NA vs. FI Cam

[SS Killer]

I was wondering if someone could state the differences between a cam for a naturally aspirated setup and a cam for a forced induction setup.

As for NA, I know you kinda keep the intake and exhaust durations about the same but forced induction you favor the exhaust, right?

I'm just trying to put a good enigine package together so I can start purchasing components instead of waiting and buying it all at once.

Thanks ahead of time for any insight you can provide!

[SoHo48]

uhh its pretty simple.. a n/a cam generally has more overlap, narrow lsa.
f/i cam usually the same amout of lift, wider lsa. a blower cam to me always runs out a little smoother. but i dont like f/i as much as a n/a n2o but thats just me.

[txarkitekt]

Hmmmm...I thought it was the exact opposite: FI cams require a tight (114-118) LSA and N/A cams can have wider (110-113) LSA...

N/A cams don't have to have even duration numbers, but I think they run better that way. FI cams should have an exahust bias. It is more favorable when running wrapped longtubes (RMT or S/C) and custom wrapped headers (for FMT).

[foggy]

Hmmmm...I thought it was the exact opposite: FI cams require a tight (114-118) LSA and N/A cams can have wider (110-113) LSA...

N/A cams don't have to have even duration numbers, but I think they run better that way. FI cams should have an exahust bias. It is more favorable when running wrapped longtubes (RMT or S/C) and custom wrapped headers (for FMT).

Txarkitekt, you have it backwards.114-118 is the wide lsa, 110-113 is a tighter lsa.
Lobe Separation Angle
Lobe Separation Angle, or LSA, is the number of degrees that separate the peak lift points of the cam's intake and exhaust lobes. LSA helps determine the cam's behavior; you can take a given set of lift and duration figures, change the LSA, and get cams with vastly different characteristics. Generally, a cam with wider LSA (112-116 degrees) offers less overlap between intake and exhaust opening and closing events. That translates into a wider rpm range, better idle quality, and higher engine vacuum, but at the cost of less torque at low and midrange rpm. A cam with a narrow LSA (104-108 degrees) offers greater low and midrange torque production, but with a narrower operating range, a choppy idle, and less engine vacuum.

For the street, you want a cam that offers a compromise--decent idle quality, respectable vacuum for operating power brakes and such, and good overall power production. separation. Again, much depends on the overall engine combination and intended use, but as a general rule, cams with a 110 to 112 degree LSA offer good power and decent street manners.

[SoHo48]

thanks....

[SoHo48]

Txarkitekt, you have it backwards.114-118 is the wide lsa, 110-113 is a tighter lsa.
Lobe Separation Angle
Lobe Separation Angle, or LSA, is the number of degrees that separate the peak lift points of the cam's intake and exhaust lobes. LSA helps determine the cam's behavior; you can take a given set of lift and duration figures, change the LSA, and get cams with vastly different characteristics. Generally, a cam with wider LSA (112-116 degrees) offers less overlap between intake and exhaust opening and closing events. That translates into a wider rpm range, better idle quality, and higher engine vacuum, but at the cost of less torque at low and midrange rpm. A cam with a narrow LSA (104-108 degrees) offers greater low and midrange torque production, but with a narrower operating range, a choppy idle, and less engine vacuum.

For the street, you want a cam that offers a compromise--decent idle quality, respectable vacuum for operating power brakes and such, and good overall power production. separation. Again, much depends on the overall engine combination and intended use, but as a general rule, cams with a 110 to 112 degree LSA offer good power and decent street manners.

meant to say thanks to this guy, dont want to be a post whore

[SS Killer]

Txarkitekt, you have it backwards.114-118 is the wide lsa, 110-113 is a tighter lsa.

That's what I thought... OK, thanks guys!

[SCIV]

I would think it would depend on what type of FI.

Turbo is not the same as centrifigual which is not the same as roots/screw

[SS Killer]

I would think it would depend on what type of FI.

Turbo is not the same as centrifigual which is not the same as roots/screw

LSA could be narrower for a S/C because there's little to no backpressure on the exhaust side but with a turbo you'd want less overlap so the backpressure wouldn't force air back out of the cylinder?

I know you want overlap on N/A so that the escaping exhaust will help "pull" the air in through the intake. But, if you have huge backpressure in your exhaust system, overlap would be counter-productive, right?

[txarkitekt]

Well, a turbo works a lot different. You are creating stresses on both sides of the motor--intake and exhaust. Air is being shoved in as the exhuast is pushed out, which spools the turbo to shove air back in...get it? SO, you would need a tight LSA so that the exhaust distribution is close to even with the pressurized intake air coming in. Not doing so would make the motor run like chit. Have you ever seen a turbo motor with a huge, nasty cam it it? Nope, didn't think so.
A S/C is a slight bit different cause it is belt driven and has nothing to do with the exhaust helping to build boost. A S/C can have a slightly wider LSA than a turbo because the exhaust is unpressurized and needs more of a free-flow on it's way out. You still need to have a decent amount of back pressure on the motor... S/C's would run fine with no backpressure since the boost is belt driven and it needs the exhaust to escape faster than the air coming in--given it is being boosted. But, a rule of thumb: Turbo cams should have a lowest of 115 LSA up to a stock 118.5 LSA (depending on motor). A S/C cam can have a lowest of 113 LSA to a 118.5 LSA. And an N/A cam can be anywhere from a reverse-split 108 LSA to a 118.5 LSA.