How much overlap does 1.8 rocker arms add?
#1
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How much overlap does 1.8 rocker arms add?
I plan to use the Lingenfelter GT1 cam with a maggie mp112? 218/229 duration @ .050 lift .561/.561 lift w/1.7, .594/.594 lift w/1.8 rr 113 CL. I’m not concern about making the most horsepower. I love the sound of the cam and somewhat wide split. I will use crane 1.8 rocker arms. It's has -2.5 overlap with the 1.7's, what would it be with the 1.8? It's my understanding that the small maggie doesn't like positive overlap.
The car is a 2003 corvette with a 6 speed and has the following mods. Stock long block, Intercooled Maggie with 3.0” pulley ~8lbs, K&N/blackwing filter and ported TB, Crane 1.8 rocker arms and Crane 7.25 push rods, PAC 921 springs, 1 3/4" longtube headers/X-pipe with no cats, Z06 exhaust and HPTuner. Thanks!
MIKE
The car is a 2003 corvette with a 6 speed and has the following mods. Stock long block, Intercooled Maggie with 3.0” pulley ~8lbs, K&N/blackwing filter and ported TB, Crane 1.8 rocker arms and Crane 7.25 push rods, PAC 921 springs, 1 3/4" longtube headers/X-pipe with no cats, Z06 exhaust and HPTuner. Thanks!
MIKE
#2
11 Second Club
I was actually thinking of this the other day. So the valve overlap you have is -2.5* at .050" tappet lift. Which is at .085" valve lift with the 1.7:1. So with the 1.8 will the same overlap be at .090" valve lift. Or it might be more complicated than that.
#4
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Well, in the overall aspect yes. The valve opens & closes at the same time. At the normal points of measure, .050 & .006, it will be slightly different. As the valve moves at a different rate of speed the duration & overlap numbers, at the same spots of measure, will a bit bigger with the higher ratio rockers.
They open the valve a little more in the same amount of time.
They open the valve a little more in the same amount of time.
#6
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#8
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This^^ by the maths, it looks like 1.8's add 2-3 degrees duration at 050 valve lift vs 1.7. On the cam, you're at .0294" lift on a 1.7 rocker and .0278" lift on a 1.8. if your ramp rate is say 50 degrees, this is about a degree every .001" between 006 and 050 valve lift. So, roughly 1.5 degrees more duration at 050 due to being on a slightly lower spot on the cam at 050 valve lift.
#9
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This^^ by the maths, it looks like 1.8's add 2-3 degrees duration at 050 valve lift vs 1.7. On the cam, you're at .0294" lift on a 1.7 rocker and .0278" lift on a 1.8. if your ramp rate is say 50 degrees, this is about a degree every .001" between 006 and 050 valve lift. So, roughly 1.5 degrees more duration at 050 due to being on a slightly lower spot on the cam at 050 valve lift.
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#11
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Saying it adds the duration & overlap is true. But.. It only looks like that because the ratio has been changed but still use the same point of measure as the other rockers. Say 240@ .050" duration w/1.5 rockers. The valve will have 240 duration at .075" valve lift. Swap to 1.7 rockers. If the duration at the valve is still measured at the same point, .075" valve, of coarse it will look bigger/more duration/overlap. Just like looking at duration numbers .006" vs. .050". If the point of measure at the valve was adjusted for the new rockers to .085" valve lift the duration/overlap would be exactly the same as the .050" lobe numbers.
So. Really all of the events happen at the same time, open/close, max lift, overlap. The valves are opening/closing at a higher rate of speed to get the extra lift in the same amount of time.
#12
TECH Junkie
Changing rockers will change the valve lift at .006 at .050 as stated above. The time at max lift will be the same, but the max lift values are different so this in itself is a little misleading. The closing area will be exactly the same yes, but everything in between is skewed by the rocker change.
If I did my math correctly, assuming a generalized 50 degree ramp from .006 to .050, you take the lobe of .050 *1.7=.085 OR .050*1.8=.09
.09-.085=.005 difference in valve lift when the cam is at .050
assume linear ramp, (.050-.006)/(50 degrees)=0.00088 " valve lift/degree cam duration
.005/.00088=5.6818181818
5.68181818/1.8 rocker ratio=3.156 degree difference at .050
You are correct that values at min and max lift will not change, but everywhere else will be skewed. If you measure in terms of valve lift, any point of lift for the 1.7 rockers will increase in duration with the 1.8 rockers. INCLUDING comparable max lift. Max lift for the 1.7 rockers is .561. The valve will spend more time at .561 with the 1.8 rockers because the valve continues to travel after that point. Another couple degrees at "max lift" for the 1.7's would necessitate a cam swap, but the same is being achieved with a rocker swap.
If you graph the valve behavior with each rocker, the graphs will look different because the larger ratio extenuates the behavior. It is literally just a multiplier for the cam lobe. So it will behave like a marginally larger cam because the overlap everywhere is increased at any given lift. The entire profile of the cam appears to shift the graph, making the "meat" of it wider, while the end points look the same. If the horizontal is degrees duration, and the vertical is lift, place a line at any lift and you can clearly see the duration widens. They are all minor changes, but they are present none the less
If I get some time over my lunch break, I'll include a graph so I don't just sound like jibberish.
If I did my math correctly, assuming a generalized 50 degree ramp from .006 to .050, you take the lobe of .050 *1.7=.085 OR .050*1.8=.09
.09-.085=.005 difference in valve lift when the cam is at .050
assume linear ramp, (.050-.006)/(50 degrees)=0.00088 " valve lift/degree cam duration
.005/.00088=5.6818181818
5.68181818/1.8 rocker ratio=3.156 degree difference at .050
You are correct that values at min and max lift will not change, but everywhere else will be skewed. If you measure in terms of valve lift, any point of lift for the 1.7 rockers will increase in duration with the 1.8 rockers. INCLUDING comparable max lift. Max lift for the 1.7 rockers is .561. The valve will spend more time at .561 with the 1.8 rockers because the valve continues to travel after that point. Another couple degrees at "max lift" for the 1.7's would necessitate a cam swap, but the same is being achieved with a rocker swap.
If you graph the valve behavior with each rocker, the graphs will look different because the larger ratio extenuates the behavior. It is literally just a multiplier for the cam lobe. So it will behave like a marginally larger cam because the overlap everywhere is increased at any given lift. The entire profile of the cam appears to shift the graph, making the "meat" of it wider, while the end points look the same. If the horizontal is degrees duration, and the vertical is lift, place a line at any lift and you can clearly see the duration widens. They are all minor changes, but they are present none the less
If I get some time over my lunch break, I'll include a graph so I don't just sound like jibberish.
#13
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Probably an easier way to say it, it makes the valve curtain look like the cam lob is more aggressive. Say, an EPS lobe vs XER lobe.
#14
TECH Junkie
100% what I was getting at^^
Thanks for turning my mumbo jumbo into something that made sense, I just couldn't find the right words to make it happen.
Thanks for turning my mumbo jumbo into something that made sense, I just couldn't find the right words to make it happen.
#15
11 Second Club
Nice write Maroon. I like how you found the duration difference.
"Poor mans cam" does put it together well. Lol.
How about this with the overlap. So with the higher ratio rockers you will seem to get a little more positive overlap but the valves are moving faster which means less time at that point. Where lower ratio rockers would seem to have less overlap but the valves are moving slower which is more time at that point. Would this even it out as to no real overlap change to be seen by the motor?
"Poor mans cam" does put it together well. Lol.
How about this with the overlap. So with the higher ratio rockers you will seem to get a little more positive overlap but the valves are moving faster which means less time at that point. Where lower ratio rockers would seem to have less overlap but the valves are moving slower which is more time at that point. Would this even it out as to no real overlap change to be seen by the motor?
#16
TECH Junkie
No, because the time spent with more overlap at .050 would translate to the same amount of time spent between .050 and .060 lets just say.
Like darth said, it will just act like a more agressive profile. The ramp rates get multiplied and everything moves a little quicker. Since nobody really cares about lift at .050 compared to .200 when it comes to airflow, the faster you can get the valves up into that .200+ zone, the better.
Walking the fine line between best performance (ideally would just be snap open to .600 then snap close) and durability is a tough one. Factor in the mass of everything and you have to start to worry about valve control, spring lift, etc etc. That's why the XE-R lobes perform so well but usually get a bad rap around here. The have very fast ramps and a smaller cam can behave like a traditionlly ramped larger cam, but those fast ramp rates are hard on everything from lifters to springs, they're noisy, etc.
In this particular case, I think 1.8 rockers are totally fine to add to this engine, and will probably help with some performance gain but it will be fairly small.
Like darth said, it will just act like a more agressive profile. The ramp rates get multiplied and everything moves a little quicker. Since nobody really cares about lift at .050 compared to .200 when it comes to airflow, the faster you can get the valves up into that .200+ zone, the better.
Walking the fine line between best performance (ideally would just be snap open to .600 then snap close) and durability is a tough one. Factor in the mass of everything and you have to start to worry about valve control, spring lift, etc etc. That's why the XE-R lobes perform so well but usually get a bad rap around here. The have very fast ramps and a smaller cam can behave like a traditionlly ramped larger cam, but those fast ramp rates are hard on everything from lifters to springs, they're noisy, etc.
In this particular case, I think 1.8 rockers are totally fine to add to this engine, and will probably help with some performance gain but it will be fairly small.
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Originally Posted by wannafbody
So you need to factor the additional lift to make sure your springs open far enough?
#20
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The comp 921 spring should be fine. 135/400 .650 lift.
https://www.hinsonsupercars.com/p-96...26921-kit.aspx
I am assuming you meant Comp because PAC does not make a VS with a 921 part number.
https://www.hinsonsupercars.com/p-96...26921-kit.aspx
I am assuming you meant Comp because PAC does not make a VS with a 921 part number.