Inliners International Forums General Discussion Engines Rocker ratio?

After doing a search on rockers I've got a question.Having never played with the rocker ratios before, always ran what ever ratio came stock. I see that a lot of you are running 1.7s instead of the 1.75s. I understand its an availabilty thing but what affect does it have on the engine other than changing over all lift?

It will also increase your duration, both valves being open togeter longer.

If you have a cylinder head that flows better @ a certain lift, & your lower ratio rockers arms are not getting there to that certain lift, you are missing that little extra flow.

Going to a higher ratio rocker arm will get you to that peak flow number so you can take advantage of your higher lift flowing cylinder head.

I know I am going to a 1.8 ratio, because my camshaft I bought (choices were few) does not bring the valve open enough to the lift I want.

MBHD

Which one increases the duration?

No high flowing head here.

How tall is a stock valve cover? I'm running the intergrated head and the valve cover is different then the regular head.I'm thinking I might run into clearance problems with the poly locks.

Greetings . . .

The idea of rocker arms and "Increased Duration" can be counterintuitive if not kept in context. Higher Ratio rocker arms cannot change the basic valve events of a given cam. The lobe doesn't push on the lifter any sooner or raise any longer.

Many cam grinders talk about 'advertised duration' as measured at .020 valve lift. In the old old days 'advertised duration' generally described the valve events directly from open to close. Some consider only the duration at .050 valve lift as useful for comparing one cam to another.

Consider a Chevy 216 cam with an advtersied 250 degrees of intake duration (@ .020 valve lift).

On this particular cam the valve doesn't reach .020 lift until the valve has been off the seat for 16 degrees of crank rotation with a 1.5 ratio rocker. With a 1.7 ratio rocker this event occurs 3 degrees sooner, and the valve stays above .020 for 3 degrees longer on the down side as well. So the effect is an increase in the 'advertised duration' from 250 to 256.

And the effect at higher lobe lifts is even more prounounced because of the multiplicative effect of the higher ratio. Again from the 216 Chevy cam described above. With 1.7 ratio rocker arms the valve reaches .050 lift some 6 degrees sooner then with 1.5 ratio. This has the effect of increasing the 'usable duration' by 12 degrees of crank rotation.

But let's not forget that there is downside to this increased duration - because as Roger Huntington always points out "everything with a cam is a compromise, you don't gain something without giving up something in return".

In this case the higher valve lift results in an additional 4 degrees of 'significant' overlap with both intake and exhaust above .020 valve lift. As you probably know any amount overlap effects low RPM response, lugging ability etc. Low end response is further impaired by reaching .020 exhaust valve lift sooner duing power stroke and later intake valve closure below .020 lift during the compression stroke.

regards,
stock49

Very nicely put!

G10-250,,,

Higher ratio rocker arms give you increased duration,(not by much)

For a stock type set-up just stay w/stock ratio.

When I bought my Crane Gold 1.7 ratio, that is all they had at the time listed for a Chevy six cylinder.

W/the stock cylinder head, the valve guides are a little short,if you increase the ratio, it will wear out/put more side load on the valve guides.

With the guides I am using on an aftermarket cyl head,they are much longer.


MBHD

Wow, thanks thats a lot to chew on. I thought there had to be more going on.

The rockers I'm looking at are the Comp Cams Magnum roller tips in 1.72 ratio.

I'm going to use the Ferrea manganese bronze valve guides and Series 5000 stainless valves.

Where can I learn more about this topic?

thanks

G10-250,

Two links to articles on rocker arms (also search for interesting articles at these sites):

http://www.compcams.com/Community/Articles/Details.asp?ID=153327675
http://www.mid-lift.com/

Thanks that mid-lift site is great.

The chevy performance site has got some great tech articles on this topic:
Rocker Efficiency
Ratio Effects

It's important to keep in mind that fixed fulcrum rockers behave differently then the sloted ones:


Fixed fulcrum rockers (216, 235, 261, GMC 270 & 302) have tendency to fade as they lift since the rocker face is actually receding from the valve stem as traverses its arc. Stamped slotted rockers exhibit the opposite effect, with rocker ratio increasing at higher lifts. Many roller rocker setups claim to be constant ratio.

In the end the only way to tell what is going on with a particular package is to put a dial gauge on the valve and actuate the valve train.

regards,
stock49

Thanks stock49

Rocker geometry has always intreged me. Having stated with a 216 then going to a 270 GMC in the 60's. Never being able to afford the best "stuff" I inched along with stock rockers for years. With the advent of higher lift cams; the welding of stamped steel ones was necessary. I continued to replace guides because of the pushing effect of the stock rockers.
The decision to make roller rockers was easy, the building more difficult for a low budget racer. Finding a shop to machine my design for the rocker and stand did come however with a trade of some electrical work for the basic parts.
Purchasing roller tips from Isky was accomplished buy promising only 16 were to be made. Chrysler adjusters were bought at a swap meet. Torrington bearing were bought from the now defunct King Bearing and I now had roller rockers. They ended up 1.63 to 1 and have never failed. I have run the same guides for over 10 seasons and they used to make 2.
If making rockers for an GMC inline remember they are almost vertical and parallel to the valve.
The fun is having your own set and aaccomplishing something not store bought. I glad some are now available...Good Luck

On GMC 270-302 rockers;I measured the lift at the tappet and the valve several times.The actual ratio with the stock sheetmetal rockers is 1.41,most cam grinders list it as 1.5,so your losing a little lift in this case if you go by the cam grinders figures.
I also experimented with rocker geometry on the same 302 by altering the rocker position when the valve is closed by changing the position of the pad that contacts the valve stem.I found almost no difference in lift,maybe 10 thousands max at the valve.My final postitioning of the rockers was based on keeping the rocker to valve contact reasonably in the center of the valve stem.This was done on a cam having .440 actual measured lift at the valve.

Tony, I have found the use of lash caps to adjust geometry to also help. I went to 10 degree valve keepers with recesses many years ago. Keeping the "wipe" of the stock rocker in the proper place on the valve was sometimes tough to do. A compromise at best. I never used a roller cam because of spring pressure. An engine running up to 6000 rpm just didn't seem to need it. Good Luck

Wow,jimmy six thats quite the accomplishment making your own roller rockers,my hats off to you.

Anyone else read through the Mid-Lift site? I finely had a chance to sit down and read through it.

Man its deep into rocker geometry.I like the idea of setting the rockers to mid-lift,it makes since.

G-10, Thanks. I agree with the aticle on the Mid-Lift site. Great stuff. When I started my design I thought I was alone. About a year later I saw 2 others had done the same thing because of the lack of any manufacturer doing them. Joe Fontana had done it back in the 80's and Don Ferguson was doing a set at the same time as me. Don's and mine were very close. I wish I had thought of stepping down the adjusting nut to make the push rod shorter. I used 2 torrington bearings side by side with a gap for oil which he didn't. Joe made his own adjusters from set screws, something I would never thought of. Hey necessity is the mother of invention.
When people see the Clifford 292 manifold on the side of my 302 GMC, some still can't figure out how I made the round ports in the manifold.This is what using an old style engine is all about...Good Luck...J.D.

J.D., I sat down this evening and drew up a roller rocker for my 302. I'll get the material tomorrow and get started next week while the machine shop is closed for X'mas. I have to admit, I did look at the pictures of the rockers you built before I started on mine. I know the machining would be much easier if the rocjkers didn't have the offsets in them. Ron

Where's the pic's? I couldn't find them.

Where can I find what changing rocker ratio does to the duration?

G-10, The pics were of a grouping I sent to Ron of a lot of the "stuff " I've collected/used/built over the years plus the LSR stock head GMC engine I'm in the process of assembling.
From what I remember the duration at .050" was only a few degrees.

Thats why I couldn't find them.

If someone will tell me how to post a picture I'll give it a shot on the rocker are assembly

Wish I could help,but I have no clue.Sorry.

jimmy six #35, this is how I post pics here:

I opened a free account at:

http://photobucket.com

Then I upload the desired pics to Photobucket website (I usually reduce their size). When I'm writing a post, I simply copy the “image address” from the picture I want to post (I choose the “Img” option) and paste it in the message. I suggest clicking “Preview Post” to confirm everything is OK.

Example:

is actually “{IMG}http://i109.photobucket.com/albums/n61/rofc/I-6GM250.jpg {IMG}"

I hope it helps..