DIY Inlet Manifold

[Jimmyd]

Ok girls help me get this straight in my own mind.

My logic tells me that the best way for a manifold to work is to have a clear flow of the mixture from the carb into the head. Now I have 36mm bike carbs but the ports on the head are oval and are appox 35mm x 31mm. It would be way to difficult for me to make the inlet tubes match both the head ports and the carb's so does it matter if I have the manifold ports bigger than the head ports. I'm thinking this means there is an obstruction but reckon I'm thinking crap as the engine is sucking the mixture in rather than the carb blowing it in. So I can't really have the manifold ports to big the engine will only suck as much as it can through the ports in the head it's only if the air was blown in it would be an obstruction.

Does that make any sense?

Also if using a megajolt with bike carbs can you simply link the vacuum take off on the back of the carb usually used to balance the carbs or a petcock to the MAP sensor on the megajolt or do you have to have vacuum pipes in the manifold?

J

[flyerncle]

What a question,Simple terms, the piston's going up and down in the engine sucks fuel from the needle in the carbs.
To get the best performance it should be matched as closely as possible to the inlet ports on the head,not always possible but make sure the manifold flange and gasket are as closely matched to port size as possible, this way the fuel/air is not hitting the edges of the head and causing turbulence and in effect wasting power.

It is possible with care to open the ports up but only to match the gasket,but be carefull. Take off casting marks and polish as much as poss.

There is nothing mega scientific in it and the airspeed across the venturi in the center of the carb rises and causes the fuel to be sucked out through the jet.

Hope this makes a little sense.

[Jimmyd]

Flyerncle you've explained it better than did, just the way I was seeing it in my mind!!,

"make sure the manifold flange and gasket are as closely matched to port size as possible, this way the fuel/air is not hitting the edges of the head and causing turbulence"


I may need to think about some head work.

J

[flyerncle]

Which head Jim ?
joking mate.

[les g]

on locost builders there was a thread about manifolds for a zetec and somebody had posted the size tube that when when squeezed in a vice deformed almost exactly to the manifold plate of a zetec if thats what you are using
cheers les g

[big_wasa]

40mm Id is very close to zetec port size.

[Jimmyd]

Currently fitting carbs to a CVH engine for IVA/current cost purpose. So was wondering how easy it would be to modify the standard manifold to fit bike carbs. After a bit of cutting I think a small adaptor welded on to each branch to allow carb rubber fitting is much easier than starting from scratch. In the pics below I've just slipped rubbers on to see how close they are in alignment and I'm surprised!! there really isn't a whole lot of fettling to be done. So anyone got a bit of thick wall 45MM ali tube going spare to make the adaptors?


J

[nicholas garn]

That looks good, nice and simple. are you going to machine the internal branches of the inlet manifold, to prevent turbulance as the air fuel mix is sucked into the engine? I mean where the rubber hose meets the cutdown manifold. Hope that makes sense!

I'm thinking of doing the same for my engine but thats a while off yet.

Nick

[Jimmyd]

The plan is to weld a circular mouth to the existing branches and then create a smooth internal flow to reduce the turbulance. By that stage the carb should be sitting very close to the new mouth with very little rubber between so I don't think it'll be a problem.

[flyerncle]

An idea, I think Shorttrack do a laser cut plate to the dimensions of the gasket for the engine,make the ports and gasket match the head and fit some steel tube to the plate that suits your carbs that way the fuel goes straight in and no bends and they will be equal in length the HP gain may be little but every little helps as I hope this post does.

Good luck.