not the THROTTLE BODY SPACER!!!!!!!!!!

well htink about this: if it frees up/creates torque then you can use less gas to reach X speed in the same time than it took without the mods.

yes intake and exhaust only really help at higher rpm (also sounds cooler/looks better under the hood :P ) it may help some at low rpm but don't expect a miracle/it to pay for itself...

It's about velocity and momentum, just because there's a wide open space doesn't mean it flows optimally at a given speed.

Think of regular sized cans of coke and mixer/mini sized cans of coke, and line up a table top full end to end, the mini sized are obviously easier to push lengthwise than the full size, they have less mass. Air has mass, we easily forget that, but at the speeds exhaust gases would like to escape the cylinders it matters. Put on pipes that are too big and you are having to push full sized cans of coke, they will get moving slowly and resist piston pressure, and the maximum speed you can move them is reduced because the pressure is spread over twice the area, whereas the mini cans will get moving quickly, and velocity will develop quick, quickly relieving pressure on the piston and evacuating the cylinder more quickly. Of course if you go down to too small a size then the wall friction is the limiting factor as for how fast you can push, that's when you "need" bigger pipes.

However it can be a touchy balance, most cars do benefit from opening up the stock exhaust size a little, like one size, 2-2-1/4 not 2-3" however on the header/manifold, you're usually better off going smaller. The greater volume after the collector is beneficial because the gases will slow down from losing heat energy and from friction, so having space for them to move slower yet still move the same volume per minute is the idea. But at the head, velocity is crucial.

The cost of the performance parts outweigh any money you will save via better mileage. I have a 2005 mustang with a high performance tune, cold air intake, high flow exhaust and 4.10 gears. My mileage in the city has not changed. I have yet to test it on the highway. I know others have gotten increased mileage with under drive pullies, and an alum driveshaft but both of those items cost is over 1000.00.

Well not necessarily if you pick them right... I was paying $65 for a generic muffler, they tended to last 12-18 months before rotting out. So I thought, screw that, I'll go for a semi performance one, looks a bit better quality, so I bought a Walker Sound-FX for $70.... wow, talk about a lot of difference for $5 bucks, uncorked the the motor nicely, began seeing 23mpg high tanks instead of 21 high mpg tanks, and it's lasted 5 years so far. So it's outlasted 3 generic ones at $65 a pop, and I've put about 60K on it, so it's saved around 113 gallons, at only $3 a gallon average, that's still $339... so $70 bucks pays $539, that's a deal right there.

Yes I totally agree. If you can find high performance parts cheap especially ones that reduce rotational mass or friction they will increase mileage. There a number of 500hp supercharged fox bodied mustangs getting 20mpg highway. In a mustang forum I frequent one guy went with a 4.6 to 5.1 stroker kit and boasts 30mpg highway.

That's the reason I got a lightweight flywheel for my car. It was only $165 from eBay. It takes less power to accellerate in lower gears, so I can shift at lower rpm and still accellerate at an acceptable rate in city traffic.

Hmmm, I maybe didn't explain quite so adequately why bigger exhausts from the collector back are more okay than having bigger header primaries. The situation is that in the header, the mass of gas in each tube has to be started from a standstill every second revolution of the engine, whereas after the collector where the flows combine, the flow of gas is more constant, I know you can feel the pulses at idle, but at higher RPM it's closer to a steady stream. Therefore it doesn't matter so much about the volume and mass of gas that has to be stopped and started. To go up the rev range quickly though, there is some limitation imposed by the mass of gas that has to be accelerated, but it's not as bad as starting it from a standstill every time as in the header pipes.