Exhaust Systems :

No Techno-babble this time guys. Lets talk power over. What about a few more ft-lbs of torque from your gas burner?

Yes. We will start from the back. …you guessed right. The exhaust system.

No other area of modification will yield as much power-per-rupee as reducing exhaust Restrictions and it's the only performance avenue that does not have some draw back.

So how do we get more power modifying the exhaust system. First get the feel of what an exhaust system does in your car.

Anyone who has breathed through a snorkel will easily get the idea that the optimal exhaust system is none at all; after all, who would want to breathe through a tube? And wouldn't any engine make more power just by removing as much of the exhaust as possible? Surprisingly, the answer is no.

High-performance headers, mandrel bent exhaust piping, high performance mufflers, are all potent horsepower tools in the hands of the seasoned enthusiast or tuner.

Before I get you really excited and have you charge out to build you a header, hang on and get some basics right.

The ambient pressure into which an engine breathes is referred to in an exhaust system as backpressure. It has an important effect on engine efficiency in two ways: Higher backpressure can literally provide greater resistance to the upward movement of a piston on its exhaust stroke. And higher backpressure can result

in more exhaust gases being left in the clearance volume (the space remaining in the combustion chamber when a piston is at Top Dead Center or TDC), leaving less room for the fresh intake charge entering. Subsequently on the intake stroke there will be less oxygen and fuel to be burned which translates to less power. Higher backpressure can also lead to greater combustion chamber heat- possibly increasing detonation, or glowing hot spots and pre-ignition.

Lower backpressure always improves horsepower and fuel economy. The difference between negligible pressure at the exhaust ports and the five or ten or even 15psi of backpressure that is possible with some factory or defective exhaust systems can be a lot of power.

Low backpressure is essentially always good (this is even true on turbo engines). So why might an exhaust system be better than open ports vented directly to the atmosphere? The answer, in part, is that the blast of exhaust gases moving through the exhausts system away from the exhaust ports at speeds of up to several hundred miles per hour have momentum or inertia. Inertia is a law of physics, which says that stopped objects tend to stay stopped, and moving objects tend to keep moving. Once a pulse of exhaust gases has been expelled from the exhaust port and is moving away from the exhaust valve at high speed, it will tend to keep moving, even when the valve closes behind it, creating a transient low pressure area behind the gases in the exhaust port and exhaust header or manifold, creating lower or negative back pressure for better scavenging the next time the exhaust valve opens.

An “open” or an un-silenced exhaust system that you will see on racing cars utilizes measured or “tuned lengths” to harness the negative pressure to scavenge or “pull out” the exhaust gases. If at that point the inlet valve is open, the fresh charge of air and fuel mixture can start flowing in even before the piston has started to make its journey downwards from top of the exhaust stroke! Now lets see where this extraction effect is lost on a silenced engine.

The most easily established loss source is due to excessive back pressure as a result of insufficient flow through the silencer. The other principle reason is that if you have a header, the collector or the exhaust pipe is too long or has an internal diameter, which is too short. On a silenced system the exhaust pipe usually ends up being 3 to 4 times longer than what is optimum. Couple this to a smaller diameter, restrictive silencer and any useful pressure waves that may have existed is damped out. So your exhaust system is more a liability than an asset!

The key to reducing back pressure is to have a silencer with adequate flow.

Unfortunately as far as I am aware silencer makers nowhere in the world give you flow data. They only provide you with pipe dia. What you need to keep in mind is that for every bhp your engine produces you need 2.2 cfm of silencer flow at normal atmospheric pressure. Think of silencer flow in terms of flow capability of the equivalent pipe size. The yardstick to use is 120cfm per square inch of pipe diameter. So if you have a silencer with 330cfm flow through a 2.25” diameter straight through perforated pipe, it will behave as a 2” diameter pipe.

Many of you will be concerned whether the pipe from the manifold to the silencer is too long. Apply rule of thumb. If you have a header fitted, measure the length of the header from the flange to the end of the header. Suppose the length is “L”. The silencer should be placed so that the moth of the silencer is at L-2” from the end of the header. The minimum dia should be 1.75”. Remember the flow equation. Pipe dia has the greatest influence on flow.

Now come to the more difficult part. All cars that you drive come with cast iron manifolds. These are dumb units that rob your car of valuable power. A properly designed header, which is tuned to the cam and the intake system and coupled with a low restriction silencer, can improve volumetric efficiencies to as much as 130%!!! Wow that is equivalent to 4.5psi boost on the intake charge. Unbelievable! Is that what you are thinking? Here is the explanation.

Exhaust systems can make power is by utilizing the transient low pressure zones of reflected shock waves moving backward toward the exhaust ports at the speed of sound (1400ft/sec in a hot exhaust) as each pulse of exhaust exits the system. Since the velocity of exhaust gases changes with rpm, at certain speeds the reflected low-pressure shock waves reach the exhaust ports exactly at the right time to help scavenge gases from the combustion chamber. If inertia and pressure wave effects at the exhaust ports coincide, this "tuning" effect is even more pronounced. If the exhaust tuning rpm range coincides with similar intake system tuning- and the cam timing and lift is similarly optimized for this same rpm range, a "sweet spot" of tremendous power can be built-in to an engine horsepower and torque curves.

Don't, however, think this effect comes free. The key to making the exhaust system work is having pipe lengths that allow appropriate pressure waves arrive at the right moment. This can only occur on a limited rpm range. But it is possible to trade off some pressure wave intensity and broaden the rpm range.

One of the easiest and most convenient ways of broadening the spread of power is to tune each cylinder or a set of cylinders for a slightly different rpm range. This is best for road cars with a band between 2500-7000rpm. Getting the length right is always a pain. There are no ready to apply formulae. It depends on what you want. On a road car a 4-2-1 system works best. They are also referred to as Tri-y headers. They produce a wider power band than 4-1 headers and coupled with a high flow silencer you should be able to get at least a 10-15% increase in power. However if you get it wrong, it will produce no more power than your stock system. I know headers made with wrong lengths that actually produce less than stock power!

This is an example of a Tri-y made for Honda City 1.5

4-1 header and collector set.

Where do you get good headers? The only person I know who makes really good headers is Raj Hingorani in Bangalore. You can contact him at powerwheels@hotmail.com

Read part 3 of this Series where the author takes us into the details of Headers and other Exhaust System Essentials.