Exhaust Manifold Design

[arseofbox]

No...Im not doing another manifold....at least not yet, but given time I will!

Ive been chatting to arc about exhaust manifold design as he needs a custom one designing for his turbo (if ever ) conversion.

The basic design is straightforward, but the thing that is bothering us is the port sizing...Most information I can find (including numerous tuning / engine design books) imply that the head port should be sized slightly larger than the port on the manifold, as this prevents the pressure wave the exhaust action creates from sucking burnt fuel back into the engine (the idea is called wave recursion I think).

Designing a manifold like that is easy, but why isnt like that on the standard item!? Ideas chaps/ladies?

 

[GhostWKD]

cant help you re the tech bits, but let me know if anything comes of this one too

 

[Oldschool]

Actually, it is the other way round. The manifold is larger than the ports. Preferably the step shouldn't be all the way round the same and rather similar to the capital D with the flatter part facing downward.
The idea is not only gas being sucked back. In conjunction with the relevant cam the aim is to suck out/through all the burnt gas, which is dependend on the revs more or less overdone on a race engine. On a normal road going engine this would lead at the normal rev band lead to excessive HC and in the long term kill the cat.

 

[ChrisUK]

Im all for killing the cat lol . I want to turbo my engine but heard its really bad due to the 11:1 compression ratio .

 

[arseofbox]

Im all for killing the cat lol . I want to turbo my engine but heard its really bad due to the 11:1 compression ratio .

Yeah...you wont be able to put much if any boost on that CR. You'll need to lower it

 

[GhostWKD]

Tom, you looked into turbo manifolds at all as yet? i cracked mine today stalling into a kerb front of car stopped, rest didnt and manifold bent overitself its cracked all round the face of it garage will weld up for me but advised me as its bad crack prob not an option and it will affect the performance going to ask van aaken how much they'd want for another manifold but guessing prob as expensive as a custom 4 branch jobbie

 

[arseofbox]

Tom, you looked into turbo manifolds at all as yet? i cracked mine today stalling into a kerb front of car stopped, rest didnt and manifold bent overitself its cracked all round the face of it garage will weld up for me but advised me as its bad crack prob not an option and it will affect the performance going to ask van aaken how much they'd want for another manifold but guessing prob as expensive as a custom 4 branch jobbie

Nope, I havent at all, Im still plugging at other stuff...

That sounds like a real bugger though mate! I think I need to read that thread, but problem is its all theory, and everyone knows theory and practice are two very very different things!

 

[GhostWKD]

will post more about it and pics tonight after work