Rocker Arm Geometry-How to Modify?

[Xnke]

Working on an idea that may or may not be workable.

Stock rockers for the engine I'm building currently are 1.6 ratio roller fulcrum types similar to the LS1 style rocker arm. I would really like to get a 1.7 ratio rocker to use for the intake valves, but not bad enough to go out and spend 800$ on converting the heads to use SBC style rocker arms and then having to make custom valve covers to fit over them.

The rocker arm itself resembles the LS1 arm, but the pedestals are independently mounted and are steel. This is the problem that others have seen in the past and have walked away from:



Supposedly this problem occurs at 0.540" of lift, while the stock rocker arms have adequate geometry to 0.700" valve lift. What I see is the rocker tip being the wrong shape, OR the rocker fulcrum is in the wrong spot.

I'll have more photos at various lifts when I get a head setup with the LS1 1.7 ratio rocker today.

[mag2555]

Your taking about a change of less then 1% here, so you would pick up about .020" more lift and about 2 more degrees over the nose of the cam.
I can't see the value in making this change as not even the opening rate will change all that much and that is where you can really pick up some power!

[Cubic_Cleveland]

So is this picture at .540" lift? Or??

I think the pedestal needs to be shorter, and possibly moved back as well. Is the pedestal on the same angle as the valves?

[John Wallace]

Your taking about a change of less then 1% here

1.6 ratio to 1.7 ratio is almost 6% change?



Now whether it is practical to do the change, I don't know.

[wyrmrider]

obviously current pic is a big non starter
let's see a pic of...
valve set at mid lift held by a valve stem seal with a screw clamp
rocker centered on stem tip with fulcrum at proper height for center of radius of rocker pad and center of fulcrum or center of roller tip- line tangent to valve stem
then figure out how you have to mod the stands and measure for pushrods
what motor, how much cam lift, what's the application, rpm etc
I'd get the "controlled induction" software first and model your system to see if this is worth it
remember to check your pushrod clearance before committing

[dave brode]

So is this picture at .540" lift? Or??

I think the pedestal needs to be shorter, and possibly moved back as well. snipped

Ditto. I assume that they are 2.2/3.1/3.4 parts. I've fooled with them on other engines too. Finicky for position. Look at them on the original application and I bet that you'll see that they need to go way back [toward intake flange] and way way down. I believe that LS pieces will be worse.

Dave

[wyrmrider]

What dave said
X2
do a mockup with a wood or plastic stnd
measure twice or in this case more than twice- cut once

[piston guy]

So is this picture at .540" lift? Or??

I think the pedestal needs to be shorter, and possibly moved back as well. Is the pedestal on the same angle as the valves?

+1 shorten the pedestal and things will come back in line. IF the 1.7 rocker has the same fulcrum to valve contact pad dimension, it should work fine.

[Walter R. Malik]

For THAT rocker arm, the pedestal needs to be a LOT shorter or the bosses shorter or both.
For a different rocker it will probably be different again.

[Xnke]

Yes, it is a 3500 v6, a derivative of the 3.1/3.4l engines. I am adapting the heads onto the older iron head rearwheel drive block. starting with the 2000 model 3400 engines they got this type of rocker arm with the 1.6 ratio, but since I am using the flat tappet cam I was looking for an option for more valve lift without increasing the duration much. The intake cam is 266 degrees duration and 0.468" lift with the 1.6 rocker, however the heads continue picking up flow to 0.500" lift. Going to a 1.7 rocker would bring the intake lift up to 0.498", this is the same as my exhaust valve lifts with just the 1.6 rocker. The limit of the current valvetrain parts is about 0.540" lift anyway. I thought that if I can make it work by adjusting the pivot height or similar that I could A-B compare on the running engine to see if there was enough gain to justify the bigger cam.

[wyrmrider]

and your retainer to rocker clearance
simulate what you want to do before spending that much work
which block, heads, etc are you melding together- lot's of choices

[wyrmrider]

and your retainer to rocker clearance
simulate what you want to do before spending that much work
which block, heads, etc are you melding together- lot's of choices

[pdq67]

Imho, not knowing any better, I figure that you would have to do the same thing us SBC guys do to put BBC rockers on our heads.

Someplace there is a picture illustrating how to do this. Hunt it up. I forget if it is in one of the old Chevy High Performance books or not?

And you do have to "dimple" the valve covers on SBC heads.

Sorry, not much help..

pdq67

[wyrmrider]

does not look like he's using studs and he'd have to use rocker tip guided rockers or fab some Isky adjustable pushrod guideplates
surely someone has put different rockers on a GM 60 degree v6 before- but year of head does matter

[pdq67]

Right, he is not using studs, but I figure the locations change be them studs or stands is all.

pdq67