MadBill wrote:Re camming for manifolds, just my top of the head opinion and hopefully CamKing, dfree or someone will chime in, but I'd expect the engine would need less overlap via later IVO and earlier EVC to prevent backflow due to lack of scavenging, and earlier EVO to make up for the presumed flow restriction. Using standard cam nomenclature, this might translate to going from say a 230°/230° on a 108°LCA advanced 4° to a 226°/236° on a 112° LCA, advanced 6°.
FWIW if someone has the specs., the early Z-28 cam made more power on open headers than the late one ('70½ on), but less through the stock exhaust.
Hi Bill, interesting topic the stock manifold and cam spec thing. I have not had to do anything for a restricted racing class that requires manifolds but have had to do plenty of street performance type engines that have had to keep manifolds either for origionality or room etc. Most of my experience here also is Ford- windsor-cleveland-B/block based.
What I have found to work best is lowish o/lap figures, intake opening around the same as with extractors but erring to the later side by a little, slightly earlier intake closing, definetly earlier exhaust closing and the biggest improvement , later on the exhaust opening and using a fairly high velocity exhaust lobe.
Trial (and error
)IME lead me to the thinking that with the restrictive nature of the manifolds work with this cam timing style because opening the exhaust later but with a quicker valve action gives less chance for the possibility of any reversion flow into the cylinder before BDC due to any backing up of gasses in the manifold or just having had an exhaust pulse near the outlet of a log type manifold(outlet biased toward the end of the manifold), I notice EX lobes like having a much sharper exhaust note(with manifolds), and low/mid range being noticably stronger but not hurting top end when compared to an earlier opening lobe with less velocity.
The reduce seat to seat o/lap definetly makes them happier down low and less peaky on the tq curve and the faster ex lobe for the closing event just helps keep the HP but also really improves the tq/feel.
Please bear in mind that these are NOT big rpm restricted race engines with manifolds, these experiences have been on street performance engines, trying to make the most TQ and best possible HP with as flat and as useable as possible performance curve and often using power steer and aircon too. Just TQ'y good performing and very street friendly performance engines that work great at low rpm but also run very strong in the 2500-5500maybe 6000rpm area. So Its not going to compare to a track engine.
Someone mentioned H/X pipes with manifolds, I DEFINETLY always use a H pipe with manifolds (and extractors btw) and in actual fact I find more gain with a H pipe on manifolds than with extractors when compared thru the same rpm range. Im thinking this is due to the manifolds more stacked up (for loss of better words) EX flow as they usually have WAY different length paths for ex flow and the fact that when you get a gas pulse at a port near the outlet then a pulse from a port just upstream of the outlet you end up getting a far more uneven pulse/flow in the manifold outlet pipes/engine pipes, so having large dia engine pipes and as big as possible H pipe as close as possible to the manifolds helps even out the ex pulsing as soon as possible. I will use an exhaust system sized to suit the HP level but as big as possible on the engine pipes leading up the the H pipe. Seems to work quite well.
I would be interested to see those Z-28 cam specs too.
Craig.
