Quote:
Originally Posted by CrazyRob
boost is the pressure of the air that isnt going into your cylinders. airflow is what you want, not boost... cams are more airflow. cooler airflow means more aggressive ign timing=more power=lower spool=more power everywhere.
imo cams and proper tuning should be a first mod.
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"airflow is what we want", i agree totally.
"cams are more airflow"......kind of...and at a different rpm range
"cooler airflow means more timing" cooler air flow means you can add pressure, or temperature, so yeah you COULD add ignition timing. but cam's themselves don't cool down the intake air temps.
more aggressive ignition timing does make more power, to a point. once you reach maximum brake torque (mbt) for a given rpm, that's it. advancing the ignition timing beyond that is just adding more pressure on the top of the piston
before it reaches tdc, increasing your chances of detonation, and actually decreasing your torque when advanced too far.
when you increase pressure you need to decrease temp or raise octane to maintain the balance. soley putting in cams alone does not do this. this is the laws of nature, physics, not my opinion.
where
p is the absolute
pressure of the gas;
V is the
volume of the gas;
n is the
amount of substance of the gas, usually measured in
moles;
R is the
gas constant (which is 8.314472
J·
K−1·
mol−1 in
SI units[4]); and
T is the
absolute temperature.
when you raise one part of the equation, you need to lower another part of the equation, otherwise you lose the = sign
the goal is to run the the least amount of ignition timing you have to in order to make good power per your motor/setup. every motor's different and you have to balance what the motor will tolerate vs what you want it to do. without that balance, it's only a matter of time before you find the weakest link.
just because you increase the duration of the camshafts on a 2.0 liter motor, you're simply changing the time (rpm) of when the motor breaths the most efficiently. however you're not changing the displacement, it's still a 2 liter motor.
as a rule of thumb, just because you increase the cam duration doesn't mean you can, or should run more ignition timing. so i disagree with that statement. changing mechanical timing doesn't warrant changing ignition timing. the two are totally different, they just share the same word, "timing"
as far as profiling the cams goes. back in the day when cams were custom ground, there was a lot of variance and imperfections and what not. now that 95% of cams are made on a CNC machine there's a lot less imperfection and they're already pretty close to perfect. but again you need to have some form of adjustability to do that and you need to know the math & purpose for doing that cam in the first place, as well as plot out what intake centerline would be optimum for your motor, the cam you're using, and what you'll be using the motor for.
then we also need to specify that we're talking about cams on boosted motors and not n/a motors because the cams are TOTALLY different.
rule of thumb is cams on a boosted motor, especially the exhaust cam is gonna be pretty mild in order to keep exhaust temps higher to spool the turbo better. on a turbo motor your POWER is not coming from displacement as it's coming from the turbo's ability to create pressure. therefore the primary objective is to run boost and run it efficiently. which means making and holding boost as efficiently as possible. therefore picking the right turbo is waaaaaaay more important than slapping a big ass cam in the motor that's definitely lacking in the turbo department.
for my 2j 240z i may even leave the usdm turbo exhaust cam and upgrade just the intake cam only, or i may leave the stock ones in all together depending on how the motor performs. however i promise i won't just slap a set of cams in there first without knowing and understanding what my motor's doing and why i'm putting the cam in there, again why i made the thread.
we can even discuss, in my opinion the "uselessness" of big ass cams on a bone stock head.
we all know motors have a bunch of different things on them, all with a job, all working together to achieve a set goal.
the job of a cam with a bigger profile and longer duration is to hold the valves open longer and change the mechanical timing efficiency of that same 2.0liter or whatever motor. however without changing the displacement of the engine, without changing the swirl effect of the combustion chamber (head), without changing the ability for the valves to seal (3, 5 angle valve job), and without changing the ability of the head to flow air more efficiently (port/polish) you're still falling short and only addressing one piece of the puzzle comprised of many pieces.
we should never just slap parts on our motors without understanding how those parts are going to affect it.
lets say you're starting out with an NA motor THEN boosting it, for example, going ka-t.
my opinion of the usefullness/uselessness of cams here changes greatly. once the TURBO "kit" with all the supporting mods are thoughtfully installed to meet it's intended goal, then i'd totally put some mild "turbo" cams in it, expecting and gaining a substantial increase in power and efficiency.
great discussion
Dave