One more time. I've checked numerous ignition maps for turbo cars (base maps for Link ECU). They usually drop from 30deg to 20deg immediately when boost is coming (110kPa) e then around 200kPa/2bars they have from 10-11deg at 9000rpm.
Why bike engines can handle so much advance in high boost??? It's strange!
Not strange, a car due to weight & high inertia of engine parts ( relatively speaking )& thermal build up, loads up the cylinders.This brings on detonation sooner than a bike engine & requires less advance....
I have consult with my friends (they make drag cars) and according to them , main reason that the bike's carry so much advance is short stroke. I don't know is it true, but I changed my mapping to less conservative (35deg at 140kPa) and bike ran very well. It's a little - U gota fight, to your right, to stay on bike .
How much boost can handle stock motor with pump gas??/
Currently I have 40° full adv. and retard 5° each 0.5bar on my B12 with busa pistons/rods. Sometimes I have 38° full adv. and retard 4° each 0.5bar... both works. Currently running 1.3 bar boost on 102oct pump gas.
Well, the 88/89 750 is the 73mm bore short stroke engine which I had in my bike to start off with. So the problems are that these engines have a 19mm wrist pin, so piston options are limited. All the 1100 's and Busa's etc use a 20mm wrist pin, so won't fit. 1052 barrels will fit straight in the cases though, but not much help without a matching crank and rods. I ended up using 05 zx10r pistons, 76mm bore, factory forged, flat topped piston. Gave a final cr of 7.2-1 though, so too low really, bit good for lots if boost. And......they still werent a 19mm wrist though. I think they were 17mm, so I had to remove the rods and get bronze bushes fitted to sleeve the rods down. Ended up at 813cc
Has someone measured piston deck height (from pin upper end to top) of short stroke 750???
Only forged pistons that I find (almost for no money) is from a 3 times bless from the God ... VAGroup. There are tons of 1.8t forged Mahle pistons - 81mm with 19mm or 20mm pins and weight of around 290 grams. I got one of older type - flat top, 20mm pin, 80,95mm and measured 20mm from pin upper end to top of piston dish. If is necessary 2mm can be shaved without any problem , so deck height of 18-17 mm is achievable.
Doesn't really matter what this measurement is anyway as you can't bore the 750 barrels to 81mm. 76mm as I did is the max. So, if you want to go this route, it's lots of work. Bandit 12 barrels bored to 81mm, then machined down by a heap to get the reguired deck height to suitthe VAG pistons, then bore the cases so the barrels will fit. To much work for a very over square bore to stroke ratio. These engines only have a 44mm stroke!
Dare I say, as you have gone to the effort of efi on this build, and if you have read my thread on ignition timing, I think you could run 14 psi on the stock comp. If you've built the ignition table well, and your tune is on the rich side as boost rises, it should be ok. But I doubt the clutch will let you load up the engine at 14psi to really test it lol If your thinking of changing the pistons anyway what have you got to loose. If they do fail, 750 pistons are cheap as anyway, so could replace with stock pistons and decompression plate. But, I will add, my own success with my turbo system is not that great, so probably not the best person to be giving advice at all really. It's your build. Do what you feel you want to. Good luck