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Rocker arm ratios

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Rocker arm ratios

Postby Backis » Fri Nov 17, 2006 2:36 pm

Hi. I was discussing whith my friend over rocker arm ratios whitch way to go high ratio type 1.8-2.0 or low1.7-1.5 on my super stock bbc engine.
I now that nextel cup engines run up to 2.0 ratios and unconfirmd pro stock figures 1.85-1.9 . I now that you gonna have problems whith valve float whith high ratio rockers which accelerate the valve faster.
Should you go for more lobe lift on cam to get desired lift or should you put more rocker ratio in to it.
which way is the correct one dragracing only.
I now that you will accelerate the valve opening as fast as you can but where is the limit and how fast can you go whitout start breaking parts.
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Postby PackardV8 » Fri Nov 17, 2006 9:35 pm

Hi, Backis,

Talk to your cam grinder. The cam has to be designed to work with the rocker ratio. If you take the fastest possible 1.6-1.7 cam lobe profile and put a 2.0 rocker on it, bad things will happen. A valve can only be opened just so fast, regardless of whether the speed comes from the lobe or the rocker.

BTW, the Cup engines using 2.0-2.2 rockers are also using titanium valves with 6mm stems. Those suckers are so light, they make a BBC valve of the same head diameter and stem length feel like a boat anchor.
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rocker ratios

Postby bigjoe1 » Sat Nov 18, 2006 1:15 pm

Most engine builders and can designers agree it is best to use higher ratio rocker to get the valve lift where you wany it.The cam designers at Comp Cams are very much aware of this, and they know what family of grinds are going to work the best with the higher ratios.Experience and trust in the cam company you are dealing with will go along way towards getting what you need. JOE SHERMAN RACING
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Postby R/T rick » Sun Nov 19, 2006 2:44 am

well that's interesting. I always though it was better to do the lift with the cam since it help keep valve train weight and strain down.
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Postby Ed-vancedEngines » Sun Nov 19, 2006 3:38 am

Same old story I say so many times but here, I say it again.

Each particular application has different needs, so there are no absolutes.

I am a fan of longer ratio rocker arms but I still do not always use them. Someone mentioned that when you are increasing your rocker ratio that you will also be decreasing the maximum rpm capability. I agree in part.

If nothing is changed but an increased rocker ratio it will in effect lower the maximum rpm capability of that combination. But for example if you were to slightly change a rocker ratio and then also swap in titanium retainers the effects of the lighter moving mass in the long side of the rocker vs the additional lift because of increasing the ratio, thereby increasing the load on the trunnion bearing, pushrod, and lifter should come very close to canceling the other out.

So By hypothetical reasoning let us say that you lost 200 rpm from the increasing the ratio but gained 200 rpm by going from steel to titanium retainers it would all be back to the same with the benefit of the increased ratio. That is if it is a benefit. On a car that is already over cammed increasing the ratio will only make it all worse.

When a benefit is achieved for an increased rocker ratio, it is usually not because of the increased lift as much as it is the increase of the effective duration from the time the valve leaves the seat and all throught the low and mid lift areas. The engine will think (?) it has more lift and more duration because of a more agressive lobe profile, but the actual duration from seat to seat is never changed at all.

Increasing the rocker ratio gets the valve off the seat and moving quicker.

In applications where you are limited in lobe lift an increase in rocker ratio helps a bunch, but the cam needs to be ground with that in mind.

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rocker arm ratios

Postby Backis » Sun Nov 19, 2006 11:53 am

Thanks for the info.
How fast can you actually go in acceleration before it goes very wrong i now its a very tricky question but how fast is it possibel to open a valve whith conventinal parts steel valve titanium retainers intake valve weight 123grams
Inch/deg
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Re: rocker arm ratios

Postby OldSStroker » Sun Nov 19, 2006 4:38 pm

Backis wrote:Thanks for the info.
How fast can you actually go in acceleration before it goes very wrong i now its a very tricky question but how fast is it possibel to open a valve whith conventinal parts steel valve titanium retainers intake valve weight 123grams
Inch/deg


It depends on the maximum rpm you wish to use. Sometimes it's not the cam velocity (inches of lift per degree of rotation) that causes the problem but what happens near the nose of the lobe. "Accelerations" of valvetrain parts cause loads and increase with rpm. Cam velocity is built into the lobe, and doesn't change with rpm.
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