MAC exhaust

I was going to weld my crossover pipe today and found so many other cracks and rust holes. So my 33 year old exhaust couldn't be fixed. My header pipes are in good shape but it looks like the OEM collectors are available. Has anyone used the MAC 4 into 2 exhaust at all ? Any fitment issues, do I have to re-jet the carburetors....etc

 

does the mac have a cross over?

 

for mac price you could probaly get used parts off of the ebay

 

The Mac system doesn't have a cross over pipe no. It seems like a decent aftermarket system though.

 

The Mac does not have a cross over and I never could get it to run as good as it did with the stock exhaust.
Yes you do have to re jet. How ever My xj 750 swallowed its valves on #3 cylinder so I think it might be terminal.
What size and model is you bike

 

81 XJ650

 

the jets will depend on the mufflers you use in combination with the pipes

 

The answer primarily depends on whether you leave the intake system (airbox, filter) stock or not. Stock intake systems rarely need re-jetting (besides to overcome the stock "lean-for-EPA-emission-testing-reasons") regardless of whether you use an aftermarket exhaust system. But even with the stock exhaust system, once you start using hi-flow air filter elements or pod filters, you're off to the re-jetting races.....

 

I plan on keeping the oem airbox setup. So, with a higher flow exhaust the fuel air ratio shouldn't need to change. I'm only concerned about the cylinder balance since there is no crossover pipe and the lack of back pressure.

 

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 maintain 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 be physically 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 gas 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.

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.

 

This is by far the most technical response yet ! Which I appreciate as a trained automotive professional. Thanks for all the advice, there's a few odds and end that I do still need, that I will contact you directly about.

 

I recently installed a Mac exhaust on my 81 750. The mufflers don't clear the shock mounts and Mac has been unhelpful about a solution. The look great and sound good. I wish someone made stock replacement mufflers. Too late now, though. I haven't had any trouble with the engine since the install. No re-jet and I have a k&n oem replacement filter. Granted, there's no emissions test in my state...

 

The issue is not really emissions, it's whether the fuel/air mixture will be overly lean, which causes extra heat build-up in the combustion chambers and can lead to problems in air-cooled engines.

 

Oh, that I know. But the plugs are a good color when I checked them. What I meant was I can't do an exhaust diag on the mixture because no one has the equipment around here.

 

You need a colortune plug, just as useful as an exhaust sniffer.

 

How bad of a fit is it? Start a thread and post up some pics.