Finding pushrod length with adjustable rockers?????
Finding pushrod length with adjustable rockers????? O.k. guys I need some help. I have Thunder's 7.400 hardened pushrods for my new engine but the heads I believe have been milled a lot. I don't know how much though. I am getting popping and back fires when I push down on the gas. It idles rough but it will idle on its own. My question is I have Comp Cams 1.75 rockers and they are adjustable. So how do I go about measuring for the pushrods with the rockers being adjustable?
I have the correct pushrod length checker.
I have the correct pushrod length checker.
Staging Lane
Joined: Sep 2008
Posts: 66
Likes: 0
There are lots of good articles on finding you pushrod length so I don't feel it's necessary for anyone to rewrite what's been written already.
http://www.circletrack.com/techartic...gth/index.html
http://www.carcraft.com/techarticles...gth/index.html
http://www.compcams.com/information/Products/Pushrods/
I don't think you're problems are pushrod length related. It sounds like your valve adjustment is the culprit.
http://www.circletrack.com/techartic...gth/index.html
http://www.carcraft.com/techarticles...gth/index.html
http://www.compcams.com/information/Products/Pushrods/
I don't think you're problems are pushrod length related. It sounds like your valve adjustment is the culprit.
I would agree. With adjustable rockers, length isn't exactly critical. The geometry may be off if the length isn't correct, but you should be able to get the proper preload with the adjustment. At least enough to get it to run right.
a) Try different lengths until you get a "proper" swipe pattern on the valve tip
or
b) Degree the cam every time you try a new pushrod length. Cam timing will change with PR length...
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That shouldn't be any different either.
Only thing that will change valve timing is lash, and hydraulic rollers have none. Rocker arm changes will change valve timing slightly too. Pushrods? Nah.
To optimize valve train geometry, first thing that needs to be determined is the center of the fulcrum point, that is, the center at which the rocker arm pivots. That height (in relation to the valve stem tip) dictates how the rocker tip sweeps across the valve tip. Then, on non adjustable rockers, pushrod length needs to be determined in order to set the proper preload on the lifter.
Adjustables are a little more forgiving.
Only thing that will change valve timing is lash, and hydraulic rollers have none. Rocker arm changes will change valve timing slightly too. Pushrods? Nah.
To optimize valve train geometry, first thing that needs to be determined is the center of the fulcrum point, that is, the center at which the rocker arm pivots. That height (in relation to the valve stem tip) dictates how the rocker tip sweeps across the valve tip. Then, on non adjustable rockers, pushrod length needs to be determined in order to set the proper preload on the lifter.
Adjustables are a little more forgiving.
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Verdad Gallardo That shouldn't be any different either.
Only thing that will change valve timing is lash, and hydraulic rollers have none. Rocker arm changes will change valve timing slightly too. Pushrods? Nah.
To optimize valve train geometry, first thing that needs to be determined is the center of the fulcrum point, that is, the center at which the rocker arm pivots. That height (in relation to the valve stem tip) dictates how the rocker tip sweeps across the valve tip. Then, on non adjustable rockers, pushrod length needs to be determined in order to set the proper preload on the lifter.
Adjustables are a little more forgiving.
Only thing that will change valve timing is lash, and hydraulic rollers have none. Rocker arm changes will change valve timing slightly too. Pushrods? Nah.
To optimize valve train geometry, first thing that needs to be determined is the center of the fulcrum point, that is, the center at which the rocker arm pivots. That height (in relation to the valve stem tip) dictates how the rocker tip sweeps across the valve tip. Then, on non adjustable rockers, pushrod length needs to be determined in order to set the proper preload on the lifter.
Adjustables are a little more forgiving.
Since I don't have a "sponsor" label or a million posts under my name, some may not accept my ramblings as credible. So here is Joe Sherman's thoughts:
"An interesting thing to do is play with pushrod length and measure the actual valve lift with the differant lengths. The pushrod that give the highest peak valve lift SHOULD be the one to use( on a race engine) but this MIGHT NOT be the one that has the best contact pattern. It can actually make quite a bit of differance in valve lift.
JOE SHERMAN RACING ENGINES"
Here is a word from Mike Jones (not the rapper, the cam grinder):
"...when you change the pushrod length until you get the most valve lift, you're also cuting the ratio at the seat. When you setup your rockers to give you a higher ratio above midlift, and a lower ratio below, you're increasing the area under the curve, and lowering the seating velocity at the valve. It makes a standard design cam act more like an Inverse Radius profile. Bigger upper lift durations, and smaller lower lift durations at the valve.
_________________
Mike Jones
Jones Cam Designs
Denver, NC
www.jonescams.com
(704)489-2449"
This is all probably well beyond the scope of this application, but definetly fun to read about.
Rocker ratio isn't constant throughout the rocker arms' arc of travel. Rocker geometry effects the rocker's arc and therefore effective rocker ratio and the amount of lift the valve sees at any given crank angle. When I said it will effect cam timing, I didn't specifically mean @.050" (although it could).
Since I don't have a "sponsor" label or a million posts under my name, some may not accept my ramblings as credible. So here is Joe Sherman's thoughts:
"An interesting thing to do is play with pushrod length and measure the actual valve lift with the differant lengths. The pushrod that give the highest peak valve lift SHOULD be the one to use( on a race engine) but this MIGHT NOT be the one that has the best contact pattern. It can actually make quite a bit of differance in valve lift.
JOE SHERMAN RACING ENGINES"
Here is a word from Mike Jones (not the rapper, the cam grinder):
"...when you change the pushrod length until you get the most valve lift, you're also cuting the ratio at the seat. When you setup your rockers to give you a higher ratio above midlift, and a lower ratio below, you're increasing the area under the curve, and lowering the seating velocity at the valve. It makes a standard design cam act more like an Inverse Radius profile. Bigger upper lift durations, and smaller lower lift durations at the valve.
_________________
Mike Jones
Jones Cam Designs
Denver, NC
www.jonescams.com
(704)489-2449"
This is all probably well beyond the scope of this application, but definetly fun to read about.
Since I don't have a "sponsor" label or a million posts under my name, some may not accept my ramblings as credible. So here is Joe Sherman's thoughts:
"An interesting thing to do is play with pushrod length and measure the actual valve lift with the differant lengths. The pushrod that give the highest peak valve lift SHOULD be the one to use( on a race engine) but this MIGHT NOT be the one that has the best contact pattern. It can actually make quite a bit of differance in valve lift.
JOE SHERMAN RACING ENGINES"
Here is a word from Mike Jones (not the rapper, the cam grinder):
"...when you change the pushrod length until you get the most valve lift, you're also cuting the ratio at the seat. When you setup your rockers to give you a higher ratio above midlift, and a lower ratio below, you're increasing the area under the curve, and lowering the seating velocity at the valve. It makes a standard design cam act more like an Inverse Radius profile. Bigger upper lift durations, and smaller lower lift durations at the valve.
_________________
Mike Jones
Jones Cam Designs
Denver, NC
www.jonescams.com
(704)489-2449"
This is all probably well beyond the scope of this application, but definetly fun to read about.
On these motors, when you change pushrod length, and nothing else, all you're basically doing is changing the preload on the lifter.
Now, using different length pushrods in the context they are, isn't the same. You would need to change the length of the pushrod as well as the height of the fulcrum point.
Whether someone is a sponsor on here or not does not make them any more or less credible. There are some very intelligent members on here, as well as some dumb *** sponsors.
That is true, with the non-adjustable pedestal mount rockers all you'd change is preload. But I'm about 90% sure that Comp's adjustable rockers are of the stud/polylock type. Those don't seat on a pedestal, but have a "floating" fulcrum in which tip height and PR length change the fulcrum's height. Those are the ones which are affected this way.
TECH Regular
Joined: Dec 2004
Posts: 440
Likes: 0
From: Alpharetta, Ga
The only way to vary wipe pattern on the base circle of the cam is through adjustable rockers correct?
That is true, with the non-adjustable pedestal mount rockers all you'd change is preload. But I'm about 90% sure that Comp's adjustable rockers are of the stud/polylock type. Those don't seat on a pedestal, but have a "floating" fulcrum in which tip height and PR length change the fulcrum's height. Those are the ones which are affected this way.
That is one reason I used the method I did, it allowed me to bench check the wipe pattern by using something that was measurable without turning over the motor. It worked well in the end.
Also, setting the pushrod length for maximum lift but potentially an incorrect wipe pattern as stated in the quote above may work for a race only motor, but I would think that valve guide wear would be an issue for a street driven car.
Also, setting the pushrod length for maximum lift but potentially an incorrect wipe pattern as stated in the quote above may work for a race only motor, but I would think that valve guide wear would be an issue for a street driven car.
That is one reason I used the method I did, it allowed me to bench check the wipe pattern by using something that was measurable without turning over the motor. It worked well in the end.
Also, setting the pushrod length for maximum lift but potentially an incorrect wipe pattern as stated in the quote above may work for a race only motor, but I would think that valve guide wear would be an issue for a street driven car.
Also, setting the pushrod length for maximum lift but potentially an incorrect wipe pattern as stated in the quote above may work for a race only motor, but I would think that valve guide wear would be an issue for a street driven car.
TECH Fanatic
Joined: Aug 2005
Posts: 1,691
Likes: 1
From: Sunny London, UK
Rocker ratio isn't constant throughout the rocker arms' arc of travel. Rocker geometry effects the rocker's arc and therefore effective rocker ratio and the amount of lift the valve sees at any given crank angle. When I said it will effect cam timing, I didn't specifically mean @.050" (although it could).
Since I don't have a "sponsor" label or a million posts under my name, some may not accept my ramblings as credible. So here is Joe Sherman's thoughts:
"An interesting thing to do is play with pushrod length and measure the actual valve lift with the differant lengths. The pushrod that give the highest peak valve lift SHOULD be the one to use( on a race engine) but this MIGHT NOT be the one that has the best contact pattern. It can actually make quite a bit of differance in valve lift.
JOE SHERMAN RACING ENGINES"
Here is a word from Mike Jones (not the rapper, the cam grinder):
"...when you change the pushrod length until you get the most valve lift, you're also cuting the ratio at the seat. When you setup your rockers to give you a higher ratio above midlift, and a lower ratio below, you're increasing the area under the curve, and lowering the seating velocity at the valve. It makes a standard design cam act more like an Inverse Radius profile. Bigger upper lift durations, and smaller lower lift durations at the valve.
_________________
Mike Jones
Jones Cam Designs
Denver, NC
www.jonescams.com
(704)489-2449"
This is all probably well beyond the scope of this application, but definetly fun to read about.
Since I don't have a "sponsor" label or a million posts under my name, some may not accept my ramblings as credible. So here is Joe Sherman's thoughts:
"An interesting thing to do is play with pushrod length and measure the actual valve lift with the differant lengths. The pushrod that give the highest peak valve lift SHOULD be the one to use( on a race engine) but this MIGHT NOT be the one that has the best contact pattern. It can actually make quite a bit of differance in valve lift.
JOE SHERMAN RACING ENGINES"
Here is a word from Mike Jones (not the rapper, the cam grinder):
"...when you change the pushrod length until you get the most valve lift, you're also cuting the ratio at the seat. When you setup your rockers to give you a higher ratio above midlift, and a lower ratio below, you're increasing the area under the curve, and lowering the seating velocity at the valve. It makes a standard design cam act more like an Inverse Radius profile. Bigger upper lift durations, and smaller lower lift durations at the valve.
_________________
Mike Jones
Jones Cam Designs
Denver, NC
www.jonescams.com
(704)489-2449"
This is all probably well beyond the scope of this application, but definetly fun to read about.
"...when you change the pushrod length until you get the most valve lift, you're also cuting the ratio at the seat. When you setup your rockers to give you a higher ratio above midlift, and a lower ratio below, you're increasing the area under the curve, and lowering the seating velocity at the valve. It makes a standard design cam act more like an Inverse Radius profile. Bigger upper lift durations, and smaller lower lift durations at the valve."
As far as "proper" geometry, I think thats debateable. Most rocker manufacturers recommend that there should be a 90* right angle formed at max lift between the valve centerline and the centerline intersecting both rocker tip and fulcrum axis (pivot length). This should put peak ratio at max lift, and like already mentioned, slower ratio/velocity as the seat opens and closes. This probably won't be the best wipe pattern, but it'll be damn close and should be easier on the seats/valve head.
But as I said in my first post, you can do either or. In this case it probably doesn't really matter.
