Valvetrain diagnosis needed
06-12-2008, 05:52 PM
#1
Valvetrain diagnosis needed Added 1.8 roller rockers & there is 0 to slightly positive pressure in the intake manifold. There are no bent rods, piston damage, blown head gaskets....etc..
The problem is that the intake valves are slightly open when the piston starts to compress the air.
Figure that either the pre-load is not deep enough into the lifter or the push rods are too long.
-If the pre-load was shallow, I thought that perhaps the lifter would have to compress before the valve would begin to open. Then thought that the late opening may also mean a late valve closing. Valve train is noisy as hell.
-If the push rod were too long, then the valve could remain slightly open & never close. However, if this were the case, figured that there would be exhaiust gases in the intake manifold & there is not. Slightly pressurized air is clean.
Previous set-up was 1.7 stock cradle mount rockers w/ 7.4" push rods. Current set-up is 1.8 stud mounted rockers w/7.3" push rods. Measured rod length using a solid lifter. Don't remember the actual pre-load, but, measured it by threads (24 thds/inch = .0416" per rotation). For example: if .060" was the recommended pre-load, then would have gone w/ almost 1.5 rotations;
.046x1.5=.062".
So, do I begin by backing off the pre-load, adding more pre-load, or are the push rods wrong? Was very careful w/ checking the push rod length. cam is .309"/.313", 114 LSA.
The problem is that the intake valves are slightly open when the piston starts to compress the air.
Figure that either the pre-load is not deep enough into the lifter or the push rods are too long.
-If the pre-load was shallow, I thought that perhaps the lifter would have to compress before the valve would begin to open. Then thought that the late opening may also mean a late valve closing. Valve train is noisy as hell.
-If the push rod were too long, then the valve could remain slightly open & never close. However, if this were the case, figured that there would be exhaiust gases in the intake manifold & there is not. Slightly pressurized air is clean.
Previous set-up was 1.7 stock cradle mount rockers w/ 7.4" push rods. Current set-up is 1.8 stud mounted rockers w/7.3" push rods. Measured rod length using a solid lifter. Don't remember the actual pre-load, but, measured it by threads (24 thds/inch = .0416" per rotation). For example: if .060" was the recommended pre-load, then would have gone w/ almost 1.5 rotations;
.046x1.5=.062".
So, do I begin by backing off the pre-load, adding more pre-load, or are the push rods wrong? Was very careful w/ checking the push rod length. cam is .309"/.313", 114 LSA.
06-12-2008, 08:11 PM
#2
Couple of things. First, whose rockers are they? Second, the rotations of the adjusting nut at the pivot point must be corrected for the rocker ratio through parallel triangles. Using your numbers:
For 1.5 rotations:
Movement of nut on stud/movement of pivot point: 1.5/24 = 0.0625
Correction for rocker ratio to obtain movement of pushrod or lifter preload: (2.8/1.8) X 0.0625 = 0.097"
Finally, when you set these up, you need to establish the wipe pattern height that provide good wipe, then use that setting to measure for pushrod length. Guess I am not following precisely what you did to set these up.
How do you know the intake valves remain open (don't fully close)?
Your comment that the valve train is noisy as hell would indicate the pushrods are too short.
For 1.5 rotations:
Movement of nut on stud/movement of pivot point: 1.5/24 = 0.0625
Correction for rocker ratio to obtain movement of pushrod or lifter preload: (2.8/1.8) X 0.0625 = 0.097"
Finally, when you set these up, you need to establish the wipe pattern height that provide good wipe, then use that setting to measure for pushrod length. Guess I am not following precisely what you did to set these up.
How do you know the intake valves remain open (don't fully close)?
Your comment that the valve train is noisy as hell would indicate the pushrods are too short.
06-12-2008, 08:20 PM
#3
Humm if I were you, I would:
1. Just use the lifters you are going to use
2. Get an adjustable pushrod
3. Verify wipe pattern of rockers
4. Use the adj pushrod to find the length where you just make zero lash with the lifter and the rocker arm
5. Take that length and then add in the preload required for those lifters
1. Just use the lifters you are going to use
2. Get an adjustable pushrod
3. Verify wipe pattern of rockers
4. Use the adj pushrod to find the length where you just make zero lash with the lifter and the rocker arm
5. Take that length and then add in the preload required for those lifters
06-12-2008, 10:59 PM
#4
Couple of things. First, whose rockers are they? Second, the rotations of the adjusting nut at the pivot point must be corrected for the rocker ratio through parallel triangles. Using your numbers:
For 1.5 rotations:
Movement of nut on stud/movement of pivot point: 1.5/24 = 0.0625
Correction for rocker ratio to obtain movement of pushrod or lifter preload: (2.8/1.8) X 0.0625 = 0.097"
Finally, when you set these up, you need to establish the wipe pattern height that provide good wipe, then use that setting to measure for pushrod length. Guess I am not following precisely what you did to set these up.
How do you know the intake valves remain open (don't fully close)?
Your comment that the valve train is noisy as hell would indicate the pushrods are too short.
For 1.5 rotations:
Movement of nut on stud/movement of pivot point: 1.5/24 = 0.0625
Correction for rocker ratio to obtain movement of pushrod or lifter preload: (2.8/1.8) X 0.0625 = 0.097"
Finally, when you set these up, you need to establish the wipe pattern height that provide good wipe, then use that setting to measure for pushrod length. Guess I am not following precisely what you did to set these up.
How do you know the intake valves remain open (don't fully close)?
Your comment that the valve train is noisy as hell would indicate the pushrods are too short.
The wipe pattern was perfect when the push rod length was established. Adjustable push rod was used against a LS7 lifter, locked so that it would not collapse. Wipe pattern was as narrow as it could be & right in the center of the valve stem. LS7 roller lifters are used.
Rocker arms are PRW. What's the max pre-load for an LS7 lifter? 7.3" push rods are used. Seems like the lifter pre-load wasn't completely accounted for in the push rod length. They were instsalled a few Months ago.
Am in the process of tuning (EFI Live) & the fueling was indicating a vacuum lead or manifold issue. When I checked the manifold presssre it was floating around 0 to just above@ idle. Something is adding pressure to the intake.
It is not a damaged piston, head gasket leak, vacuum leak. So, I thought back to the roller rockers & the noise. Thought that if the lifter pre-load was applied while the lifter was supposed to lifting the rocker arm, then perhaps the valve opens late & therefore may be closing late. The MAP issue is definately a valve train issue.
Sounds like 7.35"-7.40" push rods are in order.
06-13-2008, 10:22 AM
#6
No, they are Comp copies (PRW).
Went to Summit Racing this morning & discussed the problem w/ the tech's. The 7.30's I used were a bit short & 7.35's are a bit long. So, I went w/ the 7.35's & will have to watch coil bind closely. My diagnosis of a hanging, partially open valve may not be the case. Was just trying to figure out the very slight pressure. Thanks for the comments & thanks Summit.
Went to Summit Racing this morning & discussed the problem w/ the tech's. The 7.30's I used were a bit short & 7.35's are a bit long. So, I went w/ the 7.35's & will have to watch coil bind closely. My diagnosis of a hanging, partially open valve may not be the case. Was just trying to figure out the very slight pressure. Thanks for the comments & thanks Summit.