Some questions about itb's

[Yves]

I have a itb setup on a sbc. I was discussing things on an engine forum and what was said there made me think about certain things. I have crossram setup where there is a vacuum chamber under the runners connected to the runner with6 mm holes.

I was explained that only 1.5cyl is on the intake stroke to create vacuum and the others act as vacuum leaks. There is some sense to it as at initial start up the engine idled high given enough fuel. I also have difficulty getting iac to work on this setup and have the engine react to it.

It also makes me wonder about the way my fuel pressure regulator works with the 69kpa the engine is only pulling.

What do you guys think of this reasoning ?

It makes me think a maf is better to estimate actual airflow

[kaeman]

if your vacuum ports are below the throttle plates, then you are getting vacuum just like any other engine that share a common plenum, the biggest difference is that our engines have an idle map of about 65 to 67 kpa. if the throttle plates are closed down to control idle speed then your iac should work but will probably need to have its own manifold running to each throttle body to get air into the cylinders, or just don't use the iac and close the blades down and adjust the idle using fuel and spark. I am just using spark and fuel to control my idle speed. I have the blades closed until the engine died and then opened back just a little, added some fuel and set my timing to 10 btdc, my engine idles nice and starts easy. without idle valves or iac.
good luck