Exhaust question, Harley mufflers.

[Yard Dogg]

I'm putting Harley mufflers on my XS11 and have ordered 1 size over main jets, 140s. My original exhaust had a cross over tube, I'm just wondering if I need to have one with these mufflers. I know others have used these mufflers and they went up 1 -2 sizes on their mains, but was that with or without the crossover tube?

[SFSPECBIKER79]

harley ex

i put harley mufflers on my 79 spec and didnt have to do any jetting or use crossover pipe. been on there 2 years. bike runs same as with the stock exhaust. had to bend the muffler a little where the brake lever wouldnt touch wnen braking that all. good luck

[ambidextrous]

same as rowdy

no x-over

i didnt have to change jets, bike ran exactly the same, if not better with no changes at all

i even hammered a broomstick through to 'remove' the baffle

sounded really nice when i did that

[Yard Dogg]

I just got back from a test spin with the new mufflers on. Sounded great. Once the bike is tuned better I'll take it out and chop test with new plugs to see if I need the mains. If the stock mains are working fine, then I might add a small cross over to get more flow. Then use the larger mains. More power never hurts.

[motoman]

M ore power.......dream on, never happen. Also cross-over does NOT increase flow, does other things you may or may not understand anyway.......Gee, some people's kids.....eek.

[Yard Dogg]

M ore power.......dream on, never happen. Also cross-over does NOT increase flow, does other things you may or may not understand anyway.......Gee, some people's kids.....eek.

So what your saying, is that by connecting the two mufflers together by way of a crossover, the cylinders will not be using two mufflers instead of one right? Cross over pipes equalize the back pressure and also decrease the back pressure. Instead of having the restriction of 1 muffler per compression stroke, I'll have the restriction of 2 mufflers per compression stroke. Who doesn't understand?

But as it is, I rode the bike and am happy the way it is, so I'll leave it without the cross over.

[cywelchjr]

So what your saying, is that by connecting the two mufflers together by way of a crossover, the cylinders will not be using two mufflers instead of one right? Cross over pipes equalize the back pressure and also decrease the back pressure. Instead of having the restriction of 1 muffler per compression stroke, I'll have the restriction of 2 mufflers per compression stroke. Who doesn't understand?

But as it is, I rode the bike and am happy the way it is, so I'll leave it without the cross over.

IIRC the crossover does something about making the length of the exhaust functionally longer which has something to do with some federal regulation or other. It's something I understand Yamaha had to do to get noise level approval or something like that. I know the 400's were almost too loud stock to meet the federal noise regs for motor vehicles, and I know my stock mufflers on my 400 are a lot louder than the Jardines on my 11, and yes I know they are 30 years old, but it was just about as loud when I got it 22 years ago, and I'm told it hadn't gotten any louder since the original owner bought it new (yes I know the PO on that bike since it's my uncle). I'd be interested to hear my Jardines next to a set of stock pipes on an 80G to compare them.

[Yard Dogg]

I cut apart my old stock megaphones when they went down the tube. The cross over just made it so both the mufflers were used to exhaust instead of the one. The exhaust goes into a tube which goes past the cross over into a chamber where it can be exhausted through the muffler and the cross over to the other muffler. After seeing that, I wouldn't run stock mufflers without a cross over. These Harley pipes seem to have plenty of flow and the right amount of back pressure.

[motoman]

Old age and treachery trumps youth again!(gee...how did I know that was gonna happen) You gota love it! Cross-over does none of what your saying in this system or automotive. In the case of the XS's, you have forgotten about the "dead-fire" cyl. that takes place. To keep it simple for you without a long explanation about osscilations and such, a cross-over does nothing more than create a smoother scavenge effect and somewhat balances exiting exhaust on a dual exhaust set-up. Has nothing to do with noise levels or creating more power as you say. On the XS11"s, the stock system in good condition CANNOT be matched for flow characteristics based on design of stock intake, stock combustion chamber design, and stock cam configuration period. Just like so many other aftermarket manufactures, in this case Kerker and others back in the day sold alot of "hype" as increased performance and "cool" sound....and it worked for a certain "crowd" as does alot of aftermarket products. Just like in the automotive world, unless intake flow characteristics, combustion chamber design, and exhaust flow are matched with extensive testing, a modification in just one of those areas WILL hurt overall performance rather than enhance it. Some thirty years later, the biggest reason for exhaust changes on these bikes is because stock system has rusted out from high moisture content(humidity and such) and or combination of exhaust being acidity combined with moisture. You can bet Yamaha engineers did all necessary in ALL of those areas to enable the XS11 to trump any other manufacturers production bike.....hence in late 1977 produced the "worlds quickest production motorcycle" the world had ever known. So, advice for Yard Dogg, study a bit more pertaining to anything high-performance, as it may come back and bite you.....no offense intended, just "old age and treachery" advice...glad to hear your among many who have made good use of the Hardley take-offs, as they are a very cost efficient alternative.

[Yard Dogg]

+1 to motoman.

I like the way you handle us smart a-- youngsters. But I still say the crossover does increase flow, while also equalizing the back pressure, or to deal with the dead stroke as you call it. To me it's just simple physics. 2 exits = more exhaust movement, that's where I'm coming from. Even if the exhaust is only moving back and forth in the crossover. When my stock muffler blew a hole in it, I would have gladly patched it because they are the cats meow, but at the time I didn't have a welder, so I made new chambers much the same way as the originals were with a few differences. They worked out good and the power felt the same, but I had the popular clutch slippage at the time so there was no way of really knowing.

Also, my bike came stock with the crossover, and I may be wrong, but it seems my stock main jets are quite a bit bigger than those bikes which did not come with it. Kind of tells me they needed more fuel to match the increased exhaust flow.

[Larrym]

The H- Crossover Theory Of Operation

I thought this explained it well enough for me:

In the case of 4 stroke motorcycle engines, the reason for a cross over is this...

After a cylindar fires and the piston cycles up - exhausts gas vents through the Exhaust valves, along with the remainder of the explosion, exiting the cylinder into the exhaust system.

This explosion creates 2 pulse waves: a Sonic Pulse - and a Thermal Pulse.

When the faster Sonic Pulse leaves the exhaust it has created a vacuum in the pipe behind it, which air pressure will enter the pipe to fill the vacuum. This air will collide with the Thermal Pulse and cause it to reverse. The timing of this event causes exhausted gases to be returned to the cylinder while the exhaust valve is still open therein not allowing all the spent gas to escape and also polluting the environment for your next stroke cycle with fresh air/fuel mix.

This Event is call Reversion and seriously degrades performance and lifetime of valves and cylinder heads.

The Cylinders fire opposite of each other or nearly so on a Harley. So when One side is firing, the other side is idle. Having the cross over pipe allows the pulses to dissipate back and forth between the tubes instead of - or before exiting the exhaust and therein greatly diminishing the effects of reversion and so improve performance and engine life.

Yes - you see a lot of bikes with duels and no cross over. These are usually "Tuned Pipes".. meaning their length and size are tuned to work with the S/T Pulse in such a way that reversion can not return to the engine before the exhaust cycle has ended.

[Yard Dogg]

Maybe this will clear it up.

Stock HD exhausts have a crossover pipe, after-market exhausts do not. What is its purpose? Also, after-market exhausts are short or long, and with or without baffles. What is the use/function/purpose/advantage/disadvantage of each combination?

Billy Cole

Amissville, Virginia



A

Your first question is easy to answer; the second would take a book to be anywhere near complete. Let's look at the easy one.



Most Harley motorcycles have small mufflers relative to their engine displacement. Those small mufflers must also be quiet, and that combination results in limited airflow which, in turn, limits power output. A crossover pipe allows the exhaust gasses from one cylinder to flow through both mufflers, thus reducing the restriction and adding a bit of power.



Crossover pipes can also improve mid-rpm torque if they are properly located. Triumph first discovered this when they introduced an exhaust crossover pipe to their American Triumph Bonnevilles in 1964. The original purpose was to increase the airflow through the smaller mufflers that we Americans preferred. However, they also discovered that the crossover added performance in the mid-rpm range.



Aftermarket exhaust systems tend to not have crossovers because most buyers do not like their looks. Most Evo Harley owners change the header pipes simply because they think the crossover is ugly. Harley learned its lesson and now hides the crossover pipe, sometimes incorporating it as part of the muffler bracket.


This article goes on to explain how the baffle and back pressure helps the sound waves and pressure wave to keep the unburned fuel in the cylinder such as the article above states, and this process mostly works by the length of the pipe and has less to do with whether or not there is a cross over.

[Yard Dogg]

This thread may have been a little controversial, but now I fully understand why our stock exhaust systems are state of the art. With this info and having saw inside one of our stock mufflers I feel much more confident about how to tune my exhaust.

When a compression stroke is exhausted into our stock system, it flows through a tube past the cross over into the first chamber in the muffler, where it can then flow through the cross over tube and into both of the mufflers for decreased pressure, more flow, and MORE POWER! Also in the same chamber it hits the first baffle tuned to be at the correct length from the cylinder head so the correct back pressure wave can travel back through the pipe at the correct time to keep the exhausted gases from re-entering the cylinder head, also increasing power. After the first baffle, the exhaust splits into 3 separate tubes, loops back, goes through 3 more tubes again, loops back and then finally exits out the tip. The loop backs serve as the sound dampening process. The reason our megaphones are so big and have the cross over tube is to keep the maximum flow characteristics needed for high performance and to control that pressure wave while being very quiet.

This is my take on all of it. Now maybe I can do a little math and come up with a good equation to figure out where I should put a cross over tube and to see if my exhaust is the right length. This thread has been a learning experience for me. Thank you.

[motoman]

Glad you took no offense there Yard Dogg.




larrym's research was a great correct technical explanation! Yard Doggs disecting of stock system verifies why stock system IS the superior set-up. Several don't have that option using the stock system, rust, etc......bummer. IMO the best aftermarket exhaust set-up availible to us here in the States would have to be the Jardine "spagetti" header pipes and muffler set-up. Have become part of the "unobtanium" in recent years though. This solves the dead-fire sequence that take place by re-directing header exhaust to a constant even pulse rate evenly out both mufflers. The boys down-under and across the pond have readily had other very superior options availible(supply and demand) as they been trickin' out performance options WAY longer on scoots than we here in the States have. There savvy and experience with scoots IS unsurpased, as is there ingenuity and trial and error experience. That's why you have one here who is a menber down-under twistin 100+ ponies AT the rear wheel on the dyno.......availibility of parts, cams, exhaust, etc. that we don't have here. Dan Hodges here is IMO the best example of studying and totally understanding what needs to be done to get at least close to that kinda rear wheel power, considering what we have to work with here in the States. Oterwise the XS11 is what it is, and for the most part enough for the frame/suspension design to handle in stock form. Interesting trading of information and opinion to say the least though!