The Theory Behind Headers (Tubular exhausts)

Why headers? Before anyone runs out and buys stainless steel pretzels for lots of money, headers must be understood. Let me give you a simple analogy. Imagine a  train highballing down a very narrow tunnel. What happens to the air in front of the train? As it can't flow efficiently around the train, it gets pushed ahead. If you were to put a manometer on the nose of your choo-choo you would discover high pressure. And if the train is pushing all that air in front of it, what sort of condition exists BEHIND the train? Low pressure must follow the train. A partial vacuum hs been created in ordinary words. If you stand at the near end of the tunnel, behind the train, you will feel air being drawn INTO the tunnel. If you stand at the far end of the tunnel, in front of the train [not recommended! ;)] you would feel air being pushed OUT of the tunnel.

What the hell does this have to do with your Morgan?!!! Well, when an exhaust valve opens, a hot, energetic blast of hot gas is released into the exhaust manifold. For as long as it retains this energy (heat), it works just like the freight train....the gasses are pushed the pipes and a partial vacuum follows it. Some would have you think that headers are all about reducing back pressure, or reducing the work needed to push the gas out the tailpipe. NOPE. A rudiumentary cast iron manifold with suitable exhaust piping would flow as well as most heddars. The purpose of a header is to make use of the partial vacuum that follows the exhaust pulse and actually PULL exhaust out of the engine..which in turn allows for more air/fuel to enter the combustion chamber (s).

Now let's examine the theory of exhaust pulse.  The gasses are expelled from the exhaust port and travel down the header branch/runner. Behind it, there's that vacuum, or an area of low pressure. The gas pulse travels down the tube at some specific rate of speed, determined by the diameter of the pipe, the displacement of the engine, the temperature (very important) of the gas, the RPM, the cam timing, the induction flow, etc. At some point the exhaust  pulse will pass through the collector (the area where the manifold section joins the ports on that side of the engine). If all the factors mentioned work out just exactly right, the low pressure condition following the pulse will arrive in the collector just as the exhaust valve opens on the next cylinder in that bank. So when that exhaust valve opens, it will open on....(bingo!!!) a partial vacuum, and the exhaust gas will be drawn out, or "scavenged", from the cylinder faster than otherwise. Therefore, tuning the length and diameter and shape of the branches is what increases or inhibits power. Since these factors determine the time it will take for the low pressure condition to form in the collector, a given length and diameter of pipe will create the desired effect only in a narrow range of load and rpm. 

So will headers actually help your Morgan? That depends on the engine, its tuning, its year and its usage. For example, circuit racers need performance at high revs and speeds. They test their exhuast sysrtems on rolling road dynoes set to ape those needs. They then copy their systems and sell them to you. But you do not spend ALL (if any of your time, racing at top spped in ovals or circles. You need a system that is more flexible than theirs, something that will serve you well in all circumstances and the type of driving you wish to optimize. A given racing header system will give you an effective improvement in only a very narrow RPM range. What RPM range is that, you ask....well, it depends. Big, short tubes work better for high RPM, skinny long tubes for mid-range. A proper design reflects  the car, the engine and the driver. A unwise choice for your car can actually REDUCE performance in the range you want to help. The most common mistake is to buy a system that has too large a bore. Lots and lots of enthusiasts are doing that now. Sadly, there is no longer any Morgan  fousm that can knwoedlegably speak in this area. 

Another thing to look for is the system's want to keep the heat IN the exhaust system....the more energy (aka heat) kept in the pipes, the better. as higher heat will make the gases flow faster.  Insulation can best be achieved by coating the high heat secion with a modern ceramic (inside and out) which creates faster flow and simultaneously cooler temperatures in the engine bay and cockpit along with a pretty look that is impervious to the elements and major steel or stainless discoloration while keep the strength and flexibility of mild steel. Avoid wrapping your exhaust.

WATCHPOINT: Ceramic exhaust coating has become the goto standard with headers and downpipes in North America and Downunder. Been so for 25+ years. No brainer. Not only does it speed exhaust flow, it also prevents rust, and both engine bay and cabin heat. Sadly, it never causht on in the UK and Europe. Inexplicable. N.B. I would avoid chrome-looking ceramics. Both ss and chrome ceramaic-coated headers quikcly chnage colour.

Which Morgan header design is best for a road car? After reading the information above, you know that this is not an easy answer. But there are few things that are worth mentioning. Headers are severely compromised by space availability.  If your exhaust must make a sharp right angle turn just beyond the port this bend limits the effectiveness of anything you try. If the design is made with many loops and bends, it will inhibit exhaust effectiveness (bhp) for a road car. If the bore is beyond what is needed for the engine size and tuning, it will also contribute to a lower of bhp. For a racer, with a big heary chested engine and heads, I would advise differently. So, in fact, the more curves and bofre, is merely for show. 

Once again, headars are not about reducing back pressure, they are about scavenging. No header will produce perfectly even results across the whole rev and load range. Understand what the system was designed to optimize before you buy. Don't expect a miracle...and results may vary. Avoid getting involved in over complications followed by balance pipes and the like. One thing for sure is that well chosen headers WILL release more power than the extremely restrictive cast  manifolds. But ewually so, a too big system with many bends, circles and curves will lower power and the size of your pocket book.

THE HEADERS I USE I have a big musly Plus 8 (a tuned 4.8L which is merely a 4.6 with upgraded pistons and other breathing and tuning changes). I have also had a 3.5L and a 4.6 with lower (8.5) compression on a fast road cam. I have installed about a dozen aftermarket systems on various Morgans and advised almost 1000 other Moggers remotely. I have used the same basic system on all my Plus 8s and I have rolling-road dynoed them all and often, as I beleove them to be the best testing and tuning device. I use the classical MMC-designed Plus 8 headers from 1977, which were copied by almost all Morgan exhaust people for decades, often in a slightly larger bore from 1.875" ID to 2" (installed as stock from 1977 to 1986) and which the much of the aftermarket sells to this day with minor variants. BTW, I have never seen much bhp improvement (if any) by using bigger bores unless the engine has been modified to warrant it. The stock engines, after all, are limited by the size of the ports in the heads themselves, though I have found that port matching the heads to the headdars makes for a smoother idle. This is costless for anyone with a dremel. I have placed two articles on the Exhaust Index Page showing how to do it. If you want to use a bore larger than that, get yourself a set of larger engine heads with larger ports and/or consider a larger engine. Experts will look at your system and know it is all for show. :(

THE REST OF THE EXHAUST I USE  However, I have tossed the original systems after the downpipes. Downpipes come in non-lambda,  lambda and catalysts verisons. I always use twin 2" systems. After the decision is made on which of THESE variants mention, I use made-for-Morgan "flow-thru" silencers reputedly a copies of the silencer used on Plus 4 Supersports. (shrug) They come in a conservative format inside (quieter but a bit more restrictive) , or glasspacks. Both are made from stainless and I choose my suppliers carefully. Though most Morgan aftermarketeers have similar pricing, the quality can vary wildly.
I use the latter types. Then merely stainless steel pipes out the back that clear the rear wheels with stainless steel exhaust tips to give me the tone I want. I shun rear resonators. More on this here. Frankly, where there used to be many suppliers, and I loved them all who used quality ss, most UK/Europe suppliers have retired and what is there is not longer to my taste. I am now advising people to seek out only variants of the MMC system mentioned.

Exhaust Systems revisited February 2022
by Lorne Goldman


Let me start by stating that  I very much enjoy racing and NOT being behind the wheel or even with watching only (not much to see trackside). But I adore being part of the racing infrastucture, namely pit crew or pit a relay race format. In that type of race the pit crew is often more important than the drivers! And between alternate usage, there is a bit of time to cure what ails aa team member. If the cars can be kept in top running form, the chances of winning are muchly enhanced, especially if you are caring for Morgans.

However, racing cars are set up for different goals in a VERY different venue from road cars.  Yes, there are some things that can be learned from their experience and modifications, but very little to enhance a reliablty performant road car. Race cars do their stuff on perfect track surfaces, constant high speeds, a pinpoint braking need, ;little need for clearances from the track surface and no slow bumper-to-bumper traffic. They have extremely limited flexibility, something every road car vitally needs. They have no laws prohibiting sound levels. They have no need to respect environmental lawa. Most importantly they have their entire support infrastucture (whether technical or social, mere meters away. If something goes wrong  (heaven forbid!) they are pushed over to the paddock and the driver/crew have a beer and/or go to dinner.

Solo road mogging is FAR more complex and risky. It has no pitcrew at hand, a limited tool collection and lvery limited set spares aboard. So the tendency to blindly copy racer modifications is an ill-chosen, inexplicable decision for a mogging mogger. It makes the car less reliable and less susceptible to assistance from experts and friends. They have also too often abandon  the community template and datum, (as my friend George Dow used to call it). A road going mogger should be striving first and foremost, to make their Morgan more reliable. After that there is an infinite buffet of tried and true performance enhancements to choose from. My car has bested a number of made-for-racing Morgan fare. You can too, if that interest you, while STILL having a reliable road car. The exhausts will be aesthetically lovelier as well! That is NOT to say that many of the Performance Systems are not race track winners!

Should there be those who wish to design/construct a racing echaust system. It would be a pleasure to help out..but off-group