Peter Cobbold Posted May 30, 2016 Report Share Posted May 30, 2016 (edited) I think thats about right, zero vacuum or atmospheric 14.7psi causes the 1 bar MAP sensor to register 5v as vacuum is applied it seems to pull down linear to about 0.5v at atmos -1. So at gentle cruise in the graphs its about -1psi. Looking at a couple of SU's I have here HIF44 and HS6 both of those have the vacuum take off in exactly the same place as the Strombergs i.e. when the Butterfly is shut the vacuum port is closed, the microscopic drillings appear to be about the same size on both SU's and Strombergs so I'd expect them to give nearly identical readings, which is good to know if considering a carb swop and your using vacuum advance. I junked using vacuum advance long ago as I'm not tuning for economy and the risk of a failure (i.e. it activating) at high RPM's. I think the positioning of the vacuum port on the carbs is intentional as if you shut the throttle at high RPM's you don't want to suddenly throw in vacuum advance, positioning it where they are means that vacuum and any associated advance goes zero instantly at that point. Alan Local venturi action will be making that 1psi depression at the take-off. The piston probably only drops atmospheric by ca. 0.2 psi, i.e. the choke tube constant depression. So most is local to the butterfy edge. That's why positioning is crucial. That depression is supposed to go away when the butterfly is opened wider. If it does not, then inappropriate advance will occur. But its difficult to see that happening as the choke tube depression is set by the piston. But there will still be a drop over a butterfly at wot, perhaps 10% of atmos or around 1psi. Puzzling. Yes, I agree, safer for you to leave the va disconnected. Fun stuff, fluid mechanics....I need to ponder more as I too dont want vac advance at wot. You are dropping maybe 2psi across the butterlfy I think. Might be worth comparing pressures before and after buttlerfly? Peter Edited May 30, 2016 by Peter Cobbold Quote Link to post Share on other sites
Peter Douglas Winn Posted May 31, 2016 Author Report Share Posted May 31, 2016 Peter fantastic information I will translate it and give it to him when I retutrn the car. On my bog standard Stromberg TR6 (1971) I did everything myself for the first 25 years. I have had the 9,652 parts in my hand three times round. What does this say:- I have dirty hands and a good set of tools but no way am I an expert on tuning any type of engine. I will have to be careful and avoid the impression that I (the screwdriver) man am telling him how to do his work. I went to him for a very special reason but that is not part of this forum. Quote Link to post Share on other sites
Peter Cobbold Posted May 31, 2016 Report Share Posted May 31, 2016 Peter fantastic information I will translate it and give it to him when I retutrn the car. On my bog standard Stromberg TR6 (1971) I did everything myself for the first 25 years. I have had the 9,652 parts in my hand three times round. What does this say:- I have dirty hands and a good set of tools but no way am I an expert on tuning any type of engine. I will have to be careful and avoid the impression that I (the screwdriver) man am telling him how to do his work. I went to him for a very special reason but that is not part of this forum. Peter, Three times ! - once was enough for me You'll be able to present him with that cruise depression reading. That cannot be taken on a rolling road and must be measured when driving on the road. The reason is most of the horsepower requirement is due to wind resistance. And tyres. Peter Quote Link to post Share on other sites
Peter Cobbold Posted May 31, 2016 Report Share Posted May 31, 2016 Its very difficult to read the depression as the top of the graph, at atmospheric pressure, is unclear. There are 14 to 15 divisions in total, close to one per psi, atmos press being 14.7psi. If the carb recording is one division off atmos, that's 1psi or 50mm Hg. That's enough to actuate a capsule. The SU vac-adv take-off is on the piston side of the closed butterfly, not covered by it. Peter CORRECTION: 50mm Hg is not enough. Vac Advance capsules are specified in INCHES Hg. see page 11: https://supertrarged.files.wordpress.com/2015/11/tr6se-34-tuning-for-cruise1.pdf Peter Quote Link to post Share on other sites
Peter Douglas Winn Posted June 1, 2016 Author Report Share Posted June 1, 2016 Hi! I love abbreviations I presume the following have been correctly understood. Unfortunately nobody (I also) have written by the first abbreviation what it means. = BPM ( Beats per Minute) MPG = Multi Purpose Garage. BHP = British Home Pride CO = Constitutional Orders BTDC = Bad Timing Don't Care AFR = Antiphase Feeling Relaxed UEGO = Under Evaluation Getting On IWE = International Weekend USB = Under Stationary Bombardment BAM = Britsh American Music MAP = Music Appreciation Prize TC = Twin Carburetors OK = All correct PI = Plausible information DCOE = Double Carbon Oxygen Environment WOT = What's Over There? TPS = Tappas psi = peter's stomach indigestion If I have made a mistake please feel free to correct it. Peter Quote Link to post Share on other sites
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