Split duration camshafts and D-jet

[lmcchesney]

Premise: type IV heads tend to have excessive heat. The source of the heat is from inefficent exhaust head flow, or duratin of flow. Exhaust head flow is dependent on exhaust valve curtain area, port geometry, amount of lift and duration of lift. Split duration cam shafts provide for increased exhaust duration. Split lift cams provide for increased exhaust lift. D-jet has problems with the vacum signature of increased overlap.
Reviewing flow bench results, I don't see significant increases of flow in Type IV exhaust valves in lifts above .350- .400. Increased duration while maintaing stock overlap (12° BTCS and 4° ATDC) requires the exhaust valve opening to occur about max. piston speed on combustion stroke (Exh dur 275° with EVC at 4°ATDC places EVO at 91° BBDC).

The questions:
Being kind of new to this I have a question. I heard many people say that D-jet and speed density FI in general don't like overlap because it screws with the vacuum signature. Is that just at idle or is it throughout the power band?

Dave (DNHunt Mega Squirt 2.0L 914 Club USA)

How much Exhaust duration can you have without negatively impacting the compression(Power) stroke?
How much overlap can D-jet handle at: Idle? Mid speed? High speed?
(excluding effects of Hermholtz waves since we cannot change them).
Should you use just increased duration, or just increased lift or combination?
If combination, what mixture?
How much influence do we believe tunned header will effect the combination (now we can use hermholtz waves since we can tunne headers)?

Thanks,
L. McChesney

[raygreenwood]

The most restrictive part of the equation...as you just described....is the valve and the valve pocket, not the cam. As you noted, increased lift after a certain point does nothing for flow. D-jet already has a negative problem with overlap. The web 73 actually loses some of the compression for you, which is why running lowered compression drops the efficiency so much...and runs you hot. D-jet actually likes a small amount of overlap. Thats the positive part. Overlap...to a certain degree actually smooths the vacume signature by allowing less of a "pile-up" at the intake port. It can also run you lean by over scavenging intake charge if the overlap becomes too great. This will also run very high exhaust temps. Have careful portwork done and put on a more efficent header. I would be interested to see what a few extra degrees of overlap would do as well, but its really a waste if the port is a restriction. Ray