I don't believe we've heard the last of NHRA on the muffler
rule. 1998 noise limit regulations only apply to three NHRA classes:
Super Comp, Super Gas and Super Street (when competing at national
events). How long it will be until these regulations extend to
other classes is anybody's guess! To monitor the noise levels,
NHRA will position sound meters 50 feet from the racing surface
in two locations. Maximum noise levels have been established for
the starting line and the 60 foot mark. If the car fails these
tests, sound levels will be checked with the car parked in neutral
and the engine running at 3500 RPM. Personally, I will miss the
unmuffled roar of a highly modified race engine, but I think we
will be faced with similar regulations for other classes sooner
than we would like! Considering this, let's see what kind of impact
this will have on the racer.
Advantages of a properly designed exhaust system include: horsepower and torque gains at low and midrange RPM, better awareness of the cars operation (noises etc.) and less driver fatigue.
Disadvantages of an exhaust system include: small horsepower losses at high RPM and additional cost. Talking with Andy Scheu from Unlimited Performance, a properly designed system will cost approx. $800 (final cost may be higher or lower depending on the car and options selected).
Whatever goes in, must get out! - Let's take a minute to consider what it takes to empty and fill a cylinder during one cycle of operation (4-stroke engine). After ignition takes place, the burning, expanding gasses push the piston down. At the end of the power stroke the exhaust valve opens and exhaust gasses escape under pressure. After the piston passes bottom dead center, it moves upward forcing exhaust gasses out of the open valve. Near top dead center, the intake valve opens - this is the beginning of the valve overlap period. Both the intake and exhaust valves are open at this point allowing fresh air to begin entering the cylinder as the last of the exhaust gasses exit. Shortly after top dead center, the exhaust valve closes and the intake stroke begins. As the piston moves down it creates a low pressure (vacuum) in the cylinder and intake manifold. Above the throttle plate, air is subject to atmospheric pressure (14.7 psia @ sea level). This difference is pressure causes air to flow into the cylinder during the intake stroke.
Exhaust system design - if the exhaust system is properly designed, the velocity of the escaping exhaust gasses (during valve overlap) will have five times as much effect in creating airflow into the cylinder, than the piston going down on the intake stroke!
Too big, or too small - proper design of an exhaust system starts with primary header tube diameter. Small primary tubes work well at low RPM, but can create restriction and impede the flow of exhaust gasses at high RPM. Large primary tubes work well at high RPM, but can decrease the velocity of the exhaust gasses at low RPM. Both situations can cause a power loss. For an example of this, compare two different headers for the same engine - one meant for towing or RV applications and the other for racing. Choose your headers wisely! Exact selection of primary tube size should be based on exhaust port flow at maximum valve lift. The required diameter of the exhaust tubing after the collectors can be calculated using a formula or without this info, using tubing sized equal to, or within ½% of the collector diameter will produce satisfactory results. A balance tube is also recommended.
Mufflers - to do their job, mufflers produce restriction. If sized properly they will reduce exhaust noise and still allow the engine to produce within 1% of maximum horsepower. I recently read an article that suggested mufflers should be purchased by their flow rating. Tests showed that using a figure of 2.2 cfm per horsepower will allow you to calculate your engines exhaust flow requirements. Example: an engine develops 650 hp with open headers. 2.2 X 650 = 1430 cfm total. Divide 1430 by two, each muffler should be capable of flowing 715 cfm minimum. Don't cheap out on the mufflers, this is where power can easily be lost. Happy hunting!