A listener discussion about HP & torque

We got this email from listener Bryan Skinner, and thought the emails back and forth would make a good blog post. From Bryan: I stumbled upon your Podcast in search of finding an enjoyable conversation on motorcycling and I've really been enjoying listening to you both. Question: The issue I'm confused about is how can a Yamaha FJ-09 which is 847cc have so much more horsepower than my Honda VTX1300 cc ? My 1300cc has 57HP and 75 torque verse the FJ-09 847cc and 115HP and 65 torque. Doesn't cubic inches relate to Horsepower. Is it because the Yamaha is more modern than my 2005yr Honda VTX 1300 ? James: Bryan,
Thanks for writing. I think we’ll queue this up for an up coming show but I’ll respond here as well.
Displacement is only part of the equation. It’s really about squeezing quantities of gas and air and making it explode. I don’t know the numbers off the top of my head but I’d guess that the VTX tops out at around 6000 RPM. The FZ-09 probably doubles that number. It has less displacement but it can produce power more often. There are many other factors that can contribute such as compression ratio, power loss to friction in the motor, restrictions in the air box, other design decisions made by the engineers to produce the “feel” that they want out of the engine. RPM and compression are probably the biggest contributors.
On the upside, you’re probably getting better gas mileage AND buying cheaper gas than FZ-09 owners.
Chris: And if I may expand with some technical details to flesh that out...
Horsepower, while useful for bench racing and marketing, is a mathematically contrived measurement that's got a lot of factors going into it. In its basest form, calculating HP is simply a matter of taking the torque at an RPM, multiplying it by that RPM, then dividing that number by a fixed value of 5252. 
In the case of your VTX 1800, the I've seen docs suggesting that the torque maxes out at 120 foot pounds. To make the math easy, let's assume a good, flat torque all the way to redline. Your redline is 5750. So the resulting formula is:
T x R / 5252 = hp
120 * 5750 / 5252 = 131.4 , theoretical maximum horsepower.
In the real world, the VTX 1800 gets closer to about 105 hp with around 100 ft lbs of torque. This is because the overall breathing inefficiencies, the torque falling off at higher RPM, and mechanical drag in the engine, etc. 
According to this chart ( http://images.motorcycle-usa.com/PhotoGallerys/05PCTorque.jpg ), the VTX 1800 is good for 98 ft lbs maxed at 3700 RPM, and it starts falling off after that. So, 3700 RPM is where the engine makes its PEAK EFFICIENT POWER (which is different than maximum power) - the formula would then be:
98 * 3700 / 5252 = 69 max EFFICIENT horsepower. 
Using that chart, at max rpm of around 5500, the engine is putting out around 80 ft lbs.
80 * 5500 / 5252 = 83.8 horsepower.
No slouch by any means, especially in the "feel my arms stretch" department. 
So, as you can see, real-world numbers and theoretical, marketing numbers can vary GREATLY. 
You can modify those real world numbers a bit with better exhaust and airbox flow, tuning, etc.
But horsepower, as a thing, is fully contrived and 99.9% completely made up theoretical BS. 🙂
Me...? I'll take a nice big torque number a lower or mid-level RPM range over theoretical horsepower all day long. It's why I like twins and triples so much more than I4s. They develop their torque in lower RPM ranges, typically, and feel more spry around town.
The triple in question revs much, much higher and has a very different feel, and can produce higher contrived, theoretical (and very real) horsepower, but your VTX 1800 is going to FEEL so, so much more powerful in real-life RPM and driving speeds. Because.... it is. The Triple will have to rev much higher and feel more frenetic to develop its higher overall power.
Hope this helps. 🙂

Rowe Electronics (via Aerostich) PDM60

You often hear "just put in a relay" when people talk about adding power connections to a bike, but what does that really mean? Many people find it sufficient to simply hook up their electrical gadgets directly to a battery, or to some "key-on" power source. This may be fine enough for something as low-draw as a GPS or satellite radio, but for things that draw more power, this is usually a recipe for long-term disaster. The usual solution is to add a relay-controlled power circuit. Why? A relay is just a fancy switch that, for the actuator part of the relay, typically draws very little power. This lets you tap into an existing power source - your tail light circuit or your ignition switch circuit - to power the relay. Doing so puts very little additional stress on the bike's wiring. The battery is then connected to the switched part of the relay, allowing high-draw devices full power without harming the bike's existing wiring. relay_diagram_02A typical need for a relay circuit might be when a rider wishes to install a set of driving lights that draw as much as 15amps. That's not the type of power requirement that would safely be provided by tapping into most of the bike's existing circuits. The graphic shows a basic relay circuit. This allows you to have the relay do all the "heavy work" while not impacting the bike's generally small and fragile wiring; if you were to hook up the larger road lights to the existing headlight circuit, you would likely overload that circuit causing blown fuses and overheated wires. A simple circuit to be sure, and it is very limited. What if I want more flexibility? Excellent question. The next upgrade to this circuit would be to replace "device" (in the graphic) with a fuse box. A fuse box will take current coming into it from the relay (or battery if directly connected) and spread it out to a number of fused circuits. This allows you to run several additional items into one centralized, convenient location for power, and to protect those additions via fuses. This has been the standard for... well, as long as there has been power systems in homes, cars, boats, industrial buildings, etc. Fuses and more modern circuit breakers are the standard protection for electrical devices. That can end up with a lot of additional wiring and space being used by the fuse block and relay. Some bikes accommodate the extra pieces better than others. A modern, electronic power distribution system may be the next best solution for some riders. Enter, the Rowe Electronics PDM60. adWpEKsThe PDM60 module replaces the fuse box and the relay system with a simple-to-use, compact, highly sophisticated electronic circuit controller. This intelligent device sense electrical shorts and, rather than blowing a fuse, simply turns off power that circuit. When the short is resolved, the PDM60 turns power back on to that circuit. No more burned wires, blown fuses, and best of all, dangerous shorts and burning wires are all but eliminated. The PDM60 wires up directly to the battery and the various protected circuits are powered on and off by 'trigger' wires. In its normal configuration there's one positive (12v+) and one ground (frame or 12v-) trigger, 6 total circuits ranging from 5amps to 15amps, and two circuits with a delayed shut off. This allows quite a bit of flexibility when deciding on what to hook up and how it should be handled. Most riders will need a simple setup where the PDM turns on the circuits when the bike's key is turned on. With something like this, the PDM would be hooked up directly to the battery, and the 12V+ trigger would be hooked up to anything that comes on with the bike's ignition switch, such as the tail light, head light, aux power connector, etc. The PDM draws about 1milliamp for the trigger, so it will have no practical impact on any existing circuits. During my installation, I chose to have the 12v+ trigger hooked up to my tail lights, and the PDM's main connections hooked directly to the battery. As for the circuits, I chose a 5amp delayed-off circuit for my phone charger plug, a 15amp instant-off circuit for my dedicated tire-pump plug, and a 5-amp instant-off circuit for my GPS connection. I mounted the PDM directly into the storage tray under my seat, and ran the main power wires to the battery through holes I drilled in the storage tray. The PDM comes with eyelets preinstalled on the main power wires; these eyelets should fit most normal motorcycle battery connectors. The electrical accessories are connected directly to the PDM60 on one of the supplied output wires. In my case, the purple wire was a delayed-off 5amp circuit, and the red wire was the instant-off 15amp output. I connected and soldered the colored wires to the appropriate 12v positive side of the accessory. The PDM also includes leads that slide into the big connector to provide 12v- (ground) connection; this alleviates the need to connect the negative side of the accessories to the battery or frame ground. Once all the accessory circuits were wired in, I tapped into the tail light wire for the 12v+ trigger. On my unit, this was the grey wire. Each of the power and trigger leads are marked with stickers. I stripped back the tail light wire's insulation and soldered the trigger wire to it, then taped it back up and put it back in its normal place. I use solder on all connections that are meant to be permanent. After years of automotive and motorcycle ownership and repair & maintenance, I simply don't trust most "tap connectors" or twist-n-tape connections. That's really all there was to it. The unit works as expected and as described, and the total installation took about 30 minutes. I forced-tested the ground faults by taking the output circuit wires and grounding them to the battery's negative post. The PDM shut off power to those circuits immediately without damage to any wiring and without any smoke, sparking or any other dangerous dramatic events. The unit I installed is a few years old. The newer models are firmware updatable and are software programmable; this allows the user to select specific amperage ranges for the various circuits, delayed or instant-off control of the circuits, and which circuits are controlled by the 12v+ or 12v- (ground) triggers. It's a very flexible and simple system, and I like it very much. The unit retails for $199. At nearly $200 it's considerably more expensive than a $12 relay and a $50 fuse block. Many riders might wonder why they should choose it. I can't and won't speak for Rowe Electronics on the matter, but I will offer my opinion. I've installed relay and fuse circuit systems on nearly every bike I've owned. In every case, I had to design the routing of wires, the placement of the fuse block and relay mounting, and in every case, it took me considerably longer than a half hour. In addition, relays can be compromised by moisture, and fuse blocks can corrode in high humidity or if they get wet. The PDM60 is waterproof, has no "moving" parts (covers over fuses, switch actuators in relays, etc), and is also small and easily mounted out of the way. The PDM is safer, less complex to use and is fully self-regulating. It's also fully CANBus compatible. And you'll never need to worry about keeping spare fuses around, or finding a Radio Shack if your relay craps out. Is that 'worth it' to you? I can't say. To me, it is... at least for a bike I intend to keep for longer term, or on which I want to rely for long distance travel. It's also very, very cool. That has certain value. The PDM60 is manufactured by Rowe Electronics and is distributed by several wholesalers and retailers, including Aerostich, the company from whom I got this unit and who is the world-famous manufacturer of the RoadCrafter and Darien series of riding gear. Rowe Electronics directs users to AltRider as their primary supporter and distributor of the PDM60.