Badfrog Posted June 25, 2010 Report Share Posted June 25, 2010 Hi guys, The TR4 sports a 25D4 Lucas dizzy. Late TR3A/B with the 2138 cc block have a DM2 dizzy. Both are set at 10° dynamic as stamped on the bobweights. Are they interchangeable? Badfrog Quote Link to post Share on other sites
Brian-Northampton Posted June 25, 2010 Report Share Posted June 25, 2010 (edited) Hi Badfrog, According to the Moss site - yes they are: Moss As a double check, I went to the Starchak site and looked up the two numbers (25D4=40795 and DM2P4=40735) and both appear to have the same advance curve. What surprises me most about the settings is that it appears that maximum advance is acheived at 1200 RPM at the dizzy - 2400RPM at the engine. That seems awfully slow to have the maximum advance....... anyone know why?? Bri. Edited June 25, 2010 by Brian-Northampton Quote Link to post Share on other sites
stuart Posted June 25, 2010 Report Share Posted June 25, 2010 Probably best person to answer your query is Martin Jay Distributor doctor. Distibutor Doc Stuart Quote Link to post Share on other sites
vivdownunder Posted June 25, 2010 Report Share Posted June 25, 2010 Badfrog, the Lucas 25D4 dizzy p/n 40795 was just an evolutionary product introduced during TR4 production. There's no reason why the DM2P4 Dizzy p/n 40735, which was fitted to 86mm (2138cc) engines up to the early TR4's, won't work perfectly well. TR4 dizzy specs were - max advance (crankshaft) 11 degrees BTDC, vacuum advance 5.5 degrees, automatic advance commences 350RPM. However any engine upgrade such as a different camshaft or raised compression would see either dizzy needing to be recalibrated to suit. Martin Jay, The Distributor Doctor, would have chapter and verse on this at email martin@distributordoctor.com Regards, Viv. Quote Link to post Share on other sites
TR4Geoff Posted June 26, 2010 Report Share Posted June 26, 2010 Hi Badfrog, According to the Moss site - yes they are: Moss As a double check, I went to the Starchak site and looked up the two numbers (25D4=40795 and DM2P4=40735) and both appear to have the same advance curve. What surprises me most about the settings is that it appears that maximum advance is acheived at 1200 RPM at the dizzy - 2400RPM at the engine. That seems awfully slow to have the maximum advance....... anyone know why?? Bri. Hi Brian, FWIW, when Martin Jay set up my Dizzy, the maximum advance came in at 3K RPM. Engine fairly standard with 4A cam and 87mm pistons with head slightly shaved. Geoff Quote Link to post Share on other sites
Badfrog Posted June 27, 2010 Author Report Share Posted June 27, 2010 Hi guys, Thanks for the infos. I was pretty sure I could do the swap but a bit of advice never hurts. The only minor difference is the shape of the alloy part going into the pedestal. I want to use it as pure original mechanical spare while my 45D4 is Pertronix-equipped (real good). Alec P. told me Martin Jay will be at Malvern. There will be a thorough chat on dizzy matters then. See you there, if I can pass Calais...... Cheers, Badfrog Quote Link to post Share on other sites
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