BSM Exhaust with Crossover Tube

BSM Exhaust with Crossover Tube

Postby cshores » Wed Apr 23, 2003 1:20 pm

I decided to get a set of the BSM exhaust for my 79 cbx. The system comes with a crossover pipe, and is supposed to fit the 79 OK, even though the 79 stock system does not have a crossover pipe. Anyone know the effect that the crossover pipe is supposed to have on performance ?


Postby EMS » Wed Apr 23, 2003 2:15 pm

Well, Craig(?), you are throwing out quite a challenge here. This issue is almost a subject for a dissertation in internal combustion engineering.
In general, a crossover pipe in an exhaust is part of the overall engine tuning. One of the most difficult parts of IC engine tuning is providing performance over a range of rpms. Most engine designers pick a range where they expect the vehicle is mostly operated in and tune it for max performance in that range. Exhaust back pressure and resonance is a very important part in achieving an optimized exchange of burned and fresh load in the cylinders. Basically, you are trying to use the gas flow in the exhaust to "suck out" burned gas from a cylinder as soon as the exhaust opens and also use the back pressure to prevent too much fresh load escaping through the still open exhaust valve during the overlap period. So the goal is to have the cylinders in a multi-cylinder engine interact with each other through the exhaust. Of course, this can only be achieved in connection with the other tuning parameters, e.g. ignition and valve timing. The more cylinders you have going into a common exhaust, the more room you have to play. And there are always trade-offs. Gains in one rpm range may be made on the expense of losses in another range. You want to make the gains in the "useable" range. Numbers on paper don't always reflect that. Best example are the Harley "straight pipes'. Over all the noise that these things make, the riders completely miss the fact that their bike is now slower than it used to be because it has less low end torque - something you need to accelerate fast from 0 to 70. What good do 90 ft-lbs at 6000 rpm in a HD do you? Also, huge horesepower numbers are good to sustain high vehicle speeds, but do not indicate performance available in the street riding mode. This space does not allow to go too deep into this, but I would throw out the following statements: The crossover pipe on the late model (81 and 82) CBXes was added to increase low end torque for improved roll-on performance (low end torque). This does not necessarily mean, you will get more low end torque on your 79 by putting a BSM w/crossover on. You most certainly will alter the state the bike is tuned for , i.e.: changing the point and maybe value of your max HP and the shape of your torque curve. Then again, the whole thing may not be of any consequences to the performance of the bike the way you ride it. One thing though, your carbs should always be well adjusted to the exhaust your are using. You want to avoid both wet plugs AND holes in the pistons.
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BSM Exhaust

Postby Jim-Jim » Wed Apr 23, 2003 6:33 pm

I would say, if you have the time and inclination to put into it, to go through the bike with the exhaust you now have and note everything you can about acceleration and pull at various rpms and anything else you can think of about how the bike performs with your present setup (make LOTS of written notes; trust me, you won't remember). Check your plugs to insure that everything is burning OK while you're doing this. THEN, change over to the new BSM exhaust and duplicate everything you did and see if you can notice or record anything *significantly* different. It's an interesting idea and we would all like to know how it turns out. :P
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Postby cshores » Fri Apr 25, 2003 8:25 am

Thanks for your suggestions. I will try to do a seat of the pants before and after. I knew that the reason for the crossover was pressure pulse communication, I just did not know where that was suposed to increase torque at.

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