MolonLabe

Ok, I'll bite. Grab a cold one out of the fridge, lets chat. I'm not an automotive engineer but I did stay at a Holiday Inn last night.

Actually, a bigger tube does NOT necessarily allow exhaust to flow faster. In fact, bigger exhaust ports in the cylinder head do not necessarily allow exhaust to flow faster either. You are correct that exhaust volume is what you are after, but you are making a common mistake in forgetting that exhaust velocity creates most of the volume. Velocity is paramount. So, why wouldn't bigger ports/headers allow for more velocity? Two reasons that are actually one reason-- turbulence and exhaust pulses. To really understand turbulence and exhaust pulses it really helps to have a PhD in fluid dynamics, which I don't. But I understand this much-- at a certain point the four exhaust tubes of the header turn into one tube at the header collector, and the length of those four tubes is carefully engineered so that the puff of air from each port follows the one before it and is in front of the one behind it. All these puffs of air, exhaust "pulses", create an effect called scavenging, where the effect of all these pulses in a row actually sucks out spent gases on the exhaust stroke of each cylinder, because all the pulses are heading out the exhaust at high velocity and they suck along the next pulse in line, kind of like revese supercharging. H-pipes and X-pipes were invented for the sole purpose of helping that scavenging effect. This is also why equal-length headers are superior to unequal-length headers.

But anyway, if your exhaust pulses bump into each other instead of following each other out the exhaust, the scavenging effect is lost and you have lost exhaust velocity. This can happen if your exhaust is too restrictive, like if you put a Geo Metro muffler on your 5.0 Ford engine. But it can also happen if your exhaust is too big, like if you put an exhaust that is meant for a supercharged 426 Hemi on your 5.0. This is because when the port/exhaust tube is too big, the amount of air moved by the cylinder isn't great enough to create a nice puff of air, a nice pulse, that will rocket its way out of the exhaust at high velocity. Instead it will swirl around inside the exhaust tube, trying to fill up all that tube, and when it expands and fills up the tube, it loses velocity and creates turbulence, eliminating scavenging and filling up your exhaust port with already burned gases. All you did with your huge exhaust is create the world's most expensive and overbuilt EGR system.

To take advantage of a bigger exhaust system requires a much bigger puff of air, which will fill up the bigger exhaust tubes without losing velocity. For small displacement engines the only way to creates that bigger exhaust pulse is with RPM. You will see big exhaust on drag racing engines of 302ci displacement, because they are set up for high rpm horsepower and their powerband doesn't have to make as much power down low. Unless you have 4.56 gears and plan on shifting at 7,500 rpm, this doesn't help the powerband you want for a street car. If you put the big exhaust on a street car 5.0 you will lose torque in the low rpm range and you'll hit your redline before your engine has started puffing enough air to get good exhaust scavenging. You've given up torque down low and gotten no benefit out of it in the high rpm range. Your engine will look and sound absolutely badass-- and you will run high 14s at the track.

If you are building a street car, midrange torque is paramount. Midrange torque is everything. Build an exhaust that maximizes torque by producing good scavenging and good exhaust velocity through a wide powerband, and your biggest problem will be keeping your tires from going up in smoke when you try to launch!

SilentNoise

I've known a few people to have bad experiences with Mac headers and the finish peeling off - that was a few years ago , and maybe they've changed a few things since then, but whatever.

Holley's Hooker headers come in 1 3/4" longtubes, but I agree, for a mostly stock motor - hell even if you've swapped heads, I wouldn't worry about much more than installing 1 5/8" headers on a 5.0L. I've never seen anyone need to go that big.... even on a blown 5.0L

sick95gt

My MAC's warped in about a year and I drove my car about twice a week. They were for the gt40p's. I don't know if anyone else had the same experience, but I didn't even drive my car a lot or very hard and they warped quick.

Quik

they are a cheap product with thin flanges