harlequin Posted May 13, 2020 Report Share Posted May 13, 2020 Good afternoon All I intend to send a spare cylinder head to the machine shop to be refurbished, it is a 1974 US head with the smaller exhaust valves and the ERG port that will be blanked off. The plan is to get all the work done on the head before I start work on the car so that it's not in bits for several weeks. My first thought was to get it skimmed to give a compression ratio of 9.5:1 which is what my engine has at the moment, but then as modern fuel is less then perfect for our cars I wondered if there would be any advantage in having a slightly lower ratio of say 9 or 9.25:1. I would appreciate any advice that anyone may have on this My TR6 is my every day car and is fitted with Stromberg carbs, we often use it for touring so would like to run it on supermarket petrol rather then better quality fuel that is not always available George Quote Link to post Share on other sites
Andy Moltu Posted May 13, 2020 Report Share Posted May 13, 2020 No you should be fine with 9.5:1. If it pinks you can get your dizzy remapped to restrict low speed advance. Quote Link to post Share on other sites
Nigel Triumph Posted May 13, 2020 Report Share Posted May 13, 2020 My CP series TR6 runs at a CR of about 10:1. It goes very well on 99 octane Tesco or Shell fuel. Fill up with other super unleaded (BP, Esso etc) at 97 octane and the engine is a bit less happy with that. Touring Ireland a few years ago, I could only get 95 and had to regard the ignition slightly to stop it pinking. I agree with Andy, CR 9.5:1 is good. It will run perfectly on 97 octane and should cope with bog standard 95 unleaded. Nigel Quote Link to post Share on other sites
matt george Posted May 14, 2020 Report Share Posted May 14, 2020 My ex-US TR6 is on twin SUs and has various mechanical upgrades, with a compression ration of 9.8:1. It runs fine on any grade of fuel, never had any pinking. As an aside for the OP, getting the ex-US head skimmed down to a suitable height to achieve much higher compression will be very expensive – around £300-400 at a rough guess. So I bought a UK spec cylinder head instead and built that up. If you haven't selected a machine shop yet, Ivor Searle near Ely is very good. Matt Quote Link to post Share on other sites
harlequin Posted May 14, 2020 Author Report Share Posted May 14, 2020 (edited) Thank you gents. I shall go for the 3.146" head thickness which will give me 9.6:1 with my engine's +20 overbore. I have had pinking issues that I have got around by retarding the ignition a bit, I am hoping that the work on the head will improve things and reduce oil consumption and as Andy suggests if I still have pinking I can have a word with the Distributor Doctor about the advance curve. Matt Thanks for the recommendation of Ivor Searle, that is where I was going to send it via a friend who is in the trade, I have been given a very fair quote for a complete job consisting of crack testing, skim, guides, exhaust seat inserts, plus new valves and springs. The really annoying thing with all this is the head on the car at the moment was sent to a local company to have new guides a couple of years ago and rather then fitting all new guides they fitted liners to the ones that they said were out of spec and left the rest. They took so long to do this that I ran out of time and had to accept the rubbish job or not have a car for a European trip we had booked, and I have never been happy with the way the car is since George Edited May 14, 2020 by harlequin Quote Link to post Share on other sites
YankeeTR5 Posted May 14, 2020 Report Share Posted May 14, 2020 The factory PI cylinder head with a compression ratio of 9.5 to 1 had a deck height of 3.4". The number you posted is going to give much more I think! Not sure about the fuel mixtures in the UK, but when my son & I did this last summer with the 74 (we swapped on a spare PI head I had) we ended up chasing problems for awhile around run-on, mixture and timing. I mention this as I'd stick at 9.5 to 1 or less. You'll feel a performance bump over what you have for sure. Quote Link to post Share on other sites
John L Posted May 14, 2020 Report Share Posted May 14, 2020 This might give you a cross reference of the different heads, https://www.chriswitor.com/cw_technical/head_applications_chart.pdf You may have to check the casting and stamped numbers on the front of the head, as this list covers more or the 2500 cars than the US TR's John Quote Link to post Share on other sites
harlequin Posted May 14, 2020 Author Report Share Posted May 14, 2020 2 hours ago, YankeeTR5 said: The factory PI cylinder head with a compression ratio of 9.5 to 1 had a deck height of 3.4". The number you posted is going to give much more I think! Not sure about the fuel mixtures in the UK, but when my son & I did this last summer with the 74 (we swapped on a spare PI head I had) we ended up chasing problems for awhile around run-on, mixture and timing. I mention this as I'd stick at 9.5 to 1 or less. You'll feel a performance bump over what you have for sure. Oops yes I got that completely wrong should be 3.416" not 3.146" (note to self 'pay more attention ') Thanks George Quote Link to post Share on other sites
harlequin Posted May 14, 2020 Author Report Share Posted May 14, 2020 Hi John That's a good bit of reference material I was quite pleased to see that the head I am getting done is one of the better ones. Someone had suggested getting the earlier, bigger valves fitted but after a bit of trawling t'interweb it appears the smaller exhaust valves give a better flow, so I shall stick with what I have George Quote Link to post Share on other sites
mtrehy Posted May 14, 2020 Report Share Posted May 14, 2020 I do quite a few of these and it's worth going for shorter push rods after the skim. If you've got it in bits then stick a decent cam in it and followers at the same time. Also bore size and piston height affect CR - worth checking that nothing odd has been done to it before you skim. Personally I burette the lot and skim from that rather than just a nominal 3.4" but I accept that it is likely you'll be fine... Quote Link to post Share on other sites
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