Pete Ashby Posted November 12, 2016 Share Posted November 12, 2016 Iv'e been having a bit of a think recently :yawn: (yes I know but it interests me) about the relationship between optimum dwell angle vs optimum coil saturation for 4,6 and 8 cylinder engines and it all gets a bit muddled in my head so an open question feel free to chip in Question: As the number of cylinders increase the dwell angle must decrease (as there is still only 360' on the rotor) and if that assumption is correct then will the optimum coil saturation density also proportionally decrease with increasing numbers of cylinders Pete :confused: Quote Link to comment Share on other sites More sharing options...
radiomike7 Posted November 12, 2016 Share Posted November 12, 2016 (edited) Correct Pete, that is why many V8s have a second set of points which immediately close once the first set have opened to increase the dwell angle rather than having to wait for the first set to close. Edited November 12, 2016 by radiomike7 Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted November 12, 2016 Share Posted November 12, 2016 (edited) Also those distributors like for example on Rolls military B60, B80, B81 and Jaguar J60, wartime British vehicles with certain screened distributors (inc. Daimlers) have two sets of points with half the number of cam lobes, and one set of points operating half the number of cylinders. regards, Richard Edited November 12, 2016 by Richard Farrant Quote Link to comment Share on other sites More sharing options...
Pete Ashby Posted November 13, 2016 Author Share Posted November 13, 2016 Thanks for replies Radiomike and Richard, these would seem to confirm the conclusion I came to......... that for optimum coil density to be achieved (variously quoted between 60% and 70%) a four lobe cam is the ideal configuration set to give between 37 and 42 degrees of dwell. So this leads me to wonder about 6 cylinder engines which only have one set of points, it would seem that they operate at sub optimal coil density ??? or is this compensated for by closing the the points gap down? Pete Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted November 13, 2016 Share Posted November 13, 2016 Thanks for replies Radiomike and Richard, these would seem to confirm the conclusion I came to......... that for optimum coil density to be achieved (variously quoted between 60% and 70%) a four lobe cam is the ideal configuration set to give between 37 and 42 degrees of dwell. So this leads me to wonder about 6 cylinder engines which only have one set of points, it would seem that they operate at sub optimal coil density ??? or is this compensated for by closing the the points gap down? Pete Pete, It would seem you are thinking too deeply. There are thousands of 6 cylinder engines running with single sets of points without issues. What the most important thing to adhere to is keeping the points adjusted to the specified gap for the that engine or distributor type. Quote Link to comment Share on other sites More sharing options...
Pete Ashby Posted November 13, 2016 Author Share Posted November 13, 2016 Pete,It would seem you are thinking too deeply. There are thousands of 6 cylinder engines running with single sets of points without issues. What the most important thing to adhere to is keeping the points adjusted to the specified gap for the that engine or distributor type. :-D don't worry Richard I'm not having a brain melt down, I have to admit to a certain amount of Devils Advocate here, as you know I've had a number of six cylinder motors all of which performed perfectly well with single point set ups. What started me down this line of thought was the relationship between dwell time and coil saturation and how various manufactures addressed the problem,as an aside while rooting around for information I found reference to dual point distributors for six cylinder engines unfortunately what they were fitted to wasn't recorded. As you mentioned in a previous post it's a common set up on V8's but I hadn't ever come across a six cylinder dual set up any idea what they were fitted to? Pete Quote Link to comment Share on other sites More sharing options...
landrover nick Posted November 13, 2016 Share Posted November 13, 2016 The 28 HP Bedford engine when fitted in the 15 cwt Bedford MWR uses a different type of distributor with two sets of points and no vacuum advance /retard .would this be because it was a screened distributor although it would still be expected to perform on the road the same as any other Bedford 15 cwt . Nicky Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted November 13, 2016 Share Posted November 13, 2016 :-D don't worry Richard I'm not having a brain melt down, I have to admit to a certain amount of Devils Advocate here, as you know I've had a number of six cylinder motors all of which performed perfectly well with single point set ups. What started me down this line of thought was the relationship between dwell time and coil saturation and how various manufactures addressed the problem,as an aside while rooting around for information I found reference to dual point distributors for six cylinder engines unfortunately what they were fitted to wasn't recorded. As you mentioned in a previous post it's a common set up on V8's but I hadn't ever come across a six cylinder dual set up any idea what they were fitted to? Pete It was not me but radiomike7 who mentioned V8's, but they are different in that they do have 8 lobes on the cam. I mentioned examples of straight six and eight cylinder engines with dual points and 3 and 4 lobe cams respectively, here are a list off the top of my head, of military ones; Rolls Royce B60 (6 cyl) and B80/B81 (8 cyl) [military and civilian versions] Jaguar J60 (6cyl) [4.2 litre ohc] Daimler Dingo and Armoured Car (different engines) WW2 Humber armoured vehicles with screened dist. (6cyl sv) Bedford QLR and MWR with screened distributor (6 cyl 28hp) Austin K9 with screened distributor (6 cyl 4-litre) Commer Q4 with screened distributor (6 cyl ohv) too name but a few .......... Quote Link to comment Share on other sites More sharing options...
Pete Ashby Posted November 14, 2016 Author Share Posted November 14, 2016 The 28 HP Bedford engine when fitted in the 15 cwt Bedford MWR uses a different type of distributor with two sets of points and no vacuum advance /retard .would this be because it was a screened distributor although it would still be expected to perform on the road the same as any other Bedford 15 cwt .Nicky I guessing Nicky it may be more to do with reduction of RF interference in this case, see also Richards list of 6 cylinder vehicles fitted with dual points. Pete Quote Link to comment Share on other sites More sharing options...
LarryH57 Posted November 14, 2016 Share Posted November 14, 2016 Must there also be some correlation between the size (circumference) of the distributor, the spacing of the lobes and the amount of travel that the rotor arms does for any given engine speed? Whoever invented all this before computers was a genius! Quote Link to comment Share on other sites More sharing options...
john1950 Posted November 14, 2016 Share Posted November 14, 2016 In some cases engine improvements and critical calculations were carried out by people who had not seen the engine or piece of equipment that they were working on. Like Sir Stanley Hooper, who was a mathematician he got 10%more power out of a Merlin without seeing one or Mrs Shilling.who improved its ability to perform in combat. Quote Link to comment Share on other sites More sharing options...
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