Design your own Extractor Manifold for you ride.

[RichieOzzie]

Ever looked at the Manifolds on F1, or Rally car engines, and how they really look neat, and business-like????, well you can make your own, but unless you have access to hydraulic pipe forming & bending equipment, it may not be as pretty, but will work close enough!


As most of us view the Std Manifold with dis-taste, and longing for something that doesn't hold the car back, I figured people out there in GM land may want to "Cobble their own together".



NOW..!!, bear in mind that the dimensions or measurements have to actually fit the engine, and miss all the parts of the car we do not want to get hot... Aka, the oil-pan, Filters etc on its way to the exhaust pipe union.

The way I did it was to get flexible plastic conduit pipe, and make up a "MOCK" version of the calculated one. Glued it together with plumbers pipe weld glue, and then offered it up to the engine in situ. I found that a little heat made the pipe semi harden, which let me remove it when I had routed it around the engine bay, and then, CAREFULLY removed it as one item, took it to the bench, researched the angles and straights to buy, i.e 30degree bends, 45 degree bends etc.

Then, make the Real thing on the bench with real steel. Mark the alignment and order with a marker. Take it all to someone who can weld it together, and thats was it.

Back to the ride, and bolt it on.

MAKE SURE... you have strengthened any corners well, remembering its the inside of the tube that has to be smooth, and webbing/bridging on the outside with supports where the OEM has them is a good idea to stop it all being shaken apart.



So, if you want to try your own hand at header/exhaust manifold design, I have reduced down some of the maths involved to these simple equations and tables that should get you in the "ballpark" zone.

The real formulas are much more complex, but the complex stuff like gas temperature etc, is reduced down to constants here.

These simple formulas are not the end all solution for the ultimate in header design, but they are better than a mass produced "One-Size-Fits-all" and they will get you in the zone.

Even if you don’t want to design your header, you can use this information to sort through the design specs of a bunch of "off-the-shelf" headers to help pick one that is the most likely to work well on your motor or personal specification.



The first step is to calculate the length of the primary tube. The formula for Primary length is:

Where:
P = Primary Length
ED = 180 degrees plus the amount of degrees before bottom dead center that the exhaust valve opens
RPM = the RPM that the header is tuned to work best at. See Specs on engine power output for this info, remembering Std is what you are reading here.

You can roughly calculate primary internal diameter with this formula:
ID = (The square root of cc/(P+3) x 25) x 2.1

Where:
ID = Inside Diameter
cc = Cylinder Volume in cc
P = Primary Length



Having a tube with a slightly larger cross-sectional area than the exhaust port is a decent starting point as well.

If you wanted to design a Tri "Y" or an interference branch style header, you first figure the best overall primary length by using the above equation.

Make the length to the first "Y" junction from 13-16 inches. Subtract this from the overall primary length to find how long to make the tube from the first junction to the main collector.

To find the inside diameter of the first junction use the equation we last used to find the ID of the primary pipe. From this diameter we can determine the diameter of the next branch using this equation:

Where:
ID2 = the inside diameter of the secondary primary
ID = the inside diameter of the first part of the primary
The collector should ideally be a merged collector with an included merging angle of 14-20 degrees.


To find the diameter of the collector, this formula can be used to get you in the ballpark Zone:

Collector ID = (the square root of cc x 2/ (P+3) x 25) x 2

Where:
cc = cylinder volume in cc
P = primary length in inches


When designing a header for low-end power and street use, you typically want to tune the header for the rpm of the estimated torque peak. For forms of racing that need a useful powerband like road racing or short circle tracks or perhaps a serious streetcar, you may want to tune the header for somewhere between the torque and power peak. For all out drag racing with a close ratio gear box in a light car you might tune for the rpm of the power peak.



For a fun mental exercise, try calculating what should work with a stock engine with a stock cam at various streetable rpm ranges, and then compare your findings with typical off-the-shelf headers...., afterwards, “design” some headers for the same engine with available performance and racing camshafts at higher but realistic RPM ranges. See the big differences? Now do you wonder why you rarely see any market headers on the cars in the all-motor class?
Header/Exhaust Manifold Design Trends..?



Although these equations are what many engineers use when designing a header/exhaust manifold, they don’t take into account many of the recent design trends for header design that are being proven to work quite well. Many of these latest trends cost a lot more to make and are not likely to be found in an off-the-shelf production header. These trends are proven power adders or powerband wideners which makes designing a custom header incorporating these features more and more worthwhile.

Some of the latest design trends are: stepped primary tube diameters, anti-reversion chambers, merged collectors and venturi collectors. A stepped primary diameter steps up in primary diameter two to three times over the length of the primary. Measurements like 1.75 inches to 1.875 inches to 2 inches are common in high revving import motors. Usually these steps go in lengths of 7 inches or so. By making the propagation of waves and refractions as discussed last month more gradual, stepped primaries generally give a wider powerband with no loss of top end power. Most engines with larger camshafts respond well to stepped primaries.

Anti-reversion chambers are controversial. These chambers are areas in the primary tube with a larger ID over a short distance usually about 5-7 inches away from the head flange. The chambers sort of look like "goiter bulges" in the primary pipe. They are supposed to damp out the return of the reflected acoustic wave to prevent the short-term spike in primary tube pressure around the exhaust valve on overlap. Whether they actually do anything is a fierce source of debate among header designers.

Merged collectors are the best for power production and width of powerband, they are exceeding difficult to make

A venturi collector has a necked down area just past where the primary tubes merge. Generally this is a cone-shaped neck down with a 7-10 degree taper with a megaphone with a similar taper stepping the collector back up to the full diameter of the exhaust pipe. For most compact cars, the venturi goes from 3 inches in diameter at the merge down to a 2 3/8-inch venturi, back up to 3 or more inches to the exhaust pipe. Sometimes a short reverse cone is added to the end of this megaphone before the exhaust pipe starts to add yet another back pulse to help broaden the powerband further. The purpose of this venturi is to speed velocity and create a stronger low-pressure rarefaction(the reduction of a medium's((Exh gas in this case)) density, the opposite of compression/pressure) at the exhaust valve without reducing flow much. Some header builders use a short primary tube for good top-end and use the venturi collector to help support a broader powerband.