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Author Topic: "Doing it Right" or "How to Build a Functional Café Racer"  (Read 120811 times)

Offline Corsair

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #70 on: Nov 09, 2012, 19:39:45 »
I am well and truly impressed! Very nice write up, clear and easy to understand. I knew a lot of what you wrote due to my experience with other IC engines (cars...Gasp!) and how you put it, would make a fine teaching manual. Ever think about doing that (putting this in book form)?

Nice job!
Robb

Offline Sonreir™

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #71 on: Nov 09, 2012, 19:48:36 »
Isn't the optimum angle 14 degrees? (included angle)

I think it depends on the type of collector.  If I remember correctly, the merge style uses a sharper angle than the baffle type.  Somewhere around 10° (plus a few for merge, minus a few for baffle), I think?

Quote from: crazypj
BTW, like your math on the carb sizing for 360, I had hell of a time getting people to listen when I said 30mm instead of 32mm Mikuni's, I knew they didn't want the truth that 28mm was optimum on 350/360  ;D

Thanks.  :)
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Offline Sonreir™

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #72 on: Nov 09, 2012, 19:53:20 »
I am well and truly impressed! Very nice write up, clear and easy to understand. I knew a lot of what you wrote due to my experience with other IC engines (cars...Gasp!) and how you put it, would make a fine teaching manual. Ever think about doing that (putting this in book form)?

Nice job!
Robb

To be perfectly honest, I don't think I know quite enough to to write a book.  Believe it or not, a lot of the info here is the "basic" stuff.  It's a quick summary to help people understand what's going on.

If you're interested in reading about these things in more detail, I highly suggest any of the performance tuning handbooks by A.G. Bell.  William Denish does a very good book about tuning HDs, but the same details apply to other engines, more or less.

Also... a little off topic as to what we've been discussing so far, but Corky Bell wrote a book called, "Maximum Boost".  It's all about turbocharging and it was the first book I ever picked up that had to do with engines.  I got into bikes and engines and such because I was interested in how turbos worked.  So I read that book and kind of worked my way backwards...
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Audaces fortuna iuvat.

1977 Honda CJ360 - Café SOS - Stage One™, Café SOS - Stage Two™
1976 Puch Maxi - APuchalypse Now
Suzi T500 Cobra Resto

Custom Gauge Graphics
Custom Wiring Harnesses

DTT Red, White, and/or Black 360 Club - Better than those Blue guys

Offline BLSully

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #73 on: Nov 10, 2012, 10:07:35 »
+1 for Maximum Boost. Even if you don't plan on doing a Forced Induction  setup, Corky explains lots of complicated stuff in a relatively easy to read format.

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Offline teazer

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #74 on: Nov 10, 2012, 11:10:09 »
SPD claim that their 12 and 15 degree collector angles make most power, so I suspect that the 14 degree figure is probably correct. The larger the collector angle, the larger the flare angle needs to be in the transition.

Offline gregajo

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #75 on: Nov 13, 2012, 15:56:09 »
Quote
To be perfectly honest, I don't think I know quite enough to to write a book.  Believe it or not, a lot of the info here is the "basic" stuff.  It's a quick summary to help people understand what's going on.

If you're interested in reading about these things in more detail, I highly suggest any of the performance tuning handbooks by A.G. Bell.  William Denish does a very good book about tuning HDs, but the same details apply to other engines, more or less.

Hi, thanks for your posts and the reference of the book by Bell.  I will definately check it out.

I have been wondering about where to get information to learn how to "do the tune".  The concept of "tuning" seems to cover a great deal of different aspects of a motorvehicle and a motorcycle in particular; everything from engine internals to apsects of intake and exhaust to frame design and suspension.  It seems that "tuning" is both a science and an art and as such appears to be something that some people grow to understand well while others struggle with it.  I know that a great deal of this information exists on this forum site as well as others out there.  Maybe there is a way to put some of the expert advice and vast experience that exists among this forums members into some kind of book?  It would be an interesting (and challenging) project. 

Offline teazer

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #76 on: Nov 13, 2012, 16:18:19 »
The key to understanding tuning is to keep in mind that engines are a compromise - or a series of compromises.  What you have to do is to define your objective for the project and then understand the starting point and develop a plan on how to get from A to Z.

So you want more power eh?  How much more?  Do you want top end or all the way through the rev range stump pulling power? how large is your budget? Do you have access to a good machine shop?  Are parts available?  Is there enough metal in the right places to make your vision possible? and so it goes on.


No point starting with rusted solid CB250 and set an objective of 100HP for example. Or even 50 or 40 or maybe even 30. As you start down the path you have to continuously ask where you are going.

You may for example want to just clean things up and blueprint teh motor to get teh best out of what teh manufacturer designed.

Or you may want to add more pulling power or more top end. More torque needs more efficiency and larger bores are an easy way to go, as is higher compression.  Cams help to add back top end, so does porting.

Some engines respond well to changes and others not so much.  It's an ongoing process of development to get a bike to satisfy your needs and like any good relationship that takes work and time and understanding.

The hardest part is making sense of all the info out there - some good and much of it bad or not relevant.

Get a copy of the AG Bell book and read it cover to cover. repeat and then start reading everything you can about your bike and what other people did and see if you work out what worked and what didn't and pretty soon, you'll have a plan. It will change with time, but you will have a plan. 

Offline crazypj

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #77 on: Nov 13, 2012, 17:39:55 »
If you have the later CB250 (same engine as CB360 but smaller bore) it's possible to take them out to almost 400cc (I have a CB390  ;D )
It is a hell of a lot of work though
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Offline Sonreir™

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #78 on: Nov 16, 2012, 21:55:21 »
Applications - Forced Induction
OK... so we're going to diverge a bit.  I picked up this baby earlier in the week:

Understandably, I currently have boost on the brain, and so I want to write a bit about forced induction.  When you gotta write, you gotta write...

Forced induction, for those unfamiliar with it, is the addition of compressed air into the engine in order to raise volumetric efficiency.  By introducing air at a pressure level that is higher than atmospheric, we can add more fuel, which gives us more power.  This takes one (or more) of three different forms:  Turbocharging, Supercharging, and Nitrous Oxide.

There may be no replacement for displacement, but volumetric efficiency isn't a bad substitute in my opinion.  Dollar for dollar, you're unlikely to find another modification that generates as much horsepower as forced induction.  That's not to imply this is a cheap prospect though, only that it has high value when planned and executed well.

One of the other things I really like about forced induction is that its a bit of a "game changer".  Many of the rules and best practices we've been covering over the past few pages don't apply.  Well... let me clarify...  They still apply, but the priorities change.  In theory, if you double the amount of air your engine can ingest, you can also double your horsepower (this never works in practice, but stick with me).  So are you really going to worry about intake lengths to get an extra couple of percentage points at a specific RPM when it's possible just to up the boost and generate even more?  Probably not.  I'm not saying that boost is king and should be the only consideration, but there's no denying that it will change your approach to your build.  Think of it as a reshuffling of priorities.  Boost can mask all manner of ill designs, but it shouldn't be thought of in that way.  As with all engine modifications, they work best with a decent foundation and that foundation often means following the principles we've laid down so far.

Finally, the feel of a forced induction engine is something that everyone should experience at least a few times.  It has a very non-linear torque curve and that's damn good for generating smiles.

For the purposes of this post, I'll be sticking to turbochargers.  Superchargers are similar in operation in that they add compressed air into the intake, but how they get the energy to compress the air does differ.  Turbochargers rely on compressed exhaust gases to spin a turbine that then compresses the intake air (at a slight loss in efficiency).  Rather using the exhaust gases to spin a turbine, superchargers use power from the engine (usually a belt that's run directly from the crankshaft) to spin the turbine.  Efficiency wise, turbos hold the edge.  Superchargers are often easier to fit and implement, however.  They also don't usually suffer from lag (I'll get to lag in a bit).  For the rest of this post, it's safe to assume that the information applies to both turbos and superchargers, except where exhaust stuff is concerned.  I'll cover nitrous oxide in a future post as the way it operates is much different than turbos or superchargers and it comes with its own set of concerns.  One of the nice things about NO2 is that you can use it in conjunction with turbos and superchargers, but we'll cross that bridge when the time comes...

Selecting a Compressor
OK... so one of the first considerations in a turbo build is the size of the turbo, itself.  There are about a dozen different variables that affect a turbo's operation and often you have control over many of them.  Turbos can often have the same housing size but then the turbine and compressors will be sized differently.  Even with similarly sized turbines you get things like trim and diameters for the inducer and exducer.  I'm not going to go into those things in a lot of detail, but just be aware that you often have some measure in customizing a turbo to fit your needs.

What I will talk about is called a "compressor map".  This map displays the efficiency of a given turbo (and all its related variables) in producing a certain level of boost for a certain mass of air.  Depending on your engine displacement, the amount of air your engine desires can be measured.  For instance, a three liter engine will use more air than a one liter engine, regardless of the specs of the turbo.  However, it's theoretically possible to boost the intake pressures on the one liter engine so that it's using the same amount of air that the three liter engine would normally ingest.  The ability of the turbo to produce this level of boost is what's measured in a compressor map.

Before we go too far down this road, we need to know how much air our engines use before the turbo comes into the equation.  The first step is to figure out how much volume of air your engine is using.  It may be handy to get this into an Excel sheet (or similar) because you'll need to tweak this number and recalculate a bit later on.  Luckily this is fairly simple.  First, take your redline RPM value and divide by two (for two strokes, just use redline).  Multiply this value by the displacement of your engine and then convert to cubic feet.  For my engine we have 6500 RPM * 378cc * .00003531466672 (this is the conversion from ccs to cubic feet) = 86.76 cubic feet per minute.

Now that we have volume, we need to calculate mass.  As you may recall, temperature affects volume (more specifically, density) and so in order to find mass, we need to know temperature.  Fortunately, we can go back to the Ideal Gas Law to figure out all this stuff.  The equation for the IGL is P*V = n*R*T.  P is the absolute pressure (14.7 PSI at sea level), V is the volume of the gas, n is the number of moles of the gas, and T is the absolute temperature in Rankine. Finally, R is the gas constant and will can use a basic value of 10.73 for this (NOTE:  This constant changes depending on the units.  10.73 is for cubic feet PSI).  Through basic algebra, we can find the value of any of the variables so long as we know the other three.

For calculating mass, we rearrange the equation so that n = (P*V*29)/(R*T).  The "29" in this equation is the molar weight of air and it used for clearing some of the units.  By plugging in the variables our equation now becomes n = (14.7*86.76*29)/(10.73*531.67).  The resulting value gives us 6.48 pounds of air used per minute at sea level and 72°F.  This is purely academic, but it's an interesting (to me, at least) fact that pounds are only interchangeable with kilograms on earth.  Pounds are actually a measure of force whereas kilograms are a measure of mass.  Mass stays the same regardless of gravity, but pounds do not.

Anyway... back to compressor maps.  We now know that at redline, and in an ideal world, my 360 will be consuming 6.48 pounds of air every minute.  This calculation doesn't take into account volumetric efficiency, however.  Most engine will not be sucking in their full displacement's worth of air on every intake stroke.  Volumetric efficiency for most machines tends to hover around 82% for single valve engines, 85% for multivalve, 90% for "built" engines, and between 95% and 100% for full race spec.  Taking the 6.48 value and multiplying by .9 gives us a more realistic value of 5.84 pound per minute.

Now this is a value with which we can work.  And by "work", I really mean "apply more math".  The process is to now take our "actual pounds per minute" and convert this to a "corrected pounds per minute".  The purpose in correcting the air flow is to take into account the change in the temperature and pressure of the air as it passes through the turbo.  In this case, we're going to assume a slight vacuum of -.5 PSI (mostly due to the expected air filter over the turbo inlet) and an ambient air temp of 72°F.  The formula for corrected flow is cf = ((actual flow)*(air temp in rankine / 545)^.5)/(absolute pressure in PSI)/13.949).  The 545 value is a constant used for correction to standard temperature (85°F) and the 13.949 value is for correction to standard pressure.  Following the math through we now have a corrected air flow of 5.67 pounds per minute.  NOW we can do something with this number.  Take the compressor map for our turbo and now draw a vertical line at the point on the X axis at which our pounds per minute value corresponds:

Plotting for our cross line from the Y axis is the simple part.  The pressure ratio can be calculated using this formula:  PR = ((desired PSI + 14.7)/(initial PSI + 14.7).  For these purposes I'm going to set my desired PSI at 10.  Note that this is PSI measured at the turbo outlet, NOT measured at the intake manifold.  In reality, you lose pressure as the compressed air makes its way along the twists and turns of the intake tract.  I like to assume a 3 PSI loss for intake inefficiencies and so boost at the manifold is likely to be around 7 PSI.  The "initial PSI + 14.7" portion is exactly the same value as we used in the corrected air flow pressures and so we'll use 14.2 (assume -.5 PSI for air filter, etc).  The pressure ratio we have is 1.74.  Graph this line on our compressor map, and we have the following:

So for our purposes, it looks like our turbo is operating at the 69% efficiency range.  This is pretty decent, but it's only half of the story.  It's where we're at a redline and we'd really like to know where we are in the rest of the RPM band.  The first step is to trace our way to the left from the intersection of our two lines until we get as far left as possible on the map.  This far left line is called the surge limit and the turbo is incapable of operating in a predicable manner if the pounds per minute drop any further.  This second vertical line we're drawing is the pounds of air per minute necessary to hit our desired level of boost.

This value corresponds to 3.32 pounds per minute.  Working backwards through all of our earlier math we can use this to calculate the RPM at which full boost becomes available.  I'll spare you the boredom of that task; the result is simply 7,600 RPM.  So... we know already that our turbo going to come on late, which isn't too bad for a bike but would probably give some drivability problems on a car.  Now that we know when max boost comes in, we need to figure out when initial spool up starts to take effect.  Spool up is when the turbo starts making usable boost, but isn't yet at the boost limit.  This is also the point at which your smile starts to form as you roll on the throttle.  For this portion, we take our maximum mass air flow and multiply it by .2 (this is a rough calculation, but it's good enough for this step).  This gives us a value of 1.13 pounds per minute.  This is also the point at which we can start drawing a line from the X axis to the point of maximum boost (3.32 pounds per minute).  We'll call this the spool threshold.  Finally, draw a vertical line from the point at which our spool threshold intersects the 1.2 pressure ratio (1.2 pressure ratio is often considered the minimum amount of usable boost.  Any less and it's not likely to give any real performance benefit). Our graph now looks like this:

We can see two things right away from our most recent graphical additions.  First, the boost threshold line stays within the islands of the compressor map.  This is VERY important.  Any significant drifts to the left or right of the islands means you have a turbo of the incorrect size.  Not only will the turbo fail to perform well, it's also possible to damage your engine and/or turbo through surging and added heat.

Second, our new vertical line tells us the point at which usable boost comes in.  This corresponds to 1.74 pounds per minute.  Again, working backwards though all the math (I hope you have an Excel sheet open) we come to an RPM value of 3,600, which is damn near perfect.  In an ideal world, this value will be 33% of your redline.  Since I'm redlining at 11,000 RPM, 33% of this would be 3,630.

Finally, in a perfect world, I'd probably select a turbo even slightly smaller than this.  You can see much of the graphed line falls in the white islands of the map.  These areas are usable, but are of lower efficiency.  To shift our graph to the right, we need more displacement, better volumetric efficiency, or a smaller turbo.  Selecting a lower level of boost would also work, but we do sacrifice power if we go down that route.

OK.  We've now verified that the selected turbo is a good match for our engine.  If it's not, we need to go shopping for turbo parts.  Consult a professional to see what can be changed in order to get the characteristics you need.  Also, remember, that all of these calculations have been dependent upon temperature and pressure.  BOTH of these things change from day-to-day (but not drastically), so this has only been a rough guide to see if our selection was correct.  Ideally, you want to recalculate based on your altitude, etc.

Plumbing
This section is going to be fairly short because most of the rules we've already covered in previous posts still apply.  There are a few considerations, however.

First, is the inclusion of an air plenum into our intake.  The purpose of the plenum is to help stabilize the air pressure so that the turbo can operate in a smoother fashion.  Basically, a plenum is just a way to expand the volume of the intake tract so that when your intake valve opens and starts letting air into the cylinder, the pressure within the intake tract doesn't drop too much.  Without a plenum, you're losing power and causing the turbo to change turbine speeds as the pressure fluctuates.  Not good.  The rule of thumb is that for four or more cylinders, your plenum should be at least as much volume as your engine's displacement.  Double the size if you're running two or three cylinders.  Quadruple it if you're running a single.  This value is the minimum size of the plenum.  The maximum is simply the minimum times 1.5.  The reason to have a maximum is because this is all volume which the turbo needs to fill.  More volume means a longer fill time which means that your turbo is less responsive to throttle changes.  This delay between the time you crack open the throttle and the time boost starts being generated (assuming you're within the usable RPM range) is called "lag".  It's not a good thing and it's not fun.  Keep it to a minimum.

Secondly, the intake tract would do well to include an intercooler.  As we've discussed before, a cool intake charge is a dense intake charge.  Increased density of air means more power.  So, intercooler = more power.  Intercoolers are generally rated by efficiency and that takes into account both the flow restriction caused by the intercooler (no way around this, but do try to minimize it) and its ability to shed heat in relation to the ambient air temperature.  Aim for 70% or better from your intercooler, with an emphasis on flow over cooling ability.

The last main plumbing consideration is the exhaust plumbing.  This is a real game changer as far as designs go.  There are several considerations when designing an exhaust for your turbo.  First, make it short and small.  This area needs to be pressurized before the turbo will spool.  Having a long exhaust with a large diameter pipes makes this process take longer and contributes to turbo lag.  Second, if possible, adjust the length of the headers so that the exhaust pulses reach the turbo in an even and measured fashion.  This mainly applies to 180° twins and all of the triples.  You may need one of the pipes to be longer/shorter than the other in order to facilitate this.  In my own design, the right cylinder's exhaust header will be about twice the length of the left in order to time the pulses appropriately.

Finally, the tail pipe after the turbo should be free flowing as possible.  We're a lot less concerned with things like inertial tuning and acoustic tuning when it comes to turbos... It's all about flow.  It's not uncommon to see just a short, open pipe to channel the exhaust away from heat-sensitive components.

Fueling
OK... when it comes to fueling you basically have three options.

The first and, by far, best option is electronic fuel injection.  Carbs are just not as precise as an EFI system, especially for things like turbos.  Chances are, not a lot of us will go down this road because of the cost and difficulty in implementing them on our old bikes and so I won't really cover it.

Option number two is a draw through carb system.  This means a single carb handles all the fueling and the turbo sits in between the carb and the engine.  The turbo sucks the air through the carb and the fuel is delivered to the cylinders via a splitting-type manifold.  This is probably the easier carb method to implement, but you lose a lot to gain that ease.  First, forget about using an intercooler or plenum.  The fuel will pool in these components and lead to undesirable side effects (such as your engine exploding) .  Secondly, the transitions between the throttle positions tend to be more extreme and this can lead to uneven running.  Draw through systems are very hard to tune.  Finally, having all that air flow through a single carb can cause icing.  As the fuel evaporates, it causes the carb to cool.  Often this cooling can be severe enough in order to freeze the moisture in the air.  Stuck throttles are not fun.  I don't recommend option number two.

Finally, option number three is a blow through system.  This allows us to continue to make use of individual throttle bodies (e.g. carbs) for each cylinder and also allows for the inclusion of an air plenum and intercooler into our design.  This choice does require some changes, however.

The first of these changes is a fuel pump.  Your fuel pump must be capable of delivering at your maximum boost + 5 PSI.  In my case of 7PSI, I need a fuel pump that can put out at least 12PSI.  In addition, you'll need something called a rising rate fuel regulator.  The job of this part is to ensure that the fuel is delivered at a constant pressure in relation to the air pressure.  This regulator will sense the pressure in the air plenum and always ensure the fuel pressure is a bit higher (you can set this level and I'm going to opt for +3PSI over sensed air pressure).  Without a regulator and fuel pump in place, the boost from the turbo will push the fuel right back up the lines and into the tank.

The next changes may include modifications to the carbs themselves.  All carbs will require pressure lines running from the plenum to the float bowls.  This ensures a strong signal across the venturi and keeps the fuel pressurized at the same ratio it would normally see without the turbo present.  Generally these air lines are run directly from the carb overflow tubes, which would have to be plugged, otherwise.  Also, some CV carbs may require modification so that the area underneath the diaphragm is sensing the appropriate level of boost.  This often involves drilling and more air lines run from the plenum.  Get familiar with the operations of your carbs before making any serious changes.  Finally, there's the jetting.  The nice thing about a turbo is that boost usually only comes on after 3/4 throttle.  This means 90% of your tuning is just going to be selecting the correct size main jet.  Under boost you're going to want to keep your air/fuel ratio at around 12:1.  Either invest in a wideband O2 sensor or some dyno time (or maybe even both).  Get the fueling wrong and you're going to be in for some expensive repairs.

Other Considerations
There are a few other things you're going to want to review before making any significant changes to your bike's operations. 
The first thing is to look at your cam...  Turbos prefer high lift and shorter duration.  The idea is to open the door to the cylinder as much as possible and allow the higher pressures to fill it.  You're less reliant on intake velocities and now more reliant on intake pressures.  Don't hold the door open too long or all of your pressure comes right back out (or even goes out the exhaust without being combusted).  The stock cam isn't a bad choice for a turbo engine.

Also, take a look at compression and timing.  Turbos will add a lot of heat to an engine and so we need to be aware that other modifications that also add heat don't stack up.  I'd recommend not exceeding more than 10:1 static compression and even that's a bit on the high end.  I tend to be conservative when it comes to boost, though.  I don't see a point in killing a bike's low RPM operations just to generate higher numbers in a range at which I rarely ride.  Keep your compression at at least 8:1.  For timing, less advance will be necessary because the pressurized intake charge is now more dense.  The more dense the charge, the faster it burns.  The faster it burns, the sooner peak pressures are generated.  So in order to take this into account, we dial back the advance.  The rule of thumb here is 8° of retard for every 15 PSI of boost.  If you have an electronic ignition, these can often be linked with pressure sensors to handle the timing automatically.

Conclusion
Well... this post was longer than I intended and I even covered less than I hoped.  Maybe I'll expand on it at some point...

Just one closing thought before I sign off for the day...  You can calculate the rough increase in power using the following formula:

Final power = initial power * (1 + (boost/14.7 * turbo efficiency))

So for my 360 this becomes:
Final power = 38 (guesstimate after dropping the compression to 9:1) * (1 + (7/14.7 * .69))
Final power = 50.48 horsepower (this doesn't take into account the inclusion of an intercooler, which would also give a bump of around 21% to the efficiency of the turbo, giving a different possible value of around 55 horse power)

To put that 55 horsepower number into perspective, that's 387 horsepower per ton.  The Lamborghini Gallardo Superleggera comes in at 344 hp/ton...
« Last Edit: Oct 09, 2016, 02:16:08 by Sonreir »
Sparck Moto - http://www.sparckmoto.com

Audaces fortuna iuvat.

1977 Honda CJ360 - Café SOS - Stage One™, Café SOS - Stage Two™
1976 Puch Maxi - APuchalypse Now
Suzi T500 Cobra Resto

Custom Gauge Graphics
Custom Wiring Harnesses

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Offline crazypj

  • Posts: 12476
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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #79 on: Nov 17, 2012, 01:42:48 »
I have to read through a few times, interesting stuff
 I have to get 360's done so I can get back to XS700, 800 and 860.
700 is getting CX500 turbo (low compression pistons, around 6.4:1)
'you can take my word for it or argue until you find out I'm right'
I gave my girlfriend an orgasm the other night, but, she spat it back at me
 Some humans would do anything to see if it was possible to do it. If you put a large switch in some cave somewhere, with a sign on it saying 'End-of-the-World Switch. PLEASE DO NOT TOUCH', the paint wouldn't even have time to dry
 It’s not worth doing something unless someone, somewhere, would much rather you weren’t doing it  (Terry Pratchett)
CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0