I've been talking with someone of late who is building his first custom bike, based on a 1200 sportster and he wanted to know about the fitting and tuning of the drag pipes he has for it.
I thought it was an interesting subject to add to my blog, so here it is, my reasons for not fitting drag pipes to road bikes.
Important point number one, all engineering with regards to motorcycles is the art of finding the best compromise for a given purpose, be it engine, frame, fork design or anything else
Ok, lets start with drag pipes, how they work etc.
Obviously they are named drag pipes because they originate on the drag strip.
they work well here because a drag bike's sole purpose is to put out the maximum amount of power to travel over a relatively short distance, there is no consideration for achieving a broad spread of power, fuel economy etc
A drag bike motor will typically have a far larger carburettor than a road bike with correspondingly large jets, of course this is no good if the valves in the head cannot allow the high flow of gasses from the carb when they open.
For this reason, the valves also are larger than the average road bike, the cams that open the valves will also have a higher lift and longer duration of opening ('lumpy cams') to get all that mixture into the head, and out again after combustion has occurred
again, the one consideration is getting maximum power from the biggest bang, so to speak, drag pipes provide virtually no back pressure but do allow a high flow of gasses which is fine for drag racing, but not good for a road bike (I'll explain why later)
Because so much fuel/air mixture is being passed through the motor at such a high rate, it is more or less inevitable that some of it will get into the exhaust pipes in an unburnt state.
It is when the heat from the exhaust pipe causes the unburnt mixture to ignite that you get flames from the ends of the drag pipes. (again fine when racing, not so good when bimbling about town!)
The length of the pipes plays a big part in determining where the peak power occurs, as a rule of thumb, the longer the pipe, the higher up the range the power comes in (this is a huge oversimplification)
What we want from a road bike is considerably different, of course, where there are considerations of fuel efficiency and rideability (just ask anyone who has owned a Goldstar or Jota what they're like to ride in traffic round town!)
something a road bike needs in its exhaust system to allow it to run at its best is back pressure, this is provided mainly by the baffling in the exhaust system and can be thought of as being almost like a sound wave bouncing back up the exhaust.
What actually happens simply put is that not all the exhaust gasses escape from the end of the pipe immediately, some bounces back and creates a back pressure in the pipe, this prevents unburnt mixture escaping past the exhaust valve, giving a better burn, better power for everyday riding and better fuel economy.
As mentioned the length of the pipes affects this, as does the diameter of the pipe.
having pipes that are of a different length creates an imbalance between the cylinder head, this can be mostly overcome by introducing a balance pipe between the downpipes or creating a taper in one pipe so that it mimics the characteristics of the other.
So, while it is possible to get a set of drag pipes to work on a road bike, there are going to have to be compromises somewhere, if you spend a LOT of money on all that headwork (valves, cams etc) you will have a powerful bike for sure, its not going to be as tractable as it was though (i.e. it will be more peaky in its power delivery, and quite likely feel flat in some of the rev range), it will also be less fuel efficient and put out noticeably more pollutants (important to consider come mot time)
Also, other things would warrant upgrading-brakes, suspension etc to get the most out of the package,a stronger clutch too- how much do you really want to spend?
The other issue is, of course, noise, drag pipes are going to be significantly louder, again something to consider come mot time as it will be harder to find somewhere that will turn a blind eye to a set of pipes that will be very blatantly illegal for road use.
the sound will also be harsher than, for example, a vance and hines exhaust that gives a nice deep growl, but thats down to personal preference.
You could, of course, run drag pipes without the headwork, many people do, but really, it's not a good idea.
You may get slightly more power than the stock set up, but not so much as fitting a good stage one with slip on silencers, high flow air filter and rejetting
Remember that I mentioned the bigger valves to allow the flow of more gasses from larger carb etc?, well running with standard heads can mean that the mixture will be too lean(even with bigger jets fitted), this can cause significant hot spots in the head which leads to premature valve wear, in the worst case scenario, the valve stems themselves can fail, letting the valve head drop onto the piston-major engine rebuild time!
remember, in stock form, most bikes are already set up to run very lean to get past emission requirements for a multitude of countries.
All of this is a simplified explanation of the engineering reason for various pipes etc, and I have given a basic explanation of what can happen in the worst case scenario, so now I will also point out that there are a lot of people who run drag pipes without any significant modification and will tell you that they have never had any problems whatsoever, and that they get more power and a better running bike.
None of them are likely to be engineers!
