332 | Setting Up Individual Throttle Bodies - ITBs

Hi everyone

on the last webinar about ITBs, andre talked very briefly at around 8:50 how each throttle body connection isn't solid and there is some adjustability between each TB. My question is why is that ? I currently have a ITB set up for my L28 engine and wondered time and time again why I shouldn't go with is a solid connection especially to reduce the difference in opening at partial throttle. Since there is some ajustabilty and a spring between every TB I feel that at light throttle some TB will always be more open than others, making tuning at partial throttle even more difficult. This opening difference will be even greater on long engines like my in-line 6 L28 where the throttle linkage is actuated by a single cable on one end of the assembly. Is there any webinar that cover this ? Thanks! Picture of my set-up because it looks great.

First, cool build - so nice to see someone still using the L instead of dropping in some other engine.

I've only direct experience, as I recall(?), of the type that doesn't use an idle air bypass - the butterflies set the air for each cylinder.

For these, you may notice the body the throttle cable is connected to has the idle speed adjustment screw, this is because every other body is being referenced to this, both at idle and during opening. It's usually around the middle to reduce possible torsional deflection at the end. As your setup has an end link, there isn't really anything you can do, as such, for possible torsion at full throttle when the spring loads are highest, but if there should be a fraction of a degree lost it's insignificant. Even then, most of the potential deflection will be removed by correctly adjusting them*

Using a common airflow meter - I use this - set the approximate idle with the throttle screw, or a fast idle if you need oil-splash for a new cam', then check the airflow of the body with the idle screw, #6, and match the one next to it, #5, remembering to refer back to the #6 as airflow will alter with any rpm variations, then match #4 with double checking, #3, etc. When you're finished they should all have the same airflow.

Next, if any cam' bedding is finished, drop it down to the idle speed and check the throttle position sensor is correctly set up - this proceedure should be in the documentation, and may have rpm and mass-flow parameters that need to be referred to.

With the idle bypass type, I would assume they would be closed during the mechanical setting up of the butterlies - the idle air coming entirely from the cracked open 'flies - and then balanced in turn by opening them with the throttle and 'flies completely closed, possibly working to that end by gradually shutting the 'flies and opening the bleeds.

*You can use feeler gauges between the butterfly and body to roughly balance the butterflies before starting the engine.

Francis,

Each throttle in an ITB system is generally adjusted until all cylinders are flowing a similar amount of air as measured at the trumpet via a Synchrometer.

This is performed at idle.

The tool gets stuffed into each trumpet like this, sometimes with a sealing coupler as it does need to seal properly to get a good reading.

As you operate the vehicle, the tiny differences in throttle angle at rest become less significant in terms of airflow impact at higher throttle angles. For example, by the time you get to 90% throttle, a 1-2% difference between throttles will have little to no impact.