Re: Bigger rocker on the intake?
[Re: AndyF]
#2715917
11/14/19 09:01 PM
11/14/19 09:01 PM
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Joined: Sep 2014
Posts: 4,457 Washington
madscientist
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It just depends. I've had engines that pick up with more rocker ratio and engines that don't. Generally speaking the smaller the cam the more likely you'll see a gain from the rockers. If the cam is already opening the valves as fast as the engine needs then opening them faster costs power, it doesn't add power. Andy, do you think the lobe is just too fast, or maybe that fast lobe is being opened too early? I ask because if the lobe is faster, the seat timing is less, but the @.050 timing will be quicker than the slower lobe. And it will be quicker to .200 and so on. Seems to me that more aggressive lobe should be opened later. Does this make sense?
Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
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Re: Bigger rocker on the intake?
[Re: AndyF]
#2716068
11/15/19 11:24 AM
11/15/19 11:24 AM
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Joined: Oct 2007
Posts: 4,205 New York
polyspheric
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The best candidate for a higher intake rocker ratio is: 1. under-cammed 2. does not need exhaust duration bias (longer exhaust period) 3. wedge chamber with shallow stem angle and valve head close to the bore 4. has enough head capacity and lower end strength to operate at higher RPM 5. not coil bound at new lift (or new springs)
Your porter's input is very valuable here as to relative port capacity in/ex.
Boffin Emeritus
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Re: Bigger rocker on the intake?
[Re: AndyF]
#2716071
11/15/19 11:39 AM
11/15/19 11:39 AM
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Joined: Jan 2003
Posts: 14,479 So. Burlington, Vt.
fast68plymouth
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“It depends”
And.....
There’s only one way to know for sure how a particular combo will respond to an increase in ratio.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Bigger rocker on the intake?
[Re: viperblue72]
#2716575
11/16/19 11:03 PM
11/16/19 11:03 PM
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Joined: Aug 2003
Posts: 3,245 Between a rock & a hard place
cudadoug
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Between a rock & a hard place
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Update: This motor has a dual pattern cam: .619/.631 - 270*/276*
And that is paired with a rocker combo of 1.7 intake and 1.6 exhaust. Thereby reversing the dual pattern cam lift. Results are .700” intake and .673” lift.
Can someone explain that logic? I’m not the sharpest tool in the shed, but doesn’t that work against heads that are 66% intake to exhaust air flow??
Thank you in advance for any info!
Last edited by cudadoug; 11/16/19 11:05 PM.
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Re: Bigger rocker on the intake?
[Re: cudadoug]
#2716613
11/17/19 06:18 AM
11/17/19 06:18 AM
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gregsdart
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The intake valve is larger requiring more lift to see maximum flow. The exhaust is opening with what, 100 psi pressure? Plus it has the big advantage of exhaust scavenging. So intake and exhaust are two totally different animals. When a blower is added to the mix, now you have much more force to fill the cylinders. And a lot more exhaust volume to expell. That is why forced induction engines will perform better with a somewhat larger exhaust valve and a bit smaller intake.
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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Re: Bigger rocker on the intake?
[Re: viperblue72]
#2716645
11/17/19 11:05 AM
11/17/19 11:05 AM
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polyspheric
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You’re gonna gain more by the extra duration on the exhaust than a bit of lift
This^^^
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Re: Bigger rocker on the intake?
[Re: gregsdart]
#2717408
11/19/19 04:39 AM
11/19/19 04:39 AM
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Joined: Apr 2015
Posts: 14 Sydney Australia
sr440
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The intake valve is larger requiring more lift to see maximum flow. The exhaust is opening with what, 100 psi pressure? Plus it has the big advantage of exhaust scavenging. So intake and exhaust are two totally different animals. When a blower is added to the mix, now you have much more force to fill the cylinders. And a lot more exhaust volume to expell. That is why forced induction engines will perform better with a somewhat larger exhaust valve and a bit smaller intake. Exactly! I am always amazed when a motor makes good power with a cam with a single pattern. What are the chances of the intake & exhaust needing the same pattern????
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Re: Bigger rocker on the intake?
[Re: sr440]
#2717423
11/19/19 07:57 AM
11/19/19 07:57 AM
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Joined: Jan 2003
Posts: 9,963 Frostbitefalls MN (Rocky&Bullw...
gregsdart
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Exactly! I am always amazed when a motor makes good power with a cam with a single pattern. What are the chances of the intake & exhaust needing the same pattern???? [/quote] Compression ratio has a big effect. A low compression engine ,say 8/1, still has usable cylinder pressure farther down the bore than a. 16/1 motor so the 16/1 motor can get away with a lot more exhuast duration. The decrease in working pressure of the 16/1 motor occurs twice as fast since the volume in the cylinder doubles twice as fast.
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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