match porting and polishing a 12th (1 Viewer)

[jb BJ73]

I was thinking of making a new manifold in the future, maybe something with runners. Not 100% sure how I'd seal the ports up, would probably need to get a plate made up with grooves machined for o-rings then weld the rest to that. But that won't work if I radius the ports on the head now. Maybe a bigger chamber/plenum would be enough of an improvement.
Any change to the manifold will mean getting longer fuel lines made as well.
But looking at the factory manifold it doesn't seem to bad, especially if the ports are smoothed out.
The butterfly is for shutting the motor down. But there's a fuel cut lever on the pump so I reckon I'll use the edic setup from the 3b to shut it down. That or just run a cable to the cab

Dougal is that 16% ignoring equations? So if the port is for example 30mm wide (that's a guess from memory without a ruler handy) then the radius would be 4.8mm across? I'm guessing your using radius to describe the port edge and not as part of a circle equation (half the circumference).
Not really able to improve the top and bottom of the ports much, the inside edge of the manifold is pretty close to the ports. Would doing the outside edges of the outer ports be beneficial?

Thanks for the input guys, there's some great info here.

 

[gerg]

So what Dougal said about radizing the mouth opening will essentially eliminate an invisible restriction in your intake runner. For the time it would take to do and the chances of doing it wrong it's definately low hanging fruit. As far as individual intake runners there is some serious challenges to doing that on a large displacement low revving motor. To tune the pulses to achieve even the 3rd wave to increase your intake charge would be a long runner indeed. Long enough to be a major obstacle. If you were to do a plenum go for bell mouth trumpets and try to eliminate and changes in direction of the intake air charge. Plenum sizing and runner length is a huge topic and will serously take you down a rabbit hole. I've been down that hole and it never stops. Plan on a simple smooth flowing plenum. Lots of cool examples of top mount intercoolers with incorporate plenum so, but top intercoolers have challenges of their own. Some do look like they would flow really well. Honestly your plenum isn't so bad all things considered.

 

[jb BJ73]

Yeah as far as intake runners go was only thinking of making them long enough to make it kind of like what the guys with HZs and HDs are doing.
I think radizing the port openings will be the best option, and will work well if I decide to make a bigger plenum in the future. Gotta learn how to tig weld first anyway

 

[Dougal]

The issue with tuned length intake on a turbo engine is the sonic speed (and hence frequency) keeps changing with temperature.

I'd just radius them out and free up any obvious restrictions. Will that factory intake manifold handle high boost or will it balloon out and lose sealing against the head?

 

[Dougal]

Dougal is that 16% ignoring equations? So if the port is for example 30mm wide (that's a guess from memory without a ruler handy) then the radius would be 4.8mm across? I'm guessing your using radius to describe the port edge and not as part of a circle equation (half the circumference).
Not really able to improve the top and bottom of the ports much, the inside edge of the manifold is pretty close to the ports. Would doing the outside edges of the outer ports be beneficial?

Thanks for the input guys, there's some great info here.

Yes 5mm radius (half of a 10mm circle) would be good for a 30mm wide port.

 

[jb BJ73]

Thanks mate, I'll get it close to that.

High boost on that manifold, yeah I'm not too sure. It's a rubber gasket kind of like a rocker cover gasket that is compressed a certain amount. I'll see how I go. If it leaks then I'll just have to look at changing it.

 

[mudgudgeon]

A simple way to port match head and manifolds is to spray paint on both sides of a gasket, then before the paint fully dries, bolt the manifold to the head. Unbolt it and you will have a matching paint outline of the gasket on both the head and the manifold.

Just make sure you use a decent gasket that matches the gasket you intend to run with.

 

[Dougal]

I think it's better to have a step from the head to the manifold. This lip acts as a sort of one-way valve to slow down pressure pulses back into the head.

Plus the amount of thermal expansion you get in an uncooled manifold (vs a water cooled head) means matched exhaust ports are not going to stay matched as it heats and cools.

Intake side. Sure match them up.

 

[gerg]

Yeah I think that is known as an anti-reversion step. Been a long time since I thought about porting stuff. Steps like that are used on the intake side as well on heads ive seen, but your plenum isnt designed that way. This is a great example of why porting gurus charge so much for their craft. Its a compilation of all these little tricks working together that no one really ever spills the beans on to improve flow. Its alot more like a secret guild than mechanics.