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DBW Throttle assisted Boost Control

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Bit late posting, but when checking the valve to piston clearances, don't just do it at TDC, but also a few degrees either side as sometimes the closest positions are a little to one side ot the other +/- 10 degrees will be plenty.

If you are doing a dummy build, use light springs (inners?) and a DTI so you can gently push down the valves and measure actual clearances at the different points. If you're up to it, you can make a graphic representation of the valve lifts at their different degrees of rotation and the piston position(s) at 1 degree increments before and after TDC. With that, you should be able to figure out howmuch clearance you have to work with.

Don't forget to allow for valve clearances, as well.


I'm actually in the midst of building a dummy engine to measure these things in high resolution. (1 degree increments) The trick is to mount the variable valve timing system, and trigger it to show the range of adjustment (Of clearances) at critical points.

On the boost control front, I've been cutting up my existing manifold to add 2 waste gates with more direct access to exhaust flow:

Nice to see you are making progress.

How are you running a single WG on a twin scroll setup? Is the WG only tapping into a single exhaust tract or are both tracts going into the single WG?

If it is the first scenario, then your boost control will be problematic as you are only bypassing half the exhaust gas - any gas on the second tract / scroll will get to the turbo un-impeded.

If it is the second scenario, then you are loosing any benefit of a twin scroll turbo / manifold as the 2 exhaust tracts merge at the WG meaning that the exhaust pulses will now be present on both tracts due to the overlap at the WG.

And - is the manifold actually a twin scroll manifold?

Yes, originally a single WG was used with a divider on the seat face to control pressure on both tracts.

The waste gate will open when the boost target is achieved. Prior to that, the spool benefit of the twin scroll design is fully captured. Once the WG is bypassing exhaust gas, does it really matter if the plenum remains divided? You are trying to REDUCE the drive pressure on the turbine at that point.

My issue was that I couldn't reduce the drive pressure enough.

The Steed Speed manifold design originally a single scroll setup, so we have data points with a wide range of turbos on these engines. People have then converted to the twin scroll manifold, and seen very good improvements in spool. Everyone that has used this manifold, has struggled with boost creep. (And applied various band-aids to try to resolve the issue)

By adding the 2 WG in the positions shown, I'm now increasing the surface area exposed to direct flow by nearly 300%