SBC Shaft Mount Geometry with different int/exh lifts

[lekid]

Hi guys,

I have always setup geometry the way that Scott Foxwell recommended which is to make sure the rocker is 90deg to the valve at mid-lift.

The issue I am having this time is my new setup will have 0.065" more lift on the intake compared to the exhaust. I run a 23deg head on a SBC where the stand is 1-piece. How would I setup geometry on that?

I was suggested to do in the middle of both to be best for both.

I was also recommended to run a lash cap to compensate for the difference in lift. If I go with that method, that means that I would need a lash cap that is half the difference in lift, right? That would be around 0.030" anybody making a 0.030" lash cap for 11/32" valves?

Thanks in advance!!

[GM-DR]

I would say you might need different length pushrods for intake and exhaust if you are attempting perfection---

[panic]

Mid-lift will provide a different motion path at the valve with different rocker arm designs.
With anything but Miller-based geometry (in which the rocker shaft center bisects a line drawn through the center of the pushrod ball and the roller axle) what you gain at the valve you lose at the pushrod. This is generally considered to be an acceptable compromise, but the error magnitude will vary depending on the exact vertical location of the shaft - different not only among manufacturers but even among models from the same source (Crane). Just what is this dimension? They won't tell you, and I suspect it's not consistent among rockers from the same source.
What is the goal:
1. highest average lift (area under the curve)
2. lowest side thrust in the guide
3. lowest thrust when spring pressure is maximum
4. smallest sweep across the stem tip
5. highest lift at maximum vacuum point of intake stroke (estimated at 2-3° after the piston's maximum velocity point, generally between 72° and 78° ATDC depending on the rod ratio)

[steve316]

You could grind the tip of the valve steam off .030. or split the difference on the stands

[MadBill]

I'd say split the difference. Most manufacturers only offer push rods in no less that 0.050" increments, which would be 0.075" or more at the valve.

[foxchassis]

I would say you might need different length pushrods for intake and exhaust if you are attempting perfection---

I use different length push rods on my 18 degree heads with .030" difference

[lekid]

Thanks guys!

I will split the difference!

I totally expect to have 2 different length pushrods.

[Cogburn]

You should compensate for any difference but I usually just leave the exhaust tips up .015-.020"(mainly so the people that get their hands in there later don't screw things up too bad).

[kimosabi]

Most rockers are unsuitable for mid lift/90deg geometry. I don't know if I would compromise too much on the rocker geometry if this is a high end build. How much total lift at the valve?

[lekid]

0.825 / 0.760 lift base on lobe x rocker ratio (before lash and deflection)
0.795 / 0.732 after lash

[panic]

The pushrod isn't the problem, and won't solve the problem.
AFAIK there is no .030" lash cap (too fragile).
If you can get the vertical position you want from stand height, you need to either:
1. shorten the exhausts (but you may cut through the hardened end), or
2. shorten the intakes .030" and add a .060" cap

[turdwilly]

I would say you might need different length pushrods for intake and exhaust if you are attempting perfection---

I use different length push rods on my 18 degree heads with .030" difference

Pushrod length isn't used to set geometry on shaft systems as it is with stud rockers. With shaft systems, you will find resultant pushrod length after setting the geometry via varying stand height.

[lekid]

Or, since I still have to order the intake rockers for the shaft mount (need different offset for new heads), order 1.65 instead of 1.7 to give me 0.800 instead of 0.825 and make things less uneven?

Or am I overthinking this?

[MadBill]

The latter.
Besides, an engine that really needs the high lift flow could drop 15 HP with a change from 1.7 to 1.65.

[DaveMcLain]

I can't believe that small amount of difference would really matter enough to measure performance wise.