Pushrod Length and Lifter Preload
With the stock OEM lifters you have a lot more room for error than Comps. We recommend stock lifters for all users that are planning on revving less than 6300rpm, because it could save a lot of headache in the future. The stock lifters have a range of +/- .080 and the Comps about 1/2 that requiring a much more accurate setup. The Comp cams lifters are also noisier than stock. Comp touts them as having better high RPM performance. However, if you aren’t going to use high RPM, then there will be no point.
The stock pushrods are 7.020 and most companies list their aftermarket 7.050 pushrods as a “stock length” replacement. Comp cams measure their pushrods different than some companies by not taking into account the final radius on the ends of their pushrods. This makes their pushrods measure about ~.010 longer than some (like intense). ZZPerformance likes to run our pushrods on the short side to avoid the chance of lifter pump up. Lifter pump up is when the oil pressure raises so high that the lifter actually keeps the valve from closing all of the way to get the most compression in the cylinder as possible.
The stock head gaskets on a Grand Prix are .062 with most people running .052 as an aftermarket alternative. Some go as small as .045 and some as large as .074. These are all within range of what a lifter can accommodate for and do not usually require pushrod length change.
Some aftermarket cams can have a smaller base circle than the stock cam. This reduces lifter preload by the amount that the radius is smaller than the stock cam. Most aftermarket cams have the diameter .010 to .030 smaller than stock with the milder cams being closer to .010. The XPZ intake lobe with a very custom profile has the base circle diameter around .070 less than stock. When measuring the diameter of your aftermarket cam compared to a stock cam, keep in mind it’s the radius, not the diameter that you need to use in the math.
Aftermarket heads can have different valve lengths. There is no way around this if you recut the valve seats. If the valves have close to a stock tip height, then it probably doesn’t need to be factored in.
The ZZP heads use valves that are .100 longer than stock. We do this to correct for rocker angle when going to a larger cam. Normally the lift of a stock head is around .400 and most users of aftermarket heads have lifts around .550. Leaving the starting position of the rocker the same and opening the valve another .100-.200 causes and extreme angle on the rocker. Our use of longer valves allows higher valve lifts with correct rocker geometry, but requires the customer to use the supplied rocker shims to rise up the rockers with the valve tip.
If you were, for example running our heads with stock pushrods, lifters, rockers, and cam the additional valve tip height would use up the lifter ‘squish’ and hold the valves open all the time causing zero cylinder pressure and a no start condition. To properly set this up you need “rocker pedestal spacers”/”rocker shims” (supplied with our Stage heads), shorter pushrods, or finally a different rocker setup.
When all is said and done you need to add up the changes in head milling, block decking, head gasket thickness, pushrod length, valve tip height, camshaft base circle lobe diameter, and rocker arm type. This will allow you to calculate the length of pushrod needed.
To check for lifter preload or lifter squish you need to first get to zero lash. Turn the engine over so that one valve on a cylinder is open. Since this one is open, you know the other is closed. This means the lifter is sitting on the base circle. With that lifter on the base circle loosen the rocker bolt and then retighten with one hand while spinning the pushrod with the other. This way you can tell when everything first makes contact (the valve is touching the rocker tip, the pushrod is touching the rocker & and lifter and the lifter is touching the cam). At this point of first contact you are at zero lash. Everything past this point is lifter preload. Continue to tighten the rocker bolt until the bolt hits its snug point. This is how much lifter preload you have. Each 1/2 turn of the rocker bolt is ~.050 of lifter preload. Your goal is to have between 1/2 turn and 1.5 turns if you have stock lifters or 3/8 turn to 1/2 turn if you have Comp lifters. As you can see the stockers have a lot more room for error.
If you have too much preload and you have exhausted the adjustment of the lifter, the valves will stay open and the car will not start due to lack of compression. If you have too little preload, the valve train will be noisy and you risk exploding a lifter and having a very costly repair.
Remember, once the engine is heated up things change a bit. Parts grow and you will have to calculate in ~.005 pushrod growth. Also each pushrod with vary +/- .005 making things critical in high performance applications.