| Author | Message | ||
     Bomber |
All -- I'm tearing into the motor, in preparation for big fun this year, but I've a question -- a search of the KV got me close, but I'd like some advice -- Here's what I"m doin' - 2000 M2 - 1250 kit (cyclinders and pistons -- 10.5:1 compression ratio - XB heads - Andrews N8 cams so, am I gonna need adjustable pushrods? thanks for the backup -- I appreciate it! | ||
| Justin_case |
If your new cams have more lift you'll need shorter push rods. Adjustables simplify this. | ||
| Bomber |
Justin -- right as rain, you are -- the lift on the N8s ism as my grandpa woulda said, pert near the stockers -- more duration, and juggled points of max lift, tho just figured I'd check with someone that may have build the same beast, is all thanks for the backup! | ||
| Kilroy |
N8's can be used with stock pushrods. I put n8's in my '00 M2 and have a few hundred miles with no problems. N8's are "drop-in" cams, meaning it is not necessary to switch out any other valvetrain components. Wouldn't shorter pushrods cancel out the higher lift anyway (if matched correctly/incorrectly) | ||
| Justin_case |
Kilroy, would changing connecting rod length change the stroke of a crankshaft? Nope.The pushrods need to be changed to retain correct rocker arm geometry, otherwise the valves might never close! Talking Hi-lift here. | ||
| Kilroy |
The base circle diameter of the cam remains the same on the n8's versus stock. When the lifter is on the base circle the valve is closed. The "lifting" area of the cam only has to do with when the valve opens and how long it stays open. A bigger cam will, for example, open the valve sooner, open it a little more, and/or keep it open longer than stock. The cam/lifter/pushrod still has the same geometry when the valve is in its closed position in this case. | ||
| Djkaplan |
I was under the impression adjustables were only needed for solid, non-hydraulic lifters. More lift with the same base circle doesn't require a shorter pushrod, btw. | ||
| Panic |
If the lift change is within the hydraulic range of adjustment it will work with the original pushrods. A solid lifter cam with more lift (but no other changes) requires a different pushrod unless the base circle has been reduced by 1/2 of the lift change. Same base circle: pushrod length reduced by 1/2 of lift change. | ||
| Sportyeric |
Hydraulic lifters have about a 100 thou range. Theoretically, a stock pushrod is probably sized to have the plunger halfway down when cold, which is also the traditional goal when adjusting adjustables. This allows for a 40 thou growth in cylinder height as the engine reaches operating temperature. And it allows for gradual extension of the valve stem towards the rocker as the valve seats get hammered further into the head over time. If any cam change has the same base circle then the stock pushrods should be all that are required. Except that a decent engine build should include adjusting the squish area by using thinner gaskets or machining down the cylinder height to get a proper squish. By shortening the distance from the cams to the head, and consequently the rockers, the pushrods could become too long to allow the valves to close fully even without a cam change. I doubt, however, that the change in cylinder height would be enough to eat up the wiggle room that is build in to the design. With my up-coming build, I intend to use a spare pair of adjustable pushrods to measure and ensure that a stock pushrod will not bottom out the plunger. I have been told that as long as there is 15 thou left, that will allow for future valve seat recession and will otherwise allow the lifter to act almost as a solid lifter. By avoiding adjustables, I avoid possible oil leaks from collapsing covers. I avoid the possibility of the adjustables coming loose. And I avoid the cost of purchasing both the pushrods and the covers. But I lose the ability to change cams at random or inspect lifters periodically without pulling the rockers. |
