V-twin Tech Elements of Power Part One: Modifications for the 2007 96-inch Big Twin Engine
April 9, 2007
Filed under Uncategorized
Welcome to the first in a series of articles aimed at giving you a useful selection of modifications for the new 96-inch Big Twin engine. This is also intended to varify the points made in the previous issue’s story, Torque—and how to get some.
These modifications should give you useful information that will create practical and efficient changes on your bike. Throughout each step, we’ll retain stock reliability and real-world driveability at the least cost and the least amount of trouble—emphasizing increased performance throughout the normal rpm range that most riders use on the street.
For this first article, we tested the stock engine and then the most common bolt-on modifications, an air cleaner and mufflers. A majority of new Harley owners quickly switch the stock mufflers and air cleaners for aftermarket replacements. With a variety of reasons for doing this, the prospect of improved performance is nearly always one of them.
I have dyno tested every new air-cooled Harley engine since the iron Sportster and have found about 5 horsepower lurking beneath restrictive stock air cleaners and mufflers. There were also consistent gains in midrange power and rideability. The engines simply and clearly ran better after air-cleaner and muffler transplants, complimented by small carburetion corrections.
Well, here we were with another new version of the Big Twin Harley motor and a bunch of letters from readers asking us how to get more power from it. American Rider editor Buzzelli and I agreed that we should begin with baseline testing of the stock engine and then bolt on a set of accessory mufflers and a more open air cleaner. We chose a K&N filter assembly and a set of Harley-Davidson Screamin’ Eagle slip-on mufflers for this initial work. The K&N air cleaner is a good representative of the many high-flow units available today; and the SE mufflers are street-legal and typical of what many riders are looking for.
So with expectations and parts in hand, Ken Freund and I traveled to County Line Cycles and met with owner Arman Gevorkian. Arman allowed us the use of his Dynojet 150, which is an excellent tool for the testing we had in mind.
I first established the stock engine’s baseline performance. Four runs were enough to show that our test engine was consistent and that we could proceed. It was also making more torque and power than any stock Big Twin I have tested.
Here is a simple list of the performances of stock Big Twin engines I have tested over the past couple of decades:
n Evo 80 inch (1984): Torque—65, Horsepower—57
n Twin Cam 88 (1999): Torque—71, Horsepower—59
n Twin Cam 96 (2006): Torque—86, Horsepower—68
As you can see, peak power has risen with time and displacement. Compared with the original Evo Big Twin, the Twin Cam 96 has gained about 20 percent more horsepower. The displacement difference is also 20 percent.
Torque has increased even more; the gain is a generous 32 percent. Given the increasingly stringent emission limits that The Motor Company works under, this is an impressive accomplishment.
On With the Tests
After establishing our baseline, we started changing parts and testing the resulting effects on our engine’s performance. We tested four configurations:
n First, the stock motorcycle
n Second, stock plus K&N air-filter kit
n Third, stock, the K&N and a set of Screamin’ Eagle mufflers
n Finally, stock air cleaner and the Screamin’ Eagle mufflers
You can examine our test results on our chart. Those results are mixed and may surprise you. For instance, the K&N filter’s low restriction allows the engine to develop significantly more peak horsepower; however, it reduced power between 1,800 and 3,500 rpm—not by much, but it was a loss just the same.
The Screamin’ Eagle mufflers had little influence on peak power because they are legally quiet and therefore somewhat restrictive. However, they increased torque and power in the very useful 1,700 to 3,500 range.
The combination of K&N filter kit and Screamin’ Eagle mufflers delivered a solid 5 horsepower gain near redline. However, that power is almost useless since very few of us turn our engines that high. Also, the K&N’s power loss in the mid-range was still present.
Bottom Line (for Now)
If you’re looking for increased mid-range power, don’t bother with either change. The small gains we recorded would not be noticeable except in a high-rpm drag race and then only barely.
The Screamin’ Eagle mufflers combined with the stock air cleaner was the most effective practical modification. The mufflers delivered the greatest power gain in the rpm range we actually use, such as when we pass a truck, climb a grade, or get up to speed entering a freeway or toll road. Still, the gain was probably too small to justify the cost.
It is likely that the K&N stock replacement filter element (Part No. HD-1499) would have given better results than the large version we had, the reason for this being that Harley has carefully tuned the resonance of the stock air box and we lost that tuning when we removed it. The K&N replacement filter fits inside the stock air box and should not affect the resonance of the box.
I’m sure we’ll find more power with future testing. However, as always, there must be compromises—the engine will be a bit louder, and it may become more sensitive to gasoline quality. Compromises in reliability will be minimal because it just doesn’t matter how fast you go if you don’t get there.
Rideability is, for many of us, at least as important as torque and horsepower. As we go forward on this quest of ours, let’s keep in mind that these things should be fun and easy to ride.