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V-twin Tech Elements of Power 2: Testing a SuperTrapp Kerker 2:1 exhaust system

Rider Magazine
May 8, 2007
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This is the second of a continuing series of articles about how to increase the useful performance of the new Harley-Davidson Twin Cam 96 engine. We are not going to pursue peak power at high rpm; rather, our intention is to find simple and practical ways to improve this engine’s performance in the middle of its rpm range. Specifically, we will look for greater performance in the range of 1,500 to 4,000 rpm (beginning from about 40 mph in sixth gear) where Harley riders spend the vast majority or their riding time.
Previously, Round One
The goal of our first article (April issue of American Rider) was to establish the stock performance of our ’07 FXD and then to see what improvements we might get with a typical free-flowing air cleaner and less restrictive mufflers, the kind of modification the average Harley rider does. We did our testing on Barnett Tool & Engineering’s DynoJet 150.
After the baseline tests, we fitted a K&N air filter and retested. We then refitted the stock air filter and added a set of Screamin’ Eagle (SE) slip-on mufflers to test again. Finally, we left the SE mufflers in place and re-installed the K&N for a final series of runs. When finished, we had results for each of the aftermarket parts as well as a record of their collective influence on our test engine.
I was pleasantly surprised at some of the results. First, it was clear that Harley had done some very nice work in getting excellent performance below 3,000 rpm. As the testing demonstrated, the stock air cleaner plays a major role in that performance. It was only above 4,000 that the superior airflow of the K&N began to matter. In the important and more useful 1,500 to 4,000 rpm range, the stock air cleaner was clearly superior. The second surprise was how well the current Screamin’ Eagle slip-on mufflers worked in this same range. In my prior testing, they produced generous torque below 3,000, where the louder old SE mufflers (no longer offered by Screamin’ Eagle) tended to perform relatively poorly.
We achieved the best midrange performance in that initial testing by simply installing a set of Screamin’ Eagle slip-on mufflers. The K&N’s higher flow capacity made no difference below 4,000 rpm and very little above that. It seemed likely that further improvements would include other modifications, such as new camshafts, to increase airflow through the engine.
Testing, Round Two
Our FXD, with the K&N and SE slip-ons, had been on the road continually since the previous test. Its greater-than-stock torque, while only slight, was obvious and welcomed. It responded smoothly to throttle input and the moderate increase in sound level over stock was acceptable and pleasant. The next logical step would be to fit and test a larger, higher-flow exhaust system.
Past experience has led me to believe that less restrictive exhaust systems with larger muffler volume outperform any of the small slip-on muffler designs. While larger individual or dual exhausts have made the most peak power, the 2-into-1 (referred to by Kerker as 2:1) systems typically perform better below 5,000 rpm.
SuperTrapp’s 2:1 systems have been consistently at or very near the top in peak performance and usually ahead of the other offerings below 4,000 rpm. A major advantage for SuperTrapp systems is their adjustable disc/baffle system that can be altered to fit the needs of the engine and the desires of their owners. Installing a larger number of discs increases airflow, peak power, and noise. Fewer discs can increase midrange power while producing a more moderate sound level. For all these reasons, we chose to test one of SuperTrapp’s updated 2:1 system, the Kerker 2:1 SuperMeg (MSRP $545).
The SuperMeg system is the latest and best version of the SuperTrapp 2:1 Harley pipe that has been on the market for more than 20 years. The first version was a direct result of testing Jerry Branch and I did with an early FXR in 1985. Much has been learned between that time and now. The first version was made of stainless steel, either brushed or polished. Harley owners were reluctant to buy these systems because they were not chromed steel—never mind their performance advantages—and they tended to immediately discolor.
This latest development of the SuperTrapp 2:1 incorporates most of the lessons learned from the system’s extensive time in the field. It has a 2.5-inch collector diameter instead of the original 2-inch collector; and the baffle’s perforated tube is 2.5 instead of 2 inches. Both of these changes improve airflow and, therefore, high-rpm performance when attached to a correctly modified engine. They have relatively little effect on the performance of an otherwise stock engine, increasing output by only a small margin.
The two most important improvements, at least from a marketing point of view, are the greatly improved fit and finish of the system and its heat shields. The SuperMeg’s heat shields hide the normal bluing of chromed steel exhaust pipes and eliminate the familiar SuperTrapp melting of shoe rubber onto the rear exhaust pipe. The SuperMeg system is by far one of the finest fitting and finished systems I have worked with.
However, installation of this system is not something I recommend for the occasional mechanic. It is difficult partly because of the excellent and close fit of the heat shields and because of poor and incomplete instructions. The details don’t matter beyond saying that some confidence on the part of the installer is required. However, any experienced and careful Harley mechanic can easily fit this pipe.
The Testing
After mounting our faithful FXD Dyna Super Glide on the DynoJet 150, we made four runs to establish that the power of the SE/K&N combination was still producing as it did last time. Assured that all was well, we installed the Kerker 2:1 SuperMeg system with 18 of the included 20 discs and ran another set of four runs. In my years of testing I have found no real improvement in power with more than 18 discs. The variations between all eight dyno runs were small and typical.
The SuperMeg did not significantly improve dyno performance over the SE slip-ons at any rpm. Their superior airflow was simply not noticed by our otherwise stock Twin Cam 96-inch engine. We would have to improve airflow in other parts of the engine system in order for the SuperTrapp exhaust to make a difference in power output.
However, on the street things are rather different. The SuperMeg system improved drivability very noticeably. The engine is smoother and more responsive. The exhaust note is also lower pitched and quieter, and the engine did not seem to run any hotter. We’ll need to make other complementary modifications to fully realize the performance potential of this system—modifications we fully intend to make.
EFI Notes
It is entirely possible and even likely that our testing was affected by the stock EFI’s (electronic fuel injection) limits. While the new EFI is equipped with oxygen sensors that make it self-adjusting, there are limits as to what it can do. It may be highly probable that the increased airflow provided by our modifications exceeded the stock EFI system’s ability to completely compensate.
That will be our next test effort—to see if we can improve performance by tuning the EFI system. The stock engine is set to run very lean and very hot. We should be able to substantially improve performance and cooling by adding a bit more fuel.
We are currently in the process of gathering the different EFI tuning systems and will publish a run-down of these.
After we do more work with EFI tuning, we’ll take a look at cams and displacement. Cam design is critical to the running character of an engine. If a cam is designed for absolute peak-power performance, it will severely and negatively affect low and mid-rpm performance. Most aftermarket cams do just that and they often actually reduce performance where the average street rider wants or needs it most—between 2,000 and 4,000 rpm. Our cam testing will be focused on improving this midrange power, not what happens above 5,000 rpm.
As I stated in the first article, bigger engines can make more midrange power. It is possible to take the Twin Cam 96 to 103 cubic inches with a bore job. We intend to do this. Displacements over 103 inches require a complete engine teardown and case boring. The expense of doing so takes it outside the practical scope of our testing.
Harley has very successfully optimized this new Twin Cam 96 engine to perform its best in the normal rpm range virtually all of us use. Increasing that power will require careful alterations and testing. The more powerful version we are working toward should probably be a bit louder than stock. On the other hand, it will no doubt run cooler—much cooler—and give fuel mileage nearly the same as stock. I hope you will follow our efforts.

Special thanks to Barnett Tool & Engineering for the use of its DynoJet 150 dynamometer. For information on Barnett products call 805-642-9435, or visit www.barnettclutches.com. This article was featured in the June issue of American Rider magazine, available now.

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