Supercharger cam suggestion?

[WreZ]

Tossing up the idea of going for supercharging rather than turbocharging and a main concern was which cam to use. I was wondering if anyone who has installed a supercharger could tell me if supercharger cam's have the same requirements of valve overlap (or lack thereof) that turbo cams do and if i could just use a p90 head with a turbo cam and run about 8lbs of boost safely using that setup.

[yellowoctupus]

I'm going to keep the stock cam in mine, as it's still going to be a street car. If you're going for all out drag, or race applications, it'd probably be better to swap in the turbo cam. Turbo engines are designed for max horsepower (as opposed to blower engines, for max torque), and as you know, part of the horsepower equation is RPMs. Since they are trying to create high torque at a high rpm, at low rpms the pressurized inlet charge of a blown engine will force it right through and out the open exhaust valve during the overlap time. This is of course good for scavenging, but can be detrimental to Catylic converters and adversely affects gas mileage. (I got a bit of this from A.Graham's Forced Induction Performance Tuning)

[jc052685]

Supercharger cams are typicaly a N/A or Nitrous grind I believe.

[BRAAP]

I’m only going to touch on the basics. For a more in-depth and easy to understand read on this topic, this book is wonderful;

 

http://www.themotorbookstore.com/supercharged.html

Picture courtesy of Motorbooks intl.

 

 

None of this applies to Turbo charged engines whatsoever! Due to the ever changing back pressure to intake boost pressure ratios, Turbo sizing, flow variations between the intake and exhaust tracts, etc, the ideal cam timing events for a Turbo are not as clear cut and easy to define. Turbo engines are a dynamically changing beast and very VERY difficult to narrow down optimal lobe profiles, (I feel only small handful of people in the world could come close, and even then, they will tell you its art, not a science).

 

Back on topic, Blower cams!

Generally speaking, optimal cam timing for Nitrous and supercharged engines are similar, though not identical. Reason for that is the combustion chamber is being force-fed more air and fuel via an outside means, yet that cylinder has to expel that extra burnt charge on its own, no help. Where they differ is during valve overlap, (blower is forcing more air and fuel in the chamber which exaggerates scavenging during valve overlap, N2O doesn’t do that so much.)

 

On to cams for supercharged applications…

Off-the-shelf cam profiles for supercharged applications “generally” have more aggressive exhaust lobe, (more duration and lift) vs the intake lobe. Supercharged cams also tend to have less valve overlap, i.e. wider lobe separation/LSA because the incoming charge is being forced in, and with a tight lobe separation, (lots of valve overlap), you end up over scavenging the cylinder. The blower is now pushing the spent gasses out the exhaust during valve overlap as well as fresh oxygen and fuel rich charge. Less overlap is required to achieve the same scavenging.

 

I say these off-the-shelf cams “generally” have more duration and lift on the exhaust side because most people are bolting a supercharger on their N/A engine, even though some will drop the compression ratio which is a good thing when adding a blower, they generally leave the head alone, i.e. ports and valve sizing is optimized for N/A applications. As such, running a more aggressive exhaust lobe profile helps the engine rid of the extra spent gasses that were “blown” in by the supercharger.

 

Lets throw a wrench into this mess and speculate that you had the head built specifically for a supercharger! A head built specifically for supercharging/N2O will have much more attention paid to the exhaust side of the head. Again, the engine is being force fed the air and fuel by the blower yet that same cylinder does not have any extra help to expel the extra spent charge, so biasing the exhaust valve and port helps allow that cylinder to expel the spent gasses. The exhaust side of the head would receive lots more porting, a larger valve, etc. Sometimes, depending on the application, a smaller intake valve to allow more room for an even larger exhaust valve. In the N/A applications it is common for the exhaust valve to be in the realm of 75% the size of the exhaust valve. In a supercharged/Nitrous application, depending on the boost levels, that intake to exhaust valve size ratio becomes much closer to 100%. 85-90% is quite common. How does this affect the valve timing events? In preparing the cylinder head specifically for a supercharger, now instead of just throwing more lift and duration at the exhaust valve like those "off-the-shelf" blower cams, the exhaust valve timing events can be more ideal, less of a compromise, allowing for even just a little more power for a given boost level.

 

I’m not going to get any deeper into this as that information is out there, in print and on the net. Suffice to say, that camming for a supercharged application is much easier to get “right” than it is for a Turbocharged application. Though the efficiency the turbo affords is hard to beat..

 

Hope that helps shed some light on blower cams.