Over the holidays I was thinking about this topic and how it seems you don't really have a plan which imho shows in some things you wrote.
If you're serious about this and unless you're just randomly checking your options I'd suggest you stop and take a step back. The only approach that makes sense is to know exactly what you need (not what you want) and have a concept. To get there you'll have to answer yourself more questions.

But before getting there, you say you aim for 85-90 then 90-100hp. If the base is a 750 engine you should know that the 72hp that are on paper are actually 60-64hp at the rear wheel which can be seen on different dyno charts. Getting +30hp out of a 750 engine is not realistic and even if you can get over 20hp out of it the result will be an engine which is for sure not for everyday use. Forget max hp, that's not what you're after. If you'll be riding the bike on the street you want torque. If you aim for torque a hp increase will come with it automatically.

This should answer the question if a 750 crankshaft can handle 90-100hp. You will not get in that situation.

If 96mm pistons fit on a 750 engine ? The original ones are 88mm, the 780cc big bore kit is 90mm, the 900cc engines have 92mm pistons. 94mm pistons on a 900cc engine result in 944cc, 96mm in 960cc.
Take a 750 cylinder base gasket, measure the outer diameter of a 900cc cylinder and you'll see that doesn't make sense.

A 900SS engine has a 43mm inlet and a 38mm outlet valve, so can you fit a 44mm valve in a 750 head - no.
You can combine a 750 racing valve and a 900SS valve but you'll need a washer for the spark plugs. Lift and overlap have to be considered such as the clearance to the piston, even more if the pistons have a high dome and the cams are aftermarket.

900SS cams are not worthwhile losing time with btw. You can also read BikeBoy`s tests.

Every modification to an engine, exhaust or carburetion system will influence its performance, narrow or widen hp & torque output range and shift it up or down the revv range. You need to be clear about what you want. Some modifications only work in combination with others and some shouldn't be combined. The more there is done the more the whole characteristics of the engine is impacted and chances are you'll end up with an engine that shows a great result on a dyno but doesn't meet your expectations on the street.
For instance "lightening all the sprockets / flywheel and max porting on the heads incl racing cams" + large valves etc imo doesn't really coincide with a street used 750 engine.

I never really understood the appeal so many guys show for "race" parts. "Race" means the parts are designed for max performance at WOT and a revv range you will hardly hit on the street. Other parts like race brakes can also be the opposite of an improvement because they ideally operate at temperatures which you will not reach under normal riding conditions.

Coming back to the beginning; some questions you should ask yourself are:
- how is your actual riding style and how would an engine need to be to match that ?
- what is your budget ? A full engine modification can easily cost as much as your bike is currently worth. While the crankshaft will do you'll need aftermarket conrods, in consequence a crankshaft rebalancing, new crankshaft bearings etc etc
- does it make sense to use a 750 engine as base or directly go for a 900 ?
- how "street legal" do you want to stay ? Most mods may not be legal anyway but things like large valves do increase noise emission in a noticeable way
- do you want a bullet proof engine or just max power output (if it's the first skip a dual spark plug conversion)

sorry, got a bit carried away ...

G.

95HP was possible in the last 750 TT1 race Engines.
Just take a tone off money and build a Race Engine with a weekend lifetime

Race engines measure hours not miles!

I have an 87-F1 with some slight performance mods and it's very perky. Quite fun to ride too. But 90hp from a Paso engine will be extremely expensive and no fun on the street. Just as an example of what can be done to an air cooled twin, I have built a monster motor for my Multi. 1098cc 12:1 compression, knifed crank, slight porting on the heads, custom dyno tuning and she makes 95hp and 94 ft. Lb torque. It's street bike and it's a wonder to ride. Torque is where it's at for the street.

Hi G.,
That was a excellent recap of good old hot rod logic. I always thought for street use that a engine taken apart and "blueprinted" ie highly balanced, very good tolerances etc would be about as good as it gets for the everyday street.My experiences down that path has been with BMW's and I expect that a 750 would be the same.Regards Arch.

ok, it will take some time! Have a nice job!

Stick to what the experts say.Faloon and others such as ed milich have spent a lot of time building these engines for street / race , and are receptive to sensible emails. merry xmas and happy new year to all you duc enthusiasts. rgds J

PS No disrespect intended to any of the above posts,but there are so many variables involved in tuning a performance engine that unless you have considerable experience ,it can quickly become a nightmare

Thats a 30 year old book,a lot, no make that everything has changed since then

Ed I would listen too, when he knows you well enough to speak to you

Looking forward to watching this project progress. It's great that you can bounce your ideas and thoughts around in a forum like this.

I would agree that you should study the work of engine builders and tuners mentioned in this thread. Check their websites and examine the motor recipes they have used to see what modifications will achieve your goals.

Good luck!

Best wishes by the way!

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Holy crap G ! you sure don't look like what I expected er, nice bike