I think there's a lot involved there.
Not having compared an SP1 to SP2, it would make sense that the intake port diameters would match the throttle body runners and that would get all upset.
The injector length is different, so you'd have to buy 4 SP2 injectors as well, unless they also came with the T-B's.
And I don't know if an SP1 ECU will work with SP2 T-B or injectors.
May as well get the SP2 heads to make everything fit and work right.
But that's what projects are for.
If you do it, let us know how it goes. Knowledge is power
From a theoretical side, the smaller SP1 T-B's should allow higher velocity air flow than the SP2's.
This is ideal for low to middle throttle settings.
On carburated engines, too large a throttle bore meant flow stagnation at lower revs and throttle openings.
So on an engine with huge throttle bores, if you wacked the throttle opened at low revs, it would spit back when the flow stagnated.
Also higher flow velocity means greater opportunity for ram effect at lower revs.
This was one of Suzuki's secrets to mid-range power on it's GSXR's for years.
Honda increased the SP2 race bike T-B's to 62mm from 54mm for greater top-end HP.
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