RobNew Posted January 30, 2004 Share Posted January 30, 2004 A mechanic was looking at my engine with a strobe last night, and found that it was running at 31 degrees ahead of TDC at 1200 RPM. He was amazed and said that he had never seen anything like it and that he couldn't believe that it was set up correctly. I've since found the engine build notes, and it was built to be 36 degrees BTDC at 4000 RPM so I'm going to take it back for a test at the proper RPM. Can someone explain to me why you would want the ignition to fire BTDC at all, surely the gases are at their most combustible at TDC? And secondly, as the ignition is so far advanced, should I be paying any special attention to anything like value seat regression or fuel type/additives used (currently LRP but it's rapidly disappearing from the forecourts ☹️ ) Thanks for your help, and sorry if my question shows a complete lack of understanding of engines, but I'll never learn unless I ask Rob. Link to comment Share on other sites More sharing options...
Graham Perry Posted January 30, 2004 Share Posted January 30, 2004 If you could identify the engine type it would help with someones answer, as there are significant differnces between ignition advance settings on different engines Link to comment Share on other sites More sharing options...
Jason Plato Posted January 30, 2004 Share Posted January 30, 2004 What engine is it Lotus 7 Club Speed Champion 2003 South Wales Area Organiser C7 TOP Link to comment Share on other sites More sharing options...
RobNew Posted January 30, 2004 Author Share Posted January 30, 2004 Sorry that was a bit of a basic mistake! It's a Ford Kent x-Flow 1700 Other details that I'm not sure are relevent - ported cylinder head - one piece steel valves and steel rocker posts Hepolite pistons Piper 285/2 camshaft (108deg timing) any other details needed? Rob. Link to comment Share on other sites More sharing options...
Sheds Moderator Posted January 30, 2004 Share Posted January 30, 2004 In principle you fire the spark before TDC because the gases take a finite time to ignite and produce the power pulse. If this happens too late the piston is already back on the way down when the power arrives. So normally you fire around 8 or 10 BTDC, the gases start burning, the piston arrives at the top, power pulse arrives, down goes the piston. Engines have a centrifugal advance (old ones at least, new ones are electronic) so that at high speeds the spark comes earlier still because the piston's travelling faster. In the same way you open valves earlier than you would think and there are occasions when inlet and exhaust valves are open at the same time (this is termed "valve overlap" and racing engines have a lot of it). This is linked to the fact that this is a physical system and gases flowing round have mechanical inertia, meaning that it doesn't happen instantaneously. Hope this isn't too "granny sucks eggs" but it you think about the 4 stages of the 4str cycle, especially the transitory bits between stages, you will appreciate why. Link to comment Share on other sites More sharing options...
RobNew Posted January 30, 2004 Author Share Posted January 30, 2004 Thanks or that explanation, makes perfect sense. So my next question is why was the 1ltr Mini Metro that we also looked at, 16 degrees advanced, and my engine was 31 degrees. Obviously I appreicate that mine is slightly higher performance but why does that mean that the timing has to be even more advanced? Rob. Link to comment Share on other sites More sharing options...
Roger King Posted January 30, 2004 Share Posted January 30, 2004 If you have 3D mapped ignition, the throttle position sensor will add lots of extra ignition advance under these conditions to compensate for being on light throttle. This is exactly the same thing that used to happen with old fashioned vacuum advance systems which always had to be disconnected before checking ignition timing. If you don't have 3D ignition, which distributor do you have? If you have one of Caterham's 'unleaded' (used from around 1989 onwards) ones, a figure of 36 degrees at full advance would equate to around 27 degrees at 1200rpm (although these distributors vary wildly). The 'unleaded' distributor should be set to 14 degrees at 900rpm, but will then give nowhere near 36 degrees at 4000rpm which will cause a loss of performance. Unless you are running 3D ignition, 31 degrees at 1200rpm is far too much and may cause damage. It's always possible that the timing has simply been set incorrectly, so get it reset if necessary. It is very important to check the timing throughout the rev range though. We failed to get further than half a mile into the 1000 Lakes rally one year because a mechanic checked the timing, found it was retarded at 4000rpm and adjusted it. In fact the advance mechanism had go jammed and freed itself up as soon as the car went off the line, thus adding in a load more timing and blowing the engine. If the mechanic had checked the whole ignition curve rather than just one point he would have seen that there was a problem. Link to comment Share on other sites More sharing options...
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