Big valves and D-jet

[DDS]

Will ported big valve (44 and 38mm) heads cause a mixture problem with D-Jet provided I make no other changes?

TIA



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-- Dave
Pics of my '73 project here: http://members.tripod.com/mike4g/dds73_b4pics.htm

[Dave at Pelican Parts]

That's a very good question. I can see how the stronger pulses from the larger valves might affect the D-jet in a bad way, but then again they would be evened out a lot by the plenum (the manifold). The engine will probably be breathing better at any particular level of vacuum than the stock engine does, so chances are you will have to do some adjustment to the system. Check Brad Anders' web page http://members.rennlist.com/pbanders for lots of details on how that can be done.

--DD

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Pelican Parts 914 Tech Support

A few pics of my car: http://www.pelicanparts.com/gallery/Dave_Darling

[pbanders]

Porting, big valves, or TB modifications affect the volumetric efficiency curve of the engine. If they do nothing more than change the peak of the curve and not the shape, then changing the overall mixture will compensate. If you change the shape of the curve significantly (e.g. change valve timing and lift), you're likely to have problems. The ECU uses four waveform generators to develop an overall curve that is used to correct the mixture. If you were REALLY SMART (not me ), you could modify the waveform generator circuits to replicate the new Ve curve for the engine.

My guess is that with big valves you are probably increasing the Ve peak a bit. Richen the mixture (e.g. increase fuel pressure, add TS2 ballast resistance, adjust the MPS part-load screw) to compensate. Use an air-fuel meter to do the adjustment.

Brad Anders

[DDS]

Thanks. D-Jet is intriguing. At what point do you suppose the mechanical parts of the system, (throttle body, runners, plenum, OK, injectors too) become the impediment to flow? It would be interesting to see how much one could get out of the stock induction plumbing. I'd need a new brain I think (FI), as I am not smart enough to change the curves either.

Brad, the D-Jet info on your site is amazing.

[John Rogers]

Several knowlegable 914 people have told me the D-Jet is limited to about 6200 to 6500 RPM maximum on a 2.0l engine. From looking at the size of the throttle body and the intake runners I would agree with that.