Quick ? about 1.6 RR's and cam spec's
02-06-2003, 05:21 PM
#1
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Quick ? about 1.6 RR's and cam spec's
I know how to find a cam's lift with 1.6 rockers compared to 1.5's. My question is, do the 1.6 rockers also increase the duration like they do the lift ? If so, do you figure it the same way ? Thanks......
72Demon
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02-06-2003, 07:49 PM
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Actualy the will change the lift because the higher ratio will have your valves reach .05 lift 1/3 of a thousands earlier than 1.5 at true 1.6 and 1.5 ratio. Not a significant amount but yes it does increase duration most likley (guesstimate) less than one degree.
02-06-2003, 09:36 PM
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Turb0racing is correct. With a 1.6 rocker the valve will reach 0.050 mils of lift before that of one with a 1.5 rocker. However, the increase in duration is very small and depends on the cam lobe shape. Also, when a cam manufacturer states duration at 0.050 mils, it is tappet lift not valve lift. So the duration they state is independent of the rocker size you use. That's what I was getting at.
02-07-2003, 04:54 PM
#11
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Well, it's a little misleading. Most people talk about duration at 0.050" lift. Higher ratio rockers will increase duration at 0.050". Seat-to-seat timing does not change, as has been stated.
Rich Krause
Rich Krause
02-07-2003, 05:21 PM
#12
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Originally posted by 68BIRD
Yeah, it can get confusing.
1 mil is defined as 1/1000 of an inch.
1 mil is the same as 1 thousandth or "thou" of an inch.
So,
50 mils = 50 thou = 0.050 of an inch.
Hope, it makes sense now.
Yeah, it can get confusing.
1 mil is defined as 1/1000 of an inch.
1 mil is the same as 1 thousandth or "thou" of an inch.
So,
50 mils = 50 thou = 0.050 of an inch.
Hope, it makes sense now.
Last edited by arnie; 02-07-2003 at 05:25 PM.
02-07-2003, 06:56 PM
#13
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Originally posted by rskrause
Well, it's a little misleading. Most people talk about duration at 0.050" lift. Higher ratio rockers will increase duration at 0.050". Seat-to-seat timing does not change, as has been stated.
Rich Krause
Well, it's a little misleading. Most people talk about duration at 0.050" lift. Higher ratio rockers will increase duration at 0.050". Seat-to-seat timing does not change, as has been stated.
Rich Krause
Using Cam Dr. data from one of (y)our favorite hydraullic roller lobes, at .050 lobe lift the duration is 224 (crankshaft) degrees. If, however you looked at duration with the valve at .050, the "valve duration" becomes 240 degrees with 1.5's and about 241.5 degrees with 1.6's. At these low lobe lifts (.031-.033) you are still in the ramp area. Of course that's why .050 lobe lift was chosen for comparisons.
We do calculations and buy cams based on .050 lobe lifts. Well, if you want to do very accurate calculations, you use actual measured lift/angle numbers like those from Cam Doc.
So, my interpretation is that duration at the valve only increases a degree or two with a change from 1.5's to 1.6's but that duration is about 15-16 degrees more than the ".050 duration" we banter about.
What really happens is the area under the lift curve increases, with the greatest increase at the higher lifts. Of course that's where the valve is open for the least amount of time.
In the lobe cited above, area under the curve increases 3.26% with 1.6's over 1.5's (320.0 deg*sq-in vs. 309.9 deg*sq-in).
Because airflow thru the valve is approximately proportional to the area of the lift curve, my feeling is that the flow increase is about the square root of the ratio of the Rocker Ratio. 1.6/1.5 = 1.0667. The sq root of 1.0667 is 1.0328 or about 3.28% more.
02-07-2003, 07:24 PM
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Originally posted by OldSStroker
I agree that the ".050" thing is misleading. Fairly standard industry practice rates cam lobe duration at .050 of lobe lift, not valve lift. So, with 1.5's that's .075 at the valve and with 1.6's it's .080 at the valve.
Using Cam Dr. data from one of (y)our favorite hydraullic roller lobes, at .050 lobe lift the duration is 224 (crankshaft) degrees. If, however you looked at duration with the valve at .050, the "valve duration" becomes 240 degrees with 1.5's and about 241.5 degrees with 1.6's. At these low lobe lifts (.031-.033) you are still in the ramp area. Of course that's why .050 lobe lift was chosen for comparisons.
We do calculations and buy cams based on .050 lobe lifts. Well, if you want to do very accurate calculations, you use actual measured lift/angle numbers like those from Cam Doc.
So, my interpretation is that duration at the valve only increases a degree or two with a change from 1.5's to 1.6's but that duration is about 15-16 degrees more than the ".050 duration" we banter about.
What really happens is the area under the lift curve increases, with the greatest increase at the higher lifts. Of course that's where the valve is open for the least amount of time.
In the lobe cited above, area under the curve increases 3.26% with 1.6's over 1.5's (320.0 deg*sq-in vs. 309.9 deg*sq-in).
Because airflow thru the valve is approximately proportional to the area of the lift curve, my feeling is that the flow increase is about the square root of the ratio of the Rocker Ratio. 1.6/1.5 = 1.0667. The sq root of 1.0667 is 1.0328 or about 3.28% more.
I agree that the ".050" thing is misleading. Fairly standard industry practice rates cam lobe duration at .050 of lobe lift, not valve lift. So, with 1.5's that's .075 at the valve and with 1.6's it's .080 at the valve.
Using Cam Dr. data from one of (y)our favorite hydraullic roller lobes, at .050 lobe lift the duration is 224 (crankshaft) degrees. If, however you looked at duration with the valve at .050, the "valve duration" becomes 240 degrees with 1.5's and about 241.5 degrees with 1.6's. At these low lobe lifts (.031-.033) you are still in the ramp area. Of course that's why .050 lobe lift was chosen for comparisons.
We do calculations and buy cams based on .050 lobe lifts. Well, if you want to do very accurate calculations, you use actual measured lift/angle numbers like those from Cam Doc.
So, my interpretation is that duration at the valve only increases a degree or two with a change from 1.5's to 1.6's but that duration is about 15-16 degrees more than the ".050 duration" we banter about.
What really happens is the area under the lift curve increases, with the greatest increase at the higher lifts. Of course that's where the valve is open for the least amount of time.
In the lobe cited above, area under the curve increases 3.26% with 1.6's over 1.5's (320.0 deg*sq-in vs. 309.9 deg*sq-in).
Because airflow thru the valve is approximately proportional to the area of the lift curve, my feeling is that the flow increase is about the square root of the ratio of the Rocker Ratio. 1.6/1.5 = 1.0667. The sq root of 1.0667 is 1.0328 or about 3.28% more.
Sorry if I confused things.
Rich Krause