I did a frame off Mini resto and complete mild street mod engine rebuild some 25 years ago, so I am familiar with Minis.
Recently bought a 1990 Mini Mayfair 1000, Japanese Spec, genuine John Cooper Garages 'Anniversary Model' Conversion, ergo arguably a well coordinated Stage 1 equipped 998. My thoughts are related to my Mini and getting the performance as good as possible while making sure it is a dependable and smooth daily driver that gives me the most bang for the buck, so fuel economy is a consideration.
I have read a ton of info here, and would like to throw in my 2 cents and welcome feedback.
My Mini came with a 65D dizzy with no vacuum connection so I started studying the pros and cons of manifold vacuum vs ported, which have been presented here with proponents on both sides. My dizzy vacuum advance is stamped 80 200 8, which I understand to mean vacuum advance starts with 80 mm hg (3.15 in hg) and gives a maximum advance of 8 degrees at 200 mm hg (7.87 in hg).
One of my premises is that at idle and deceleration the mixture is lean, and a lean mixture burns slower and therefore needs more advance to burn more completely and properly. Conversely, I believe that under load the mixture is richer, burns faster, and needs less spark advance to avoid detonation or pinging. In the end, whatever timing curve is best suited for your particular engine and driving conditions is what you should use, but here I am looking at my Mini.
My 1 1/4" SU at the back side of the engine has a ported vacuum port, right under the throttle plate, so it's vacuum at idle is zero, and would come up to a maximum of maybe 8-10 in hg briefly when revved up, then drop back down to zero once RPM leveled off. I had no manifold vacuum port, so I put a Tee in the brake servo line to get a feed, this has about 18 in hg at a no load idle, zero at wide open throttle, and up and down between revving up & down.
I finally decided to do a rolling test as sitting there revving the engine does not simulate real driving conditions. Timing and valves have been adjusted (12 &13 for intake and exhaust respectively), carbs balanced and idling great around 800 RPM, LCB that flows into a prexisting CAT into a single R40 box at the rear. Engine showing 88,882 km, 55,229 mi. with compressions reading 150 PSI accross the board, give or take a couple of PSI, and about 10PSI increase with a 'wet' test. It's a low compression block per 'Guess Works', so methinks the internal mechanicals are more than acceptable for her age.
Teeing into the ported vacuum results were very similar to the stationary test, with a brief uptick of vacuum up to about 8 in hg at the start of acceleration, quickly reverting back to zero as RPM leveled off. Seems to me that under acceleration, where the SU piston upward movement is retarded specifically to enriched the mixture, extra advance under these conditions seem less than ideal to me as I am not trying to extract every last bit of performance possible and prefer to avoid the possibility of 'pinging'.
Teeing into the manifold vacuum the idle and deceleration results were a bit lower overall while driving than the stationary running results. As expected manifold vacuum dropped to zero on hard acceleration, and was the highest on hard deceleration.
Since my premise was idle and decelleration requires more advance for leaner mixtures, and richer loads require less advance, I am going to run manifold vacuum to my own dizzy. Idle wise there is no really significant difference in the idling characteristics. I can always adjust base timing to accommodate the extra vacuum advance at idle should I so wish.
Thanks and look forward to calm, balanced, and reasoned responses.
Edited by gdcarpenter, 14 April 2020 - 07:26 PM.
