Ratio rockers and calculated valve lift gains ?

[87BlazeLS]

Hey , Been pondering a new set of Roller rockers for the monte for next season install.

I have heard that an easy upgrade is to install 1.6 ratio rockers in place of the stock 1.5
So how would you determine the amount of gains (o.oooo" ) of valve lift gained by
such a swap. Even include 1.7 ratio as these are also available for SBC chevy 350.
My motor has the tall valve covers already so hitting anything is a low chance.

So my senses tell me that to calculate this you would take the cam lift and mutiple
the rocker ratio by 1.5 or 1.6 or 1.7 to determine true valve lift in the head ....

Would this be correct ?

 

[drogg1]

Yes. Alternatively, if you know the valve lift with 1.5 rockers you can still find it if you switch to another ratio.

Example:

Cam lift max = 0.320"
Using 1.5 rockers we get valve lift = 1.5 * 0.320" = 0.48"

If we know the valve lift is 0.500" with 1.5 rockers we can find the cam lift and then the valve lift with 1.6 ratio rockers:

cam lift
Valve lift with 1.6 = 1.6 * (0.500" / 1.5) = 0.533"

 

[565bbchevy]

A few things I would want to check if changing to higher ratio rocker arms which is why I would want to do this on a stand.
Piston to valve clearance, retainer to rocker arm clearance, coil bind and also the pushrod through holes in the heads for any binding.
Also I would want to verify that the pushrod length is still correct for the change in geometry.

 

[87BlazeLS]

I see, Thanks for your input.... I need to look at my cam profile to see just what the numbers are.
It's not too aggressive. And I was thinking of going 1.6 anyway.

Thanks 565bb, You bring up some good points. Normally these stock gen 1 350 motors can except
a lumpier cam without much more than better springs and rockers. As long as you do not go over overboard. For Me I would replace the springs and the rockers at the same time.
I just needed data on the ratio conversion.

Thank you for your input as well.

 

[pontiacgp]

if you have the stock stamped 1.5 rockers now they are never 1.5. They normally range from 1.43 to 1.48. Even with a decent set of 1.5 roller rockers you'll notice the differemce

 

[87BlazeLS]

Yup, And I already priced a kit with upgraded pushrods , roller tipped rockers from Jegs.com
They are 1.52 ratio and deliver consistent lift. There are a set of 1.6 ratio that has the fulcrum offset
to keep the push rods aligned so bind and scrub is a goner . These are rather pricey.
The jegs set above is right at $170. New springs would be another Few hundred.
So maybe $400 or so just to upgrade a little. You know I may be better off to just save the funds
to purchase a set of Aluminum heads where I can work on them off the motor. Then swap.
I need a bit better CR than the current 8.5 to 1. So thinner head gaskets are in order.
And I can get a rebate back when I sell the stock heads. So for a bit more than 2x what
upgrading these heads cost I could get into a decent pair of after market ones. Purchased
when on sale with free shipping of coarse....

 

[565bbchevy]

I would only go with a full roller rocker if you want to see an improvement the roller tip style still have a lot of friction since it is still a ball and socket mounting like the factory and really only eliminates the scrubbing on top of the valve, you will actually see a horsepower gain with a full roller over stock rocker arm.

 

[565bbchevy]

You know I may be better off to just save the funds
to purchase a set of Aluminum heads where I can work on them off the motor. Then swap.

If this is affordable for you it would probably be a better option and usually you get a few chamber size choices depending on the head manufacturer and can run a good quality head gasket and still raise your compression plus it makes the swap faster since you can have the heads set up for upgraded springs etc. ahead of time.

 

[87BlazeLS]

If this is affordable for you it would probably be a better option and usually you get a few chamber size choices depending on the head manufacturer and can run a good quality head gasket and still raise your compression plus it makes the swap faster since you can have the heads set up for upgraded springs etc. ahead of time.

So tell me this what does the different dome sizes in cc's mean exactly. if smaller cc's say equals increase in quench and CR ?
and does larger cc's say have an opposite effect ?
What would the weight of an assembled aluminum head be in comparison to the stock cast iron?
The reason I ask is my back is not very good, and the lighter Aluminum head would be much easier
for me to install.
And what you said above is exactly my same thought.
SO< How much would the stock heads be worth ? They are in great condition
and less than 3K miles on them.

 

[565bbchevy]

For stock heads I don't really know what there value would be but I don't imagine it would be much more than a few hundred especially for used ones regardless of miles especially since the SBC market is flooded with all kinds of affordable aluminum heads for only a few hundred more.
As far as weight difference that would also depend on the head manufacturer but I would estimate between 20-25 lbs. lighter per head over a cast iron version.
As far as piston cc the easiest way to explain it would be use a flattop piston with no valve reliefs as your 0 baseline if the pistons have valve reliefs or a dish this would be measured in a + cc dome volume and the higher the number the lower your compression will be with the same head.
When you have a domed piston that rises above the deck this would end up being - cc dome volume and the higher the number here the higher your compression will be with the same head.
Then you have the head chamber volume also in cc the smaller the number here will increase compression accordingly.
Depending on the cc of your current cast iron heads will tell you your options for going to a smaller chamber to increase your compression ratio.