Header Tube Size and Performance Expectations

Hello all. Because aftermarket exhaust systems are becoming harder to find, using different models and year systems, then modifying them to fit, is on my list. I have a complete 1985-1987 Kawasaki ZX600 Yoshimura System, and the exhaust ports have the same spacing as the GPz550 Head, so it will work there. It also fits under the engine, so that's golden as well. Header tubes are slightly larger than the Moriwaki race System and Supertrapp System I currently have. My question might be simple, but I just want to be sure. Going from one size header tubing to larger or smaller diameter tubing, what would the the noticeable performance difference if everything else remains the same(length, exhaust outlet, bend...)?

I will get the O.D. measurements this evening.

Thank you in advance.

the peek torque will move up in the rpm's. maybee 700 rpm?
just try it and see. pay closer atension to the low rpm changes. u may loose some bottom end.

Sorry it took so long

Thanks Mark, I was thinking a loss/movement in bottom end with bigger tubes would be a result.

I have read that an increase of 1/8 of an inch in diameter will move the torque peak up between 650 and 800 rpms. Likewise a decrease of equal size will decrease the same amount.

In general, and I do mean in general, I think the following are basically true regarding exhaust systems:

•Longer primiaires tend to increase power below the engine’s torque peak and shorter pipes tend to increase power above the torque peak
•Large diameter pipes tend to limit low-range power and increase high range power
•Small diameter pipes tend to increase low-range power and to some degree limit high-range
•"Balance" or "equalizer" chambers between the exhaust pipes, tend to flatten the torque peak(s) and widen the powerband.

These are the header pipe o.d. measurements from the exhausts I have, measured along the down pipe straight section.

1.375" Moriwaki Race
1.390" Supertrapp Street
1.380" Yoshimura Street(pictured above)
1.380" Stock GPz550

Never measured them before, and the Yoshimura will be going on a street bike.

back in the day, they would build dif pipes to suite dif. tracks. smaller primaries and or longer primaries for more technical tracks ,for more bottom end, comming off the many turns. and other pipes that would suit the faster tracks.

Thanks for your feedback.

I have heard that a true merged collector makes a big difference in power too. Takes up a lot of ground clearance for racing though.

Would love to compare all systems, but I don't have unlimited access to a dyno.

Corey,

The level of build of the engine, the particular carburetors you are using, the cams and cam timing you are using, etc...... can all have an affect on how a particular pipe change will make your bike perform at various RPM ranges. Simple truth is you are going to have to swap the pipes and see what you end up with.

Why are you swapping?
What are you trying to accomplish with the swap?

Larry C

Larry,

A little history first. I have had no other streetbike since my first 1985 GPz550 back in 2003, and have progressed to owning 5 more in various states of repairs needed. I could have had much more, but space limitations stopped me, and besides I like being married. I only plan on having 3 running, as one is a dedicated race bike, one is a daily commuter, and another will be a daily commuter for visiting friends to ride. The race bike has been a race bike since the first track day in 2008, and has progressed with more than 6 visits to the same dyno, from stock(no air box or stock exhaust) to 615cc, Moriwaki Race, Megacycle #478-60 Degreed Cams, +2mm intake valve ported head, larger CV Carbs, to where it is now. I am coming up on 12 years of the same model GPz550, and have no plans to ditch them for another bike.

Before I installed another exhaust on the street bike ('85 GPz550), and before I measured the pipe diameters, I posted this question to see what I might expect for a change/movement in power. The only testing I have done that I can really feel is swapping the Supertrapp Street with the Moriwaki Race from an all stock engine, with only K&N Pod Filters, Stage 3 Jetting, and Accel Coils and Wires, CA Emission removed. Just for the record, I always run the Supertrapp System without any discs and without the end cap.


Here is a few of the Moriwaki Race System:




The headers on the Yoshimura looked to be a lot larger in diameter that the Moriwaki, and Supertrapp, until I measured them above for the first time, so I guess I jumped to conclusions, which is about all the exercise I get.

Without dynoing the two or all systems, "seat of the pants" or the "butt dyno" is all I can go by, but I agree without the dyno readings it's all hearsay.

Exhaust midpipe and outlet openings will make a difference too.

Too much info Larry?