Join Date: Aug 2010
Location: N-East Central USA
FYI: Lobe spacing (phasing cams), VVT
LSA /LCA spacing (ACCORDING TO David Vizard)
LCA=107° or 108° is the best for Esprit ‘107’ cams with 35.5mm inlet valves. The same engine, but w/36.8mm inlets needs LCA=111°
Comparing to any two-valver, four valve heads exhibit a huge low-lift flow.
*The greater the low-lift flow is in the relation of the displacement of the cylinder being fed, the wider LCA needs to be.
*If you bore and stroke your engine, the LCA has to be tighter.
*Increasing the CR requires the optimal LCA to spread, and decreasing CR tightens it. For every CR above 10.5 add 0.75° to LCA
*The faster the intake valve is accelerated off its seat (high lift lobes), the wider the LCA needs to be.
Dyno testing reveals two factors are evident:
1. There is a narrow range of LCAs over which the engine delivers its best.
2. It is better to err on the tight side rather than the wide side. Lower LCA [-1° or -2° from optimal] delivers better torque at the lower rpm, but TOO TIGHT an LCA produces overall negative characteristics, like choppy idle and diminished low speed output. Just how much of a low-speed manners have been lost ultimately depends on the overlap used.
Turbo engines don't like big overlap values. It is difficult to build boost while the back door is open (I.E. overlap, when both valves remain open).
Before the boost onset, the engine acts as N/A and large overlap is beneficial for scavenging.
When the turbo comes in, the large overlap hurts your performance.
Ideally, it would take two LSA settings to cover the whole rpm range. On DHOC engines, one can achieve it by phasing the intake cam.
VVT technology was developed for this purpose.
~~~~~~~Simple cam tuning rules for BOOSTED engines:
Advance intake and exhaust => more low-RPM power, less high-RPM power
Retard intake and exhaust => more high-RPM power, less low-RPM power
Less overlap => lower EGTs, faster turbo spool, less fuel consumption
More overlap => higher EGTs, slower turbo spool, more fuel consumption
*The more retarded in relation to the crank the whole event, the more top end,
*the more advanced = the more bottom and mid range power
So actually you'd end up better off taking the cams in unison either 2* each "towards each other" or "away from each other" (depending on what you want to do with the overlap) and depending on if you're widening to allow a more aggressive tune and if you want more top or mid-range with fast spool...
Last edited by MRDANGERUS; 09-07-2017 at 09:49 AM.