CTX700 in Vetter Fuel Economy Challenge

gregsfc

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Schultz Engineering - Custom Motorcycle Parts and Renewable Energy Products

I have a theory and have been trying to prove it spite of Fuelly data, with all else being equal (all else being equal is the hard part of proving theories) that the 670 parallel twin can be a more economical power train, at least with respect to the CTX700 manual (manual version no longer available in U.S. or Canada) and probably the NC versions as well, than is the 471 twin that's in the new Honda CB500s; nearly as economical (all else being equal) as the CBR and CFL 250/300s; and pretty much as good or better than any power train out there for a fully highway capable bike. The reason this is so important to prove is that in eco motorcycle circles, it has been assumed and even proclaimed by well-respected designers and engineers that the smaller the displacement and the less rated horsepower, the better achievable mpg, because horsepower is the enemy of fuel economy, and I agree with that second part, but one has to consider also how much horsepower is being generated going down the road; not just the peak rated horsepower of the engine itself. Maybe an engine the revs lower with a little more displacement and peak rated horsepower could travel down the road generating the same or less horsepower as a screamer. That's what I think; but can't prove.

So for a highway capable motorcycle, these guys are assuming and have even said that we need something on the order of a little bigger than 200 cc's and, for a scooter, maybe just a little more due to the slightly less efficient drive train. The requirement for power at the event is to have a vehicle that can achieve 70 mph sustained in a 30 mph headwind as a highway capable bike. I think that requirement is about right. But the horsepower proclamation that has been made is that the best design would be 17-28 rated peak. Now the contest has already shown this belief and proclamation to be flawed. A diesel motorcycle rider; with a fairly big guy riding, hooked up to a 667 single cylinder diesel naturally-aspired, mechically injected, light weight, Hayes diesel engine on a modified KTR650, that can generate 31 hp rated and 35 peak ft lb torque has won this event and other times has been right there with two, perfectly streamlined, cut-down low Ninja 250 streamliners. Fred Hayes rides the diesel, and I think his best score has been at or about 175 mpg; and Vic does the best usually of the two Ninja guys (he's smaller built than the other Ninja fellow and much smaller than Fred). Vic has accomplished 187, but the best he's done when Fred was also present the same day was around 181; barely beating out Fred the last time they both ran, but Fred hasn't ridden in the last few years. His streamlining is not as perfect for mpg purposes; he used the same near bike configuration to break a land speed record for a diesel motorcycle, and his bike is much heavier. So my theory is that this engine strategy can also be used with a gas powered engine and compete very well against a screamer. If someone would just build a more car-like powered bike with larger displacement, more horsepower, but with a design for great mpg, and flat torque built in. It'd be right there with the Ninjas and Honda 250s. Anyway, that's my theory.

And somebody did. Honda did...Enter the near-perfect candidate; the 670 parallel twin. Maybe just a little less displacement and lighter bike would have been even more perfect, but close enough to prove a point...Right? Peak torque coming at 4700 RPM or so, but still really good torque down at 3,000 and even lower due to its flat curve. Peak rated horsepower maybe up to 47; the CTX at the wheel probably more like 43 and peak torque close to 42. Much higher numbers than the believed perfect engine for a high mpg bike. I'm a 5'8 145 lb guy with a short torso on a low riding cruiser style bike. I should be able to do well against these smaller stock bikes with easy throttle inputs and keeping RPM not much over 3000. That was my plan...

I've been to this controlled fuel economy contest twice with my CTX700 to see some amazing streamlined bikes and also to try and prove my point. The problem though, even though I've gone on forums and tried to encourage other owners of economical bikes to come and ride is that I can't get other stock bike riders out there to the event and compare them against each other. I'd like to have some BMW 650 singles and 799 inline twins to ride with; and a bunch of 250s of different brands; Honda 471 twin; maybe a Burgman 400; one of those Burgman's finished at 80 in one event not too many years ago. In 2014 I went and rode my CTX700 and accomplished 96.9 mpg. I beat all other stock bikes by 20 mpg or more; it's still published at Craig Vetter's website, but there were very few competitors of the stock variety and no modern bikes like I wanted to run against. I came within 2 mpg of beating Craig Vetter himself who was on a perfectly streamlined Honda Helix but that was an aging machine that was losing efficiency. The only pure stock bikes/scooters in the challenge that could be of some competition that year was a DR200 and a Vespa 200. They actually did not get disqualified on speed; they were able to maintain 72 mph and not get passed for several miles on the interstate stretch, but I beat them to death on mpg even though they met the performance standard that year and were much, much lighter and smaller. Small riders to boot.

I just came back from the 2017 event. I did not do nearly as well @ 86 mpg; speeds got up to 75 this year and sustained for several miles, and then went down to around 72, for an 18 mile total interstate stretch with a total 95.3 mile run that included some back roads and a couple small towns. I still beat all other stock bikes by 8 mpg or more. There were only three of us and one was a liter plus that got 46. This year there was a Honda CBR250R rider in the ride. He got 78. I beat one fully streamlined Honda Helix Clone, and I beat a partially streamlined Kawasaki Ninja 250 by 3 mpg, but that bike still needs some work as its in the middle of streamlining rebuild with a legend rider.

So will they get the theory now. There is more than one way to do this...I'm not sure. The stock CBR250R rider is a bigger framed guy than me and I noticed he wasn't riding as steady as I do when I followed him. That was until I passed him and got up toward the front with more steady speed riders. Another Honda 250 rider scored 126, but he designed and built and installed a tail that gets him way up there, however, even before the tail, he had a history of good scores over 100. So this thing is a hard thing to prove. Too many variables. I think I'm below the Honda 250/300 all else being equal, but above everything else being built today, comparing stock to stock.
 

FezUSA

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Was hoping to come to the event this year, alas my middle child had the nerve to become another year older this weekend :eek: Not that I'm anywhere near those numbers at that speed, and I'm reasonably confident that some of it has to do with the weight and shape of the rider (me)!! Even when running at a steady 45mph I'm "only" getting about 76mpg. Once I go over 65 it starts to drop quickly to low 60's. Still, my other vehicle choice is the family truck/RV hauler at 13mpg so I'm happy and winning!!!

Congrats on a good run though, glad it wasn't too hot.

Maybe next year, be sure to remind us again.
 

sendler

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The Honda 700 engine has fantastic fuel efficiency when ridden carefully and would probably be neck and neck with the 500. Both of which can get within 15% of the much smaller 300/ 250 due to their long and longer stroke and low and lower rpm cam. Just think what the efficiency would be if they made a 335 out of half a 700 engine.
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There are one each standard and DCT faired CTX700's leftover at my local dealership if anyone is interested at $6,000 for the standard shift.
 

Michael Moore

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The big advantage to longer stroke that I see is a smaller bore and more compact combustion chamber. Stroke is a small factor in fuel efficiency; I'd rather have a shorter stroke with a better combustion chamber/milder cam and ports for fuel efficiency than a long stroke engine with a poor chamber/ports.

Power/efficiency is predominately determined by the combustion chamber/ports, not the stroke.
 

sendler

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The big advantage to longer stroke that I see is a smaller bore and more compact combustion chamber. .
You said it yourself in the first sentence. For any given displacement, a longer stroke will have a more compact bore with much less area in the combustion chamber to waste heat. And extra leverage on the increased radius of the crank arm for more torque at lower rpm.
 

Michael Moore

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Nope, the most important part is all above the cylinder/head gasket. You can make a wretched combustion chamber with a long stroke engine or a good one with a short stroke engine. Put overly-large ports and a wild cam with too large ID on the exhaust pipe in a long stroke engine, killing the mean gas speed, and it will run worse (be "less torquey") than a short stroke with proper ports and a smaller ID exhaust.

What part of motorcycle sport benefits from lots of low-end torque? Trials. Clearly a good trials 4T engine should then be a long stroke one, right?

1976 TL250 Honda 74 x 57.8 mm
1977 "long stroke" Miller 305cc - 74 x 71 mm
1981 RTL 360 82 x 68 mm
TL200E (Seeley) 66 x 57.8 mm
1987 RTL 250S 70 x 64.9 mm

Odd, not a single long stroke in the bunch, though Miller's big displacement 305 from the TL250 was gotten by stroking (probably because there wasn't enough meat in the stock TL liner for much more of a bore) but still no where near being a long-stroke engine.

Gordon Jennings pointed out the long-stroke fallacy about 50 years ago in a Cycle World article, using as an example the English Ford 105E which had a bore/stroke of 81 x 48 mm. He pointed out that it was known for being "torquey" even though it was very short stroke.

GJ also pointed out that the common "torquey" long stroke motors (remember, this is mid 1960s) had big flywheels, small bores, small valves, small ports, small carbs, mild cams, mild compression.

So take your short stroke motor and give it "big flywheels, small bores, small valves, small ports, small carbs, mild cams, mild compression" and you'll have something that most, if not all, people won't be able to tell is different from something with a longer stroke.

Sure, if you want ultimate power than you'll go with a short stroke so you can fit bigger valves and get a higher RPM for a given feet/sec piston speed. But that doesn't mean that you can't get the same engine to be "a torquer" if you optimize everything for that "torquey" operating range.

ETA: my two-stroke 1976 325cc Bultaco Sherpa T trials bike is VERY torquey, but the bore/stroke is 83.2 x 60 mm. Oddly enough, it has massive flywheels, a small carb, mild ports, and low compression (and exhaust/inlet lengths designed for low RPM power).

cheers,
Michael
 
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sendler

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Ok... Too bad I mentioned torque in the second part of my reply. Torque for a given displacement happens either way. With the increased force of a bigger bore. Or the increased leverage of the longer crank radius. When I say the Honda 500 engine is long stroke and the 700 engine is longer stroke, it is relative to the average motorcycle engine which are generally designed for high specific power output and feature oversquare engines. The 700 is undersquare .9125. Which is fairly unique in motorcycles these days. They chose to do this for a reason. Fuel economy. A smaller bore has less surface area in the combustion chamber to waste heat. That was the initial decision to be made by the engineers which included members of the Honda automotive engine design team. Then of course the valves, ports, manifolds, and cams were drawn from there to to go with this. Also to feature fuel economy.
 

gregsfc

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There is no way I can enter this debate from a technical standpoint, as I am not engineer minded nor do I have near the understanding to explore these two competing theories of efficiency as a matter of the engine design approach; but I do have enough practical sense to add that if it were true that the Moore assertion was more correct (no pun intended) than the Sendler assertion , then there would be many examples of 650 cc ish motorcycles that could achieve better mpg than the Honda 670 parallel twin, but I don't know of a gas-powered motorcycle in the history of production, in this class, that can achieve the mpg of the Honda 670 cc parallel twin. I can regularly achieve mid-to-upper 70s on my long commute to work and back in warm weather. I can achieve low 80s in a short, back road trip; and I'm not a hypermiler. I'm pretty good at riding efficiently, but I don't employ any hypermiling techniques with anything I ride or drive.

I do question Sendlers optimum displacement assertion using the approach Honda has taken with the 670 twin; not saying he's wrong, because again, I can't back up any of my thoughts with anything technical, just thinking out loud. Although I agree that 670 is probably too big to be the optimal displacement using this flat-torque-type approach to maximize fuel economy on a motorcycle, especially if Honda applied this engineering architecture to a smaller/lighter bike, but it seems to me, due to the low-revving nature of my bike; when you consider that my bike peaks horsepower at only at or about 43 on the dyno, versus upper 20s for 250 cc class engines, that it would take more than 350 cc's to design a bike to achieve the highest possible mpg and still be strong enough to go 70 mph in a 30 mph head wind and to be an acceptable bike for the masses with respect to acceleration and top speed. I mean, at least for a production bike that has curb weight from 350-400 pounds. Wouldn't it take closer to 500 cc; maybe something like a 480? Just interested in anyone's thoughts on this, because I've often wondered about this myself.
 

sendler

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and to be an acceptable bike for the masses with respect to acceleration and top speed. I mean, at least for a production bike that has curb weight from 350-400 pounds. Wouldn't it take closer to 500 cc; maybe something like a 480? Just interested in anyone's thoughts on this, because I've often wondered about this myself.
You have nailed the key point. Most people want at least 45hp and some extra weight. So the CTX is perfect where it is. Looking closer at the Fuelly data, even though the usual riding habits of all of the posters could be better for FE, these habits are probably comparable across these models. And it does look like the CTX is averaging a few percent better than the CBR500R.
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Yes, 22 hp would be too small. But it was just handy to say "half of a 670 engine" since the design work has alredy been done. The reason to recommend something smaller than 670/ 45 hp for even better fuel efficiency comes down to the brake specific fuel consumption maps which always show the best fuel consumption versus output at high throttle openings at the rpm just below the first torque peak. Honda made a bold decision to design an all new low revving, ultra low torque peak fuel economy engine for a motorcycle. Their motorcycle division is the world leader in fuel concious design with their range of PCX, 300, 500, and 700 engines.
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But it's hard to use large throttle openings if the resulting power makes you go way too fast. This is why I have to use PulseNGlide even with the measley 25hp CBR250R. Here is a random BSFC map of a car engine just for reference. Read it like a topo map with the best FE found on the highest ground.
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BSFC Saturn.jpg
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Michael Moore

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Greg, what I was getting at is that if a person looks at the bore/stroke ratio and then says that "this engine is of that type based on only this one factor" they are ignoring that the powertrain is a system, from the opening of the air box to the far end of the exhaust, and has to be evaluated as such.

The Honda 73 x 80 bore stroke is not going to have as much ultimate power potential as an SV650 with 81 x 62.6 because it eventually won't be able to breathe as well due to the space for larger valves being limited by the smaller bore. But I don't see why with a different head/exhaust/intake/map the Honda (in street legal trim) would have any problem matching the power of a stock SV650. Road race 4T singles with a similar or longer stroke and a much heavier piston redline at 9-9500 RPM so the Honda could spin a lot higher if it can breathe enough. However, at that higher power level the Honda would get similar fuel economy (or lack thereof) too.

That works the other direction too, I'm confident that if Suzuki wanted to they could make a low power/high mpg SV650 by changing everything but the bore and stroke to optimize the package for those requirements. It is like my example of the 325 Bultaco two stroke: you can make it with great low end and a flat torque curve and about 19 hp in trials form or you can set the same bore/stroke engine up with porting/pipe/carb/ignition (and a much shorter fuse) for short track with nearly 50 hp (my long stroke 2015 KTM 250SX 2T engine with 66.4 x 72 makes 50 hp).

The SV650 was designed for higher power/less mpg and the Honda lower power/more mpg. With the low RPM limit of the Honda I can see them possibly choosing a longer stroke with lighter pistons to give a lighter reciprocating mass that might help reduce engine vibration. Since the 670 is a parallel twin the long stroke (giving a smaller bore) allows a slightly narrower top end which with the CTX foot peg location helps to avoid pushing the rider's legs farther away from the bike center line.

Also keep in mind that for many people motorcycles are a lifestyle accessory so they tend to be built with the goal of satisfying peoples' Walter Mitty daydreams which probably aren't often about high fuel economy. Honda went out on a limb with their new engines designed for economy. There probably aren't a lot of (or any) competing economical bikes from other manufacturers because they don't see a demand for those kinds of bikes. I think that Honda may be trying to build a niche market with these bikes instead of filling an existing one.


But it's hard to use large throttle openings if the resulting power makes you go way too fast.
Which is one of the reasons why for my return to street riding I bought a Honda 670 and just gave my SV650 project to a friend. The Honda goes plenty fast enough for me.

I think a good 250 twin/350 single would have sufficient power for a solo rider street bike with reasonable performance and economy. The Kawasaki EX250/300 used in the Vetterized streamliners will be about 28-32 hp depending on size/year. I was going to make the SV engine into an SV325 with similar 30-32 hp but then Honda came out with the 670 which let me avoid all the modifications the SV engine would need to be turned into a single for my FF project. The Honda is heavier but it has more power with a nicer power delivery. The DCT was a plus as it makes it easier to relocate the feet to new positions.

cheers,
Michael
 
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