Where does the big sound of a cam come from?
03-07-2006, 07:27 AM
#1
Where does the big sound of a cam come from? I looking for a cam for my LS1, and am trying to figure them out. I want a cam that will work well with a blown engine(mines not blown yet, but will be eventually). But I also want a cam but the big "old school" sound. Where does the sound come from in a cam? LSA? Duration? Lil' help please.
03-07-2006, 08:33 AM
#3
If the lope at idle is what you're referring to, that's caused by overlap. Overlap is the duration that the intake and exhaust valves are open at the same time.......quick and dirty explanation, someone here will elaborate I'm sure. I think overlap would be bad in a FI setup though, you'd be bleeding off all the benefits of forced induction wouldn't you?
03-07-2006, 08:37 AM
#4
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Overlap/Duration/LSA
You really can't build an engine that is powerful N/A and blown at the same time.
It's better to put all your eggs in their respective baskets.
You CAN build it with a good blower cam now... but you'll be down on power until 'eventually' comes.
BTW, Lope isn't neccessarily best for blown applications.
You really can't build an engine that is powerful N/A and blown at the same time.
It's better to put all your eggs in their respective baskets.
You CAN build it with a good blower cam now... but you'll be down on power until 'eventually' comes.
BTW, Lope isn't neccessarily best for blown applications.
03-07-2006, 08:59 AM
#6
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Originally Posted by FunkyTownFBody
Took me too long to type!!!
Yeah overlap isn't really desirable for FI, that's why most turbo guys are running reverse split cams.
Yeah overlap isn't really desirable for FI, that's why most turbo guys are running reverse split cams.
too much overlap though will hurt boost down at low RPM's, which is different from reverse split or any kind of split
03-07-2006, 09:10 AM
#7
The LSA, and also I think the ramp rates (which I suppose affects the overlap?) have an effect. Reason is, you can get a lope without a huge cam. My T1 (.558/221 112) lopes pretty decently without much duration. The Hotcam does too. But I know some of the larger, longer duration cams with the X-ER lobes don't lope quite as much.
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03-07-2006, 09:15 AM
#8
Overlap:
Overlap definition, is the time period when both the exhaust valve and the intake valve are open at the same time. The exhaust valve needs to stay open after the piston passes TDC in order to use the vacuum created of the exiting exhaust gases to maximize the amount of exhaust gas drawn out of the cylinder. The intake valve opens before TDC in order to use the vacuum created by the exiting exhaust gases to start drawing the intake charge into the cylinder.
This sequence of events above are controlled by the duration and LS (Lobe separation) of the cam. On a typical N/A motor this is essential since you have no pressure being developed on the intake side to push the charge into the combustion chamber. The problem with this event is a turbocharged motor will create a larger amount of backpressure on the exhaust side. Due to this event the above definition will not apply. Reason being is, when the intake valve opens at BTDC, the burned gasses in the chamber will exit out the intake since the pressure is lower than the exhaust. Since this is true you would not want to open the intake valve until the piston has started going down, ATDC. This will lower the combustion chamber pressure till it's below the intake manifold pressure.
To calculate the overlap of your cam simply follow these steps below:
**Example turbo cam:**
Duration @ .006 218/212
Lift .544/.544 lift
Lobe Separation (LS) 114
Add the intake and exhaust durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2
Overlap is -6.5 Degrees of overlap
**Example N/A cam :**
Duration 236/242
Lift .568/.576
Lobe Separation (LS) 112
Add the intake and exhaust durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2
Overlap is 7.5 Degrees of overlap
Overlap definition, is the time period when both the exhaust valve and the intake valve are open at the same time. The exhaust valve needs to stay open after the piston passes TDC in order to use the vacuum created of the exiting exhaust gases to maximize the amount of exhaust gas drawn out of the cylinder. The intake valve opens before TDC in order to use the vacuum created by the exiting exhaust gases to start drawing the intake charge into the cylinder.
This sequence of events above are controlled by the duration and LS (Lobe separation) of the cam. On a typical N/A motor this is essential since you have no pressure being developed on the intake side to push the charge into the combustion chamber. The problem with this event is a turbocharged motor will create a larger amount of backpressure on the exhaust side. Due to this event the above definition will not apply. Reason being is, when the intake valve opens at BTDC, the burned gasses in the chamber will exit out the intake since the pressure is lower than the exhaust. Since this is true you would not want to open the intake valve until the piston has started going down, ATDC. This will lower the combustion chamber pressure till it's below the intake manifold pressure.
To calculate the overlap of your cam simply follow these steps below:
**Example turbo cam:**
Duration @ .006 218/212
Lift .544/.544 lift
Lobe Separation (LS) 114
Add the intake and exhaust durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2
Overlap is -6.5 Degrees of overlap
**Example N/A cam :**
Duration 236/242
Lift .568/.576
Lobe Separation (LS) 112
Add the intake and exhaust durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2
Overlap is 7.5 Degrees of overlap
03-07-2006, 09:47 AM
#9
overlap is when both the intake and exhaust valves are open.
i believe its main benefit is known as the scavenge effect. this causes fuel/air to be drawn in faster, like a vacuum, at higher rpms. as we all know, more fuel + more air = more hp.
however, at lower rpms this is more or less a waste of fuel/air because you aren't getting a totally clean burn.
i believe its main benefit is known as the scavenge effect. this causes fuel/air to be drawn in faster, like a vacuum, at higher rpms. as we all know, more fuel + more air = more hp.
however, at lower rpms this is more or less a waste of fuel/air because you aren't getting a totally clean burn.
03-07-2006, 10:12 AM
#10
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I agree with all the above. A wider LSA will contribute to better performance with a S/C application especially at lower rpm's simply because it reduces the time the blower has to push the intake charge out the exhaust, but there is a limit to the LSA you would want to run and get good high rpm performance. I also believe that with FI you have to pay closer attention to DCR and LSA will affect DCR quite a bit.
There is one other thing I have noticed that contributes to lope that hasn't been discussed. It's the by-product of overlap, reversion and it's effect on the MAF. Reversion causes the MAF to get weird measurements. I first noticed this when I put my car into speed density to cal my VE tables. The lope was reduced markedly and the car ran extremely smooth.
I've also read of some running huge cams had increased drivability quite a bit by going speed density and using the MAF for a paperweight.
There is one other thing I have noticed that contributes to lope that hasn't been discussed. It's the by-product of overlap, reversion and it's effect on the MAF. Reversion causes the MAF to get weird measurements. I first noticed this when I put my car into speed density to cal my VE tables. The lope was reduced markedly and the car ran extremely smooth.
I've also read of some running huge cams had increased drivability quite a bit by going speed density and using the MAF for a paperweight.
03-07-2006, 01:15 PM
#14
Originally Posted by 777
I find it funny that there is like 15 comments of the same thing saying i believe it's this, when it's stated 10 other times right before it.
Since when did anyone bring up anything about unburnt fuel??
Your right 0 times! Now that's funny... 
Last edited by Redneck Z; 03-07-2006 at 01:30 PM.
03-07-2006, 03:05 PM
#15
Originally Posted by Redneck Z
Since when did anyone bring up anything about unburnt fuel?? Your right 0 times! Now that's funny...
Your right 0 times! Now that's funny...
Originally Posted by s346k
however, at lower rpms this is more or less a waste of fuel/air because you aren't getting a totally clean burn.
also, i think everyone is trying to explain overlap in terms that are easily understandable. 777, that gom you posted probably isn't completely understood by the majority of the reading audience, hence my post following it.
03-08-2006, 04:07 AM
#16
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Originally Posted by Steel Chicken
that statement is half true. Turbos respond to reverse splits better (more intake duration) and superchargers usually respond better to more exhaust duration
too much overlap though will hurt boost down at low RPM's, which is different from reverse split or any kind of split
too much overlap though will hurt boost down at low RPM's, which is different from reverse split or any kind of split
Thank you for cleaning that up for me, that post was on about 32 hours of being awake, I can't seem to say what I mean, actually....after reading my post I'm not even sure what I was trying to say! Gotta find a new job where I work regular people hours!!! I think I shall restrict myself from posting technically related things to the hours of 4pm-12am
03-08-2006, 07:38 AM
#19
Originally Posted by s346k
perhaps i should've elaborated...you don't get a "clean" burn because some of the mixture simply exits the chamber before it gets a chance to burn, thanks to overlap.
also, i think everyone is trying to explain overlap in terms that are easily understandable. 777, that gom you posted probably isn't completely understood by the majority of the reading audience, hence my post following it.
also, i think everyone is trying to explain overlap in terms that are easily understandable. 777, that gom you posted probably isn't completely understood by the majority of the reading audience, hence my post following it.
