) and 7:1 (used in later US TR6s and Kenyan exports!) (see this site for helpful info). So I cast far and wide  and came up trumps with a 7:1 compression head from a US car (Casting number: 313248 Stamping: 218227), thanks Conrad. Interestingly this combination of numbers is not on any of the online resources, but a quick measure of the head thickness (close to 90 mm or 8.3125") shows it is 7:1. This is great as detonation should not be a problem and I should be able to run a higher boost (e.g. 10psi).

A WHIMSICAL REFLECTION:

On reflection this cylinder head has had quite a life. Cast in Canley in Coventry in the 1970s, shipped to somewhere in the US, repatriated to Nottingham in the new millennium and then returned to a machine shop (Coventry Boring) within 1/4 mile of the site of the old triumph factory in Canley for a refurb!