How Milling Heads Affects Valve Lift
I am restoring an '81 T/A. The 301 was worn out, so I built a '77 0.030-over-400 with a 243-degrees-duration/0.486-inch-lift cam, a Performer intake, an 800-cfm carb, and headers. I ported and polished the heads and intake, and port-matched the headers. I have 100cc 6X heads and I shaved 0.050 inch off of them. Did this make the lift on the cam 0.536 inch? By shaving the heads, the cam woke up and throttle response is great. Are the stock Pontiac valvesprings strong enough for this lift?
I have a Turbo 400 trans. What stall converter do I need? What rear gear will work best for 95 percent street? Thank you for any info.
Rodney Gilder
Via Internet
Jim Taylor responds:
"Shaving" or milling cylinder heads does not increase cam lift. I understand, though, how you are getting these values. Your car has 0.486-inch lift, so you figured bringing the rocker studs and valves closer to the deck and cam by way of the 0.050 mill would increase valve lift by 0.050 inch. Though 0.486 lift plus 0.050 does equal 0.536, it doesn't work that way.
If the lobe lift of your cam is 0.324 and you use a 1.5:1-ratio rocker arm, the gross lift of the valve would be 0.324 x 1.5 = 0.486. The same lobe lift with a 1.65:1 rocker would be 0.324 x 1.65 = 0.535. If you mill 0.050 or 0.100, it is possible you will need to shorten the pushrod length by that amount. Any milling of heads, block deck, or changes in valvetrain parts prompts checking valvetrain geometry, which is corrected with pushrod length. Milling heads may also cause the head bolts to be too long.
The 243-degree duration cam you list is huge for the street. The only Pontiac valve-springs that may work would be R/A-IVs-speak with the cam manufacturer to get correct springs. It's always best to buy a cam kit that includes lifters and springs.
A Performer manifold is misapplied with your cubes, cam, and carb. I would choose a Performer RPM or Torker II. A 3.55 gear ratio with a 2,600-2,800-stall converter will work well with your combo. Thanks for your question.
When Do I Need A Four-Link?
I am not asking about the differences between four-link, ladder bars, or leaf springs, but how much is involved in the conversion and when is each needed. If I want a four-link or a ladder bar suspension, does that really require cutting the floorpans and moving the gas tank into the trunk like I've heard? Also, does it require these narrowed framerail kits I've seen, or can one be made while keeping the trunk, backseat, and gas tank all where they are? I was told a four-link was not to be installed on stock framerails. Is this true?
Another question, how much power does a car have to have before one of these setups is required? Does it help cornering or just reduce hop? If I put a 428 in my '68 Firebird and I'm having the framerails replaced in the restoration anyway, should I go for a four-link? Thanks.
David
Spring, TX
Rocky Rotella responds:
The intended usage of your particular application and overall budget will dictate the type of rear suspension you ultimately end up with in your Firebird.
Ladder bars are typically associated with drag-race applications. They involve solidly linking a rear axle to the framerails, which transfers all forward momentum to the body immediately upon launch, causing the nose to lift and the rear tires to plant. Because of their rigid design and limited range of motion, ladder bars tend to considerably stiffen the ride, ultimately lessening cornering ability.
A four-link setup is designed to maximize traction by maintaining the rear axle's position under the vehicle under myriad conditions. It uses two solid links per side and typically includes a coilover shock absorber, which improves ride quality while providing relatively good axle control during high-rate maneuvers.