TO ALL SIFUS PRO IN NA!!!!!
VALVE TIMING!!!! VALVE TIMING!!!!
Please help me... I think of this everyday I cannot solve!!! I cannot sleep and get bad headache.... I think all the time until stress the hell out of me... I take time in office to read everyday until I cannot work... I read so much until my eyes blurr.... PLEASE PLEASE PLEASE help to enlighten this NA noob... My brains are about to burst. There are a few things about NA that I find that I do not understand, or rather, there are contradicting facts...
1. Advance timing will give more low-end torque (Intake valves only; on DOHC; all else same)
Why? I know they said that if advance cam timing, it will enable the intake valves to shut earlier thus trapping more air & fuel building more pressure inside the cylinder. Fine. Acceptable. But what the heck? By doing this, if the intake valves shut earlier, the engine might not suck in enough air & fuel if the piston haven't reach the BDC. So doesn't this actually reduces the amount of air-fuel and pressure inside the cylinders?
2. Retard timing will give more top-end power (Intake valves only; on DOHC; all else same)
Why is this? People said that by retarding the cams, the valves will shut later thus allowing for more air-fuel to enter the engine. But making it shut later will also mean that the valves will open later! Then the air-fuel could not enter the cylinders yet when the piston start to move down! And also, doing this will miss the chance to make use of overlapping with the exhaust stroke (scavenging effect) which is suppose to help draw more air-fuel in.
3. High duration & lift cams will improve top end power, reduce low-end torque and severe the fuel consumption.
I understand about the top-end power and low-end torque compromise. But why does this affect the FC so badly? Suppose that we have a high duration and lift cam engine, at idle and low-revs, the high duration and lift will let the air-fuel escape back to the intake manifold because the valves cannot shut off when the pistons start moving up. Isn't this suppose to improve FC because we actually use less air-fuel? Doesn't this reduce the effective displacement of an engine? For example, a 2.0 L engine might have an effective displacement of only 1.8 L.
This also cause the power/expansion stroke to have a longer movement downward compared to the compression stroke simulating an atkinson cycle engine which is more economical. Just like the camry 2.4 engine.
Help help help help me please... I'd be really grateful if anyone can share your knowledge...
simple..
answer for 1: its all because of the velocity...and flow efficiency...imagine u were on a road with several traffic light that have same distant....and the light turns into green as u cruise by slowly with minimal traffic....that was on low rpm...but when try to drive past all the light in high speed on sam traffic u can past some light..but not all because u have to stop/slow down to wait the slow-next green light...short time frame for green light also making u slower when high traffic...the capacity of traffic was even small because all of them would be slower and slower since too many traffic
this means the engine starving on high rpm..compared to low rpm relaxed condition...and u cant go fast
answer for 2: same traffic light on that stretch...but one day u were cruising like normal,suddenly one of the light on red longer......even the green was longer by the time u start to move the next light have already turns red..have to wait long again before move on slow speed cruising....but if u drive fast enuf u can go through past all the light easily.....so when even high traffic capacity travelling at high speed all of you can co through the light sap sap sui....but imagine got hoggers and all have to drive slow in heavy traffic..sure cannot go through all the light..still stuck at some...
this mean the engine was good at high rpm...compared to low rpm "hogger"....u have to go fast but if u go slow then it wasn't smooth...takes time to accelerate after slowing down...
answer for 3: why FC affected so much???stock camshaft lift and duration have been calculated by the engineer to balance power,torque and acceptable fuel consumption in mind....when u change to higher lift+longer duration..but this means higher fuel sucked.....
coz higher lift+Longer duration sucked more fuel+air during intake stroke....
but longer duration waste the sucked fuel+air during compression stroke because the mixture already escaped through the late closing exhaust valve....there u goes wasting money on fuel....on idle......
still,since the powerband move to higher rpm...u have to rev higher to get the car moving from stop...rev higher means higher FC...
more,since it was higher lift+longer duration to get more power by allowing more fuel+air during intake at higher rpm..and u have drive it on high rpm to maintain the powerband...it simply means higher FC in any way u drive a modified camshaft car...
be in mind....a camshaft only have theorytical optimum range around 2000rpm range....like 3k-5k range...2.5k-4.5k 8k-10k rpm range....so if u get "this much" in specific rev range...u cant get the same on other rev range....this is why v-tec,mivec,vvt,vvti,dvvt etc etc invented.....to give a broader optimum range of a camshaft.....to the engine
