Supercharging

Yeah, a Roots blower is positive displacement. Every rotation can move a certain volume of air. But, if the throttle is closed, it will not be getting any air to move, therefore no boost (just parasitic drag losses). The throttle must ALWAYS be the determination between boost or not. Otherwise how would one control the engine output? [emoji3]

During low load operation, the blower is just drawing power from the engine to mix the intake charge. Only during heavy load will the throttle be open enough to allow enough airflow to produce boost - a positive manifold pressure.
 
suck through as per the SR picture, drive it behind the alternator in line with the starter chain, but may change when the bike gets stripped
The more I look at that SR setup the funnier it looks. He's mounted it upside-down compared to a "typical" roots setup. The blower output faces away from the motor, requiring a lot of plumbing to redirect towards the motor again.
It's possible a pulley-up arrangement might not have fit in the frame.
Every other supercharged roots setup I've seen is done with the pulley at the top so that the output faces the cylinders.

Have you seen any information about the build? I've only been able to figure out that it belongs to a guy named Gary in Australia.
Found a better picture of his plumbing and an alternate drive pulley arrangement.
 

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I wonder about the 180* crank on the xs400. The overlapping intake stokes and only one carb/changer would cause one side to run leaner. This has been discussed on another thread in the past.
 
The more I look at that SR setup the funnier it looks. He's mounted it upside-down compared to a "typical" roots setup. The blower output faces away from the motor, requiring a lot of plumbing to redirect towards the motor again.
It's possible a pulley-up arrangement might not have fit in the frame.
Every other supercharged roots setup I've seen is done with the pulley at the top so that the output faces the cylinders.

Have you seen any information about the build? I've only been able to figure out that it belongs to a guy named Gary in Australia.
Found a better picture of his plumbing and an alternate drive pulley arrangement.

looks like he's put a counter shaft on that.

the reason is maybe he needs enough tubing for the large plannum thats needed for a single

All I know is the guy is from OZ, love to be able to pick his brains.
 
I wonder about the 180* crank on the xs400. The overlapping intake stokes and only one carb/changer would cause one side to run leaner. This has been discussed on another thread in the past.

I think there will be so much gas in there on tap, it shouldn't be a probelm.

most online info points to the 180 cranks being the best for superchargeing as it evens out the pulses. 360 cranks are basically a big single split into 2, which is over come by having a large plennum chamber
 
Have there been many 180* crank supercharged parallel twin bikes/motors done? Most I have seen are 360* v/twins or 4 cylinder bikes. I did see a cb500 which I think did come with a 180* crank stock but they said they re-phased the motor. My guess is they changed it to 270*
 
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I can't actually think of any completed supercharged bikes with 180 cranks either.

I agree with drewps though. The 180 crank should work pretty well.
Smaller required manifold volume and possibly less leakage past the rotors.
 
My first thought is that the manifold pressure will vary greatly with a 180 crank. Pressure will spike and drop dramatically, with #2 not receiving nearly the same charge that #1 got. I'd expect that a large plenum would help reduce this effect, but not eliminate it. The problem with a large plenum is that the mixture will have a chance to separate, especially at lower engine speeds.
 
The manifold volume would cycle twice as often but the blower would only have two move half as much air each time.
Supposedly suck-through roots blowers greatly increase atomization of fuel in the mixture, possibly compensating for any mixture separation after the blower.
Tuning a single-carb setup is already hard enough. Adding the blower in there only complicates it.

Have you considered doing a dual-carb blow-through setup, drewps?
The carbs would need some modifications and have to be sealed but it'd eliminate many of the issues mentioned so far.
A blow through setup will also be much safer compared to a suck-through and its explosive gas mixture.
I believe most race rules require significant guards added around the blower to protect the rider in the event of a backfire and explosion.
 
The manifold volume would cycle twice as often but the blower would only have two move half as much air each time.
Supposedly suck-through roots blowers greatly increase atomization of fuel in the mixture, possibly compensating for any mixture separation after the blower.
Tuning a single-carb setup is already hard enough. Adding the blower in there only complicates it.

Have you considered doing a dual-carb blow-through setup, drewps?
The carbs would need some modifications and have to be sealed but it'd eliminate many of the issues mentioned so far.
A blow through setup will also be much safer compared to a suck-through and its explosive gas mixture.
I believe most race rules require significant guards added around the blower to protect the rider in the event of a backfire and explosion.

there's no room for blow through, unless it's EFI

Just won a rebuilt 1 1/4" H2 SU carb tonight

won't be starting this till summer as lots to think abaout and lots of engineering to do
 
From what I have seen and have been told to make it work like it should a re-phase should be done. That would also be a first for these bikes I think:) It will be awesome to see this finished. :bike:
 
Maybe worth sourcing another motor if you are replacing the pistons etc and running new gaskets throughout and changing so much stuff around. Its like a rebuild so you may as well keep your nice new motor as it is and rebuild another one? Just a thought then you can get some good cash for your original motor.
 
Maybe worth sourcing another motor if you are replacing the pistons etc and running new gaskets throughout and changing so much stuff around. Its like a rebuild so you may as well keep your nice new motor as it is and rebuild another one? Just a thought then you can get some good cash for your original motor.

400 engines are going for £300 now:yikes:
 
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