Mikuni HSRs worth all the fuss?

So Im running two SU HS6s with DX needles on a slightly built B20 (F head, nice header, slightly hot cam) and Im wondering whats the truth on Mikuni HSRs. Big power gains + efficiency gains? Or just efficiency? 42 vs 45 vs 48? I know they?re a lot easier to tune and have a far better overall design. Just wondering if worth the cost to upgrade, or to just get the old SUs professionally tuned and deal with the quirks.

If I'm remembering right, the HSRs are for motorcycle and other small engines... I've seen the dyno charts from v-performance on these, and honestly doesn't look like it's worth the money for a proper kit.



This one was a direct comparison between stock SUs (green) and the 42mm mikunis (red). They didn't specify what the blue line was, perhaps larger mikunis.

Wasn't a B20 being dynoed though. I think in their tests it was a B-series BMC engine with a hot cam and a mega-ported head, so I'm not sure you'd see the 13 hp difference in a lightly done-over B20. Also consider that peak power in the 'road test' condition for the dyno was 105 mph, so I'd assume they were revving this engine pretty hard.

EDIT: For what it's worth, assuming you're running a pretty stock head (not super shaved or ported or with big valves), you'd probably be better off sticking with the SUs.

Thanks for the advice! Definitely don?t want to spend money if it isn?t worth it. Ive learned to not exactly trust V Performance after some of the stories Ive heard about the shipping on their supercharger or carb setups. Guess Ill put the money into maybe putting a P1800 rear axle and disc rears later.

there is a book " tuning bl A series motors" about mini motors by Vizard. There is one chapter just on air cleaners and bellmouths. The important one to you is s chapter just about getting SU's to flow more, almost as much as a weber sidedrafts. There are a lot of little cheap mods, like cut the throttle shaft in half and use buttonhead screws. Most of the data was for 1 1/4 or 1 1/2 sus but would still apply any of them.
He air flowed everything he did to show any improvement.
Well worth the read.