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Your Performance Questions

I just bought my first bike.  How can I improve its performance?

The best place to begin a performance upgrade on your Harley is with the exhaust system.  The number one reason I hear for replacement is the desire for that glorious, traditional, deep-throated rumble of a Harley V-Twin.  While having the "right" sound emanating from your bike may be a priority, there are other important things to consider when choosing a system.  To maintain a wide, usable power band, a certain amount of back-pressure in the system is required.  Back-pressure is achieved through a variety of restrictions.  Where these restrictions occur, and to what extent they happen, determines how efficiently an exhaust system will work with any given performance combination.  For the Big Twin, I recommend the Screamin' Eagle baloney cut mufflers.  This product will give your bike the sound everyone looks for, plus the baffling system is capable of covering requirements from the first step of performance through aggressive high performance for the street.  An added bonus -- it is also one of the least expensive exhaust systems on the market.

The next step is the air cleaner.  The stock air cleaner is adequate for your Harley's factory engine combination.  But as you improve your system, the air cleaner can be upgraded easily by installing the Screamin' Eagle air cleaner kit. 

With the addition of these two items (mufflers and air cleaner), your engine's breathing ability will improve to the point that the carburetor will have to be re-jetted to take advantage of all that extra air.  There are several after-market kits such as a Dyna Jet available.  The stock CV carb yields a wide power band, good fuel economy and the ability to adapt automatically to any altitude change you might experience during your rides. With the installation of the Dyna Jet kit, it is capable of out-performing any carburetor on an 80” Big Twin up to 6000 RPM.  This is an economical alternative to expensive after-market carbs. 

Why is the length and diameter of exhaust systems important?

The length and diameter of exhaust system pipes is an important tuning factor.  Basically the longer the pipe, the more torque you obtain.  Short pipes, on the other hand, benefit horsepower.  This is true for normally aspirated motors.  Supercharged motors have their own idiosyncrasies that are much too complicated to cover here.  Having matched lengths of pipe is important if you’re trying to get optimum performance from your motor.  Nearly all the pipes on the market today are designed for aesthetics with no concern as to engine performance, which is why you see different, unequal lengths, etc.  It seems like the consumer thinks that if they "look pretty" and are "the louder they are," they'll buy them. 

Stepped headers (increasing the diameter of the pipe in stages over a certain length) are a good way to get maximum performance.  But to get the most out of this style of exhaust requires that the lengths and diameter be matched to your motor combination, which takes time and study.

Are there advantages to staying with small valves?  What about polishing the ports?

There are many reasons to keep both intake and exhaust ports small.  On the intake side, small ports with rough surfaces are more efficient.  They increase the velocity and enhance the atomization of the air/fuel mixture for a more complete burn during the combustion cycle.  While it may take more labor and skill to make a small port perform at peak efficiency, small ports of the correct shape can equal the CFMs of an inefficiently designed, large, hogged-out port.

On the exhaust side, small ports are also more efficient in increasing velocity.  However, these ports should be polished smooth rather than being left rough, to increase the scavenging flow of exhaust gases.  In addition to flowing more CFMs, polished exhaust ports transfer less heat to the heads which helps prevent engine-damaging detonation.

Is Cross-over necessary?

The crossover on an exhaust system has several benefits.  In a stock configuration (such as the Softtail), the mufflers are extremely restricted.  The crossover allows the gasses from one cylinder to bleed off excess pressure to the other side.  This increases performance by decreasing backpressure.  Also, a crossover helps decrease exhaust noise by having the second muffler share a percentage of the load.  Locating the crossover approximately seven inches from the exhaust valve enhances performance, allowing that seven inches of header pipe to retain good exhaust gas velocity, and reducing the pressure at the crossover.  Another benefit of crossover is when you install freer breathing mufflers  -- they will reduce the intensity of the reversion wave back toward the cylinder.  Crossovers located a long way down the system and close to the mufflers (such as with a Dyna) have very little to do with performance and are there mainly to reduce noise.

Is there a difference between tuning for acceleration performance and tuning for higway cruising?

Yes, tuning for acceleration performance is different than tuning for a "steady state" (maintaining a specific cruising speed) performance.  Acceleration of a motor benefits from being slightly on the rich side as opposed to what is needed in the steady state.  Of course, when running rich, you reduce your mileage capabilities.  When jetting on a dyno, you need to stop going richer when significant gains are no longer being made.  For example, on one of my bikes (an 80' EVO producing 92 HP for the street), we stopped making significant horsepower gains with the main jet at 190.  I richened it up to a 210, but only gained about two HP.  This 2 HP gain didn’t influence the over-all power band and wasn't significant enough to justify the reduced mileage the 210 produced, so I went back to the 190.

What is "overlap?"

With stock or extremely mild cams, the exhaust valve is closed (or nearly so) when the intake valve begins to draw in the new air/fuel charge.  The norm for performance cams is that while the exhaust is closing, the intake is opening.  The period of time when both intake and exhaust are open is called "overlap."  Depending on the RPM range you are running with an unrestricted exhaust systems, the intake charge can be pulled across the top of the piston and out the other side by the velocity of the exiting exhaust gasses.  This greatly increases emissions, hurts mileage, and (if there are any air leaks in the exhaust) can create a secondary burn in the exhaust system.  Also, in a non-restrictive exhaust system, you have and event that can occur called reversion.  At lower RPMs, the velocity of the exhaust can create a negative pressure in the cylinder and a reverse pulse wave returns the exhaust gasses back into the cylinder, polluting the new intake charge.  In extreme cases, I have seen where this reversion was so strong and the period of time both valves were open was so lengthy, that the exhaust blew the incoming charge clear out of the cylinder, back through the intake and carb to the point where fuel was dripping off the air filter profusely. 

By increasing backpressure, you greatly reduce the power of any reversion wave.  Be aware, though, that even with stock cams having no overlap, reversion can still re-enter the cylinder and pollute the incoming charge.

I have a fuel injected ’01 Electra Glide with a Stage One and Samson fish tales. I usually ride solo at freeway speeds. I’m thinking about Andrews TW21 or TW26 gear drives and Samson True Duals. Without going to a 1550 kit, what are my other options?

Both those cam choices are good for a heavily loaded bike and two-up riding. For your requirements, I think you should consider Andrews 37G or the Screamin' Eagle 203.  They both deliver wide power bands with a top-end punch for passing situations the other two could not deliver.

When you're trying to achieve higher performance levels, the exhaust system is a very important component.  Drag pipes, fish tails, and shark fins are not performance-oriented systems, and can even significantly subtract from what you're trying to do.  I recommend that you stay with the stock header systems and get either Cycle Shack slip-ons or the Screamin' Eagle one-piece mufflers.  These are reasonably priced, and will yield good performance levels throughout your entire RPM range.

Drop John a line at jhfxr@aol.com.


© 2004 Hildreth Performance

Last Updated on 06/14/2004