I picked up a HotRod magazine the other day and found there

I picked up a “Hot-Rod” magazine the other day and found there are a number of advertisements for amazing parts I can buy for my car that dramatically improve its performance. Below are a few and some of the performance claims they made. For each item, indicate whether the claim is likely to be true, and under what conditions it might be true. I want a fairly careful explanation of of why, including calculations if necessary. Assume my car has a 2L I4, NA engine, with peak power at 5000 rpm of 132 hp, and peak torque at 3500 rpm of 122 ft-lbf.

A new exhaust system for my car (post-catalyst) that increases the exhaust pipe diameter from 2” to 2 1/2”; the claim is that this will provide a 10 hp gain;

A conical foam air filter to replace the paper air filter on my car. The claim is that this will increase the power of my engine by 7 hp.

A special spark-plug with a split ground strap; the claim is 3-5 mpg improvement in fuel mileage.

Solution

1) No exhaust system is ideal for all applications. At different RPMs, different sized exhaust tubes are more desirable than others. For instance, at low RPMs you would want a small diameter exhaust pipe to give some backpressure and increase low rpm torque, while at high RPMs you would want a large diameter exhaust pipe to be able to let out the exhaust gases quickly and easily, but keeping very little amount of backpressure. Depending on their design and purpose, all exhaust systems compromise something to achieve something else. A very common mistake made by some performance people is the selection of an exhaust system with pipes that are too large in diameter for their engine\'s state of tune. Bigger is not necessarily better and is often worse.

One of the biggest issues with exhaust systems is the relationship between gas flow volume and gas flow velocity (which also applies to the intake track). An engine needs the highest flow velocity possible for quick throttle response and torque throughout the low-to-mid range portion of the power band. The same engine also needs the highest flow volume possible throughout the mid-to-high range portion of the power band for maximum performance. This is where a fundamental conflict arises. For \"X\" amount of exhaust pressure at an exhaust valve, a smaller diameter exhaust pipe will provide higher flow velocity than a larger diameter pipe. Unfortunately, the laws of physics will not allow that same small diameter pipe to flow sufficient volume to realize maximum possible power at higher RPM. If we install a larger diameter pipe, we will have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle response and torque will suffer. This is the primary paradox of exhaust flow dynamics and the solution is usually a design compromise that produces an acceptable amount of throttle response, torque and horsepower across the entire power band.

2) The construction of foam dry filters is identical to paper filters, only instead of a folded paper membrane, they use a foam material to filter the air. Their porous surfaces mean foam filters allow more air to come through. Another advantage is that foam filters can be cleaned. Simply knocking them or washing them can remove the majority of deposits, allowing you to reuse the filter.

The disadvantage is: foam filters are less effective than fine, porous paper, and allow more particles to enter the engine. To get the same effect as with a paper filter, the foam needs to be very thick. Deeply deposited dirt can also be difficult to remove completely, meaning that foam dry filters will also need to be exchanged completely after a certain time.

A Foam air Filter can actually boost engine performance by redirecting more air flow rate to the engine so this claim may well be true.

3) No. Electricity follows the path of least resistance. It has nothing to do with a split tip or a fancy U groove The sparkplug does one thing only. It acts as the grounding source for the ignition\'s electricity and that\'s it, period, end of story. The only difference with a sparkplug is that there is a gap in the current path which makes the spark. The size of the spark is dictated by the voltage and amperage output of your coil, and the output of your coil is dictated by the saturation (dwell) time of your ignition system. You\'ll hear claims like, \"it made more horsepower\" or \"I got better gas mileage\". Well of course they did! Their engine needed a tune-up or they wouldn\'t have been changing the plugs in the first place! ANY engine would get better gas mileage and make more horsepower after a tune-up! What split tipped plugs can do is promote pre-ignition and/or detonation, especially in a high performance engine.