Peter Cobbold Posted May 31, 2011 Report Share Posted May 31, 2011 Peter the 2.5 pi {132bhp} saloons had vacuum take off on the front throttle body, a smaller bore version of the M U take off ,it wil be at full manifold vacuum so the engine wil have extra advance at idle or low load. when driving the TR5 with the vacuum connected i am aware that the throttle peddle is bearly open at 50 mph . Dave, That's intriguing. Having the va capsule active at tickover and over-run must have been normal on the saloons. It would be quite easy to 'T' into the brake servo pipe for those unwilling to drill TR manifolds. No need to drill at throttle edge then. You must have the most cost-effective economy device for a TR6 PI ever - a short length of tubing for 4-5 mpg better. Terrific. Do you have any idea of cruise AFR? Peter Quote Link to post Share on other sites
kiwican Posted May 31, 2011 Report Share Posted May 31, 2011 "Also interesting is the suggestion that the P.I. system has no transition enrichment provision" Doesn't the mixture get richer when the vacuum is low. So when cruising you have high(er) vacuum and then when you accelerate the vacum drops thus enriching the mixture? Simon Quote Link to post Share on other sites
Tom Fremont Posted May 31, 2011 Report Share Posted May 31, 2011 Tom I did not know 250s on carbs would get to cruise at 41 impmpg - that's clearly a target for the PIs and Webers!! And I'll accept that challenge too, despite the blower. Peter Peter, I haven't the least doubt I could get that mpg on a '250 with Webers with the stock cam ( 10/50/50/10 valve timing ) . Unfortunately it would serve only to prove that point as one should not expect any power increase over the Strombergs ! Although a " scientist " myself( mech. engineer ) I have no depth in IC engine fuel systems beyond my own experience. I'll just say that lean as can be, the P.I. cam exhaust positively stinks ( literally ) compared to the lower overlap ones. I've no doubt the efficiency is well below that of the '250 cam but the point was different - higher max power! Basically, the P.I. cam allows more charge into the combustion chamber but the penalty is lower mpg and more pollution . VTEC of course reconciled those diverse objectives - I guess just about anything of interest in showrooms today has that aboard... Quote Link to post Share on other sites
Tom Fremont Posted May 31, 2011 Report Share Posted May 31, 2011 You will not achieve the correct curve with any linear dizzy with Weber's it will only be a best guess Hi Neil, You have PM. Tom Quote Link to post Share on other sites
Peter Cobbold Posted May 31, 2011 Report Share Posted May 31, 2011 "Also interesting is the suggestion that the P.I. system has no transition enrichment provision" Doesn't the mixture get richer when the vacuum is low. So when cruising you have high(er) vacuum and then when you accelerate the vacum drops thus enriching the mixture? Simon Simon, In short, no. Its easy to forget that when the vacuum is high it needs less fuel to keep the mixture constant, because each cylider contains less air ( the engine has to be throttled back to get the vacuum). Peter For those puzzling over the PI set up, heres why it cant respond with transient enrichmemt. The volume of fuel per squirt is set for any given 'vacuum' by the diaphragm pulling on the fuel cam. When running at light load the engine is throttled so the cylinders fill with less air. So less fuel per squirt is needed to get the same mixture strength. The cam is lifted up by the vacuum to do just that. But the relationship between vacuum and mixture (aka air fuel ratio, AFR) is fixed. Of course it is possible to adjust that cam on the bench to give a increased AFR (ie lean the mixture) but we cant have a choice of rich or lean at any one vacuum in a running engine. Its that inflexibility that causes the transient problem. When suddenly going from throttled (cruise) to wide open throttle ('wot') engines need briefly- and instantly - a rich mixture. But the PI MU can only provide the standard set mixture and it takes time for the diaphragm springs to push down the cam and for the cam followers to allow the end-stop piston to come out of the fuel distributor etc. Its all too slow, and those six butterflies are already wide open and air is entering the cylinders maximally. The mixture if anything will go lean as the MU tries to catch up the air. But we want rich briefly - like the Weber pumps or the SU damper provide. So I think the fuel cam is set up by tuners to be rich along its length, just so that brief, transient leaning is not so great that the engine pinks. But rich is not good for economy. But all is not lost,we can make it work better. It should be possible to speed up the movement of the diaphragm and cam during that brief cruise-to-wot transition. And to speed up the removal of cruise-advance by the av capsule. That way it should be possible to run the PI much leaner at cruise vacuum, and to reduce the time that pinking occurs. I posted elsewhere how the PI mixture could be leaned at cruise with a simple solenoid valve and a small air pressure pump. The fast-acting 'transient eliminator' uses two small solenoid valves, a vacuum-sensing switch and that same air supply. The 40mpg PI TR6 is I think distinctly feasible, without sacrificing performance. Quote Link to post Share on other sites
Peter Cobbold Posted May 31, 2011 Report Share Posted May 31, 2011 Peter, I haven't the least doubt I could get that mpg on a '250 with Webers with the stock cam ( 10/50/50/10 valve timing ) . Unfortunately it would serve only to prove that point as one should not expect any power increase over the Strombergs ! Although a " scientist " myself( mech. engineer ) I have no depth in IC engine fuel systems beyond my own experience. I'll just say that lean as can be, the P.I. cam exhaust positively stinks ( literally ) compared to the lower overlap ones. I've no doubt the efficiency is well below that of the '250 cam but the point was different - higher max power! Basically, the P.I. cam allows more charge into the combustion chamber but the penalty is lower mpg and more pollution . VTEC of course reconciled those diverse objectives - I guess just about anything of interest in showrooms today has that aboard... Tom, Maybe the timed, pulsatile fuel delivery from the injector ensured rather little fuel was carried into the exhaust in the overlap? (the air nearest the inlet valve as it opened having least fuel in it) That would not be possible with carbs, hence the smell from the PI cam? Now with a blower, definitely no need for overlap! - perhaps I can get 40mpg too! I have plans... Would have been an engineer myself if I'd had the maths, but deserted to cell biology. Peter Quote Link to post Share on other sites
Tom Fremont Posted June 1, 2011 Report Share Posted June 1, 2011 Now with a blower, definitely no need for overlap! - perhaps I can get 40mpg too! I have plans... I shall expect no less with you at the helm of the mission Actually, it occurred to me that you could have it both ways with a blower and low overlap cam . Quote Link to post Share on other sites
kiwican Posted June 1, 2011 Report Share Posted June 1, 2011 Simon, In short, no. Its easy to forget that when the vacuum is high it needs less fuel to keep the mixture constant, because each cylider contains less air ( the engine has to be throttled back to get the vacuum). Peter Peter, Thanks for the explanation, what you said makes sense. I think at some point it would be interesting to put my car on a dyno and see what is happening with the AFR at various loads/speeds. Might be a good club event for our British Car Club here in Las Vegas. The more I read of your posts the more I think that an EFI set up might be in my future. My car is running fine and I dont plan to ditch anything but should I have problems down the road I think I would seriously look at the EFI. Regards Simon Quote Link to post Share on other sites
Nick Jones Posted June 1, 2011 Report Share Posted June 1, 2011 Hi Nick Thats very interesting - there's no doubt digitally mapped fuelling and timing has the legs of the old mechancal stuff for sheer tunability. But the flame in the combustion chamber will be exposed to similar compression/squish/turbulence etc so it will spread at similar speeds. So the timing settings you use would be applicable to the Lucas PI engines. How much advance can you give it, before it gets rough? Peter Peter, See ignition map attached. Not certain that's the current one but it'll be very similar. It will take alot more advance at low rpm / minimal throttle without pinking, but it causes over sensitivity to throttle and loss of progression. Takes a delicate touch as it is! Agree that without VA/3D the engine is only tuned for WOT. Surprised Triumph considered it acceptable - smacks of an accountancy solution rather than an engineering solution....... Nick Quote Link to post Share on other sites
Peter Cobbold Posted June 1, 2011 Report Share Posted June 1, 2011 Peter, See ignition map attached. Not certain that's the current one but it'll be very similar. It will take alot more advance at low rpm / minimal throttle without pinking, but it causes over sensitivity to throttle and loss of progression. Takes a delicate touch as it is! Agree that without VA/3D the engine is only tuned for WOT. Surprised Triumph considered it acceptable - smacks of an accountancy solution rather than an engineering solution....... Nick Nick, Many thanks for sharing that data. Is this what we're looking at: 1000kPascals is atmospheric pressure. (100,000Pascals = 1000 millibars.) So that column is the pressure (absolute)in the inlet manifold? Therefore: The bottom left hand corner is tickover.(low pressure in manifold and low rpm) Top right, max power; wide open throttle (atmos press in manifold, and max rpm.) Top left: wide open throttle at very low rpm: chug..chug..chug. Bottom right: over-run e.g. using engine braking down steep hill (high rpm, closed throttle, low pressure.) So we can see the pattern: At all pressures as rpm rises more advance is needed - the colour coding shows up the broad pattern from left to right. The centrifugal advance on the TR6 disy can do this. Indeed some of the lines across that table will resemble a TR6's advance curve. At a guess it will be near the top lines for a PI disy. What the TR6 PI disy without its av capsule connected cannot do is adjust the timing up and down that table. In other words it cannot adjust for load, which on the table is measured as pressure. High load at top, load decreasing as we go down. (Ignore the columns at very low rpm 900 and 1000rpm: tick over, and also the bottom row: closed throttle overrun.) Now look how the advance increases as we go lower in the table towards lower loads. Particularly interesting is the region of 2500rpm, a typical cruising rpm. WOT timing is 27BTDC. But lower down at part throttle, where we would be at cruise, the timing advances to around 40-45 BTDC. Its that advance at cruise that gives better fuel efficiency. Without the av capsule the timing (27) is effectively retarded, and the flame is chasing the piston down the cylinder wasting energy rather than exerting its max pressure on the piston at around 15-20ATDC. Thanks Nick, your table gives a good idea of where we should seek to set the cruise timing when tuning the av capsule: 40 to 45 BTDC. I confess that I dont use the av capsule, I cut it off to fit a rotary solenoid that pushes the points anti-clockwise as boost retard. But it can easily be modified to give any advance I want, triggered with a vac-sensing switch at any manifold pressure I want. Not as flexible as your electronics, but at least I'll have second line lower down that table. Peter Quote Link to post Share on other sites
Nick Jones Posted June 2, 2011 Report Share Posted June 2, 2011 Peter, Yes, you've understood it about right - except that it idles at about 60 - 65 kPa and cruise is approx 45 - 60 kPa. On (this) Vitesse gearing you need 3000 rpm for 70 though in spite of higher than standard gearing. Only really goes below that on trailing or very light throttle. It only goes into the top left if it's about to stall - hence the extra advance as a crude antistall device - works! Nick Quote Link to post Share on other sites
smizgals Posted June 7, 2011 Report Share Posted June 7, 2011 Thanks Nick, your table gives a good idea of where we should seek to set the cruise timing when tuning the av capsule: 40 to 45 BTDC. I confess that I dont use the av capsule, I cut it off to fit a rotary solenoid that pushes the points anti-clockwise as boost retard. But it can easily be modified to give any advance I want, triggered with a vac-sensing switch at any manifold pressure I want. Not as flexible as your electronics, but at least I'll have second line lower down that table. Peter Peter, Very interesting, that makes me think again about the http://www.leenapk.nl/product_details.php?category_id=74&item_id=104? to achieve the same? Stan Quote Link to post Share on other sites
Peter Cobbold Posted June 7, 2011 Report Share Posted June 7, 2011 Peter, Very interesting, that makes me think again about the http://www.leenapk.nl/product_details.php?category_id=74&item_id=104? to achieve the same? Stan Stan, Yes it might - could also be used for boost retard. I haven't looked into their tuning software but as long as it goes out to the advances Nick used it should be OK. That said, I shall be sticking with the rotary solenoid as I have to advance the timing at cruise on petrol, kill the advance under load on petrol and then re-advance above 3psi boost to compensate for slower ethanol combustion. That's the fun of dual fuelling! And I have an aversion to using digital signals and laptops etc in older engines. Peter Quote Link to post Share on other sites
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