Yup. Anything that restricts airflow into or out of an engine limits the power. No exhaust manifold or pipe, no cat converter, no muffler, would be the best for maximum horsepower. Ironically though exhaust manifolds and the exhaust pipe can help improve part-throttle or mid-RPM power. That's the intent of "header" exhaust pipes. Imagine a slug of exhaust air just getting shoved past the exhaust valve, and into the exhaust manifold, by the piston. That wad of air molecules now has momentum and will thus naturally continue moving through the exhaust manifold. Once the exhaust valve closes, the tube between the valve and that moving slug of air will develop a vacuum. When that slug of air passes a junction in the manifold - i.e. where one cylinder's exhaust tube connects to another cylinder's tube (that joint is called a "collector" typically) the vacuum will act on the second pipe. Now if the exhaust valve of that second cylinder just happens to be opening as that vacuum builds up the vacuum will help draw out the exhaust from that cylinder - reducing the "shove" needed by the piston... ergo reducing the horsepower/torque lost pushing that second cylinder's exhaust out. It's a type of resonance: getting the exhaust flow rates to line up with the length of the header tubes so that they aid each other. This occurs over a narrow RPM range normally. By varying the length & cross-sectional-diameter of the exhaust tube between the engine and collector designers can adjust this resonant RPM. Long tubes resonate at lower engine RPMs, possibly aiding towing or normal driving speeds; short tubes resonate at higher RPMs aiding top-end power. Cars with "inexpensive" exhaust manifolds (like the "log" style cast iron things used in many factory designs because they're a) cheap and b) easy to fit into limited under-hood space) don't get this resonance benefit at all... such log headers are nothing but minor, or sometimes major, resistance to the exhaust airflow and as such limit maximum horsepower.
Intake manifolds can be "tuned" as well though they act on the airflow differently. Instead of trying to have one slug of air help draw out another slug of air, intake manifold lengths are tuned to match the timing of the intake valve opening and closing. When the intake valve is open, air is drawn into the cylinder as the piston moves downward... when the valve closes, the air in the intake manifold runner (tube) has momentum and wants to keep moving towards the cylinder... except the closed valve is in the way. The air bounces off the valve and starts running backwards in the manifold, towards the big plenum chamber. It can bounce off the plenum (other air in the plenum actually) or off the throttle plate and rebound yet again towards the intake valve... if the length of the intake runner is correct, this rebound air mass will reach the intake valve just as it opens. Many modern cars use butterfly valves or other valves to direct the intake manifold airflow through short or long tubes in the manifold - more complex designs with dual air paths so they can have two resonant RPMs. This really helps fatten the torque vs. RPM characteristic of an engine, making it more versatile and more "flexible" from a driver point of view. Need to accelerate? Hit the gas at any RPM and you get good torque response anywhere in the RPM range; you don't have to first shift into the "power band" to get the engine to wake up.
Exhaust headers and variable length intake manifolds are common on today's street engines where response at any RPM is desirable. They're less useful on race car engines (especially multiple intake manifold lengths, exhaust headers are still common) where everything is tuned for max RPM power - all this extra complexity generally isn't worth it as everything is geared for minimum airflow losses into the engine. If race rules allow it, individual exhaust pipes are used (e.g. drag racing) else complex headers are used (NASCAR, IndyCar, etc) since exhaust systems must be present anyway. Might as well design one that helps at race RPMs.
Think about some of the "performance parts" add-ons over the years: bigger carbs, high-lift or long-duration camshafts, exhaust headers. Bigger carbs are pretty easy to understand: more cubic feet per minute (CFM) capacity = more ultimate high-RPM power... but too big means the idle will suck and part-throttle engine performance may not be great. Especially from an emissions point of view. High lift camshafts, or long duration camshafts, add to high RPM horsepower by letting more air into the cylinders... again though at the expense of low RPM or idle characteristics. Many "hot rod" motors (or motors for high performance boats) were hard to start, had a lumpy/shaky idle, etc. because the camshaft lobes were set for high lift/long duration to let as much as as possible into the engine at high RPMs. At low RPMs though the airflow rate into the cylinders ended up being too low to promote swirl/mixing inside the cylinder leading to uneven combustion. Often long duration camshafts actually had the intake valve start opening while the exhaust valve was still open; this is called overlap. Most engines have some overlap actually; it gives the intake airflow a chance to get started moving. Too much overlap though lets the air+fuel mix pass through the intake valve and right out the exhaust valve --> unburned fuel out the tailpipe? That's about the worst possible for emissions! Modern car engines use variable valve timing - a mechanism that functionally changes the camshaft to engine crankshaft timing (imagine moving the chain one link) so that there is little overlap at low RPMs (for good idle and good emissions) but more overlap at high RPMs (to help the breathing). Variable valve timing is yet another way to expand the usable torque vs. RPM characteristic of an engine; race engines are optimized for a narrow RPM range and thus rarely have variable valve timing. (okay, Touring Car race cars, or other race types based on production cars may have VVT since the production car had VVT... but I don't know of any "pure" race engines that use VVT)
Anything that restricts the intake or exhaust airflow not only limits maximum potential horsepower but it also reduces the engine efficiency at all RPMs. The piston has to shove the burnt air out, through the exhaust valve and through the exhaust piping... that means the piston doing the pushing is extracting power from the crankshaft. On the intake stroke, the down-going piston generates a vacuum in the intake tract; this vacuum tries to pull the piston back up. The crankshaft has to work to pull the piston down. That work is energy that isn't making it to the wheels.
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Intake manifolds can be "tuned" as well though they act on the airflow differently. Instead of trying to have one slug of air help draw out another slug of air, intake manifold lengths are tuned to match the timing of the intake valve opening and closing. When the intake valve is open, air is drawn into the cylinder as the piston moves downward... when the valve closes, the air in the intake manifold runner (tube) has momentum and wants to keep moving towards the cylinder... except the closed valve is in the way. The air bounces off the valve and starts running backwards in the manifold, towards the big plenum chamber. It can bounce off the plenum (other air in the plenum actually) or off the throttle plate and rebound yet again towards the intake valve... if the length of the intake runner is correct, this rebound air mass will reach the intake valve just as it opens. Many modern cars use butterfly valves or other valves to direct the intake manifold airflow through short or long tubes in the manifold - more complex designs with dual air paths so they can have two resonant RPMs. This really helps fatten the torque vs. RPM characteristic of an engine, making it more versatile and more "flexible" from a driver point of view. Need to accelerate? Hit the gas at any RPM and you get good torque response anywhere in the RPM range; you don't have to first shift into the "power band" to get the engine to wake up.
Exhaust headers and variable length intake manifolds are common on today's street engines where response at any RPM is desirable. They're less useful on race car engines (especially multiple intake manifold lengths, exhaust headers are still common) where everything is tuned for max RPM power - all this extra complexity generally isn't worth it as everything is geared for minimum airflow losses into the engine. If race rules allow it, individual exhaust pipes are used (e.g. drag racing) else complex headers are used (NASCAR, IndyCar, etc) since exhaust systems must be present anyway. Might as well design one that helps at race RPMs.
Think about some of the "performance parts" add-ons over the years: bigger carbs, high-lift or long-duration camshafts, exhaust headers. Bigger carbs are pretty easy to understand: more cubic feet per minute (CFM) capacity = more ultimate high-RPM power... but too big means the idle will suck and part-throttle engine performance may not be great. Especially from an emissions point of view. High lift camshafts, or long duration camshafts, add to high RPM horsepower by letting more air into the cylinders... again though at the expense of low RPM or idle characteristics. Many "hot rod" motors (or motors for high performance boats) were hard to start, had a lumpy/shaky idle, etc. because the camshaft lobes were set for high lift/long duration to let as much as as possible into the engine at high RPMs. At low RPMs though the airflow rate into the cylinders ended up being too low to promote swirl/mixing inside the cylinder leading to uneven combustion. Often long duration camshafts actually had the intake valve start opening while the exhaust valve was still open; this is called overlap. Most engines have some overlap actually; it gives the intake airflow a chance to get started moving. Too much overlap though lets the air+fuel mix pass through the intake valve and right out the exhaust valve --> unburned fuel out the tailpipe? That's about the worst possible for emissions! Modern car engines use variable valve timing - a mechanism that functionally changes the camshaft to engine crankshaft timing (imagine moving the chain one link) so that there is little overlap at low RPMs (for good idle and good emissions) but more overlap at high RPMs (to help the breathing). Variable valve timing is yet another way to expand the usable torque vs. RPM characteristic of an engine; race engines are optimized for a narrow RPM range and thus rarely have variable valve timing. (okay, Touring Car race cars, or other race types based on production cars may have VVT since the production car had VVT... but I don't know of any "pure" race engines that use VVT)
Anything that restricts the intake or exhaust airflow not only limits maximum potential horsepower but it also reduces the engine efficiency at all RPMs. The piston has to shove the burnt air out, through the exhaust valve and through the exhaust piping... that means the piston doing the pushing is extracting power from the crankshaft. On the intake stroke, the down-going piston generates a vacuum in the intake tract; this vacuum tries to pull the piston back up. The crankshaft has to work to pull the piston down. That work is energy that isn't making it to the wheels.
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