Hey Craig! :beer:you seem quite knowledgeable and experienced, so for you and Jeff (Wreckless)...
re: cams - it is my understanding that the general trend is more lift/less duration for n/a motors and less lift/more duration for turbo motors. is this incorrect?
re: rev limit - it is my understanding that generally, most turbo motors are revved to just beyond where the turbo noses over (no longer holding power). for example, iirc, my rev limit was 9k-9200 with the 8685 that started to nose over around 86-8800.
On cams - More lift is almost always better unless we're getting into component limitations (e.g. spitting shims, beating the shit out of valve seats, valvespring coil bind/valvespring fatigue failure etc) because more lift always equals more flow everywhere. Whereas duration dictates the RPM range in which that combination will make max HP. Generally speaking, more duration = higher RPM. Overlap between intake and exhaust valves is the biggest difference between camshaft choices for NA, turbo, or supercharged engines. NA engines benefit considerably from some overlap especially in higher RPM ranges, giving everyone that lopey cam chop most folks love to hear at idle. This is because the exhaust side is almost always an area of lower pressure so it's a scavenging-effect trick to 'suck' air in through the intake valve.
Turbocharged engines, on the other hand, always have more backpressure than boost pressure. This means that overlap can be detrimental and actually cause exhaust reversion *back* into the cylinder through the exhaust valves. That's an extreme case, but it's easy to over-cam a turbocharged engine to a point where the power curve is moved further to the right, but peak HP is not improved due to turbocharger limitations.
This is also why most camshaft selections for 2JZ's and other turbo engines don't have much if any lope once they're degreed properly. Cam upgrades on turbo engines generally reduce the boost pressure required to make a given whp figure and on most setups (provided the turbo's exhaust housing or turbine isn't profoundly undersized) that also reduces backpressure somewhat (again, so long as the turbine housing isn't undersized) and with reduced backpressure we have reduced EGT's. Ideally, between cams and turbine housing selection one wants to get the backpressure as close to 1:1 with boost pressure as possible, and the optimal RPM band of a given camshaft selection should be matched to a turbo's power range as closely as possible.
Ideally, the engine should be falling off cam at the same time the turbo noses over - this is why GSC and other cam makers are suggesting turbocharger sizing to match the cams involved, not HP level. Similarly, there's only so much one can push the no-overlap game before it's detrimental at very high RPM, so big ass cams like HKS 280's and GSC S3's will have lope no matter what, but that's based on the needs of big ass turbos at high RPM, which are again under the assumption that it's a big ass turbine housing with minimal backpressure.
So with those cams and big turbine housings things are great when in boost, but the off-boost torque response suffers especially in non-vvti cases due to the loss of dynamic compression. With VVTi engines, they can retard the intake cam to increase dynamic compression at low rpm/off boost and vastly improve off-boost torque and driveability. VVTi is really pretty kick ass stuff.
Similarly, we can get away with more static compression if we've got rowdy cams, due to how the advanced intake opening reduces dynamic compression. So an engine combination that had detonation on a given fuel at 18psi with stock cams, may be just fine at 18-19psi with 272's or 280's installed and the same ignition timing values. The same can also be true of turbine housings, if it's detonating with a .68 housing a 1.00 may add lag but also reduce the backpressures enough to run without detonation.
Power is moved to the right with these changes (as well as peak tq in most cases) so we can see the relationship on how moving the powerband to the right reduces peak tq at lower rpm and how that peak tq value can be used to determine what's safe on a given fuel. These days though, with E85 and such - we normally just want to make more HP and that's why the bigger cams and housings get involved.
But on gasoline, playing these games with stock-ish ~8.5:1 static compression with big housings and big turbines with big cams is how Japanese tuners have achieved 800-900hp on their RON 100 (~94-95 octane) pump premium. Such combinations add substantial amounts of lag, but it doesn't really affect the type of highway racing they do with that kind of HP, it's all high-speed roll-ons often starting at 60-70mph and exceeding 160-180mph at times as they haul ass around toll highways that are deserted late at night.
So bigass turbos like the GReddy T88H-38GK with bigass housings on 8.5:1 3.4L's with HKS 280's make sense - less lift on the cams means less valvetrain beating for those long ass high RPM pulls. This is also why Japanese tuning houses were historically obsessed with oil coolers, coolant expansion tanks, etc. Keeping a car alive for 30-45 seconds in a highway pull is one thing, keeping it alive when it's in boost for 20-45 minute blasts at 150+mph at a time is a much different beast.
Supercharged engines are at the opposite extreme, where there's little if any backpressure from the exhaust system but there's a bunch of pressure/air mass coming in the intake side. The benefits of duration for max HP on a given fuel and higher RPM performance are much the same, but ideal cams for superchargers are typically disproportionate on the intake side vs exhaust side, with a lot more duration on the intake side and less on the exhaust side. Too much duration on the exhaust side of the cam, and especially overlap, leads to boost pressure basically being blasted through the cylinder with little beneficial effect. This is why on DOHC engines with superchargers we'll often see the best results with staggered cams with a 272 on the intake and a 264 or 256 on the exhaust. If the desired RPM range dictates more duration, that duration difference tends to stay proportional (280 intake + 272 exhaust, etc) and of course overlap is tuned out completely for lower RPM engines and it's as minimal as possible to match the needs of higher RPM engines.
With NA and supercharged engines, there's a tendency to over-cam the engine just for the sake of the lopey sound, especially on domestics. So it's common for an LS powered F body or 5th gen to have a cam that'd be ideal for a 7-8k rpm build on an otherwise stock LS1/3 that doesn't see past 6k for the most part.
This moves the powerband to the right and affects low end tq, but big V8's have a lot more low end tq to spare before it's detrimental to driveability. We can see the same effect on turbocharged engines, where folks in the past have put HKS 280's in BPU cars and they sounded great and did pick up high RPM power, but the backpressure and turbo limitations show up long before the cam is done making power, so the power was moved excessively to the right and it's likely, even probable, that correctly degreed 264's or similar would make equal if not damn near the same peak HP but without the added lag.
Overcamming supercharged engines, though, just wastes power. The nature of a SC engine will mask the losses quite well so most people don't notice and they think it sounds cool, so they drive on with it. But going to a proper cam profile developed for a SC engine often shows rather impressive gains, regularly in excess of 50-75whp on a 550-650whp car.
I have also seen two different 5th gen Camaros that were virtually identical builds except for camshaft choice - one was a 6-spd whipple + longtubes car and the other was a 6-spd whipple+cam+longtubes car. The cammed car had a rowdy ass cam meant for an NA. After tuning, the car with the stock LS3 cam was barely ~15whp less than the rowdy-cammed car. The cammed car had picked up close to 70whp from the cam prior to the SC install. That is an instance where installing a proper cam for a SC profile would likely pick up ~50-60whp right off the bat.
I've added to this post throughout this morning so hopefully it's cohesive and makes sense.
I would welcome any input, as I don't consider myself a foremost authority on this stuff, and I'd love to learn more from anyone with additional insight/experience in these things.