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Need help/advice: coating the headers, taping the exhaust

S

sothpaw

Guest
Everyone,

I wanted to tap in to the considerable
corvette wisdom here:

Why is it that coating the headers to
keep the heat localized within them
IS recommended?

Why is it that the exhaust tape in the
catalogs is not recommended for
headers or exhaust?

--I put in a previous thread about painting
my LT1 mufflers. I got good advice there
to go ahead and paint. I'm going to the hardware
store tonight--I could get the paint and paint
them this weekend.

I am just nervous that
coating the mufflers will keep them a little
hotter and that might cost horsepower--
that is why I thought no exhaust tape on
the mufflers.

Thanks for help/patience!
 
Joined
Jun 20, 2002
Messages
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Location
Tallahassee, FL
Corvette
none right now :(
From a physics standpoint, the cooler the exhaust is, the more horsepower you should have... If you exhaust is cool, the hot air wants to go towards the cool exhaust, so it moves quicker, and less horsepower is robbed from the engine forcing the exhaust out...

localizing the heat is for engine compartment issues, the headers get so hot, they make the engine hotter too... Taping? not sure, I am assuming it can't withstand the extreme heats of the headers? I am not sure what kind of exhaust tape you are talking about

I have actually toyed with the idea of making an exhaust cooling system... wrapping the headers and exhaust with tubing and flowing a coolant through it... adding a couple of small radiators in front and under the car, and having a small electric pump power the whole system.... I haven't done it for two reasons... first of all, I obviously am not the only person in the world that has taken physics... so I assume that it doesn't add enough hp to make a difference big enough to warrant the aditional weight.... the second problem is packaging... what do you use to cool an entire exhaust system?... I assume the coolant would get super hot, you would probably need fans blowing at the system during slow speeds etc... sounds like a big pain in the a@@ to me.

The other idea i had for the cooling system is a dual layer exhaust... for those familiar with race boats with dry exhaust it would be something similar... basically your normal exhaust would be surrounded by a second tube... coolant of some sort would flow between the layers... again, the problem is the extreme heat, and packaging problems...

well, maybe others have the same ideas, or an idea on how to get the system to work? until next time....
 
Joined
Oct 30, 2001
Messages
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Location
Glen Burnie, MD, USA
Corvette
1986 Bright Red Coupe
Ever seen the tubes inside a heat pump? They're wrapped with what looks like thin metal tape with metal spines sticking out of one side to conduct heat away from the condenser. Something like this wrapped around an exhaust should work well, I'd think, though you'd want to either take the spines off the side just under the car or put reflective tape above the exhaust pipes.
[RICHR]
 
B

Bullitt

Guest
The reason most companies advise against the use of exhaust tape, is that the tape traps moisture into it. With the constant heating cycles that the exhaust would face with this tape and water present, deteriotation would occur. That's why most companies will not warranty their headers if exhaust tape/wrap is used.

The cooling fins mentioned, are commonly found on air-cooled engines. Take the old Harleys that did not use radiators, but instead fins to keep the engine cool. The higher horsepower numbers and technology forced everyone to liquid-cooled engines. Would these fins work on the exhaust? Possibly, but a few things have to taken into consideration. First are the quality of the welds to the header/exhaust mainfold. Poor welds will draw heat towards them, eventually causing their failure or hot spots. Second, is location. You don't want to have to always remove your exhaust when changing spark plugs do you, if the fins are too large? Third is the cost-to-benefit ratio. Fourth, would be added weight. And lastly, would the fins be as effective being enclosed under a hood?

For these reasons, high temp. coatings are more practical, in my opinion. Pricing may seem a bit high at first, but when you see it drop header temperatures by a few hundred degrees, they are probably worth it.

For my money, if you wanted to add to the cooling affects of coated headers, would be to try and evacuate the hot air from the engine compartment. We've all seen fender and hood louvers. They've been widely used to help exit underhood hot air. Also, the introduction of more fresh air into the hood would also be beneficial. Various ducts with perhaps in-line fans (found through boat retailers) should drastically introduce cooler air. The problem to solve would be making sure the air exits around the car or above the hood, rather than beneath it. You would not want to increase air velocity underneath the car at high speeds, for sure! This would of course hurt aerodynamics and cause lift.

It's a trial and error task for sure, but common sense should prevail over anything too drastic or dire.

--Bullitt
 
S

sscam69

Guest
I disagree.

the cooler the exhaust is, the more horsepower you should have... If you exhaust is cool, the hot air wants to go towards the cool exhaust, so it moves quicker, and less horsepower is robbed from the engine forcing the exhaust out...

I disagree vettelt193.

There are several things that help build horsepower with the exhaust. Cooling it off is not one of them. You want to keep that exhaust hot. Thats part of what the coats are for. The other is keeping engine bay temperatures down.

There are a lot of complex processes going on through the exhaust so somethings seem to contradict themselves in testing but there are a few general rules.

Keeping the exhuast hot helps keep the velocity of the exhaust higher than if it were cooler. Because air has mass and as it is being pushed out of the exhuast it has momentum. The momentum of this gas as it moves through the exhaust kinda acts as if you were sucking through a straw pulling out the exhaust exiting the engine.

There are also pressure differentials the help in the process called "scavenging".

When people talk about "tuning" the exhuast what they are refering to is changing the length of primary header, the collector, and total exhaust length to help with the scavenging by optimizing it.

Get coated headers and save yourself the trouble. Header wrap is a pain and absorbs liquids. The coated headers look better and compliments the engine bay.

Frank
 
Joined
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Messages
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Location
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Corvette
none right now :(
I haven't heard about high exhaust temperatures helping performance until now... the really funny thing is I found as much information on how cool exhaust helps as I did hot exhaust... The cool exhaust is on physics type web sites, hot exhaust on racing web sites... I will go with the hot idea from now on, scientists never seem to know what the heck is going on.... thanks for setting me straight sscam69
 
S

sothpaw

Guest
painting mufflers: heat retention

Yes, thanks for keeping me straight too.
Moisture absorption was what I was thinking of on the tape, I just couldn't remember.

--It sounds like the racing concensus (experience based, the best kind of knowledge) is that if anything, painting the mufflers would increase
performance as the out let exhaust SHOULD be kept hotter?

--I might ask some thermal engineer here, they might get a kick out of this. I wish I remembered Bernoilli's equation for fluid flow.
 
S

sscam69

Guest
I wish I remembered Bernoilli's equation for fluid flow

Thats the thing, Bernoilli's equations do not work for this type of flow. Those equations are based on steady state flow. The exhaust on the other hand is dynamic. The gas is accelerating and decelerating. But the concepts still do apply.

Frank
 
Joined
Jun 20, 2002
Messages
2,509
Location
Tallahassee, FL
Corvette
none right now :(
I feel like I am in school again:( Maybe what I should have been thinking about is a way to heat the exhaust up? That would be interesting....

Ever hear of a peltier? it is a heat pump designed for computer processors... you plug it in, and it has a cold side and a hot side... put the cold side on the processor, the hot on the heat sink... imagine a system like that reversed? the hot side on the exhaust, cold in the engine compartment? is it possible to actually 'pump' the heat from the engine compartment into the exhaust... giving hotter exhaust with a cooler engine?
 
B

Bullitt

Guest
Thats the thing, Bernoulli's equations do not work for this type of flow. Those equations are based on steady state flow. The exhaust on the other hand is dynamic. The gas is accelerating and decelerating. But the concepts still do apply.

The same thing can be said about Bernoulli's application to aerodynamics concerning racecar design.

--Bullitt
 
S

sothpaw

Guest
Bernoulli's equations

Yes, you are right, these are very over-used. Used when they really don't apply. But I remembered that caution, so I was going to read it along with the math itself.

--I was only hoping the math might confirm that hotter mufflers are better.

--Thanks again, I will paint them, I just might not get to it as soon as I'd like.
 

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