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MGB Three port exhaust header?


Jedi Hopeful
I don't proclaimed to be world class mathematician but can anyone explain how four cylinders equals three tube exhaust headers? I don't plan to put one on my 77 but what the he.. I had a Volkswagen rabbit when I was young and put a four into one header on.. Made a world of difference .... But what cylinder gets shorted? More British engineering?


Jedi Hopeful
I am thinking there was a question obviously asked in committee .... How many exhaust ports on a four cylinder engine and someone said.."ah.......... Three! " And everyone yes that's right... I just cannot cypher the logic! Lol

Marvin Gruber

Country flag
Center two ( 2&3) port out same spot, think on it a minute, neither exhaust at the same time and it saves space in the head. So you swap left and right with the valves. No big deal

frankssc1800-2 (1).JPGIt's call a Siamese port, it's not that uncommon, and for sure not just a British thing. On most MGs, like the MGA, MGB, A series engines found in the Midget, Sprites, Mini, etc. they are all 5 port heads, three exhaust ports, cylinder #2 and #3 share the same exhaust port, and on the intake ports #1 and #2 , and then #3, and #4 shared the same intake port. Here's a better look at the head, with a header bolted up, this was a MGB engine I built a few years back.


Great Pumpkin
Country flag
And since the firing order is 1-3-4-2, that center port is effectively one for each cylinder when being used.

Boggsy... you know Audi (and Benz diesel) have made five cylinder engines, yes? :devilgrin:


Great Pumpkin
Country flag
Boggsy, The art of designing a system for scavenging the exhaust from an engine is very high tech! You don't need 4 exhaust ports on a 4 cyl engine or 8 on an 8 cyl engine to get the best performance out of it. The head and how it's built is the key to success. Believe it or not, the stock exhaust manifold on an 1800 MGB engine is about as good as your going to get in the normal RPM range. Headers are a waist of money on a street car, any street car, their just for looks! Racing, well that's another story. Totally different approach to get those gasses out of there in a hurry and keeping everything balanced. JMHO PJ
Country flag
The reason is simply explained thusly - "Thou must count to three. Three shall be the number of the counting and the number of the counting shall be three. Four shalt thou not count, neither shalt thou count two, excepting that thou then proceedeth to three. Five is right out."
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