4-2 exhaust collector replacement

Quick question, saw in another thread that the firing order for the xj550 is 1&4, 2&3 If I replace my rusted out exhaust collector with custom pipes, would bringing 1&2 and 3&4 each separately to a muffler be appropriate? Exhaust pulses should be separated in that setup. Any reason I would need an H or X pipe to balance the two?

 

Yes. Scavenging matters, even in 4-strokes. Not having a crossover won't keep it from running, it will make it run a bit worse than it should though.

Many aftermarket exhausts are 2-2 as you described. You may need to rejet depending on what you use for mufflers.

 

Best setup is to have the cross-over........However, I have had--and have seen plenty of bikes that have had 4/2 systems put on that do not have a cross-over pipe. They still run decent, though there is often some inherent extra vibration/roughness to the feel.....not much, but if you are really in-tune with your bike, you'll notice the difference.

 

1. chacal did a really nice write up on the exhaust and how the cylinders interacted with each other in the exhaust.

4:1 Exhaust vs 4:2

 

chacal on
EXHAUST SYSTEM OVERVIEW:

As with many other systems on these bikes, the Yamaha XJ-series of exhaust systems are a mini marvel-to-behold, well thought out and engineered......so please do keep this in mind when considering aftermarket "performance" exhaust systems.

Let's start with the headpipes: many of them are actually of a double-wall design......and inner tube (that carries that gas) and an outer tube that can stay relatively cooler and thus maintains its appearance much better and longer. Additionally, they are tuned in their internal (gas-carrying) pipe diameter to match the flow characteristics needed for their intended use.....thus, for example, even though the headpipes for an XJ650RJ Seca can physically be installed onto an XJ700 engine, the inner pipe diameters do differ, and thus the swap may cause more harm than good, as it messes up with the balance of airflow thru the engine.

Pipe diameter helps determines (among other things) both the volume of exhaust gas that can be carried away, and, equally as important, the speed at which those gases will flow thru the pipe. All other things being equal, a bigger diameter pipe --- while surely capable of flowing more gas volume, will also slow down the progress of gas thru that pipe......which, in the world of exhaust gas flow --- and its important effects on intake gas flow (how well each cylinder can be scavenged of its burnt exhaust gases and allow for a full and complete incoming fuel/air charge) --- is a mighty important consideration.

By the way......although we've touched on some of the reasons above.....in regards to "swapping" headpipes between models, our best advice is: don't. Besides the whole "gas capacity and velocity" issues discussed above, there also the issue of fit: although the headpipes between a 550 and a 750 (for example) may look the same, and may actually bolt onto the cylinders heads, both their length and their unique bend angles ---- needed to follow the frame and mate up with the exhaust collector ---- will likely be different.


Next: the collector. Although these weren't used on the XJ650 Maxim and XJ1100 models --- those bikes use a crossover pipe to perform the same function --- the collector box performs a few not-so-obvious but very important functions, and does it in a very admirable manner. First, of course, it provides a transition between the headpipes and the mufflers. Second, it keeps the decibel levels down to a reasonable volume. And finally....and very important for overall engine performance.....it provides a method of assisting that all-important "gas velocity" by providing a sufficient volume of space for "warehousing" (temporarily) the exhaust gas volume, while also providing (via its unique design) the ability to assist in providing maximal exhaust gas flow.

If you've ever looked carefully at your heavy (and probably-rusty) collector, you'll notice that it is designed ("split") in such a way so that cylinders 1 & 4 (the outer two) "share" a certain internal cavity space, while cylinders 2 & 3 "share" the rest of it. This is important because of the firing order of your engine: 1-2-4-3

If cylinders #1 and 2 --- which fire sequentially (and, thus need to exhaust their spent gases sequentially) ---- were to share the same exhaust cavity, then their (sequential) exhaust gas flows would be "spaced" too close to each other (in time) and would see vastly different back-pressures (cylinder 2, which fires after cylinder 1, would see more back-pressure than cylinder 1, which emptied itself into a, well, into an "empty" exhaust collector).

To put it another way, cylinder #2 sees a lot more back-pressure than cylinder #1, because the gas pulse from #1 doesn't have much time to exit the system before the pulse from #2 is trying to squeeze itself in the same pipe. Notice that cylinders #3 and 4 have a similar situation.

The same thought goes for the other two cylinders (# 2 and 3). Notice how even the aftermarket "dual" exhaust systems suffer from the above problem, since they are really two separate system, one system being for cylinders #1 and 2 (left side) and the other being for #3 and 4. Notice how it's the "sequential" problem.....since cylinders 1 and 2 fire (and exhaust themselves) sequentially, and since those gases vent into one common chamber, that cylinder #2 is going to see more backpressure than cylinder #1 will......and same for #3 and 4.

This un-balanced situation....with different cylinders seeing different exhaust system back-pressures......can (and does) result in different amounts of burnt gas scavenging from the differing cylinders, which is not the way to get an engine to make the most amount of power.


Aftermarket 4-into-1 systems overcome this "differential scavenging" situation by re-introducing, somewhat, a "collector".......where the fours pipes come together into a common chamber, just before the muffler. This collector allows the system to maintain some amount of equalized back-pressure (and thus burnt gas scavenging), while the particular placement of the four headpipes, as they come into the collector, allows a little bit of "pull-thru" between each cylinder, helping to reduce overall system back-pressure levels, while still maintaining a somewhat equal amount of back-pressure to each cylinder.

Or, maybe, aftermarket systems do no such thing:

http://www.motorcycleproject.com/text/exhausts2.html


Of course, the final piece of the puzzle is the mufflers, which are primarily decorative pieces....they look nice, stylish to the lines of the bike, contribute almost no back pressure to the system, and provide a nice throaty (but not too loud or droning or buzzing) sound output.

So just remember that the real heavy-lifting in the exhaust system is actually done by the headpipes and the unique, well-designed (but often overlooked) collector box.

Last edited: May 19, 2016
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Sometimes it seems that Len is really レオナルド (Japanese for Leonard); as his high level of knowledge on all components of these bikes would lend one to believe he was the Lead Engineer at the Yamaha Motorcycle Division back in the developmental days.

 

Awesome, I appreciate that. I misunderstood the other post I read. I guess that one was looking at the firing order from the perspective of the coils. 1&3 spark at the same time but 3 is on the exhaust stroke while 1 is on the compression stroke. I guess I'll have to figure something else out. Maybe just buy a new collector, I think mine is beyond repair.

 

1 & 4 are on the same coil

 

The engine is essentially a pair of parallel twins. Ignition is via wasted spark.

1 and 4 are 180º out from 2 and 3, and are paired on the 1/4 coil.
2 and 3 are also 180º out from 1 and 4, but paired on the 2/3 coil.

The firing order is 1-2-4-3.

 

Waisted spark because 1 and 4 are fired at the same time by the same coil (as well as 2 and 3 by the other coil), but cyl 4 is not ready to fire when cyl 1 is. Is that correct ?

 

Correct. Just like a Brit twin.

 

Or a Yamaha XS650

 

Which is a copy of a Brit twin that was built under license (sort of)
http://www.curbsideclassic.com/blog/sunday-salon-the-yamaha-xs650-originality-is-overrated/

 

Yeah, affordable copy, great sale success.

 

Yep. and less leaky.