quattro Posted November 16, 2016 Share Posted November 16, 2016 Car is off the road at the moment so I have been reading up and up and up on tuning stuff, and the theory behind it all. One thing that is confusing me is vacuum advance. The vacuum advance unit on a Rover V8 gives 8 deg at the distributor (16 deg at the crankshaft). Of all the spark maps I have found on here, the wild ones (use at your own risk and careful not to melt your engine) only advance around 10 deg between WOT and overrun, and the mild ones only around 5 deg. Why aren't they up at the 16 deg mark? I have a feeling I am missing something here and am asking a stupid question Richard Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted November 16, 2016 Share Posted November 16, 2016 Where do you think vacuum advance operates? Asking a question to know how to answer you Quote Link to comment Share on other sites More sharing options...
quattro Posted November 17, 2016 Author Share Posted November 17, 2016 The vacuum advance works with the original distributor and advances the timing when there is a lot of vacuum, i.e. during tickover or cruise. It advances the timing by around 16 deg to start the burn quicker as there will be a leaner mixture at very low throttle openings. Leaner fuel mixture needs more time to burn. If it advances the ignition by that much using the dizzy, why don't we advance the timing on our spark map by that much? From what I can work out, it operates between 50 and 85 KPa. Above 85 it's inoperative, it advances around 16 deg at 50 and below. I would assume it's pretty much linear between 85 and 50. Just curious or confused Quote Link to comment Share on other sites More sharing options...
Quagmire Posted November 17, 2016 Share Posted November 17, 2016 My understanding was that standard setup with SUs gives vacuum advance only at cruise, not at idle or wot. The port is on the filter side of the butterfly (I thought so anyway, it's been a while). Here's a description of what I did to get my spark map: http://www.v8forum.co.uk/forum/viewtopic.php?p=99262&sid=a4b1f0b2dae52c7a14181ea0bae697a9 Economy is good and been driving around for a couple of years like this, including my recent track session at Silverstone. No blow-ups/melting yet! *Touchwood*... Quote Link to comment Share on other sites More sharing options...
quattro Posted November 17, 2016 Author Share Posted November 17, 2016 Interesting interpretation, unfortunately not the same as mine The reason the port was moved to the other side of the butterfy was to do with emmision control prior to CAT technology improving, and not anything to do with fuel economy or power. (I have read an awful lot and this seems to be the opinion of many). So I have ignored this bit in my calculations and thoughts. My dizzy had 5 15 8 stamped on it, which apparently means it starts to add advance at 5"hg (15KPa). But my map shows the amount of pressure from 20 up to 100, not vacuum. So I have interpreted this as it starts to add advance when it gets down 15KPa, i.e. when it gets down to 85 on the map. So, no advance at 100 (WOT), then starting to add from 85 down to 15"hg (51KPa) so 49 on the map. At 49KPa (on the map) it would have added 8°of dizzy angle, so 16° crank angle, and below 49 it adds no more advance. So, why are the spark maps I have been studying not 16° higher under 50KPa than they are at 100KPa? I know there is a difference between distributors and EDIS, but this seems to be huge difference and I was just wondering - why? Or have I got it all wrong? Richard Quote Link to comment Share on other sites More sharing options...
Quagmire Posted November 17, 2016 Share Posted November 17, 2016 Ah, i think I see what your saying and we are mostly saying the same thing- except for one point- you are taking the Mercury values as gauge values, not absolute. Not sure who is right to be honest, but thinking about it, you might be! I have a low rpm/high load misfire to sort first, but after that I might re-jig my spark table and see what the effect is. Hopefully better consumption! To answer your question on why people don't go to the full 16 i think its because its natural to be a bit conservative with spark incase things go pop. Quote Link to comment Share on other sites More sharing options...
Cheesy Posted November 18, 2016 Share Posted November 18, 2016 Without going through it all again (to remember what goes on) we dont have to compromise with two interacting mechanical systems to get an output that works. Also looking at your numbers wouldnt the vac advance have a full 8 deg at 15kPa going to nothing at some higher kPa as opposed to starting to add advance at 15kPa? Quote Link to comment Share on other sites More sharing options...
quattro Posted November 18, 2016 Author Share Posted November 18, 2016 I can understand being conservative Jamie, I am not even going to try this until I get a full understanding of it - which I haven't yet I realise Cheesy, that the MS is more refined and a lot more controllable than two interacting mechanical systems, but it should still work on the same principals, only a lot better. The 15KPa that it starts at is vacuum, so the top line of the table i.e. 100KPa is atmospheric pressure, and the 15KPa of vacuum means that the original dizzy would start to add advance when it gets down to the 85KPa level. As it goes down the table it will steadily add more advance until it reached 49KPa (15"Hg of vacuum) where it should have the full 16°. It doesn't add any more advance after this so the bottom part of the table (anything under 49KPa) will be at 16° extra advance. Looking at various ignition maps that I have found, nowhere near this amount of advance is being added, hence my question - Have I got this wrong? Bit confusing this tuning lark Richard Quote Link to comment Share on other sites More sharing options...
Quagmire Posted November 18, 2016 Share Posted November 18, 2016 I think you are right to be adding what the dizzy would have done, and agree with what you are saying about where vac advance should start and end. My setup does the same but because I (now mistakenly I think) used absolute values instead of thinking about the values being gauge pressures my vac advance comes in lower down the table. Doing it your way would give me higher advance at cruise for sure, which could be good for economy. To go back to the question on adding the full 16 degrees- I initially had a full 16 degrees of vacuum adv added but at some point reduced this to 13, I was getting a pinging type noise on overrun, however no amount of fuel or spark tuning eliminated it, I think it is actually some kind of exhaust rattle. I never got round to putting the extra three degrees back in... Quote Link to comment Share on other sites More sharing options...
quattro Posted November 18, 2016 Author Share Posted November 18, 2016 I wouldn't mess about with anything on my say so I'm still part way up a sharp learning curve. I will probably give it a go though, albeit on a very conservative, bit by bit attempt . I don't want to be bringing my pistons home in a jamjar. Richard Quote Link to comment Share on other sites More sharing options...
Cheesy Posted November 20, 2016 Share Posted November 20, 2016 Yes you are right about the absolute pressure bit, is you distributor from an SU setup? there is a note in the Des Hammill book about being careful using these units as you end up with full vac advance at idle Quote Link to comment Share on other sites More sharing options...
quattro Posted November 27, 2016 Author Share Posted November 27, 2016 How's this looking? I have toned it down a bit, so thought I could use it as a starting point. Richard Quote Link to comment Share on other sites More sharing options...
Hybrid_From_Hell Posted November 27, 2016 Share Posted November 27, 2016 This is a very complex subject, and I have done much studying over the years. Vac has to be taken in to context with throttle opening and load Spark advance alters quite signi icantly with Engine variant, let alone the dizzy fitted ! Much has to do with Bore /Stroke, ie a 35 can take MORE advance than a 3.9 why is unbelivably complex, please just take my word for it that spark advance is different for all rover V8s ! Its all about getting a smooth table IMHO and making the most of the 144 cells, interpolation on the current MS units makes this even more cruical looking at the map above you could remove both columns and lines !! I never even worry re 100 KPA, if I am flat out WOT I want EVERYTHING so set tops at 97 ! 3 KPA isnt going to do any dmage ! (Just a nerdy bit for yer alls ) Also bear in mind many years ago advance was tweaked on engines by playing with springs and bob weights to gain better performance /advance, MS means you are not restrained by mechanical bits whizzing around,m so 100% mimicking / mapping a Dizzy isnt actaully very good plan ! trick is to have as much advance withouit pinking, giving max BHP / Torque What is the engine, whats the MS system on it ? and manual or auto ? Nige Quote Link to comment Share on other sites More sharing options...
quattro Posted November 27, 2016 Author Share Posted November 27, 2016 (edited) 4L C/B Rover V8, 20,000 miles. P6 car, 1,300Kg, manual LT77 2 wheel drive. MS1 V.3, running fuel and spark, bought from Nige. Engine has RP4 cam, tubular headers and full Stainless system, Stage 1 SD1 heads, 35% lightened flywheel, I realise that mimicking a dizzy isn't the ideal way to tune the spark map, but to be honest, it's all I've got. In all of the mountains of information I have read about MS, 75% of it doesn't apply to MS1, and all the other 25% percent says about setting up your spark maps is to do it before VE tuning. Edited November 27, 2016 by quattro Quote Link to comment Share on other sites More sharing options...
Hybrid_From_Hell Posted November 27, 2016 Share Posted November 27, 2016 Nige ....that's me lol !!. Did you have programmed ecu????? Quote Link to comment Share on other sites More sharing options...
quattro Posted November 27, 2016 Author Share Posted November 27, 2016 Yes Nige, I know it's you There was no MSQ with the ECU, but you did email it to me mid September. I did get a spark map with it but it was very mild. This one has been tweaked up a bit with the information I have been reading. It isn't as advanced as I was going to have it, but more so than the one on the MSQ. I was just looking for any opinions on it. Richard Quote Link to comment Share on other sites More sharing options...
Cheesy Posted November 30, 2016 Share Posted November 30, 2016 It looks similar ish to mine, a well worn 4.5 (square bore stroke) so a few degrees lower on the top end, it could do with more at 50kmh area looking at it now though and the 500rpm row is pretty pointless Quote Link to comment Share on other sites More sharing options...
bodumatau Posted November 30, 2016 Share Posted November 30, 2016 Nige, I would also be interested in an MSQ for my 3,5 V8 9,35 compression, my ECU only came with some 350 cubic inch preset in it. any chance of emailing me one? Quote Link to comment Share on other sites More sharing options...
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