[longride]

Compression plays a role. High Compression motors will turn better over-all hp numbers but you may have peaks and valleys in the torque curve because the motors are alrady tuned for hp. If you have lower compression you will usually run lower boost but in doing so you will have a flatter, fatter torque curve or line as it were.

Not that I am a turbo expert, but you can't run much boost with a high-compression engine without burning holes in the pistons. The first step for running high-boost turbo applications is a cylinder spacer to lower compression and then usually C-16 gas to control detination. If you are pinging on boost, you will have a motor meltdown sooner, rather than later. Low boost and mild cam timing, and high rpm's is what makes a turbo go. Like I said, I am not an expert, but high-compression and high boost = BAD!

 

[longride]

I'm not talking insane compression, here. More like the type you get out of your standard superstock 1000cc-1400cc bike. You can run an easy 8-10ftlbs of boost out of a stock Busa motor and get hp figures in the 240 area pretty easy without burning up to motor. But in a V-Twin application, running 7-9ftlbs of boost will trigger a torque curve that should stay above 100ftlbs from about 2500rpm (or so) to 6000rpm, usually peaking in the 130 range in or around the 4500rpm range.
I've seen v-twins running flat-top weisco pistions with 8.5to1 and get solid 150hp ranges all day and night. I know a Busa owner that does his own mapping with a stock motor hit the 260 range. He's never dropped a piston and as far as I know only runs premium 93 octane pump gas. I'm not expert, either, but If you run FI motors and have a laptop you may be in pretty good shape if you know fuelling. I don't. I'm just giving my take from what I've experienced. Personally, I'd go blower. Much more consistant air pressure in application.

Busa's have low compression compared to the other Supersports ,which is why they are so popular to turbo. Much easier to turbo a Busa at 8-10 lbs., which is low boost. Go any higher than 10 and you would need a spacer and octane boost, or some type of intercooler. The way your friend is set up is fine for an occasional blast, but any extended high-speed run, and he might end up with a motor for lunch.

 

[longride]

... too many variables to make any accurate generalizations.

My rule of thumb for gas engines is: "If you want more HP get a bigger engine." Turbos just introduce too many problems. That's what happened with the turbo bikes the Japanese introduced all those years ago. Why put a turbo on a 750 to get 1000 performance? Just get a 1000.

Anyone who thinks they need a turbo on a Hayabusa is welcome to the problems.

At the boost levels the OEM's ran the turbo was more of a marketing scheme than a performance booster. The reason most guys run turbos compared to buying new or building the motor is expense and ridability. Build a non-boosted 200+ hp Busa, and you lose rideablity AND it costs an arm and a leg. With boost you retain stock rideability until the turbo spools and then hang on! I have heard of 500 HP streetable Busa's out there. Crazy!

 

[longride]

If you lower the compression on a boosted engine you lose performance until you are on boost. My brothers old Merkur was a very irritating car to drive because it was gutless until it was on boost. So you were either poking along or pressed back in your seat. It was a rush that got very fatiguing after a while. That and the fact that the POS quit running at random moments and had to be towed. I got very tired of driving 25 miles to pick him up on the side of I-5. LOL!

Which is why there are so many turbo Busas running around with stock compression and 10 lbs of boost. 250 hp with stock drivability for not a lot of cash. I was considering doing it myself until my license took a couple of hits from the friendly state troopers.