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Author Topic: Harley Exhaust Systems  (Read 1855 times)
ChrisandKristin
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« on: June 24, 2015, 10:36:53 PM »

A pretty common question from non-Harley riders is what is the deal with Harleys, Harley owners, and their pipes.  H-D don't bother using millimeters to specify stroke, or cc for displacement, because they are not metric engines.  Like the 80 cubic in Evo has a stroke of 4.25" (1/4" longer stroke than a 454 Big Block Chevy).  The Twin Cam 88 has a bore of 3.75", stroke of 4.00".  The TC103 has a bore of 3.875" and stroke of 4.375".

The Harley Big Twin has almost half the total piston displacement of a Goldwing in one cylinder, and big cylinders have to breathe.  Valves are not the problem - the intake and exhaust is.  And the Big Twin is basically choked off from the factory to make EPA happy.  Open one up and let it breathe and the Harley Big Twin will make more torque below 2,000 rpm than most metric motorcycle engines of similar displacement can produce at peak.  This is the because the metric manufacturers use high-revving OHC oversquare bore/stroke combinations.  The Harley engine is undersquare like a diesel, and the result is that in big bikes like our FLHT-series, there is rarely need to spin the engine at more than 2,500 rpm.

When it comes to torque vs horsepower, torque is more important.  An old adage in building racing engines to describe the difference is that horsepower is how fast you hit the wall.  Torque is how far you take the wall with you.  Peak horsepower can rarely be used in normal street riding because it is achieved at engine speeds that are rarely used.  Low rpm torque is used every day.  And that characteristic of the Big Twin has made it become, and remain, the most popular engine in motorcycling.  You won't win many drag races with a Harley against high-revving, higher horsepower machines.  But the usable power in the rpm range where they are most often ridden is what makes them fun because you do more in two gears with a Big Twin than you can shifting four times with a high-revving metric.

Changing the exhaust system on a Big Twin (along with the associated tuning items like induction/fuel map and valve timing) is very simple due the engine's design and serviceability.  And the most effective performance improvement for the dollars spent that there is.  So the Harley-Davidson Big Twin has become the one that virtually all the aftermarket manufacturers support, and it is also the most modded engine in motorcycling history.  It is also very easy to change final gear ratios on a Harley with a Big Twin because you can simply change either the primary or secondary in the drive train (without having to ever touch the transmission) to achieve a final gear ratio that is more suitable for the modifications done to the engine.

Hot Bike put out an article a few years ago explaining some of the basics of exhaust tuning on Heavy Breather engines like the Harley Big Twin:
http://www.hotbikeweb.com/tech/exhaustsystembasics
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bagga
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« Reply #1 on: June 25, 2015, 01:55:41 AM »

my early evo is jetted a little fatter than the later model evos and it has the butterfly kehein carb which is commonly bitched about by some people. i never had any problem with it and it gets pretty good mpg. the least i got was mid 30's pulling a trailer to sturgis one year and the best i ever got was about 56 heading north (no trailer) thru illinois with about a 30 mph tail wind and keeping it at about 65 mph. i'd like to try an S&S on it sometime but north of $500 for a new one is kind of hard to swallow and the old one works too good yet. the S&S sure looks nice tho.
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ChrisandKristin
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« Reply #2 on: June 25, 2015, 03:58:51 AM »

That is the same carb our bike has - Keihin butterfly valve.  Putting an S&S Super E on it with a 1 7/8" throttle bore would probably wake it right up on the top end but the gas mileage might suffer.  We've been been getting 50-51 mpg riding two up and the Evo in our bike has been tuned for low end.  I don't know what your bike runs at, but at 60 mph the engine in ours is spinning at 2,300, at 70 mph it's turning 2,700, at 80 mph it's turning 3,100 rpm, at 90 mph it's spinning 3,500 rpm.  The Andrews cam is a little lopey below 1,800 rpm.  Have to keep it above that riding thru town or it will tend to be a bit jerky at light throttle.  As long as throttle is being applied it pulls strong from 1,500 rpm.
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bagga
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« Reply #3 on: June 25, 2015, 11:33:22 AM »

mine is bone stock and at 3200 it's running about 72 mph. that's where the motor is happiest but the fuel mileage suffers. i prefer riding anywhere between 60-70 mph. that's fast enough for me and it's not like the interstates around GB are nascar tracks but the 70 mph signs are now up so guess we'll see if they start driving closer to 75 now.
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bagga
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« Reply #4 on: June 25, 2015, 11:56:49 AM »

chris, you forgot to mention the single crank pin that gives that beautiful sound at idle, the shovelhead is the best sounding motor of them all. you can idle them down to about 700 rpm and the sound like no other but it's kind of hard on the bearings. i keep mine around 900 or so. the evo is happier at 1k rpm.
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ChrisandKristin
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« Reply #5 on: June 25, 2015, 03:17:21 PM »

chris, you forgot to mention the single crank pin that gives that beautiful sound at idle, the shovelhead is the best sounding motor of them all. you can idle them down to about 700 rpm and the sound like no other but it's kind of hard on the bearings. i keep mine around 900 or so. the evo is happier at 1k rpm.

The Harley's crankshaft, being a multiple piece unit, is kind of unique.  The bottom end is total roller bearing - there is only sleeve bearings in the gearcase.  The Pans, Shovels, and Twin Cams use straight roller bearings, the Evos use tapered (Timken style) on the mains.  The rod bearing is triple-row roller, and they have roller cam(s).  Of course, the Evo has one cam, the Twin Cam has two.  And they are dry sump - a "feature" only usually seen in high-performance racing engines.

There are many folks that view the Harley Big Twin design as being "crude" or low-tech.  But its simplicity, serviceability and ability to be infinitely rebuilt is the reason you see Electra Glides with 400,000 miles on them and have never had the bottom end apart.
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Jarlaxle
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« Reply #6 on: June 27, 2015, 02:37:18 AM »

Harley exhaust system?  Isn't that usually a straightpipe?
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ChrisandKristin
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« Reply #7 on: June 27, 2015, 04:03:22 AM »

Harley exhaust system?  Isn't that usually a straightpipe?

No.  What looks like straight pipes on some bikes have baffles in them.
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Jarlaxle
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« Reply #8 on: June 28, 2015, 02:26:56 PM »

Not the idiot that rides  by my bedroom every night at 2am!  It's a chopper with about 18" open pipes, and emits a sound level similar to a Winston Cup engine!
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ChrisandKristin
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« Reply #9 on: June 28, 2015, 04:35:21 PM »

Well, yeah - I should've said legal pipes.  My wife had one of those - a '79 XLCH with drag pipes on it.  It was louder than a Winston Cup engine, and sounded way meaner.  But hers had 12:1 pistons and big cams in it and you could barely get it started.  But once you put in the effort to get it fired up it was sure fun to ride!  And when you opened the throttle on that bike all hell broke loose.  I raced both Z1R's and GS1100E's at Amber Green Raceway with it and never got beat.
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Jarlaxle
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« Reply #10 on: June 29, 2015, 02:17:32 AM »

Every time this idiot passes my house and blasts me out of a sound sleep, I am a little closer to supporting a requirement for unaltered stock exhaust systems on all motorcycles.
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