IR vs. OPEN PLENUM INTAKE RUNNERS

[v114]

We are experimenting with intake manifold designs on a V-twin engine for the street (7000-8000 rpm). We may need to use two carbs and the question is this:

For the manifold design, would it be better to have individual runners to each cylinder (IR) or an open area behind the carbs (open plenum)?

I have heard theories for both cases. Any experience out there for this one?

Thanks

Jim

 

[richdubbya]

In general a plenum designed well for wet flow will be smoother and more tractable at low rpm (which seems to be where harleys are run the most). It will have a more linear torque curve thru the low and low/mid range but with a loss of upper mid to top end over the IR. Most of the drawbacks of the IR are caused by backflow thru the carbs. Fuel injected IR gives you almost all the advantages of both systems IMO.

 

[1Bluegrass]

The use to which you are putting the engine would be considered for the type of inlet tract used.
The IR setup uses a carb with different airflow capacity than a plenum type, with the signal so strong that the carb sees airflow signal in both directios such that the carb will meter fuel in the reverse direction and produce a vapor mist above the air horn area. This makes jetting very specific.
In addition, the tuning can be tightly peaked in the RPM range by selecting tract length as a tuning function.
The plenum connection inlet system subdues the IR attributes to a large degree depending on the physical makeup if the inlet system. This has an effect on the carb size and the jetting since both cylinders could share the fuel/air mix.