How do 1.8 rockers affect duration?
Thread Starter
Racer
Joined: Dec 2005
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From: Northern Virginia
How do 1.8 rockers affect duration?
I understand how the 1.7 to 1.8 lift calculation is made but I havn't been able to find the
1.7 to 1.8 duration equasion...
Any help is appreciated
1.7 to 1.8 duration equasion...
Any help is appreciated
Melting Slicks
Joined: May 2000
Posts: 2,498
Likes: 4
From: Brentwood TN
The duration won't change. The cam acts on the lifter at the same point indedpendent of the rocker ratio. The amount of lift at a particular rotational point is different and the total lift will be greater with the 1.8 vs 1.7, but duration from valve on the seat to valve back on the seat will not change.
Charlie
Charlie
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Joined: Nov 2007
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From: Port Angeles Wa.
Not sure I agree
I'm new to this site, but if memory serves me correctly, the general rule was @ .050 valve lift the valve is considered functional, so by increasing the rocker ratio,the valve would become funtional a few degrees longer depending on the ramp on the cam lobe. I suppose with past experience with a degree wheel, that there could be as much as a five degree gain in funtional valve lift depending on the cam being used.
Team Owner
Joined: Mar 1999
Posts: 22,025
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From: At the beach in little Rhody
I have read it is equivalent to about an extra 4 degrees of duration (specifically for the Cranes). Seat to seat timing doesn't change, since lift points come earlier and linger longer the perceived duration is longer.
Thread Starter
Racer
Joined: Dec 2005
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From: Northern Virginia
OK, thanks, that is what i originally was thinking and why I posted the ?
b/c even w/ lift at .200 it's going to be that much incrimentally different, again lifting just a bit sooner.
so on an comp xrpm lobe if the duration at .050 lift is 224 and .200 lift is 142 and the overall duration is 277.
So I could see if you said the overall duration of 277 didn't increase because the cam starts the lift at the same time, but b/c of the increased ratio rocker arm the duration above .050 and .200 lift would increase as that lift is being achieved sooner and later on the lobe.
We're dealing with curves and slopes not linear lines, the rocker wouldn't make the lift magically jump up ~.030 higher at peak only but not affect anything else.
So if the duration at .050 increased 5* then at .200 it might increase 2*
Hopefully there's an equasion outhere. I would like to know if this makes an xe or xrpm lobe like an xer, xfi, or lsk lobe.
b/c even w/ lift at .200 it's going to be that much incrimentally different, again lifting just a bit sooner.
so on an comp xrpm lobe if the duration at .050 lift is 224 and .200 lift is 142 and the overall duration is 277.
So I could see if you said the overall duration of 277 didn't increase because the cam starts the lift at the same time, but b/c of the increased ratio rocker arm the duration above .050 and .200 lift would increase as that lift is being achieved sooner and later on the lobe.
We're dealing with curves and slopes not linear lines, the rocker wouldn't make the lift magically jump up ~.030 higher at peak only but not affect anything else.
So if the duration at .050 increased 5* then at .200 it might increase 2*
Hopefully there's an equasion outhere. I would like to know if this makes an xe or xrpm lobe like an xer, xfi, or lsk lobe.
Last edited by dmiz0420; Nov 23, 2007 at 11:47 AM.
Thread Starter
Racer
Joined: Dec 2005
Posts: 426
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From: Northern Virginia
this idea usually gets poo pooed right off the board b/c people say just get a cam w/ the lobes you need/want and use the 1.7 rockers.
but if the ramp rates are equivelent or less than an xfi or lsk while accomplishing the same lift with a milder cam and higher ratio rocker arm, What would be the difference?
Why grind a new cam if you have a 224/224 .568 lift cams when you could combine it w/ 1.8's to get the desired effect: 228/228 w/ .602 lift?
but if the ramp rates are equivelent or less than an xfi or lsk while accomplishing the same lift with a milder cam and higher ratio rocker arm, What would be the difference?
Why grind a new cam if you have a 224/224 .568 lift cams when you could combine it w/ 1.8's to get the desired effect: 228/228 w/ .602 lift?
Melting Slicks
Joined: May 2000
Posts: 2,498
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From: Brentwood TN
It occurred to me that while my answer is correct regarding off to back on the seat duration, that is not the way manufacturers spec their cams. This may stem from hype back in the day that listed off to on duration which was not as informative as duration measured from .050" off the seat to the point where the valve was .050" from being back on the seat.
That being the case, my answer is technically right but wrong in respect to the op question. Duration is affected and is marginally longer because the valve reaches .050" off the seat sooner with the 1.8 ratio. The answer above that further qualifies the answer by stating "with the same cam" is even a better answer because without knowing cam lobe acceleration rates, it is difficult to calculate duration change and may need to be measured empirially with the engine assembled and a degree wheel mounted on the engine.
Charlie
That being the case, my answer is technically right but wrong in respect to the op question. Duration is affected and is marginally longer because the valve reaches .050" off the seat sooner with the 1.8 ratio. The answer above that further qualifies the answer by stating "with the same cam" is even a better answer because without knowing cam lobe acceleration rates, it is difficult to calculate duration change and may need to be measured empirially with the engine assembled and a degree wheel mounted on the engine.
Charlie
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Brett Foote
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Joined: Mar 1999
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From: At the beach in little Rhody
If you have ever looked at the lift plots and overlap triangles, the off the seat locations won't change, the shape of the curve will increase with more area, and the area under the overlap triangle will increase.
Melting Slicks
Joined: May 2000
Posts: 2,498
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From: Brentwood TN
Team Owner
Joined: Mar 1999
Posts: 22,025
Likes: 192
From: At the beach in little Rhody
Found this, maybe it will help:
To summarize: the stock rocker's rated ratio is 1.7:1 but that occurs only at maximum lift with starting and ending ratios noticeably less and the "average" ratio being somewhat less.
The Crane Quick Lift, is also rated 1.7, but that occurs from about .300". lift to max. lift with the ratio for the rest curve being higher than rated. This higher ratio at the ends of the lift curve means the valves move faster. According to Mark Campbell, this increases performance and enhances throttle response. If you also step-up to the 1.8:1 Quick Lift you also gain the advantage of an overall, higher rocker ratio.
"If you plot a lift curve with stock rockers," Campbell continues, "then plot our Quick Lift and measure the durations at .200" valve lift; you're gonna find, depending on the cam, six to eight degrees more duration with our rockers. Compared to other, aftermarket rockers, you don't get as much, because they're a little more aggressive, but the least we have seen, plotting Quick Lift against competitors at .200" valve lift, is a four-and-a-half degree increase. When we've run on an engine dyno with the 1.7s, we're seein' 15 hp over stock. With the 1.8s we're seeing about 20."
To summarize: the stock rocker's rated ratio is 1.7:1 but that occurs only at maximum lift with starting and ending ratios noticeably less and the "average" ratio being somewhat less.
The Crane Quick Lift, is also rated 1.7, but that occurs from about .300". lift to max. lift with the ratio for the rest curve being higher than rated. This higher ratio at the ends of the lift curve means the valves move faster. According to Mark Campbell, this increases performance and enhances throttle response. If you also step-up to the 1.8:1 Quick Lift you also gain the advantage of an overall, higher rocker ratio.
"If you plot a lift curve with stock rockers," Campbell continues, "then plot our Quick Lift and measure the durations at .200" valve lift; you're gonna find, depending on the cam, six to eight degrees more duration with our rockers. Compared to other, aftermarket rockers, you don't get as much, because they're a little more aggressive, but the least we have seen, plotting Quick Lift against competitors at .200" valve lift, is a four-and-a-half degree increase. When we've run on an engine dyno with the 1.7s, we're seein' 15 hp over stock. With the 1.8s we're seeing about 20."
Melting Slicks
Joined: Jun 2002
Posts: 2,246
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From: Middletown CT
I thought I read that the 1.8 rockers were not recommended for use with fast ramps like the XER or LSK lobes. I think the reasoning was that the fast ramp of the lobe and the higher rocker ratio could cause valve train instability. At the very least, I would think that you would have to be very careful in your choice of valve springs. I'm just throwing that out there, because I don't know the answer.
Team Owner
Joined: Mar 1999
Posts: 22,025
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From: At the beach in little Rhody
Yes, I would agree that with an aggressive cam the higher ratios might be an issue. The other aspect that you have to consider is that higher spring forces now translate more force to the lifter due to the higher ratio as well. So a setup with 375 lbs. of open force from the valve spring is statically putting 638 lbs. on the lifter whereas the 1.8's will put 675 lbs. on the lifter.
Thread Starter
Racer
Joined: Dec 2005
Posts: 426
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From: Northern Virginia
ahhhh, increased lifter load...for every action.....
there's usually a dozen other reactions, and you only ever account for 6 of them, 4 won't matter then the other 2 will bit you in the ....
Yeah, I wouldn't put the 1.8's w/ an xer either, but an xe or xrpm lobe like the 224 listed above the 1.8's would give you the duration of an 224 xer at .200 (146) and carry a little more lift say .601 rather than the xer .589
From this I extrapalated (somthing I probably shouldn't do if I can't spell it) that the 1.8's on the lesser lobes would give me the same ramp of the xer or xfi or lsk (though I havn't been able to find the #'s on lobes of the xfi or lsk to compair) so therefore valvetrain stability should be maintained
there's usually a dozen other reactions, and you only ever account for 6 of them, 4 won't matter then the other 2 will bit you in the ....
Yeah, I wouldn't put the 1.8's w/ an xer either, but an xe or xrpm lobe like the 224 listed above the 1.8's would give you the duration of an 224 xer at .200 (146) and carry a little more lift say .601 rather than the xer .589
From this I extrapalated (somthing I probably shouldn't do if I can't spell it) that the 1.8's on the lesser lobes would give me the same ramp of the xer or xfi or lsk (though I havn't been able to find the #'s on lobes of the xfi or lsk to compair) so therefore valvetrain stability should be maintained
Last edited by dmiz0420; Nov 24, 2007 at 09:41 AM.
Team Owner
Joined: Mar 1999
Posts: 22,025
Likes: 192
From: At the beach in little Rhody
zo6vetteman2003 on here who uses the Crane 1.8's with a Comp cam in his Z06 very successfully, I will see if I can find him. He is out of Mass and spends more time on the Z06 tech forum.
Last edited by vettenuts; Nov 24, 2007 at 09:49 AM.
Melting Slicks
Joined: Nov 2003
Posts: 3,011
Likes: 9
From: Dublin GA
Found this, maybe it will help:
The Crane Quick Lift, is also rated 1.7, but that occurs from about .300". lift to max. lift with the ratio for the rest curve being higher than rated. This higher ratio at the ends of the lift curve means the valves move faster. According to Mark Campbell, this increases performance and enhances throttle response. If you also step-up to the 1.8:1 Quick Lift you also gain the advantage of an overall, higher rocker ratio.
"If you plot a lift curve with stock rockers," Campbell continues, "then plot our Quick Lift and measure the durations at .200" valve lift; you're gonna find, depending on the cam, six to eight degrees more duration with our rockers. Compared to other, aftermarket rockers, you don't get as much, because they're a little more aggressive, but the least we have seen, plotting Quick Lift against competitors at .200" valve lift, is a four-and-a-half degree increase. When we've run on an engine dyno with the 1.7s, we're seein' 15 hp over stock. With the 1.8s we're seeing about 20."
The Crane Quick Lift, is also rated 1.7, but that occurs from about .300". lift to max. lift with the ratio for the rest curve being higher than rated. This higher ratio at the ends of the lift curve means the valves move faster. According to Mark Campbell, this increases performance and enhances throttle response. If you also step-up to the 1.8:1 Quick Lift you also gain the advantage of an overall, higher rocker ratio.
"If you plot a lift curve with stock rockers," Campbell continues, "then plot our Quick Lift and measure the durations at .200" valve lift; you're gonna find, depending on the cam, six to eight degrees more duration with our rockers. Compared to other, aftermarket rockers, you don't get as much, because they're a little more aggressive, but the least we have seen, plotting Quick Lift against competitors at .200" valve lift, is a four-and-a-half degree increase. When we've run on an engine dyno with the 1.7s, we're seein' 15 hp over stock. With the 1.8s we're seeing about 20."
I have debated on adding aftermarket lifters, but don't want to go to the 1.8s due to excessive wear associated with "higher lift" cams
Safety Car
Joined: Oct 2000
Posts: 4,949
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From: Monticello MN
I wonder if anyone has any "personal experience" (read dyno #s) on these quick lifts to confirm their 15 hp #s (should actually be higher on an engine that can breath better)
I have debated on adding aftermarket lifters, but don't want to go to the 1.8s due to excessive wear associated with "higher lift" cams
I have debated on adding aftermarket lifters, but don't want to go to the 1.8s due to excessive wear associated with "higher lift" cams
Team Owner
Joined: Mar 1999
Posts: 22,025
Likes: 192
From: At the beach in little Rhody
I wonder if anyone has any "personal experience" (read dyno #s) on these quick lifts to confirm their 15 hp #s (should actually be higher on an engine that can breath better)
I have debated on adding aftermarket lifters, but don't want to go to the 1.8s due to excessive wear associated with "higher lift" cams
I have debated on adding aftermarket lifters, but don't want to go to the 1.8s due to excessive wear associated with "higher lift" cams
Melting Slicks
Joined: Nov 2003
Posts: 3,011
Likes: 9
From: Dublin GA
VN
I saw your reply on the other post, but in reqards to this one, I was wondering if Crane had stumbled onto somthing "special" with their "quick lift" rockers that would allow them to pick up additonal hp.
I saw your reply on the other post, but in reqards to this one, I was wondering if Crane had stumbled onto somthing "special" with their "quick lift" rockers that would allow them to pick up additonal hp.