Jfrahm

Strange. Have other people running the Raptor SC had this problem?

My assumption would be the blower hitting a wall of boost piled up against the throttle plate at lift throttle, which would require something to give. Either a boost limiting valve (not fitted) or the external belt has to slip, or the internal belt has to give way.

The shockwave would be bad news.

I thought about adding a (the) throttle body in front of the SC which should also make it possible to keep the MAF up topsides.

Maybe a comprehensive review of how this system should be deployed is in order. Swapping it for a V8 is a nice way to solve the problem and one I am quite interested in, but outside the scope of this particular discussion.

I'd be inclined to discuss:

SC head - is the blower simply too big for this application?
Belt traction - is it solved for Stage I? (slip and belt breakage might be a result of lift-throttle surge)
How should it be set up?
--Relocated Tbody?
--Blow-through MAF?
--Boost limiting valve?
--BOV or recirculation?
--Rerouted signal to the BOV?

I am also interested in possibly limiting tip-in boost. Tip-in detonation worries me. On hot days I suspect I get some with this setup. An electronic recirculation or boost control valve that prevents boost spikes and can allow boost to rise more slowly.

The issue there is that at cruise the blower is ready to deliver a great deal of boost once the vacuum drops and the recirculation valve closes. It's sitting there like a pail of bricks on top of a ladder. The engine management is set for full spark advance and fuel economy. When that boost hits you can get a flurry of knock and the timing gets pulled out and then gradually added back in again. The feeling, I think, would be a hesitation.

I am not sure this is happening but I think I have felt it on hot days. I'd like to try to datalog the knock signal. I have also thought about tying an electronic recirculation valve into a flow sensor and a water injection system. This would not allow the valve to close without water flow occurring. That'd elegantly allow for some time between WOT and the boost hit, and also prevent full boost if your water injection was not working.

I think my brother's first supercharged 928 motor died due to tip-in detonation but it could have been bad gas and hot, hot weather. Anyway it is a concern with superchargers due to the availability of full boost on short notice.

Cheers,
-Joel.

Eric_Oz_S2

Joel - that is a good point, I never thought about tip in enrichment. Are you running the stock ignition timing map? I plan to retard the ignition on the PT and WOT maps when I install my Rotrex SC, particularly in the higher RPM ranges. I will monitor with a knock sensor. I don't see how you can add 0.4 bar boost and not have an issue, even with additional fuel - especially when used for track use in our high ambients over here in Oz. If you look at the 944 turbo maps the ignition advance drops off for boost condition. Perhaps the 928M design relies on the knock sensors to retard ignition by -6 degrees in hot climates?? I am only conjecturing here, perhaps Carl could clarify. The DME has some ignition retard built in to it, but I recall the max retard is reached well before 100C.

With 40 degrees C ambient, you will get around 100 degrees C on the outlet of the SC. I guess another approach would be to adjust the intake temp ignition advance correction map so that you further retard ignition at higher temps.

Lemming

Hey Carl,

Did the belt ever get sent?

Carl Fausett

Last week already.

Carl Fausett

Another idea I heard of that has merit is trying a Porsche fuel pressure regulator for the 951 (944 turbo) on this kit, instead of or in addition to the FMU we supply. It might provide more fuel under high boost. I have a customer trying that now.