I have Hooker super comps and they are great. I had to "reshape" one of the tubes slightly to work with a roller block and a z bar kit. However, they worked perfectly with my old 289 block, and they have not rotted in 8+ years at all.
Coating is definitely not snake oil, it will slow energy transfer from the exhaust gas to the exhaust material, and thus slow energy loss altogether, resulting in better gas velocity (through less pressure loss from temp decrease). The percentage of difference is around the 10% mark though. I suspect coating the entire exhaust would yeild the best results as coating the headers just make the next uncoated section the biggest heat sink to suck energy / velocity out of the gas.
I've only heard good things from them. I hear they are one of the few headers that clears a 65-66 mustang with a t5 conversion.
Your exhaust will have a slightly higher velocity, therefore slightly more scavenging.
HOWEVER, if your headers are the right size, you have all the scavenging you need. That's why you pick the right tube diameters and lengths if you are serious.
You want the headers to act as a heat sink to get all the benefits they give.
The heat loss is where many of the benefits are vs cast iron manifolds.
If you want heat retention, you might as well port the iron and save money.
Heat loss at the header aids engine cooling, fights detonation, and shrinks the exhaust gasses so you can run smaller and lighter exhuast system behind the headers.
When the gasses come out hot behind the headers, you need to run bigger pipe/mufflers (more expensive) to handle the expanded gasses. This also adds weight which is effective HP loss!
So, if there really is a slight gain from scavenging, you lose it due to overloaded small exhaust, or heavier larger exhaust. Also from timing cut back to prevent detonation if you are running your fuel and/or compression on the edge. (Which it seems you would be thinking about all the dough you spent on coating!)
Edit:
Forgot about the mufflers...
They are meant to be located as far back from the headers as possible. The closer they are to the extreme heat, the shorter life they have. Heat kills most mufflers.
If you coat the headers and the high heat can't get out until later, you kill your mufflers quicker.
There are other things to consider I am forgetting to I am sure.
I have argued this before, but it has been a couple years.
Ratio,
You are forgeting however, that cooler, denser gas flows slower. Higher gas temperater (higher kinetic and potential energy), woud allow for higher exaust flow speeds, if that is what your point is. Also, by keeping the heat inside of the header, you are decreasing engine bay temperature, leading to a cooler inlet temperature (more hp).
Higher exhaust gas temps do cause the velocity to increase.
But the gasses also displace more area because they are expanded.
So, without larger exhaust pipe, you will cause increased flow restriction.
My point is that even though you gain positives in one area, there are 10 others that are all effected negatively.
As for the engine bay... heat in the bay means that it is not in the engine.
It's a good thing.
Hood scoops have always been used to overcome bay heat.
Like I said, smaller tubes assure higher velocity if that is what you need.
either way guys, i have a secondary tube (two singles into one thats a little bigger) thats almost complelty bent in half cause of my pitman arm. i dont think the inner coat of ceremic is gonna affect it THAT much
Don't forget that you're still pushing the same mass through the exhaust pipes, regardless of temperature. The critical gas velocity (=speed of sound) goes up with temperature, so a pipe of a certain diameter isn't necessarily more restrictive to a hot gas than it is to a cold gas, as long as the mass flow in both cases is the same.
I have Hooker super comps and they are great. I had to "reshape" one of the tubes slightly to work with a roller block and a z bar kit. However, they worked perfectly with my old 289 block, and they have not rotted in 8+ years at all.
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