Scott's exhaust theory

[scottfree]

I receive a lot of e-mails about exhaust performance on the xs and this is just my opinion on the subject. The best exhaust ever made is only going to give you a small increase in performance while a bad design can seriously hurt performance (too restrictive). With that being said the simple fact is the xs is a slow motorcycle compared to today’s bikes. You could spend a small fortune and a hell of a lot of time trying to squeak that extra 1 or 2 horsepower from a set of pipes that you will NEVER notice on the street and unless you are going out looking for other xs’s to race it doesn’t matter a hill of beans because the first kid on a stock 600R or something is going to leave you in a different time zone anyway . And to you guy’s building fighters and trackers with motor mods and suspension upgrades and such, I get it! you do it because it’s fun and it’s YOUR creation, just like the hot rodders of old did. When I have the “need for speed” I’ll go jump on the R1(180+hp), but most of the time I find myself on the best motorcycle ever made, the one I built ;D. So don’t sweat it so much and go build something cool.

[Jusupov]

Yes, I really do agree that there are very few cheap and guaranteed "gimmicks" to get "more power" out of XS650. The only such thing I've experienced and can recommend is to replace rear sprocket with > 38 teeth one.
With that you can really feel the additional "oomph" at highway speeds when you give the throttle ;D

(The theory behind is that XS650 gives an extra horsepower for every additional 100 rpm above 4000 rpm.)

[Burns]

Lowering the gearing will give you more punch of course, but fuel economy and general riding comfort will suffer.

IMHO less is more. Getting rid of weight is the best performance enhancer. There is a lot of mild steel on this sled.

[Jusupov]

Dec 27, 2008 12:44:05 GMT -5 Burns said:

Lowering the gearing will give you more punch of course, but fuel economy and general riding comfort will suffer.

These are general beliefs but might not be true. At least in all cases. Have you test by yourself?

During this year I rode 3000+ miles with my XS650 and measured miles and litres of gasoline used. The whole year average shows 10% less fuel consumption than with std. gearing. (I replaced 33 rear with 38 one.) The gearing was the only change I did this time compared to previous years. The additional hp at the same speed means less throttle opening and less fuel used. Even if the motor runs more rotations per mile.

The XS650s seem to be very variable vibrating beasts. Some vibrate almost intolerable with any rpm and others do not have much change through the rpm range. I have noticed no change in the general level of vibration when having the 500 rpm more at the same speed. But the vibrations do change the place where you can feel them. Even +- 100 rpm can make the change.

I do agree the weight is the key issue and does the change. When I lost 20 lbs of my own weight clearly my XS650 got a lot of more power ;D

[Burns]

I certainly agree that evidence trumps theory. Your gearing change worked for you and it is a very inexpensive and simple mod.

[dps650rider]

I have had a similar experience with gas mileage vs gearing. I think the main reason for this is that engine efficiency drops as the RPM is reduced. Think about it, this engine puts out the most torque around 6000 RPM. At 3000 to 4000 it's not really running that good by comparison.

In the Yamaha shop manual that I have there are several sets of very interesting performance curves and one of them is a measure of efficiency (the bottom curve in the attached). Check out the engine speed where the efficiency is highest (fuel consumption is the lowest), about 7000 RPM! Interestingly, the efficiency drops the most steeply between 4000 and 3000 RPM.

[Burns]

hmmmm. Seems rather counter-intuitive doesn't it. Extending your argument would lead to the conclusion that you get better economy at full throttle than at partial throttle.

I'm guessing that the figures are taken at wide open throttle - i.e. maximum power. While the engine is more efficient in the sense that it is extracting the most energy from the gasolene at or near the "sweet spot" for its cam/intake/exhaust configuration at full throttle I don't believe that you are getting maximum economy (miles per gallon) there.


You do not need maximum power for cruising you just need enough to overcome rolling and aerodynamic resistence.

I can see where lower (higher numeric) gearing would require less throttle to maintain speed due to torque multiplication but I also think that higher engine revs would offset that.

I'll bet that the controlling factor is cruising speed.

Very interesting.

[71XS1B]

This is an interesting thread.

While the graph does indicate that the engine is most efficient at 7,000 RPM, it still consumes over three times as much fuel at 7,000 RPM as it does at 3,000 RPM.

I think Pamcopete posted a gearing/RPM chart on this board (somewhere). It would be interesting to translate these efficiency figures (gr/BHP/H) into more usable and easily understood GPH.

Dan

[dps650rider]

To calculate the MPG or GPH would be interesting. Along with the graph above, the drag at various speeds would have to be factored in as well as the effect of throttle position. Not a simple task.

I have put almost 100,000 miles on my bike and for all of them up until this past season the engine was basically stock. I have used 32, 33, 34 and 36 tooth rear sprockets and have never seen any significant changes in gas mileage. I have also noted that the mileage is about the same if I'm cruising at 45 MPH or 70 MPH which should not be unless something other than drag is coming into play.

Based on that curve I believe that as the speed and RPM increase, the drag increases which should lower mileage but along with that the engine efficiency increases as well thus keeping the mileage about the same.

[xsleo]

the engine may use less fuel at 7000 rpm. the chart say gr/bhp/h i think that grams of fuel per h/p per hour. at 7000 rpm you make, 53 h/p at 7000 thats about 210 grams per h/p. 53 x 210 = 11130 grams per hour. at 2500 rpm you use about 275 per h/p. at 2500 rpm you make about 17 h/p 17 x 275 = 4675 grams per hour. at 2500 rpm you are using about 6455 grams per hour less fuel. at the slowwer rpms the engine may be less effiecient but uses less fuel. the engine make more power per gram of fuel at higher rpm's but that isn't where the best economy comes from. high mpgs comes from using the least fuel to get you the farthest. my clymer book in the spec sheet under performance. fuel economy is 87.2 mpg at 37 mph. so by burning about 57 percent more fuel at 7000 rpm would be guesstimating 57 percent less mpg's about 37.5 at 115 mph. so on the average tank of gas you would get 37.5 x 3.3 gallons = 123.75 miles. 87.2 x 3.3 = 287 miles. thats with a 70 to 74 bikes the 75 and later don't give this info. your mileage may vary.

[scottfree]

you guys are soooo funny ;D I think you may have missed the point of my post. PUT DOWN THE CALCULATOR AND GO BUILD SOMETHING! LOL just kidding guys

[Burns]

I love this kinda stuff.

The single greatest variable in fuel economy is speed (weight is a constant in this discussion) because aerodynamic resistence (drag) increase at the square of the velocity. It is an exponential increase. That's also why you just about have to double the HP to go from 180 to 200, those last 20 mph cost as much as the first 180.

But - back to the asphalt dyno. Let's EXPERIMENT!!

Gear the bike with ratio 1; take the tank off, fill it up, weigh it, put it back on; drive the bike on a circle course with no traffic (so you can repeat the drive) the course should be long enough to let little changes average out - you be the judge on that one; park the bike where you started; take the tank off, weigh it.

Repeat with ratio2, 3, etc. filling up from the same fuel source of course.

The difference in the weights will tell you the effect of the gearing.
Note that weight is a lot more accurate than volume since not all gasolene gallons are create equal (thermal expansion you know)

(ok, not EXACTLY since your not controlling for changes in wind, ambient or fuel temperatures, humidity, but pretty darn close)

bring beer.

[sgtsleazebag]

i hear ya scott!

[geohaydook]

I was going to go with the stock 17/34 combination, but noe I'm having second thoughts. What chain length do I need to convert to a 36 or 38 rear sprocket?

[ShakerNorm]

Burns kind of hit the nail on the head for speed and efficiency -
If you remember the old 69 & 70 Dodge Daytonas and Plymouth Superbirds in NASCAR - Ma Mopar realized that to gain any advantage over the Blue Oval boys would take a MASSIVE amount of horsepower. At those speeds (180 mph+), it would have taken an extra 500 hp to go an extra 20 mph - that's why they went the other route and used aerodynamics. The extra fiberglass weighed almost 100 lbs extra, but with no other modifications, they still gained over 20 mph! Ford and GM actually threatened to pull out of NASCAR if the Mopar boys weren't banned!

That said - I don't want the fastest bike around, or even the fastest XS.... I just want one that's reasonably reliable, safe and FUN to ride! (and keeps up with traffic, of course!) Being able to tell everybody that I've rebuilt every nut, bolt and screw on the bike is just a HUGE ego boost, too.