Hey fellas still collecting info for my 434 build.My question is are there any benifits to running different ratios for your heads?I did find a few cars that have done it.One was a 427 build with twin turbos that had 1.7 on int & 1.6 on the exh.but couldn't find out the benifits of doing that.Any advise?Thanks guys

 

If you have the proper cam lobe profile, push rod and valve springs to accept higher
ratio rockers, the benefits are quite good.

Since the rocker acts as a multiplier, it will lift the valve faster and higher at
each degree of cam lobe lift. This means the corresponding opening and
closing points above 0.0" lift will have increased duration.

It's normally good practice to install bigger rocker studs, a stud girdle, or a
combination of both to reduce flex and vibrations.

While the valve remains open longer, this will allow more air to flow in the
upper lift range where the heads flow best.

Also note: the valve timing does NOT change.

 

I don't think the valve stays open longer, does it? Just farther, for the same amount of time.

 

Intake valve @ 0.290" lobe lift on the opening ramp, to 0.290" lobe lift on the
closing ramp is open longer with a 1.7 ratio rocker, than it would be with a 1.6 ratio rocker - "effective duration".



The valve timing does not change (IVO, IVC, etc.).

I'm not sure how to explain it without drawing a diagram.

 

Ok let me take a swing at it since the valve is moving faster with the higher ratio rocker it will make it to .050 lift sooner, open more, and arive back at .050 later all because of the increased lift and valve speed this will change the duration rated at any lift other than 0. the total duration - .000 lift back to .000 will stay the same.

hope that helps

 

Yeah, I understand that. But, it's not really longer.

 

Here's a quickie diagram to help with the explanations:

Imagine the red curve being the valve profile with a 1.6 rocker ratio.

The blue curve would represent a 1.7 rocker ratio.

Notice the extended duration of valve opening at the dotted line. We'll
say that's 0.500" valve lift.

Notice again that the valve timing doesn't change. Rocker ratios can
help get a little more punch, with very little effect on idle vacuum.

http://gmthunder.com/tino/effective_duration.jpg

 

This is a trial and error thing.Changing rocker ratio and lash can dial a cam in to the last dot.It can tell ya how close your cam is for your set up and which way to go if it isn't right.
There can be gains by going up or down on the I or EX and it takes a bunch of days at the track to get the desired ratio and performance.
Unless ya got several different set's of ratios laying around and time to do it ya won't get anywhere. The best thing to do is get the right cam and a good ratio the cam is ground for and go have fun ya won't see 2 tenths unless the present cam is ALL wrong.

 

That says it all, and at a couple of levels.

 

If you're running a solid (one of the many nice things about solids) you can play with lash to see what the engine likes. If it responds well to a little tighter lash then it may well respond well to a little more rocker ratio.

Nothing is ever set in stone, even with the best of preperation.... "some tweaking required" is about as true as it gets.

-Mindgame

 

Thanks guys as always their is always good advise to be had from you guys.Have to keep you posted on the progress of this build

 

We all know he's talking about my sexy diagram

 

What I find interesting is the amount of difference between rocker arm manufacturers. (how the rockers are built), and their effect on geometry and horsepower.

If you've ever checked geometry with a few different brands on the same engine, than dynoed them, you would be suprised.

 

Now that you mention it, what's with these variable ratio rockers from Crane?

I'm going to read up and find a link...

EDIT:
http://www.cranecams.com/?show=newProduct&id=12

 

Very good point.

They all build their rockers a little differently. The only way to know what the correct pushrod length is.... is to check. And you're right, just changing rocker brands could throw things off.

-Mindgame