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| Vacuum advance or not? https://ausmini.com/forums/viewtopic.php?f=2&t=100829 |
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| Author: | Steam [ Fri Mar 12, 2021 6:42 pm ] |
| Post subject: | Vacuum advance or not? |
Probably been asked before but I cannot find what I want to know from a search. Given the Cooper S , GT, and various other Minis ran without a vac advance units (23d4, 43d4 etc) would I be better to run a higher compression 1275 engine with or without vac unit. And are the settings different with or without etc. Cheers in advance (pun intended  )
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| Author: | drmini in aust [ Fri Mar 12, 2021 7:11 pm ] |
| Post subject: | Re: Vacuum advance or not? |
Just something else to go wrong and mess up the advance curve. Performance European motors didn't use it, particularly those with Weber or Dellorto carbs. eg. Alfa Romeo GTV, Fiat 124 coupe, Lancia Beta, etc etc. |
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| Author: | simon k [ Fri Mar 12, 2021 9:20 pm ] |
| Post subject: | Re: Vacuum advance or not? |
vacuum advance is a fuel economy device only - it advances the ignition timing when the engine is not under load |
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| Author: | ben73 [ Sat Mar 13, 2021 6:27 am ] |
| Post subject: | Re: Vacuum advance or not? |
simon k wrote: vacuum advance is a fuel economy device only - it advances the ignition timing when the engine is not under load Exactly, and if set up correctly will save you fuel with no negative effects. |
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| Author: | 1071 S [ Sun Mar 14, 2021 9:33 am ] |
| Post subject: | Re: Vacuum advance or not? |
When a gallon of fuel (super of course aka premium) cost about the same as 2 middies a vacuum advance was not much of a concern.... (The local BP gave a discount if you bought $2 worth of fuel.. but you couldn't fit that much in a (non S:) Mini...  Today its a no-brainer. Cheers, Ian Although, checking the math ..maybe not much has changed... but vac advances are still worth having on a road car. |
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| Author: | mickmini [ Sun Mar 14, 2021 1:08 pm ] |
| Post subject: | Re: Vacuum advance or not? |
I don't run vac advance on my electronic dizzy on my 10.9:1 1410cc engine. It's a performance motor with a Weber that does not have a port. But that's ok, because I never run it below 3000rpm, where vac advance might actually activate. Full mechanical advance is in by that stage. If you are looking for economy on a commute where you are doing 90-100km/h cruising, the engine will be sitting above the rpm where vac advance would be effective. If you are more likely in traffic going 60km/h or less, then there may be an economy argument, because the engine will operate in the range where vac advance activates. The last mini I used for a daily commute was the mpi Mini40 I had in the Netherlands, so the question never came up. The next question is whether you would notice the fuel savings. I can't answer that because I just pump the fuel in and have fun. I take a similar view on tyres, I get the grippiest for fun, and let them wear out, get a new set. So it's basically horses for courses. What do you do with your mini, and does economy matter to you? cheers michael |
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| Author: | Steam [ Sun Mar 14, 2021 1:37 pm ] |
| Post subject: | Re: Vacuum advance or not? |
Thanks all for your input. I have just recently, for a couple of reasons, replaced the dizzy in one of my minis nd the new unit does not hve vac advance, as I had it as a spare. I will run with it and see what happens, whether I notice any difference. |
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| Author: | 1071 S [ Sun Mar 14, 2021 6:00 pm ] |
| Post subject: | Re: Vacuum advance or not? |
"...If you are looking for economy on a commute where you are doing 90-100km/h cruising, the engine will be sitting above the rpm where vac advance would be effective. If you are more likely in traffic going 60km/h or less, then there may be an economy argument, because the engine will operate in the range where vac advance activates. ..." Not really a good understanding of how vacuum works. Of course there's vacuum at idle but not much... and who cares. 3 to 4K cruise - especially with an engine like Micks - its where you're going to find maximum useable vacuum. Revs don't matter, its the degree of throttle opening compared to engine demand that counts. Max vacuum will occur going downhill at speed ... but again, who cares. Your foot may be off the throttle completely meaning there's not much airflow so not much fuel used... so not much improvement available. Pretty much any Mini uses about 20 odd HP to cruise at 100 kays. On a stock 850 that means a fairly, but not fully, open throttle to produce that much (of its available) HP. For a worked 1275+ you're using less than a third of the available power meaning a much reduced throttle opening. ... which provides a lot of room to reduce fuel usage. Carb vacuum works better than manifold vacuum (for reasons beyond my understanding) but I've effectively employed manifold vacuum when using both a DCOE and IDA Weber... Cheers, Ian |
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| Author: | GR [ Sun Mar 14, 2021 8:25 pm ] |
| Post subject: | Re: Vacuum advance or not? |
HI All Being a little older than a lot of you, you would not know about a little gauge used back in the 50s/60/70s it was called a vacuum gauge and this is what people used to get the best economy out of their driving. The more vacuum the gauge read meant that you had the throttle closed creating a lot more vacuum and pulling on a lot more advance for better economy and a lot of people used them. The vacuum advance is designed to get better economy when cruising not around town where it not very useful, it is designed for cruising at higher speeds for longer periods. The higher the speed on a light throttle the more vacuum your engine will pull, with the mini running at 100/110 k's with vacuum advance depending what deg's are in the vacuum side of the dizzy you will have some where around 60 to 75 deg's of advance in the engine, even with my range 266ss and RE13 cam they will still pull 10 to 15 ins of vacuum. This is why with programmable ignition on an off throttle we put in a lot more advance to help burn the lack of mixture in the cylinder, have a look at what modern cars do running compression ratios of 13/14 to one with knock sensors and funny ignitions curves. When I dyno an engine I do a lot of part throttle openings to see what the mixture is doing at cruising and throttle openings through the range,people go to drive on dyno,s and they take readings from low down to maxium HP not checking what is happening at part throttle openings which is very important . Graham Russell |
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| Author: | Phil 850 [ Sun Mar 14, 2021 9:04 pm ] |
| Post subject: | Re: Vacuum advance or not? |
Thank you Graham for the detailed explanation. Far better described than I would have managed
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| Author: | Steam [ Mon Mar 15, 2021 2:42 pm ] |
| Post subject: | Re: Vacuum advance or not? |
Thanks again for excellent replies, a couple of follow up questions if I may. Would the economy savings be small or large ie what percentage ? Does the lack of vac make it noticeable at cruise part throttle, ie misfire or lack of pick up etc ? And finally would a different initial setting be needed without vac? Cheers. |
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| Author: | GR [ Mon Mar 15, 2021 7:11 pm ] |
| Post subject: | Re: Vacuum advance or not? |
Steam wrote: Thanks again for excellent replies, a couple of follow up questions if I may. Would the economy savings be small or large ie what percentage ? Does the lack of vac make it noticeable at cruise part throttle, ie misfire or lack of pick up etc ? And finally would a different initial setting be needed without vac? Cheers. Hi Steam 1/ That would depend on how far you drive your car and how you drive your car,if you could drive from Sydney to Penrith on the freeway on cruise your not going to save 3 litres of fuel but every little helps 2/ not sure what you mean here on cruise or part throttle is when you get vacuum, but when you open the throttle you loose vacuum advance and go back to your normal dizzy advance. either way it will not cause misfire or lack of pick up. 3/ No you will not need a different ignition setting just remember to take off your vacuum when setting the timing. One thing the vacuum advance helps with is it helps clean up the cylinder on idle and helps to stop plugs from fouling. This is why LINDSAY'S dizzy,s work so well as they fire for 20 deg,s after firing (lomin on this forum) Some of the dizzy,s in the UK do not fully advance till 6000 to 6500 RPM no good for performance or economy. In some of the motors I build I run a locked dizzy just set the timing to full advance around 28 to 34 deg,s but you need a good starter motor. When I modify a mini dizzy I run 8 deg,s in the dizzy all in by 3000 RPM this means you will set the timing up at 16 DEG,s static this means the motor will idle off the advance and not the throttle opening, the more you can close the throttle on idle the less chance the motor will run on. Graham Russell |
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| Author: | Steam [ Mon Mar 15, 2021 9:27 pm ] |
| Post subject: | Re: Vacuum advance or not? |
Thanks GR, thats just what I was lokking for. |
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| Author: | lomin [ Tue Mar 16, 2021 9:28 am ] |
| Post subject: | Re: Vacuum advance or not? |
Hi, I certainly do not believe that you need a vacuum advance if the distributor is set up correctly. I also agree with Graham, but will expand this conversation in the direction that i note you have fitted a new distributor without vac advance. These all have their origins in China ,some are slightly modified in Uk, some in Australia, and some sold as they are manufactured, on Ebay and elsewhere , and range from $90 to $250 .They cost $550 for our kit of modified long duration dissy coil and leads. These take 5 to 7 hours per dissy to modify. The problems you will face include too much timing built into the distributor so you cannot run enough base timing, wrong curve that increases too rapidly leading to pinging, lack of heat sink paste or bush grease, rotor button not pointing at terminal on cap when firing (phasing) weights that jam, etc etc.and a weak spark output to boot.All this means you pour more unburnt fuel out the exhaust pipe. That does not help economy or performance. i don't wish to put people off buying lovely shiny new products for their mini, but there are reasons why some cost more. Cheers Lindsay Siebler |
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| Author: | Steam [ Tue Mar 16, 2021 9:41 am ] |
| Post subject: | Re: Vacuum advance or not? |
Thanks for the advice Lindsay. However I may have misled you by saying 'NEW' distributor. I have fitted a 43D4 that I have had here for a number of years. It was a reconditioned (by NMP when Henry was still there). I have the curve as tested after recon and it matches a 43D4 which is also the same mechanical advance as the 45D4 but without vac of course. Cheers.Dave. |
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