Mikuni 40phh intake runners

[BRAAP]

Tony,

I’m merely theorizing here, but after reading your input here, (good stuff by the way), I have inferred that for carbs or EFI being injected way upstream before the butterfly, that this as cast ridge/step at the manifold to head mating surface is aiding even further atomization beyond what is achieved at the venturi or injector nozzle due the drastic velocity/pressure change across that step, therefore realizing more efficiency, drivability, and power due to more thoroughly atomized fuel? (which would support injecting fuel at the port close to the valve means that step has less importance?)

 

Would that be accurate or am I chasing mythical creatures?

[madkaw]

I have learned a lot from this thread, but the best part that I learned is that I don't need to mess with this set-up----just bolt it on! That leaves much more time and energy to finish mocking up my cold air box for the triples!

 

Plus dialing them in will also take a little time--I'm sure!

[Tony D]

One thing to realize is that 'what size the gasket is, will not necessarily be what size the HEAD PORT will be' --- this is the fallacy of 'gasket patching. Because you have a gasket oversized to permit casting-shifted dissimilar components on a production run does not mean that is the size the ports need to be. We are looking at a gasket which is slightly larger than the head port. We have a gasket which is considerably larger than the intake manifold. But in reality is really only what---1mm different? While the ports looks small compared to the gasket, in reality compared to the head they are quite close.

 

Now, to Paul's question---the F1 engines inject above the intake barrels. Longer turbulent mixing helps with fuel homogenization, and this means a more even burn and more power.

 

Where the step comes in is like an 'anti-reversionary' header---the slight difference in the manifold (being slightly smaller---0.020 to 0.040" radially <I may have mistyped that in the PM.> equally around the circumfrence of the port) will aid in the intake air remaining flowing towards the back of the valve during time of long overlap, or lower speeds when reversion can dilute the intake mixture.

 

This goes for ITB style injector placement, or placement at the head. It's the airflow dynamic that is in play here, and keeping the air flowing to the head and not reversing in any substantial way. It acts like a 'dynamic check valve' to flow into the head during the sub optimal flow times...

 

Make better sense?

[luvemfast]

So I have a choice of manifolds to use with my new triple 44PHH Mikuni carb's. (bought from Sam280Z)

The Mikuni as discussed in this thread and a TWM manifold (longer than Mikuni manifold) which is the same port size as my N42 head.

My L28 engine is running an aftermarket cam (72° & .49" lift), Ø88mm flat top pistons and balanced bottom end. Currently running 40PHH Mikuni's on a Cannon manifold (which is longer again).

I'm also going to use a TWM cold air box with 50mm velocity stacks.

 

Am I better off using the Mikuni manifold as supplied or the TWM one?

Am I better off to port the Mikuni manifold, as it is not a standard L28?

 

I'm concerned about the shape of the Mikuni ports. Short runner with sharp bends and lip with reduced port exit. It goes against all the principles I've ever beleived in......

 

Note,

My car is used for club events and sprints (track days) only.

Edited January 24, 2011 by luvemfast

[Tony D]

Put them all on the dyno and find out which works best for you.

 

 

That, despite all experts opinions, is the correct answer!