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The camshaft is a magic item that increases power.. bwahahaha.. that's it!! NOT!
I guess the more knowlegable will know that the camshaft pushes the valves open and let's them close before the valves have a 'kissing session' with the pistons.
So what's with those numbers? 250, 120, 11.5mm etc?
Simply put, those numbers mean this.
What those numbers means are when the duration measurements are taken (for the intake cam at this example), at TDC, the valve has opened 1.27mm from its seat (the numbers will be a little off, depending on your rocker arm's tappet wear, or the flat portion where the cam lobe pushes the rocker arm. Japanese does it differently, and they measure duration at 1mm lift. This causes american cam duration numbers to be slightly smaller than japanese cams.
Please do not take duration numbers without lift numbers seriously.. the SAME 240-250 duration cam at 1mm lift can be spec'd at 270-290 at 0mm lift. So that's why those lift numbers are important.
The lift numbers like 11.5mm, etc. are pretty obvious, and I won't explain it too much, other than japanese spec it as tappet lift, and americans use valve lift... simply put, higher lift = less piston clearance, less allowance for high rpm (unless you mill the piston reliefs)
As to how they work.. they control your dynamic compression ratio.
Say.. you have a STATIC compression ratio of 13 to 1,
So if you use a cam that closes 50 degrees after BDC on the compression stroke, you can calculate how it affects your dynamic compression ratio.
It needs to be noted that VTEC has two stages of cam profiles (the first stage is the primary and secondary lobe, and the second is the mid lobe (VTEC), so calculating the dynamic compression ratio will vary from before VTEC to after VTEC.
I'm going to show only the mid (VTEC lobe) for an example, you can use it to calculate the primary and secondary lobe, and then average it out for a close estimate.
A full 180 degrees is from BDC to TDC, so we put those together as a percentage.
((180-50) / 180) x 100 = 72%
So your compression ratio is 72% of a full 180 stroke from BDC to TDC, so that 13 STATIC compression just got reduced by 28%
13 * 28% = 3.64, so your 13 - 3.64 = 9.36 dynamic compression ratio, which is a mild compression, and as such, you can build your engine on a higher STATIC compression to get that dynamic compression you wanted. I'm not sure exactly how much dynamic compression you can get away with on 97 RON petrol, but from what I know, it varies from 9-11. (pump petrol isn't 97 RON all the time, due to it's characteristics change due to various factors, from what little I have studied.) Also remember that the valves are still open with 1mm/1.27mm of lift when you reach that full 242 duration, so the compression ratio will be slightly smaller than the final calculation.
Now, you must wonder... if everything is just a close estimation, why bother?
The reason I've bothered with this is that AFTER you get a close approximation of your target dynamic compression ratio, you can choose a cam suitable for your engine setup. And with a few dyno runs and adjustable cam pulleys, you can get it right by advancing or retarding by a few degrees to compensate for errors in the calculation due to various factors like rocker arm wear, primary + secondary lobe duration values, etc.
That's why high duration cams usually hurt low end power on stock engines, because it kills the dynamic compression ratio... and also why on non cam -switching systems, like the proton engines and etc, it will result in a rough idle and poor low RPM power... it's one or the other for non cam-switching systems, and why NA VTEC is tunable to be very powerful for it's engine size.
You need higher static compression to use high duration cams which will give power at high RPM.. remember I said HIGH RPM, because flow suffers at low RPMs, and that compression will be pushed out the intake valve port.. that's why low rpm has milder cam lobes, to prevent this from happening.
The reason VTEC can use higher dynamic compression ratio without hurting the low rpm power 'much' is because it has that "low cam" for the lower end of the RPM, and which is why instead of earlier VTEC crossover point, the high duration VTEC cams usually are suggested by crower to open LATER in the RPM range.
Higher lift = less resistance to flow.. at the expense of clearance of the valves and pistons, so that's something to think about.
Why some people claim difference when installing aftermarket cams pulleys, even if their engine is (near) stock?
Hmm.. you know that half round shape that's called 'woodruff keys' (I think that's what they're called) that slots into the slots on the cams and the crank? That's liable to wear, and usually even the stock cam timings will go off by 1-2 degrees. Which is why sometimes cam pulleys are 'said' to gain power just after installing.. the actual reason is more like, you gained back that loss compression due to the 1-2 degree off due to wear and tear (not to mention that it's lighter, but those kind of power gains are measured in fractions of an HP, so let's not delve into that). So it can be said that the procedure of 'degreeing the cams' to ensure accuracy did that' instead of some magic item like the cam pulley.. The cam pulley simply allow adjustments to take up slack in the wear and tear of those slots in the crank and cams.
Due to the way cams are made, you can expect a little gain from aftermarket cams that have 'VERY SLIGHTLY' higher duration, but that power gain will come post VTEC.. with probably a little from the primary+secondary lobes (depending on how much different they are from stock cams). If you're thinking.. "Hey! If I plonk it cams with a VTEC lobe with 270 degree duration at 1mm of lift, I'll be able to get more power!" All I can say is... prepare to be disappointed if you're stuck on 8K rpm limit.
As for the procedures of degreeing cams.. I also know, but allow the mechanics around malaysia some leeway in trying to make a living, okay? :P
But if you want to know.. feel free to ask.. but I guarantee you won't have the equipment.. :P
Cams are magic!! NOT!
Now.. the only thing left is to bore the block and do some headwork, and calculate the compression so I can choose cams.. too bad cash flow is rather stagnant right now.. :P
I guess the more knowlegable will know that the camshaft pushes the valves open and let's them close before the valves have a 'kissing session' with the pistons.
So what's with those numbers? 250, 120, 11.5mm etc?
Simply put, those numbers mean this.
What those numbers means are when the duration measurements are taken (for the intake cam at this example), at TDC, the valve has opened 1.27mm from its seat (the numbers will be a little off, depending on your rocker arm's tappet wear, or the flat portion where the cam lobe pushes the rocker arm. Japanese does it differently, and they measure duration at 1mm lift. This causes american cam duration numbers to be slightly smaller than japanese cams.
Please do not take duration numbers without lift numbers seriously.. the SAME 240-250 duration cam at 1mm lift can be spec'd at 270-290 at 0mm lift. So that's why those lift numbers are important.
The lift numbers like 11.5mm, etc. are pretty obvious, and I won't explain it too much, other than japanese spec it as tappet lift, and americans use valve lift... simply put, higher lift = less piston clearance, less allowance for high rpm (unless you mill the piston reliefs)
As to how they work.. they control your dynamic compression ratio.
Say.. you have a STATIC compression ratio of 13 to 1,
So if you use a cam that closes 50 degrees after BDC on the compression stroke, you can calculate how it affects your dynamic compression ratio.
It needs to be noted that VTEC has two stages of cam profiles (the first stage is the primary and secondary lobe, and the second is the mid lobe (VTEC), so calculating the dynamic compression ratio will vary from before VTEC to after VTEC.
I'm going to show only the mid (VTEC lobe) for an example, you can use it to calculate the primary and secondary lobe, and then average it out for a close estimate.
A full 180 degrees is from BDC to TDC, so we put those together as a percentage.
((180-50) / 180) x 100 = 72%
So your compression ratio is 72% of a full 180 stroke from BDC to TDC, so that 13 STATIC compression just got reduced by 28%
13 * 28% = 3.64, so your 13 - 3.64 = 9.36 dynamic compression ratio, which is a mild compression, and as such, you can build your engine on a higher STATIC compression to get that dynamic compression you wanted. I'm not sure exactly how much dynamic compression you can get away with on 97 RON petrol, but from what I know, it varies from 9-11. (pump petrol isn't 97 RON all the time, due to it's characteristics change due to various factors, from what little I have studied.) Also remember that the valves are still open with 1mm/1.27mm of lift when you reach that full 242 duration, so the compression ratio will be slightly smaller than the final calculation.
Now, you must wonder... if everything is just a close estimation, why bother?
The reason I've bothered with this is that AFTER you get a close approximation of your target dynamic compression ratio, you can choose a cam suitable for your engine setup. And with a few dyno runs and adjustable cam pulleys, you can get it right by advancing or retarding by a few degrees to compensate for errors in the calculation due to various factors like rocker arm wear, primary + secondary lobe duration values, etc.
That's why high duration cams usually hurt low end power on stock engines, because it kills the dynamic compression ratio... and also why on non cam -switching systems, like the proton engines and etc, it will result in a rough idle and poor low RPM power... it's one or the other for non cam-switching systems, and why NA VTEC is tunable to be very powerful for it's engine size.
You need higher static compression to use high duration cams which will give power at high RPM.. remember I said HIGH RPM, because flow suffers at low RPMs, and that compression will be pushed out the intake valve port.. that's why low rpm has milder cam lobes, to prevent this from happening.
The reason VTEC can use higher dynamic compression ratio without hurting the low rpm power 'much' is because it has that "low cam" for the lower end of the RPM, and which is why instead of earlier VTEC crossover point, the high duration VTEC cams usually are suggested by crower to open LATER in the RPM range.
Higher lift = less resistance to flow.. at the expense of clearance of the valves and pistons, so that's something to think about.
Why some people claim difference when installing aftermarket cams pulleys, even if their engine is (near) stock?
Hmm.. you know that half round shape that's called 'woodruff keys' (I think that's what they're called) that slots into the slots on the cams and the crank? That's liable to wear, and usually even the stock cam timings will go off by 1-2 degrees. Which is why sometimes cam pulleys are 'said' to gain power just after installing.. the actual reason is more like, you gained back that loss compression due to the 1-2 degree off due to wear and tear (not to mention that it's lighter, but those kind of power gains are measured in fractions of an HP, so let's not delve into that). So it can be said that the procedure of 'degreeing the cams' to ensure accuracy did that' instead of some magic item like the cam pulley.. The cam pulley simply allow adjustments to take up slack in the wear and tear of those slots in the crank and cams.
Due to the way cams are made, you can expect a little gain from aftermarket cams that have 'VERY SLIGHTLY' higher duration, but that power gain will come post VTEC.. with probably a little from the primary+secondary lobes (depending on how much different they are from stock cams). If you're thinking.. "Hey! If I plonk it cams with a VTEC lobe with 270 degree duration at 1mm of lift, I'll be able to get more power!" All I can say is... prepare to be disappointed if you're stuck on 8K rpm limit.
As for the procedures of degreeing cams.. I also know, but allow the mechanics around malaysia some leeway in trying to make a living, okay? :P
But if you want to know.. feel free to ask.. but I guarantee you won't have the equipment.. :P
Cams are magic!! NOT!
Now.. the only thing left is to bore the block and do some headwork, and calculate the compression so I can choose cams.. too bad cash flow is rather stagnant right now.. :P
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