Was is the advantage of a 1.7 intake rocker and 1.6 exhaust rocker?
01-23-2006, 03:45 PM
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Was is the advantage of a 1.7 intake rocker and 1.6 exhaust rocker? What is the advantage of this on a max effort nitrous engine... 12:1 compression. Large solid roller cam (duration over 250 @.050) and lift on intake over .700 and lift on exhaust around .680 (because of the different ratios on the roller rockers)
I've seen this done on motors before but never knew what the performance advantage was... And my engine builder ordered my jesel shaft rockers like this. forgot to ask him why specifically....
I've seen this done on motors before but never knew what the performance advantage was... And my engine builder ordered my jesel shaft rockers like this. forgot to ask him why specifically....
01-23-2006, 04:40 PM
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Mostly from my work with the Dynomation engine simulation program, but with a couple of examples right off the dyno, plus discussions with several professional engine builders my answer is:
Because the engine loses power with too much exhaust lift ('too much' in the case of most small blocks being ~0.550" to 0.650") but most cam grinders put more lift in the commonly longer duration exhaust lobes (in the case of nitrous cams, much longer), typically paired with shorter intake durations. Thus, as you get to the bigger off the shelf camshafts, the exhaust exceeds max power lift and responds positively to a lower rocker ratio.
Because the engine loses power with too much exhaust lift ('too much' in the case of most small blocks being ~0.550" to 0.650") but most cam grinders put more lift in the commonly longer duration exhaust lobes (in the case of nitrous cams, much longer), typically paired with shorter intake durations. Thus, as you get to the bigger off the shelf camshafts, the exhaust exceeds max power lift and responds positively to a lower rocker ratio.
01-23-2006, 05:11 PM
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Yeah that was initially the reason i asked the question... Cause when you look at specs usually the exhaust lift is greater than the intake lift when using the same rocker size.... So if cams are ground this way I could understand why you might use a 1.7 on both intake and exhaust for the increased lift but i still can't figure out why you wouldn't just grind less lift into the lobes and run either 1.6's on both or 1.7's on both depending on what setup you had...
01-23-2006, 07:46 PM
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beats me! but actually i posted a similar question a few months ago that covers similar concepts, hope it helps https://ls1tech.com/forums/advanced-engineering-tech/398602-i-got-good-question-you-cam-guys.html
01-23-2006, 08:28 PM
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Thanks, yeah it did a little concerning stuff I didn't know like the 1.7 ratio actually moving less than the 1.6 which makes sense... So I can see how that would help... I'm really interested though in the difference of lift between intake and exhaust. Most cams are ground intake lower lift, exhaust higher....
This seems to reverse that principle cause with my cam now the intake is higher and the exhaust is lower.... I've always thought exhaust flow is the most critical in a nitrous motor but it would seem that is not necessarily the case in my setup... Exhaust side has smaller ports, smaller valves and now less lift.... Just wondering how it affects power production.... Thanks again though for the link cause it shed light on some stuff I didn't know...
This seems to reverse that principle cause with my cam now the intake is higher and the exhaust is lower.... I've always thought exhaust flow is the most critical in a nitrous motor but it would seem that is not necessarily the case in my setup... Exhaust side has smaller ports, smaller valves and now less lift.... Just wondering how it affects power production.... Thanks again though for the link cause it shed light on some stuff I didn't know...
01-24-2006, 07:13 AM
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It is because of cylinder pressure, as it relates to reliability. With max effort N2O, the post-combustion cylinder pressures are massive. To help evacuate all of the exhaust gases, nitrous cams have very large exhaust duration. This compounds the cylinder pressure issue: As the exhaust valve is opened sooner in the expansion stroke than normally seen, the gases are at an even higher pressure because they have not expanded as much.
All this amounts to massive stress on the exhaust side of the valvetrain on initial lift. To help with reliability, one uses a lower rocker ratio, which puts less compressive stress on the exhaust pushrods, resulting in less deflection and increased reliability.
Using a lower rocker ratio to decrease force amplification at the pushrod results in needing a taller lobe for an equal valve lift. This is very easily accomplished on a nitrous cam because of the large exhaust duration. Because the duration is so long, increasing the lobe lift doesn't have such a large effect on ramp shape and acceleration as it does on a shorter duration lobe.
The end result of this methodology is simply increased reliability.
Hope this helps.
All this amounts to massive stress on the exhaust side of the valvetrain on initial lift. To help with reliability, one uses a lower rocker ratio, which puts less compressive stress on the exhaust pushrods, resulting in less deflection and increased reliability.
Using a lower rocker ratio to decrease force amplification at the pushrod results in needing a taller lobe for an equal valve lift. This is very easily accomplished on a nitrous cam because of the large exhaust duration. Because the duration is so long, increasing the lobe lift doesn't have such a large effect on ramp shape and acceleration as it does on a shorter duration lobe.
The end result of this methodology is simply increased reliability.
Hope this helps.
