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Old 06-25-2017, 11:58 PM   #1 (permalink)
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Cheap Turbo motorcycle system?

Hi guys,

I realize a turbo is opposite to saving fuel, however I don't know another forum where I could post this.

I have a Honda Rebel 250cc. Great bike.
And I want a little more power.
It's good enough in gas mileage (I get between 66MPG full throttle, to 115/120MPG at best); but sometimes I wished I had more power for the highways.

The Rebel uses an old style carburetor, and I want to keep cost down for a turbo upgrade.
I'm looking at a small turbo, that would give anywhere between 4 to 9 PSI boost.

I don't want to change my cams, and I don't want to install a fuel controller.

I've devised a system (but haven't put it to work yet) that might or might not function properly...

The fuel goes from the tank through the fuel line (with inline fuel filter), to a splitter. From the splitter it goes to the carburetor (standard route), while the other side of the splitter, the fuel is going to a small opening in the air intake (say, a fixed jet of a fixed size), before the turbo.
Before the turbo I basically have a carburetor-like device without butterfly valve.
The fuel gets diffused in the air stream, and through the turbo, and compressed before it hits the carburetor air intake.

The more the turbo sucks the air intake, the more fuel it will suck in, basically enriching the bike's stock settings.
I will think I'm running a bit rich on idle, but I hope it'll work out nice over the rev-range. A jet gives me the flexibility to connect it to a choke cable, and adjust it as I ride, to see what size jet I need, and if the system is fixed, or if I will have to adjust the fuel supply depending on the throttle position or boost.

I'm hoping this system would work, by leaving my carburetor fully stock, and have only one jet to work on before the turbo.

My question would be:
- Will a turbo blow back the fuel through the fuel lines (eliminate the vacuum)?
- Will there be any danger in fuel combusting when it gets compressed in the turbo?
- Will my motorcycle be able to handle the extra air? (it has a 9,2:1 compression), but is aircooled, so I suspect the PSI to go up to 199PSI at full load, peak HP.
- There might be a small chance I can use a single fixed jet?


Last edited by ProDigit; 06-26-2017 at 12:06 AM..
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Old 06-26-2017, 01:29 AM   #2 (permalink)
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not my area of expertise, but I would assume you'd just put the carb - possibly a bigger one - before the turbo and rejet to get the mixture correct.

I've seen such setups before, though don't ask me if letting an air/fuel mixture run through a hot turbocharger is a good idea or not.

I'd assume running boost on an air-cooled engine wouldn't be the best idea for it's longevity.

Were it me, I'd be attaching a smallish electric motor (like an etek) that I'd only turn on when I needed a boost for overtaking or hill climbing on the highway. Ah, heck, I'd probably try and make it fully hybridized so save gas too.

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Old 06-26-2017, 02:26 AM   #3 (permalink)
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Modifying just the carburetor is not going to work, simply because the turbo will need a lot more fuel at a certain rpms when under load, than when running without load.
You'd either run the setup stock (so without load it'll run fine) and have it run too lean at load,
Or run it fine under load, but too rich without load.
Some additional fuel supply system needs to be added, to make it run well under load.

Electric motors aren't viable, as some people have tried anything ranging to a leaf blower. And a 1000W leaf blower only added between 3-5HP to a 1.8 liter, 135HP car. Not worth it. The Rebel's alternator only makes a few Watts of extra power, so I'd have to swap out the head and tail light, for leds, and can install a maximum of about 60-70W blower.

A blower may be more effective on smaller engines than on larger ones... But I'm expecting the differences to be minimal.

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Old 06-26-2017, 02:33 AM   #4 (permalink)
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I didn't mean an electric supercharger. I meant powering the wheel directly.
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Old 06-26-2017, 04:25 AM   #5 (permalink)
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Turbo on a Rebel 250. Complete waste of time. This type of question has been asked many times for similar small bikes. It'll never work unless you spend alot more than the bike is worth. ALOT more. Be a heck of alot easier and cheaper to just get a 500. I've been riding and working on motorcycles for 30 years. Just trying to save you alot of wasted time.
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Old 06-26-2017, 05:03 AM   #6 (permalink)
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Could you elaborate on the price?
The Garrett GT06 seems to be the best match, however, if the GT12 is somewhat restricted on the intake, it would work well too. The GT06 is almost $680, and the gt12 is 170. Big difference!

I'm looking at clones of the GT06.
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Old 06-26-2017, 12:36 PM   #7 (permalink)
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That video talks about how VW get 20% more economy from their turbo exhaust.

I'd just rig it up and get it working to prove the 'experts' wrong. I think this whole field of turbo-charging for efficiency is a completely new area.
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Old 06-26-2017, 02:38 PM   #8 (permalink)
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In the case of the rebel, I will not see any increase in efficiency.
In fact, a turbo on an engine ALWAYS reduces mpg, not increases it.
A 1.4L NA uses less fuel than a 1.4L turbo.
It ready depends, but an eco turbo engine uses up to 20% more fuel.

They're looking at, if you'd compare engines HP wise.
A 1.4 L Turbo has about the same HP as a 1.8L NA, but with the same gearing uses around 5-10% less fuel.

It's all based on the fact that most cars spend their time idling, or cruising. In a scenario of start stop traffic, the turbo will have worse mpg than the NA.
Idling or cruising at a speed under where the turbo starts boosting, the turbo engine uses less fuel, because it has lower cc.

At HP peak rpm range under load, usually the turbo uses a lot more fuel.

Turbo engines are only efficient if they are used to boost engines when they accelerate, but are idle when cruising.

A too small turbo, will rob the engine of power (HP) and focus power at lower RPM (torque), and is best paired with taller gears.

A large turbo, only kicks in at the higher rpm, under load, and is able to increase HP, at the cost of a mild reduction in torque at mid RPM.

Adding a turbo to an engine, Always will reduce MPG, but it can make sense in the case of a rebel, that only makes 25HP, and top speed of 75-80MPH, if more top speed is needed.
If well tuned, the turbo can double Hp, however, if HP would just increase from 25 to 35HP, the bike's top speed would increase from 75/83 MPH to 85/93MPH, which would be enough for the interstate.

Considering the Rebel's weak clutch, it won't be able to handle a GT12's added power. And a GT06 would only increase acceleration, not top speed.

Then there is a problem with the fuel vapors injected before the turbo.
They might actually dissolve the oil in the turbo housing, and I fear it igniting when the turbo gets really hot (a possible issue on the GT06 at full boost, due to it's small size and low heat dissipation. Not an issue when just compressing air though..).

injecting extra fuel in the air duct after the turbo, might be harder to do. Some people have just used an electric fuel pump, to pump in fuel under pressure in the carburetor...
The problem there, is that the bike runs rich (air gets robbed by the turbo, and too much fuel gets added by the pump), and at load, the turbo still runs so lean that the engine starts stalling.

The proper way is to get a fuel management system installed, and that is expensive.
I was hoping for a cheaper solution :/

Still more to read up on, but with prices of the GT06 so high, I don't think this will work out (the gt12 would only offer boost at around 5000+ RPM, but I'd have to restrictions it's boost, not to blow up the engine, basically making the GT12 not an option; while the GT06 gives extra torque, while not giving extra top speed, not what I'm looking for...)

If they had an other brand turbo that was in between a GT06 and a GT12, that could offer a solution?
Still, no workaround the fuel injection system.

Last edited by ProDigit; 06-26-2017 at 02:50 PM..
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Old 06-26-2017, 03:24 PM   #9 (permalink)
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It could be actually possible to increase overall efficiency with a turbo, but it's easier to do so with direct injection since it allows leaner burn. Anyway, in an air-cooled engine which actually relies on richer mixtures to cool the pistons down, I would rather not to impose a higher thermal load. When it comes to motorcycles, even though I am aware of some that had been turbocharged, the space limitations on them makes it quite troublesome to fit a turbo on them.
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Old 06-26-2017, 04:09 PM   #10 (permalink)
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Quote:
I have a Honda Rebel 250cc. Great bike.
And I want a little more power.
It's good enough in gas mileage (I get between 66MPG full throttle, to 115/120MPG at best); but sometimes I wished I had more power for the highways.
Turbos are generally configured as suck through and blow-through. If the turbo blows through the carb, that must be modified.

My best suggestion, if you need power for highway cruising instead of up throught he gears, is hie thee to the Aerodynamics subforum.

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