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Bigger rocker on the intake? #2715830
11/14/19 05:43 PM
11/14/19 05:43 PM
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Between a rock & a hard place
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cudadoug Offline OP
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Between a rock & a hard place
I’m interested in on thoughts regarding...

Bigger rocker on the intake of a single pattern cam. Advantages?

1.6 intake/1.5 exhaust. Or in the case of the motor I’m working with, 1.7 intake/1.6 exhaust. 360-1 Indy heads if that matters.

I’m considering talking some of the lift out via the rockers, thus my question.

Discuss...

Re: Bigger rocker on the intake? [Re: cudadoug] #2715874
11/14/19 06:52 PM
11/14/19 06:52 PM
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AndyF Offline
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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.

Re: Bigger rocker on the intake? [Re: AndyF] #2715917
11/14/19 08:01 PM
11/14/19 08:01 PM
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madscientist Offline
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Originally Posted by AndyF
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
Re: Bigger rocker on the intake? [Re: madscientist] #2715926
11/14/19 08:22 PM
11/14/19 08:22 PM
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AndyF Offline
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I don't know what the actual physics are. I do know that I've chased rocker arm ratio on the dyno several times. It costs a ton of money to do and the results are iffy at best. I think it cost me more than $5000 last time I chased rocker arm ratios. Rocker arms cost close to $1000 per set these days with shipping and hardware. So if you round up four sets and then pay for the dyno time to run all the tests you can easily burn thru $5K. Last time I did that I found zero power. I'm not in a hurry to do it again.

Re: Bigger rocker on the intake? [Re: AndyF] #2716068
11/15/19 10:24 AM
11/15/19 10:24 AM
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polyspheric Offline
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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.


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Re: Bigger rocker on the intake? [Re: AndyF] #2716071
11/15/19 10:39 AM
11/15/19 10:39 AM
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So. Burlington, Vt.
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fast68plymouth Offline
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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
Re: Bigger rocker on the intake? [Re: fast68plymouth] #2716146
11/15/19 03:58 PM
11/15/19 03:58 PM
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junction city oregon
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viperblue72 Offline
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A fair amount of lift is lost on the intake side of the 360-1 due to the pushrod angle. My bet is that you’ll end up with close to the same lift intake/exhaust at the valves with the staggered rockers.

Re: Bigger rocker on the intake? [Re: viperblue72] #2716575
11/16/19 10:03 PM
11/16/19 10:03 PM
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Between a rock & a hard place
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cudadoug Offline OP
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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 10:05 PM.
Re: Bigger rocker on the intake? [Re: cudadoug] #2716581
11/16/19 10:14 PM
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viperblue72 Offline
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What is the lobe lift on the cam? You’re gonna gain more by the extra duration on the exhaust than a bit of lift.
Your lift at the valve after lash will be about the same on intake/exhaust with that cam. You lose about .020-.030 lift on the intake side due to the offset rocker.

Re: Bigger rocker on the intake? [Re: viperblue72] #2716597
11/16/19 11:29 PM
11/16/19 11:29 PM
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cudadoug Offline OP
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Lobe is .412” intake / .420” exhaust.

Last edited by cudadoug; 11/17/19 12:57 AM.
Re: Bigger rocker on the intake? [Re: cudadoug] #2716613
11/17/19 05:18 AM
11/17/19 05:18 AM
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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.

Re: Bigger rocker on the intake? [Re: viperblue72] #2716645
11/17/19 10:05 AM
11/17/19 10:05 AM
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polyspheric Offline
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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: polyspheric] #2717002
11/18/19 01:09 AM
11/18/19 01:09 AM
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cudadoug Offline OP
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It’s all clear now. Thanks to all for the input!

Re: Bigger rocker on the intake? [Re: gregsdart] #2717408
11/19/19 03:39 AM
11/19/19 03:39 AM
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sr440 Offline
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Originally Posted by gregsdart
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????

Re: Bigger rocker on the intake? [Re: sr440] #2717419
11/19/19 06:21 AM
11/19/19 06:21 AM
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dthemi Offline
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Every mopar I've added intake ratio to has performed better.

Re: Bigger rocker on the intake? [Re: sr440] #2717423
11/19/19 06:57 AM
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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.


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