Exhaust/Headder Tech

[MIGHTYRAM97]

exhaust system/headder tech...courtesy of www.centuryperformance.com






Why Header Wraps Suck ... and Why Header Coatings Are Recommended


► The Facts
► Background Info
► Test Results
► Headers Oxidize
► Step Headers
► Basic Knowledge
► Closing Thoughts



Coating Companies:

Sanderson Headers
Airborn Coatings
Jet-Hot Coatings
HPC Coatings
Back To:
TECH SECTION



Special Thanks to Jet-Hot® coatings for some of the charts and information listed here.




Many times I've been asked about, or have commented on the use of header wraps. This issue is a real pet peeve of mine. Good or bad about a product I'll give my opinion based upon direct use and fact.

THE FACTS:

Header wraps are designed to keep the heat in the header to improve scavenging of the cylinders. Keeping the heat in the header allows the exhaust speed to remain high. (the right idea)
There are no header manufacturers that I know of that will warranty their headers if any header wraps are installed on their products.
In most cases the header wrap damages the headers beyond repair. (I will explain below)
If you run a lean mixture, you "may" see a slim performance gain using header wraps. A rich mixture may show slim to absolutely NO gain in performance.
If you do not mind replacing your headers and header gaskets regularly, and you like that ugly look of a wrapped header, go ahead and use the heat wrap.
BACKGROUND INFORMATION:

In the past, almost all NASCAR and other racing engine builders used the header wraps for the added power gains. But, after having to replace the headers after each race due to the wrap being about the only thing holding the header together, they do not promote the practice any longer! They now utilize the thermal coatings that are chemically and electrically applied to the headers. Those include Airborn, Jet Hot, HPC, and others.

Imagine having to replace a $1200.00 plus set of headers after each race weekend! Few but the most financially well-off race teams can afford to do this. But, it is also in the downtime for remaking a custom set of headers. Most custom header makers do not have copies readily available.

I believe that the wraps are good to protect various items from heat, but not to hold the heat in the header. For example: you can use the wrapping for the protection of fuel and oil lines, wiring, etc.

Cool air needs to be around the header, and insulating it with a wrap to hold exhaust heat in makes the header material temperatures near molten. When you wrap the header you trap the heat in the header, but also in the material that needs to breathe to dissipate heat for it's own survival.

Engineers, Metallurgists, and other experts out there will state that there is no way that the material can fail because it can withstand, and it was designed to withstand, the internal temperatures of exhaust gases. TRUE! But, when the header is not allowed to cool so as to dissipate those extreme temperatures that the wrap is controlling, you have now developed a heat absorption that compares to thermal friction which will will continue to gain in temperature beyond the normal exhaust gas temperatures (EGT's). This is the same as with most any insulation.

Try this experiment ... launder a load of bath towels and then dry them. Immediately pull them out of the dryer and just toss them in a snug pile on your bed. Now leave them there for a day and then open them. You will find that there is still a considerable amount of heat left in the center towels. This heat, even though the outer towels and bed are normal room temperature have been able to contain their heat. This is a simple thermal insulation test, but with your headers you have an internal heat supply coming from the engine. The heat on the outside portion of the header material is trapped between the warp and the header and will continue to fatigue the header. This build of heat is amplified by the wrap. Towels do not need to breathe, header material does.

The EGTs stay the same but the properties of the header material changes in a way of amplifying the temperatures because of the insulation. This action goes against normal laws of thermal dynamics, but this effect is fact, and you have to pull the ears off most engineers before they believe you. This is the trouble with plenty of education, but NO "common sense"

Here are a few temperature readings taken at Daytona Motor Speedway.

Below are the test parameters and results using Jet Hot® coated and uncoated headers:

(10 Laps, same engine and car with identical headers, one standard, one Jet Hot® coated. Engine is ran between 6,900 and 7,500 RPM and temperatures are measured immediately after the last lap with the engine idling at 2,000 RPM with identical sustained EGT's of 850º.)

MEASURED AT:
Jet Hot Coated No Coating DIFFERENCE
1" from engine port (on header) 300º 750º 450º
2" above header 210º 300º 90º
1" above floor pan (in car) 115º 165º 50º

Pretty impressive difference, and any of these coatings do not damage the headers, the coating becomes part of the header. Most of the other coating brands are comparable to these figures.


Something else that few Racers and Car Owners realize:
Headers Oxidize!

Under normal use and even more with higher EGTs and header surface temperatures, headers will oxidize and material is actually removed from the headers. This means they get lighter and weaker.

Try these actual test numbers:
Mild Steel (1010) uncoated header exposed to 1200º F. in normal air will have a weight loss percentage of roughly 25% with only 10 hours use at this temperature.

Stainless Steel (410) uncoated header will have roughly 8% weight loss in the same 10 hour period.

A coated mild steel header will have NO weight loss at temperatures up to 1200º F. In fact it will actually gain a bit of weight! Between 1300º F. and 1600º F. the coating will begin to show signs of mud cracking or like the look of lacquer checking. However, limited diffusion takes place between the coating and the substrate, producing a very thin film of iron aluminide, which continues to inhibit oxidation.

Now, use the info above and then add the heat stress generated by the wraps. What do you see? Remember that the wrapped metal cannot cool properly and the header wrap is causing the material to super-heat and pre-mature failure.

[MIGHTYRAM97]

On A Side Note:

Do you understand the physics of exhaust flow in regards to step-headers?

Step headers are designed to allow controlled and efficient exhaust gas expansion at a selected engine RPM. What does this mean? As exhaust gases are exiting the engine into the header they are still burning and rapidly expanding down the primary tubes. If you design the header to take advantage of the pulse expansions of the exhaust gases you WILL gain horsepower. But, you'd better do it right, or that $1200.00 or more for a set of headers is a waste of money. When you set up a step header, you'd better know where you want the maximum power at, since steps are RPM specific to each engine combination.

I'd better say this now ... a step is an increase in tube diameter.

Here is an example:

A basic Chevrolet 350 cubic inch engine ... 3.48 stroke, 5.7" connecting rod length, 4.0" bore, 10.5:1 compression ratio, standard 2.02 / 1.60 valves ... designed for 6500 RPM operation. Cam specs are: .500" lift, 235º duration @ .050" lift, 112º lobe centerline.

For the average Joe Bracket Racer this would be a standard 1-5/8" or 1-3/4" primary header primary tube size, depending on vehicle weight and where you want your power. Here is the "exact" header size for a racing engine operating at 6500 RPM. Note: This is for max power at 6500 RPM!

HEADER TYPE PRIMARY TUBE DIAMETER PRIMARY STEP LENGTH SECOND STEP DIAMETER SECOND STEP LENGTH THIRD STEP DIAMETER THIRD STEP LENGTH
Straight pipe 1.757" 31.9" - 34.4"
2-Step header 1.757 " 14.0" - 16.5" 1.882" 15.4" - 17.9"
3-Step header 1.632" 14.0" - 16.5" 1.757" 6.8" - 9.3" 1.882" 6.1" - 8.6"

Collector Specs:

3.221" Diameter with a 18-23" length (2nd best = 9-11.5" length)
Acoustical Tuning Specs:

best wave = (- negative pressure) = 5,000 - 6,000 RPMs
bad wave = (+ positive pressure) = 2,018 - 3,518 RPMs
wav-lag degrees = 3.2º - 6.3º
This is a "Optimized" header design for an engine operating primarily at 6500 peak horsepower operation.

The reason for adding the above step-header specs was to to let you know that the header has areas that the expansive exhaust gases compress as they pass through the header. Or should I say the gases are not allowed to expand. As we all know, compression of air, or in this case exhaust gases, you will generate heat. Anywhere there is not the proper step you will get added heat.

Of course there is no "Perfect" header design unless the engine was to never change RPM. With an engine that has operating use from idle to well over 6,000, 8,000 or even 10,000 RPM, there is no specific header that is best for an entire power band. You set up the step header for the RPM that the engine will spend the most operating time.

You can call this compression of exhaust gases (or restriction) that of thermal dynamic friction. You cannot have true friction from a gas against a solid, but you can have this compressed gas as a source of added exhaust heat.


BASIC UNDERSTANDING:

Because of the expanding gases, if you have had the experience (as I have) to see first hand where a header with a wrap fails, it is typically where a step would be. I have had headers in my hand where literally the only thing keeping the header in one piece was the wrap. Whole chunks of the header material was gone, melted away. Talk to most "engineers" (I use that title with caution) and they will probably tell you that aliens stole the metal as the car was racing around the track! They say this because the wrap is causing problems that are not part of normal metallurgy and thermal dynamics theory <key word>. The header warp allows temperatures between the wrap and the header to turn the material molten.



IN CLOSING:

We know that header manufacturers will NOT WARRANTY a header which has had a wrap installed on it. We know that I have personally seen, tested and inspected headers that have been destroyed after running wrapped headers on a perfectly tuned engine.

Just because an item or product is advertised for a specific use does not mean it does not do what it is promoted as, but it also does not mean it is the best choice either. We have all seen the recent TV commercials on all of these "medical miracles" that do such feats as fast weight loss and re-grow your hair. How often have we laughed at the side affects, like the most common being penile failure. Oh sure, I want to take some drug if my hair starts falling out (yes, it is currently all here) that makes my sex life go to hell. NOT !!! Those pharmaceutical companies can pucker up and kiss my "bald" backside! I'd rather have function than hair.

The same can hold true with other products. Heat wraps are great, just do not use them ON the header. Use them on a device or component on your vehicle that you are trying to protect from header heat.

If you want a true thermal barrier that will reduce thermal loss in the header, reduce under hood temperatures, and make a few horsepower in the process, have your headers coated by one of the many companies available (links to a few are above). Most all of the coating companies offer inside and outside complete coating as standard. DO NOT allow your headers to only be coated on the outside, this is not a complete or quality process.

Some other considerations about the coatings. Once the header is coated, it's permanent. That means that if you screw up your header, it is not an ease job to repair and most header companies do not want to even attempt a repair. The coating permeates into the header material which makes welding processes on a coated material less than adequate. This is why you always coat the headers last, after all modifications to the header are made, and you are 100% sure the header will fit your vehicle.


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