H-D Exhaust

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ericjon262
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H-D Exhaust

Post by ericjon262 »

I've been thinking about engineering a new exhaust for my Fatboy, partly for power and fuel economy, but mainly because I have a welder and don't like all the chrome on it(the bike). The Harley brings an interesting challenge that I hadn't thought about for exhaust design. The Twin Cam V-twin has a 45 degree bank angle with a single crank pin, therefore, front cylinder fires, then 315 degrees later the rear fires. 405 degrees after the rear, the front fires. this is why Harleys have their characteristic thump. Quick and dirty math(based on degrees of crank rotation) says that the front primary should be about 12.5% longer than the rear in order for the pulses to meet at the collector end to end. All that's fine and good, but now you end up with differences in primary length, which has it's own affects on cylinder scavenging. I went ahead and ordered up a copy of Scientific Design of Exhaust and Intake Systems, it's an older book, but is packed with info, that really hasn't changed much for the purposes I need. It should be here later this week.

http://www.amazon.com/gp/product/083760 ... PDKIKX0DER

Image

Looking at the stock exhaust, you can see the front is longer from the factory, but probably almost twice as long, which would be as much of a detriment as having them equal. my current plan is to have the front primary follow basically the same path as stock, and have the rear run down the outside of the rear cylinder, and meet up with the front just behind the transmission.

Image

this should bring the difference in length much closer to my 12.5% mark than the stock design, and look nice too. next step is to start making calculations.
"I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
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Re: H-D Exhaust

Post by The Dark Side of Will »

ericjon262 wrote:engineering... ...for my [Harley-Davidson]
I don't understand.
ericjon262 wrote: Quick and dirty math(based on degrees of crank rotation) says that the front primary should be about 12.5% longer than the rear in order for the pulses to meet at the collector end to end.
That's true only at one fundamental RPM and integral multiples thereof.
ericjon262 wrote: All that's fine and good, but now you end up with differences in primary length, which has it's own affects on cylinder scavenging. I went ahead and ordered up a copy of Scientific Design of Exhaust and Intake Systems, it's an older book, but is packed with info, that really hasn't changed much for the purposes I need. It should be here later this week.
Good choice.
The different primary lengths will result to some extent in each cylinder having a different shaped power curve. That means that the cylinder that makes more low end torque will pull more air at low RPM and the other cylinder will pull more air at high RPM. This makes tuning both cylinders to run at their best impossible unless you have independent mixture loops for each cylinder.

Before you get wrapped around the axle about primary length, consider that the manifold pictured below only gave up ~15 HP to fully developed long tubes on a 790 HP engine.

Image

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ericjon262
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Re: H-D Exhaust

Post by ericjon262 »

The Dark Side of Will wrote:
ericjon262 wrote:engineering... ...for my [Harley-Davidson]
I don't understand.
from the aspect that Harley's are rather poorly thought out designs, or that there are thousands of off the shelf pipes available?
The Dark Side of Will wrote:
ericjon262 wrote: Quick and dirty math(based on degrees of crank rotation) says that the front primary should be about 12.5% longer than the rear in order for the pulses to meet at the collector end to end.
That's true only at one fundamental RPM and integral multiples thereof.
I'm basing this on degrees of crank rotation in between cylinder firings, so, whatever RPM the primary tubes are optimized for, the front should be 12.5% longer for optimal merge in a 2-1 collector, The Gas slug shouldn't slow appreciably on the added 12.5% distance of pipe, but I'll look into it.
The Dark Side of Will wrote:
ericjon262 wrote: All that's fine and good, but now you end up with differences in primary length, which has it's own affects on cylinder scavenging. I went ahead and ordered up a copy of Scientific Design of Exhaust and Intake Systems, it's an older book, but is packed with info, that really hasn't changed much for the purposes I need. It should be here later this week.
Good choice.
The different primary lengths will result to some extent in each cylinder having a different shaped power curve. That means that the cylinder that makes more low end torque will pull more air at low RPM and the other cylinder will pull more air at high RPM. This makes tuning both cylinders to run at their best impossible unless you have independent mixture loops for each cylinder.
the book is very technical, but a very good read. a good friend of mine let me borrow his copy years ago, and I'd been meaning to pick my own copy but hadn't had an excuse to do it.

The bike is fuel injected and has one O2 sensor per cylinder, so I imagine the fueling can be skewed much in the way you can an obd2 car.

The bold text is what I'm trying to avoid, because with the cylinders performing differently, they will work against each other to some degree.

The Dark Side of Will wrote: Before you get wrapped around the axle about primary length, consider that the manifold pictured below only gave up ~15 HP to fully developed long tubes on a 790 HP engine.
<snip>
I'm not really getting wrapped around the axle on anything yet, right now, just considering action plans. the exhaust pictured, to me looks kinda like it following a similar idea to an "exhaust termination box". I see nothing terribly wrong with the design in a situation where space inhibits the use of a better design, but in my case, I have plenty of room to work with.
"I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
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Re: H-D Exhaust

Post by The Dark Side of Will »

I was making a joke about not a lot of good engineering being applied to Harleys.

The coolest exhaust I ever heard on a Harley was an apparently equal length header with about a 1" long collector.

You want the primaries the same length... you just do. In theory you can do things with different length primaries but this is the real world... KISS.

However, Harleys may not need collectors at all. Both firing intervals are longer than the exhaust valve duration, which means that it's basically impossible for the cylinders to meaningfully interact in the collector. I know a guy who used to race them, though, so I'll ask him what worked for exhaust.
ericjon262
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Re: H-D Exhaust

Post by ericjon262 »

The Dark Side of Will wrote:I was making a joke about not a lot of good engineering being applied to Harleys.

The coolest exhaust I ever heard on a Harley was an apparently equal length header with about a 1" long collector.

You want the primaries the same length... you just do. In theory you can do things with different length primaries but this is the real world... KISS.

However, Harleys may not need collectors at all. Both firing intervals are longer than the exhaust valve duration, which means that it's basically impossible for the cylinders to meaningfully interact in the collector. I know a guy who used to race them, though, so I'll ask him what worked for exhaust.
From what I've read, they seem to respond to a 2-1 exhaust better than a true dual. esp with cams. I'm all about any advice from someone who has btdt. got a link to the 1" collector?
"I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
The Dark Side of Will
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Re: H-D Exhaust

Post by The Dark Side of Will »

mk e wrote: sorry, I just saw your message.

Yes, what you're saying is pretty much correct I think. For Hp they like separate exhaust, normally open step headers....but the last bit of the pipe can act like a collector or be a megaphone as it is a reflection point that can be tuned. Burns sells 2:1 H-D exhaust but I've never seen it beat dual for peak hp...... it can beat dual for average power over some range or maybe peak torque but even that is rare.

As I understand it cylinder to cylinder isn't a huge effect on most engines. All the Dynomation sim stuff assumes it's 0...but that's not exactly right. Some engines can put the collector into resonance so 1 aids another, but normally it more about not harming another like is a common problem on 90 degree crank V8 engines with H-Ds 1/2 between that 2 points.
ericjon262
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Re: H-D Exhaust

Post by ericjon262 »

well, that gives me something to think about. I'm going to try and do some more research on odd fire engine exhausts and see if I can find any other ideas.
"I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
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