treyZ28

A long story short-
A car discussion turns into a turbo vs blower discussion.

My friend states that a propperly setup turbo application use the same kind of cam a blower motor.

I disagreed saying that a turbo setup was like a blower, however the turbo has one more "variable:" spooling the turbo. Wouldn't it make sense that the turbo setup could be optimized?

My second reason for thinking the camshafts would differ i the off the shelf cams made for turbo setups

Looking here at some of their most common turbo cams- they seem really "weird" relative to na and blower cams.


So I figured this would be a great place to ask since there are a ton of qualified engine builders around here

So lets do this 100%

Relative to an NA cam (I think we've disected the NA cam to the point where its a good relative starting point)
How do blower cams varry and how do turbo cams varry.
I read rich's little write up about "overcamming" a blown motor which makes perfect sense as does the added exhuast and no need for overlap as you would in an NA situation. Perhaps you can complete this puzzel and then turbo one for me/us?

Thanks
Trey

NightTrain66

From what I have seen, most BLOWER cams have a wide split duration (exhasut duration 6-14 degrees larger than intake).

Most TURBO cams are reverse splits but the specs are closer (intake duration 2-8 degrees larger than exhaust).

I have bever ran a TURBO or BLOWER so I am not familiar with EXACTLY what the IO, IC, EO, EC differences are (if any) between them.

Both are trying to trap cylinder pressure so they are USUALLY on wider LSA's than N/A set ups. I have seen some LARGE turbo set ups with cams that have 106 LSA's and the theory was that WHEN MAXIMIZED, the turbo would only make "so much" HP (depending on size) and they were using the tight LSA to INCREASE overlap and allow some unburnt fuel to enter the exhaust, build more heat here (since the fuel is burning in the exhaust) and spool up quicker. Since the turbo size was the limit on HP, they were gonna make the same peak power and were just gonna gain some down low.

I am sure others can describe more differences but these are my takes on the differences between the 2.

NightTrain66

JordonMusser

with out getting into too much detail, the reason why they must be different is because of exhaust vs intake pressures. on a blower, you have positive intake pressure, and during periods of overlap you can blow the intake charge right out of the exhaust(with a NA motor, the overlap period helps pull intake charge in).

with a turbo motor, you are working with exhaust back pressure also. Depending on the turbo, anywhere from a big reverse split, to a single pattern. It relaly depends on what kind of back pressure to boost pressure ratio you have.

94bird

Jordon nailed the main driver between the intake/exhaust split right there. Stock turbo motors can easily have twice the exhaust pressure before the turbine housing as the boost they are building in the intake port. A freer flowing setup however can cut this down substantially and come closer to wanting a single pattern cam.

The other poster mentioned tighter lobe separations to spool up the turbo quicker. That's also correct, but I wouldn't normally recommend it for a street car.

For a sleeper street car I'd keep the duration quite mild, less than 220 @ .050" for most street LT1s, get an aggressive lift, and work on minimizing the pressure drop in the intake tract and exhaust system to optimize the system.