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what are the major benefits from higher ratio rocker arms and what application do you benefit from them the most?

what are the drawbacks of higher ratiorockers.

I have a bbc twin turbo engine i need a rocker system for and I found a new set of t and d shaft rockers but the ratios are 1.85 and 1.75. This is for a twin turbo dual purpose race and lake boat.
 

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Well, the main reason people jump up in rocker ratio is for an easy bump in lift without touching the cam.

For example say you have a 500 intake lobe with a 1.7 rocker, that puts your gross lift at .850 gross lift. (.500 X 1.7) .... jump up to a 1.8 rocker (1.8 X .500) and you get .900 lift.

I believe your duration increases a bit as well when increasing the ratio.... this may or may not be beneficial, depending on the application. I don't know boat grinds, rules of thumb etc.... I prefer going fast on land, lol... I've been 80mph on the water, fast enough!!!

Probably more stress on a longer arm, but with a T&D shaft setup, you are good to go...... that's all I'll ever run. Top notch product. Nice stuff.

My 2.

Cheers.
 

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Engines are a compromise. The biggest advantage of a higher rocker ratio is to allow big lift at the valve, while keeping the velocity of the heaviest parts in the valvetrain (lifters, pushrods) to a minimum.
Maximum ratio allowed will be dictated by rocker body length. It comes a point when moving the pushrod seat in the rocker towards the fullcrum (raising ratio) compromises rocker design and strength.
Remember, it isn't quite as easy as just upping ratio. You need to design the cam profiles/valve train for this to take maximum advantage. That is why allot of times guy's go up in ratio and lose power. Not that the thing might not have liked the extra lift, but it may have caused some control issues as it will also speed the valve up for a given lobe lift.

As far as speed in a boat, 110 in a 20ft jet is allot differant than 140 in a 28 footer, which is allot differant than 150 in a 52 footer!
 

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As stated increasing rocker ratio is easy way to increase lift without changing the cam. Just remember that as you increase the rocker ratio it increases the column load on the pushrod. Pushrods already can be the weak link in the valve train so don't skimp on pushrod diameter or wall thickness.
 

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While there will be an increase in valve opening accelerations there is no change in duration. Duration is a function of the cam and remains unchanged by anything that happens at the valve. The valve will open and close at the same time but the rate of acceleration of the valve will be greater. The end result is valve movement that is comparable to what you might see with a more aggressive lobe profile without the stress on the lifter associated w/ lobes that are really fast.

Most of my experience is w/ Fords and not all engines will gain w/ a ratio change. I've seen some that didn't respond and some that picked up a tenth or more. The ones that seemed to work best were ones that only the intake ratio was increased on. It depends on the complete package, really well developed engines may not change much.

Good luck.
 

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While there will be an increase in valve opening accelerations there is no change in duration. Duration is a function of the cam and remains unchanged by anything that happens at the valve. The valve will open and close at the same time but the rate of acceleration of the valve will be greater. The end result is valve movement that is comparable to what you might see with a more aggressive lobe profile without the stress on the lifter associated w/ lobes that are really fast.

Most of my experience is w/ Fords and not all engines will gain w/ a ratio change. I've seen some that didn't respond and some that picked up a tenth or more. The ones that seemed to work best were ones that only the intake ratio was increased on. It depends on the complete package, really well developed engines may not change much.

Good luck.
.... and to the others stating there's no change in duration.


Well, let me tell you what Comp Cams states:

"It's not a dramatic amount, but an increase of .10 in rocker ratio (e.g. 1.5 to 1.6) can yield 1-3 degrees of increased effective duration." ;)

Great article in the back of the latest Hot Rodding issue, page 82-83. Take a looksy.

Cheers.
 

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.... and to the others stating there's no change in duration.


Well, let me tell you what Comp Cams states:

"It's not a dramatic amount, but an increase of .10 in rocker ratio (e.g. 1.5 to 1.6) can yield 1-3 degrees of increased effective duration." ;)

Great article in the back of the latest Hot Rodding issue, page 82-83. Take a looksy.

Cheers.
The key there is the word "effective".

As stated earlier, the increased rate of lift from higher ratios will increase the duration measured at .050" - the lift that is typically used to represent the point that any significant flow starts to occur.

The cam will start to move the valves and stop moving the valves at the exact same duration regardless of rocker ratio.
 

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The duration at the cam will stay the same at all points and valve opening and closing will occur at exactly the same time. The valve will experience a change in duration from point to point between the opening and closing events, but the opening and closing events remain unchanged. The valve will accelerate differently due to the change in the point to point measurements which gives it the effect of a lobe profile change.
 

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thats right, How can the total duration change?,,, simple It can't. what changes is area under valve,, which effectively gives a greater duration, with an increase of RR (& less with decrease) at the different points 020 050 200 etc,,,
 

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.500 +/- cam lobe on a conventional core is about as much as you should go. The lifter drops out of the block farther and the cam core starts to get weaker. Hence the reason to use a higher ratio rocker to get more lift. Cheaper than going to a bigger core like a 55mm and such.
 
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