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MG MGB Technical - Tourist Trophy vs cast manifold

Is anyone using the Moss Motors Tourist Trophy Stainless Steel exhaust header? How's the performance on your B or how do you rate or compare it to the original cast iron manifold? I need to invest in a manifold but what's the best choice?

Thanks,
Brian
Brian

The tubular manifold from other sources is supposed to make for better breathing, but I didn't notice any difference when changing from that to a standard cast manifold. It's only likely to make any significant difference when accompanied by improvements in breathing on the carb side, head porting and combustion chamber changes.

I changed from the tubular to the cast when replacing the exhaust with a stainless system, as at the time it didn't seem to be straightforward to keep the tubular manifold and just replace the rest of the system.
Paul Hunt

Hello Brian,

I used ANSA steel headers and it was fine with my B. But I could not rate a real improvement with a standard engine, except for a more sporty exhaust sound.

ANSA has an excellent quality headers like their free-flow exhaust system..

Cheers,

Jean G.
Jean Guy Catford

Brian,

i changed from the stock carst iron manifold to a grid out modified one that improved things a little on the modified engine. Then i installed a Peco header that i matched to the ports of the head (modified to ralley race stage with 1 3/4" SU's etc., Abingdon ST stage 6) and the engine was able to reach 6500 rpm and more much more freely.

I still doubt that this mod is necessary on a stock engine, but if there are some stage 2 to 4 improvements been done to the motor, it is worth to have one installed.

With the V8 (stock SD1 with Holley carb) the change to the the RV8 style headers did a remarkable increase of drivability, so it seems, that the usual exhaust manifold on the 4 cyl. seems to be a very propper design you can stay with, as long as there had no other mods been done to the engine. With the V8, things are different, i learned.

Ralph
Ralph

Brian-
It is true that a decent tubular steel exhaust manifold might be obtained for less money than that of extrude honing and / or electropolishing an Original Equipment cast iron exhaust manifold, but tubular steel exhaust manifolds have much thinner walls that can resonate under the fluctuating pressures inside of the exhaust system and thus are much, much noisier. At certain parts of the powerband, they can actually resonate into an annoying ringing sound. In addition, tubular exhaust manifolds are welded assemblies, and their welds have been known to crack under the repeated stresses of heating and cooling. Further potential problems of misalignment of the exhaust runners with the exhaust ports, and mounting flanges not being on the same plane, all contribute to making the purchase of a tubular exhaust manifold into something of a gamble. The Original Equipment cast iron exhaust manifold has none of these problems. Because of the lesser heat conductivity of cast iron as well as the decreased surface area, the electropolished exhaust manifold will radiate less heat into the engine compartment. Its greater mass will also have the side benefit of reducing noise to a level notably less than that possible with any tubular steel exhaust manifold.

I sincerely believe that an LCB (Long Center Branch) 1 ¾” diameter tubular steel exhaust manifold will not flow any better than an electropolished Original Equipment cast iron exhaust manifold if it has the same basic design. It can also be beneficial to electropolish exhaust ports, thus reducing carbon buildup that results in the creation of airflow turbulence, as well as the advantage of less heat being conducted into the cylinder head as a result of the decreased surface area of the interior of the runners.

It is important to understand why the pre-1975 Original Equipment exhaust manifolds and LCB (Long Center Branch) 1 ¾” exhaust manifolds have nominally the same performance. The cross sectional sizes of their internal passages are nominally the same, thus the velocities of the exiting combustion gases passing through them are nominally the same. This high gas velocity is critical to power output at low engine speeds because the greater the velocity of the exhaust gases, the greater their inertia. Due to the high degree of directional inertia, the exiting combustion gases continue to flow exclusively in the direction of and out past the exhaust valve even though the intake valve is opening. In order to properly understand this phenomenon, one needs to view the cylinder as an being an extension of the combustion chamber. The combusting fuel / air charge exerts pressure upon the piston crown, accelerating the piston down the cylinder. It must be understood that due to the geometry of the crankpin and connecting rod, the piston is decelerated as it passes 90o After Top Dead Center, this geometry-induced deceleration becoming increasingly severe as Bottom Dead Center is approached. However, combustion pressure causes the combusting gases continue to accelerate downward, their inertia causing them to pile up on top of and thus increase the pressure of the atmosphere immediately above and upon the piston crown. Due to this inertia effect, at Bottom Dead Center the atmospheric pressure at the roof of the combustion chamber is actually less than the atmospheric pressure immediately atop the piston crown. Because all forces in nature tend to equalize, at this point the pressurized atmosphere atop the piston crown expands upward, increasing its upward inertia as it accelerates toward the roof of the combustion chamber. If the exhaust valve is open to the point that it has sufficient airflow capacity, the inertia of the exiting exhaust gases will remain sufficiently high enough to literally scavenge the atmosphere from inside of the cylinder, creating a partial vacuum of as much as 7 pounds per square inch less than the ambient atmospheric pressure outside of the engine. This in turn allows the incoming fuel / air mixture to be pushed in not only earlier, but also at a higher velocity (and thus a larger quantity of fuel / air mixture with better fuel atomization) by the greater ambient atmospheric pressure outside the engine, thereby increasing power output. All other factors being equal, a larger diameter exhaust manifold would decrease this critical velocity, and with it, its benefits.
Steve S.

Thanks, these are helpful suggestions. Do you think cast or steel tubular type will work better with an aluminum head? I've heard the aluminum head runs a little cooler. Will that make much of a difference?
Brian

Brian

John Twist answered this wuestion at one of his tech sessions. He felt the early manifold is very large for the engine and gives very good performance for street engines. he did not feel that headers added any noticeable difference.
Bruce-C

Steve, thanks for your insight. I tend to agree regarding the stock pre-1975 headers performance (partly from reading your knowledgeable posts in the past). I'm sending my pre-1975 manifold out to be ceramic coated very soon. My 80LE runs a Peco system (not the big bore one) and I do have some questions. Would you recommend a different head pipe, stainless steel for instance, instead of the stock head pipe? Also, considering the interia characteristics, is there any difference in exhaust flange gasket choices? My engine is basically stock with a Weber DGV (although might install a set of SUs that I have.
Rick Penland

This thread was discussed between 05/12/2008 and 15/12/2008

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