Blower Cam profiles vs Nitrous and Unblown
— contributed by Badassengines.com
Nitrous pretty much IS supercharging. It is simply a chemical way of putting more “air” (oxygen) into the engine. Whether you do it with pressure (a blower) or with a chemical (nitrous), you MUST maintain a given air/fuel ratio for it to burn. A blower forces atmospheric air (containing oxygen to burn more fuel) into the engine, and nitrous does it with a dense, oxygen rich (3 times more than what the atmosphere contains once it hits 550 degrees of temperature in the cylinder). Either way, you need to meet that oxygen with fuel to make power. Air doesn’t burn, fuel burns. They key to making power is making the engine burn more fuel. That can be done with a blower, nitrous, or more cubic inches to suck-in a bigger gulp of air (and fuel). With that, the effect on cylinder pressure is also the same in any 3 of those scenarios.
The differences between nitrous / supercharged engines, and naturally aspirated engines is the cam profile (and the compression in some cases). The basics of what you want in a nitrous (or supercharger) friendly cam is a wider lobe separation than what most “performance” or “race” cams have. Anything with a 112, 113 or 114 degree lobe separation is great for blowers and nitrous. Can you use a performance cam with a 110 or a 108 LSA? Of course, but you won’t get as much of the benefit from the power adder as you would with a wider separation angle.
” Hot cams” narrow that angle down to gain more of the scavenging effect at higher RPM’s from what is called “overlap”. That overlap is what gives the engine that rumpity rump sound that everyone thinks gives the engine more power. Overlap is the time between when the exhaust valve closes and the intake valve opens. It is when both valves are off the seats at the same time. It is also what causes cams to not “come in” until higher RPM’s. The more overlap, the higher the RPM range will be with that cam.
Cams with wider lobe separation angles “come in” sooner and pull harder at lower RPM’s. They also retain or “contain” the cylinder pressure of superchargers and nitrous much better at lower RPMs.
If you run a blower (or nitrous) with a true race cam that has a 102 or a 104 LSA you will blow most of the blower boost (or nitrous and fuel mixture) right out the exhaust. That is wasted energy that could have been inside the cylinder pushing on those pistons. This same effect is also true for LSA’s with anything less than 110 degrees when using nitrous or a supercharger. Lift and duration don’t have much effect on it. Blower cams however like lots of duration because the longer you hold that intake valve open, the more fuel and air that blower will push into the cylinders.
Keep in mind, this is a HUGE subject, and I am not going to write a book so everyone can have free educations in this stuff, but I’ll share as much as I can and try to touch on as much as I can in a short amount of time. With that, naturally aspirated engines need narrower LSA’s at high RPM because with more overlap, that unburned mixture blowing out the exhaust PULLS (siphons) a bigger charge of burnable mixture back into the cylinder at higher RPM and actually raises the cylinder pressure (and “effective” compression) without the use of a power adder. Again though, this is why “big” cams make the engine a turd at lower RPM’s (because they are bleeding off cylinder pressure because the scavenging effect hasn’t began because the engine isn’t at a high enough RPM yet), so “big” cams don’t make the engine come alive until 3,000, 4,000 or more RPM. This is also why “race cars” have high RPM stall converters, super low gears. They leave the line and run down the track at high RPM’s. That’s the ONLY place those engines make their power. A 750HP engine at 4,000 RPM will usually get its butt kicked by a 400 HP engine at say, 2,500 RPM because a 400 HP engine will have a “smaller” cam (less overlap), and will make more torque (which is what moves the vehicle) at a lower RPMs.
Some true race cams with lots of overlap and narrow lobe centers don’t “come-in” until 5,500 or so RPM. They have a very narrow power range (5,500 RPM to 8,500 RPM). When the power comes in, it is explosive, but again, the engine will be a complete turd if it isn’t up in it’s working RPM range where that scavenging effect of the cam comes into play.
The problem is, people with “hot rods” love that race cam “sound”, and unfortunately they think that sound is a sign of an engine making power. It ain’t!! Especially in a street engine. This is why there are so many rumpity rump cars out there with guy’s that “claim” they have 500 or 600 HP engines, yet at the track they peel off a 14.97 quarter mile time, while the guy that knows what he is doing has a car that might not be so rumpity rumpity, and peels off an 11.90 quarter mile time in full street trim. The cam can (and most certainly does) make or break the car’s performance. Just like carburetors, bigger is certainly NOT better!
When you are running nitrous or a supercharger, the rule of thumb is you usually want to stay away from cams with a lot of overlap and go with a cam that has a wider lobe separation. If you want to learn more about this stuff, check out my Heads, Porting & Valve Trains DVD set.
Aaron Johnson – Badassengines.com