Another Angle on the Nitrous Camshaft Selection Process!
The Camshaft as a Thermostat...
Every day at ProCar we get several calls inquiring about and orders taken for custom cams. Most are bracket or street related, but as our Corvette goes faster, the number of ‘serious' cam recommendations for guys using heavy nitrous doses has skyrocketed. Our view of how a cam works in a power-adder application has changed, and we thought we'd share some of our views here. As always, experiment at your own risk- and feel free to share your findings with us. If opposing options have proved beneficial to other combos, again, feel free to write a column. It's how we all learn.
First off, let's quickly dust off some basics, as this really isn't a beginner's article. There are primarily four families of pushrod, overhead valve camshafts. The most basic and flexible of these is the good old hydraulic. It uses no lash, (more on that later) uses engine oil to maintain resistance against the pushrod, and can actually out accelerate solid flat-tappet cams by a small margin. The entire valve train is relatively is cheap and basic. Are there negatives? Yes- you cannot rev them very far past 6800 or so, as the lifters and springs cannot handle that stress (due to various reasons, not the least of which being lifter pump up) equaling bad news for a nitrous car that will occasionally lose traction and hit the rev limiter. Overall valve lift is also limited, as the multiplying rate of the spring will compress the lifter via increases in rocker ratio, netting a loss in duration and lift.
Solid flat tappets are lighter; do not rely on engine oil inside the body and use lash, or a clearance in the valve train to adjust the acceleration rate of the lifter. The initial area of the ramp that gathers the lash is soft, so increasing the clearance gets the lifter ‘smacked' by a more aggressive part of the ramp. Too much lash will ultimately cause premature valve spring fatigue and lead to the possibility of ‘edge riding' the lifter, flattening the cam.
To eliminate edge riding and allow a high spring pressure, roller tappets were developed to facilitate fast and precise valve timing. The acceleration rate of the lifter and valve is only limited by the pushrod and spring's ability to maintain the profile. Hydraulic rollers are left behind because their oil-filled lifter can't withstand the spring pressures needed to make real power. They are commonly used in street machines and production vehicles, and to 6500 rpm they're a great choice for a big inch N.A. motor. If you're looking for that hot EZ Street cam, or something for a 632 on two fat stages, you're going to need the solid roller.
The next row of numbers on the cam card that need to be explained are the LSA (lobe separation angle) and the ICL (or the intake centerline). The LSA is the physical difference between peak lift of the intake and exhaust lobe, it is ground into the cam and is permanent. The ICL is the phase of the cam, or where the intake centerline is positioned relative to BDC. By adding timing to one side of a lobe, we change the LSA, because the centerline of the lobe has been shifted. By advancing or retarding the cam, we affect all the timing events, as they are linked.
When shopping for a nitrous cam, especially a solid roller, it becomes evident the various cam grinders are hell-bent on providing you with huge exhaust lobes and wide separation angles. Why? There are several reasons, but the most critical negatives they're trying to avoid are pumping losses and low rpm backfire. As you ram more oxygen and fuel in the chamber, the extra burnt stuff has to get out, and the earlier exhaust opening event provided by the wider separation angle and extra exhaust duration gets that done. By advancing the exhaust lobe only, the lobe separation angle is increased, cutting down on overlap, or the period both valves are open. This is important when nitrous is introduced at a low rpm- on a car with a tight street converter and a full throttle switch. If the hit is too big for the combustion chamber to swallow and burn, (due to low rpm) the flame front will climb the intake when the valve opens, messing up some nice shiny parts. Isolating these events by spreading the centers (advancing the exh. and retarding the intake) keeps the flame where it should be, in the chamber.
So, if the catalog grinds available do a good job of utilizing the nitrous and protecting parts, why would we want to re-invent the wheel? Simple- many of those grinds are over ten years old and a lot of new thinking has changed the way we look at camming an engine.
As stated, one of the negative byproducts of nitrous is the need to start dumping the crap left behind after combustion earlier and for a longer time. In an extreme circumstance, the pumping loss that would result from inadequate exhaust timing- the piston wasting energy pushing out hot exhaust- would eat away at the power gained from the nitrous. Perhaps too much burnt charge would be left behind, again defeating the purpose of squirting the stuff in there- intake charges need to be neat and clean. Many of the grinds available to us take this into account, often to a fault. They hang exhaust lobes on these things that would do just fine in a 706, and advertise them as the correct item for a 572. Cylinder head efficiency has come a long way since everyone's idea of what makes the right nitrous cam was formed in the early 90's.
Tuning the closing side of the intake and the opening side of the exhaust lobe are two of the tools we have to determine how much heat the combustion chamber will see and ultimately maintain. After all, heat is what we're after, and why we shoot the spray in there- to release all those sexy little British Thermal Units from the fuel and ram that piston down its' naughty little hole - Ahem. As we said, there are opportunities to maximize these events, to make the heat work for us without advancing the timing or burning parts. While the intake closing point will determine when the charge begins to compress, the exhaust opening is what lets the heat out, effectively ending the useful portion of the power stroke. Cams with wide centers and a ton of exhaust duration will do two things; close the intake valve later and open the exhaust sooner, both reducing the work the engine can get done via heat with the intake charge.
Recent trends that have worked in not only our car, but also those we are familiar with have included shorter, somewhat retarded intake lobes (relative to LSA, keeping the intake opening delayed as long as the proper closing point allows it, helping fight off dilution while increasing dynamic compression) and significantly shorter exhaust lobes, combined with a slightly later, or retarded exhaust opening and centerline. This closes up the LSA, but preserves the exhaust closing point, further staving off intake dilution. Port improvements over recent years have allowed later intake opening without penalty; their velocity lets you start them later but they'll catch up well before peak piston speed. Opening the intake later also helps stave off the hood gymnastics- a little.
In all but the most extreme cases the size and rpm range of the engine should dictate the exhaust lobe; I feel that whether it is sprayed or not is a secondary consideration. This isn't to say that the nitrous doesn't matter; it's still a big concern- but the lobe should be sized in direct relation to the volume the air pump is capable of moving, first. The presence of nitrous only adds another element to this. For example, our 615” Raptor headed (cast by Pro-Filer and sold by Reher-Morrison) Top Street engine carries .050 events smaller than those suggested by R-M for naturally aspirated use- and we burn 4.5 lbs. of nitrous per run. Why would a cam tailored to their suggestions- on motor no less- be less effective than our current grind that went in the opposite direction? It has to do with the type of transmission, gear ratio and average engine speed we run. Since we average less high rpm (time wise, during the run) than a 5-speed car, average hp over 7600 doesn't mean nearly as much as average power from 6600-7500. A naturally aspirated engine spinning (an average of) 700 rpm higher on a pass will need a bunch different cam than one spinning lower speeds.
I know, lots of elements are getting added, but follow along- it all comes together. If we were running this engine to 8700 rpm 4 times, we'd cam it much differently than an engine that will see 8000 twice- (Powerglide) and hang at or near the shift recovery rpm for an extended period. A high rpm engine likes many of the same provisions that a nitrous piece does; wide centers, big lobes, big lift. The problem is, many nitrous cams began as high rpm grinds that had additional nitrous compensation added to them, resulting in overkill. The too-wide centers, late closing intakes and huge exhaust lobes allow for a higher, wider, broader torque curve above its peak, but the peak is often so far down on power the perceived benefit is an illusion, at least for a car with a 1-2 (or the 2-3 in a TH-400) recovery rpm of 65-6800 rpm or so. The increased rpm flexibility is good news for an engine destined for a light 5-speed car, but bad for an engine that spends considerable time at a specific rpm, accelerating slowly away from peak torque. Some (all a bunch smarter than me) feel that a lower ‘natural' torque peak as set up by a more conservative camshaft can be easier on pistons, rings and valves than a larger one that sends torque peaks higher up the rpm band. A higher, broader, but overall weaker/slow accelerating torque curve can become very aggravated once nitrous is added, and lugged. Often detonation can set in before the correct plug ‘heat' readings can ever be reached, baffling the tuner how he's detonated the lands out of it. Timing curves in these engines often severely cushion the 1-2 shift, hurting acceleration.
The solution may lie in providing the engine with more ability to accelerate via shorter timing events, helping it pull up and away from the shift on its own. A slightly tighter LSA (removing the duration from the exhaust opening pulls the centers slightly closer- we have no beef with the exhaust closing point) will also set up a hard accelerating engine through the upper- midrange. The last thing a nitrous fed engine wants to do is sit at a steady state, and anything we can do to help it accelerate up from the rpm drop on the 1-2 will pay off in et and reliability.
How does the engine maintain its high rpm horsepower if the timing events have all been trimmed so? Won't speed and back half incrementals suffer? So far, this hasn't been the case, at least with us. Today's stiffer pushrods, larger lifters and increased rocker ratios have increased the high lift area the valve actually sees- so much so that less gross duration is required to run through 8200 rpm. Our average power or acceleration through our ‘base' rpm range hasn't softened-it's been much better- even up top, but I must admit shift rpm's that were once just a bit out of range cause power to fall off a cliff now. Since we don't normally see that engine speed, it doesn't matter- it might have looked bad on the dyno, but it's indisputable the car benefited from better upper/mid averages.
One final point- we've been able to generate heat in the plugs we've never
previously seen without detonation. The car pulls harder through the 660',
virtually jumping off the gear change, so we think the more aggressive heat
marks point to greater work being accomplished. (No timing has been added over
previous baselines.) By trapping the charge, working to keep it pushing longer
on the power stroke, and trimming timing events to the minimum, we've arrived
with a combination that runs harder, more predictably, and with greater