Yeah, so I did some clearance checking, and i currently have .147" space between the exaust valve and the piston relief. The intake is over .300" because that's how thick the putty was. We need to fix this

So doing the math with my cam lobe standing at .324" (exuast) and the intake at .334" (intake), I did some math and came up with two plausible solutions.

1) I could run 1.6:1 ratio rockers and get .519" (rounded up for saftey) gross lift which would leave me with 0.114" clearance. still a bit much no?
or
2) I could run .050" over rods (7.850") w/stock rocker ratio which would yeild .561" gross lift which would leave me with .072" clearance...

Oh no! we've passed that magical number: .080" !!!

Mind you that's without the headgasket installed yet that has a compressed thickness of .015" which would technically leave me at .087" clearance - which would be within the .080" Comp Cams and other sites seem to recommend.

Opinions wanted!

Cheers

 

How do you figure longer pushrods will increase valve lift?

 

adding .050" to the pushrod would be the same as adding .050" to the cam lobe if I'm not mistaken. The valve lash would remain the same (hydraulic flat-tappit) so the rocker would be pushed up .050" higher, which in turn would compress the spring and vavle 0.075" down further w/ stock rockers

I multiplied (.050" + .324") * 1.5 and got .561". I found the difference between my current clearance and this sample one

The .015 head gasket would actually yeild another 0.0375" clearance between the valve and the piston's valve relief, but I thought it better to just use .015 for an extra margin of error

edit: (it would work out to an actual 0.1095" clearance between the piston and the valve total with the headgasket installed, but i'd rather be on the safer side and use just .015 for my calculations which left .087" clearance)

 

"adding .050" to the pushrod would be the same as adding .050" to the cam lobe if I'm not mistaken."

ONLY IF your push rod keeps the valve .050" from completely closing, which obviously isn't practical. Push rod length has no affect on lift. It's changed to ensure proper valvetrain geometry-correct contact between the rocker tip and the valve stem. You're thinking too hard.

 

Changing valve lift by changing rocker ratio isn't a direct change to the piston to valve clearance. It's not just a change at max lift but through the entire duration of the cam lobe. Increased lift during partial duration depending where the piston is in the cylinder at the time may increase or decrease the PTV distance. The only way to know how much the PTV clearance changes is by changing the rockers and putting putty on the piston to recheck it.

As mentioned above, pushrod length is only to keep the proper valvetrain geometry. It has no effect on valve lift.

Increasing valve lift with a cam or rocker change needs other measurements checked. Will the springs bind with the increased lift? Will the bottom of the spring retainer hit the top of the valve guide etc.

Typically with a stock type piston and cam grind, increasing the lift with 1.6 rockers is still well within the PTV limits. I see increased rocker ratio as a poor mans cam swap. If you want that extra lift, just buy a cam with that lift.

 

okay - im going to replace the rockers - i stopped in at my local speed shop and they told me the same thing about the geometry and explained it really well to me... another problem with it is it changes the angle at which the rocker is on the valve - and can really screw it up. I really am happy with the cam i chose for this car... i want the same cam, but i want to just let a little more air in the smogger heads until i can afford a good set of aftermarket heads. I think ill try the 1.6:1 rockers (i know i have plenty of clearance to the pistons... thanks for the help

 

I had an "RV" cam in a 350 with 882 "smogger" heads. Going from 1.5:1 to 1.6:1 rockers made absolutely no difference in 1/4 mile time or MPH. The heads were overwhelmed with the RV cam and the 1.5's, let alone the 1.6's.