Valve lift question

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Boneswell

Boneswell

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I have an LA360 with a lunati retrofit roller cam and hydraulic lifters and EQ magnum heads. I ordered up some prw 1.6 roller rockers from Hughes but was shipped 1.65 instead.
This is completely irrelevant because I'm shipping them back come Monday for the correct parts but it does pose the question of how much lift is lost at the valve due to valve train geometry. I suppose I could wait until I get my heads from the machine shop and get a rough number but I'm curious if someone smarter than me has already figured this out.
 
1.65 will give you more lift than 1.6. You decide how much lift you want and assemble accordingly. You start to lose when all the parts begin to wear out.
 
magnumdart said:
1.65 will give you more lift than 1.6. You decide how much lift you want and assemble accordingly. You start to lose when all the parts begin to wear out.
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magnumdart said:
1.65 will give you more lift than 1.6. You decide how much lift you want and assemble accordingly. You start to lose when all the parts begin to wear out.
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That's not what I'm asking. After you calculate 1.5 or 1.6 or whatever, has anyone measured the lift at the valve and subtracted that from the cam lift? Such as...a .500" lift cam with a 1.5 rocker measured at the valve will not be exactly .500" lift because of imperfect valve train geometry.
 
I measured a loss of about .015" at the pushrod tip-due to the small block mopar lifters at a wider than straight angle to the pushrods. so that X a true 1.5 rocker would be a loss at the valve of .022. then there is what the rocker arm ratio actually is?
 
Marcohotrod said:
I measured a loss of about .015" at the pushrod tip-due to the small block mopar lifters at a wider than straight angle to the pushrods. so that X a true 1.5 rocker would be a loss at the valve of .022. then there is what the rocker arm ratio actually is?
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Yes, I'm sure it's exponential with the rocker arm
 
Which is why you will have to measure everything if you are trying to figure clearance. Push rod length will matter also, as it will change the ratio. In Marcos example the .0225 loss would be true with a 1.5 rocker at any cam lift if the pushrod loses .015. The rocker arm will always be 1.5.
 
and some say stock rockers measure 1.4, and rocker C may be 1.45, and rocker whatever will be whatever, maybe some more than advertised

DSCF0227.JPG
 
magnumdart said:
Which is why you will have to measure everything if you are trying to figure clearance. Push rod length will matter also, as it will change the ratio. In Marcos example the .0225 loss would be true with a 1.5 rocker at any cam lift if the pushrod loses .015. The rocker arm will always be 1.5.
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Obviously this question is subjective and too full of variables. Even if I had an exact answer, how much of that is even relevant one the engine is running? Thermodynamics and material deflection will most definitely cause some changes. This really has little relevance to my build. My math is sound, I have a machinist I trust and this is a fairly low compression street engine. As I said, thus is just hypothetical.
Thanks
 
Boneswell said:
That's not what I'm asking. After you calculate 1.5 or 1.6 or whatever, has anyone measured the lift at the valve and subtracted that from the cam lift? Such as...a .500" lift cam with a 1.5 rocker measured at the valve will not be exactly .500" lift because of imperfect valve train geometry.
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A .500" valve (not cam) lift with a true 1.5 rocker, would have .3333---" cam lobe lift. Because of the lifter angle, the trig function shows the cam lift seen at the rocker to be .3234---", which would make the valve lift .485". The bigger the lobe lift, the more loss and a smaller lobe is obviously less loss. The rocker ratio will have no effect on the amount of loss if the rocker geometry is correct, i.e. correct pushrod length on a magnum and correct shaft height on the LA. Fwiw, the lifter/pushrod angle is not as problematic as it would seem.
If you were looking to get 1.6 at the valve, you should have kept the 1.65's. You would have been a lot closer.
 
B3RE said:
A .500" valve (not cam) lift with a true 1.5 rocker, would have .3333---" cam lobe lift. Because of the lifter angle, the trig function shows the cam lift seen at the rocker to be .3234---", which would make the valve lift .485". The bigger the lobe lift, the more loss and a smaller lobe is obviously less loss. The rocker ratio will have no effect on the amount of loss if the rocker geometry is correct, i.e. correct pushrod length on a magnum and correct shaft height on the LA. Fwiw, the lifter/pushrod angle is not as problematic as it would seem.
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Very interesting. I'm not an engineer but I get really excited about engine dynamics. I have loads of engines under my belt (Subaru, SBC, rotary's, 2 and 4 stroke motorcycle engines) but this is my first mopar. I like learning about and overcoming design flaws as well as exploiting their strengths. The FABO archives have been a pretty serious wealth of info.
 
Boneswell said:
Very interesting. I'm not an engineer but I get really excited about engine dynamics. I have loads of engines under my belt (Subaru, SBC, rotary's, 2 and 4 stroke motorcycle engines) but this is my first mopar. I like learning about and overcoming design flaws as well as exploiting their strengths. The FABO archives have been a pretty serious wealth of info.
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What valve lift are you trying to achieve? The 1.65 rockers you have will get you very close to 1.6 lift numbers. I'd try them before sending them back.
 
B3RE said:
A .500" valve (not cam) lift with a true 1.5 rocker, would have .3333---" cam lobe lift. Because of the lifter angle, the trig function shows the cam lift seen at the rocker to be .3234---", which would make the valve lift .485". The bigger the lobe lift, the more loss and a smaller lobe is obviously less loss. The rocker ratio will have no effect on the amount of loss if the rocker geometry is correct, i.e. correct pushrod length on a magnum and correct shaft height on the LA. Fwiw, the lifter/pushrod angle is not as problematic as it would seem.
If you were looking to get 1.6 at the valve, you should have kept the 1.65's. You would have been a lot closer.
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Also, I feel like I might be pushing what I want for lift anyway. My cam is .535/.550". With a 1.6 that's .584" and 1.65 is .604". The springs will be fine but I left the heads unported so i could do a before and after comparison. I think I might be at the limits of what my heads flow.
 
Scratch that. I just threw out numbers on the exhaust side. That does put me pretty close to what I want with the 1.65.
 
Boneswell said:
Shooting for around .580
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With the 1.65 rockers, you should be right around .571" valve lift on the intake side, accounting for losses, which is where you would theoretically be with a 1.6 rocker and no losses. A 1.6, with losses, would only give you .557" valve lift.
 
have tried 1.6 rockers over 1.5 on a few engines on the dyno, never gained much, if any. I guess we picked good cams from the get go
 
Marcohotrod said:
have tried 1.6 rockers over 1.5 on a few engines on the dyno, never gained much, if any. I guess we picked good cams from the get go
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I have EQ magnum heads. 1.6 rockers are factory. I'm not even sure if you can run a 1.5 without machining for clearance.
 
well I was giving an example of engines that came stock with 1.5 rockers. and my point is that whatever engine you have, an extra .020" lift is not likely to give much more power if you have the right cam for the combo to begin with
 
Marcohotrod said:
well I was giving an example of engines that came stock with 1.5 rockers. and my point is that whatever engine you have, an extra .020" lift is not likely to give much more power if you have the right cam for the combo to begin with
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Exactly. The question only came up because the wrong rockers were mailed to me. Or so I thought. The guy at Hughes told me that because of the funky pushed angle, a 1.65 calculates much closer to 1.6. As soon as my heads are finished, I'll measure the valve lift with the 1.65. I'll know for sure then!
As for the cam selection, I feel I'm pretty close for what I want. I've never used a roller cam so we'll just see what happens. To be honest, I don't have any major needs like good mileage or vacuum. Only shooting for wicked burnouts and relative reliability. I'm also trying to light that fire in my 12 year olds eyes with a muscle car.
 
Marcohotrod said:
well I was giving an example of engines that came stock with 1.5 rockers. and my point is that whatever engine you have, an extra .020" lift is not likely to give much more power if you have the right cam for the combo to begin with
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A higher rocker ratio is not a crutch for a poor cam selection. It can make a good cam better or a bad cam worse. And, the additional lift has very little to do with it. Interestingly though, there are a lot of people who get twisted out of shape if they are missing .010" lift at the valve, but the geometry can be a wreck and that's acceptable. But, you are right that the .020" extra lift won't make much of a difference in power. On a really good head, there probably wouldn't be more than 5-7 cfm difference at full lift.

The benefit is the extra area under the curve, much like the advantage of a roller cam. The valve opens quicker, and dwells longer, allowing the engine to breathe better without increasing the seat timing and negatively affecting lower RPM power. Bigger cam power with smaller cam driveability. Btw, correcting geometry does the same thing, so just bolting on a higher ratio roller rocker without getting the geometry right can hurt power. Did you adjust the shaft height when changing rockers?

If you aren't making more power with a 1.6 rocker over a 1.5, then the cam is probably too big already, or you don't have a stable valvetrain to take advantage of the increased rpm potential. I rarely use a 1.5 ratio anymore, and always recommend a 1.6 or higher on a small block.
 
B3RE said:
A higher rocker ratio is not a crutch for a poor cam selection. It can make a good cam better or a bad cam worse. And, the additional lift has very little to do with it. Interestingly though, there are a lot of people who get twisted out of shape if they are missing .010" lift at the valve, but the geometry can be a wreck and that's acceptable. But, you are right that the .020" extra lift won't make much of a difference in power. On a really good head, there probably wouldn't be more than 5-7 cfm difference at full lift.

The benefit is the extra area under the curve, much like the advantage of a roller cam. The valve opens quicker, and dwells longer, allowing the engine to breathe better without increasing the seat timing and negatively affecting lower RPM power. Bigger cam power with smaller cam driveability. Btw, correcting geometry does the same thing, so just bolting on a higher ratio roller rocker without getting the geometry right can hurt power. Did you adjust the shaft height when changing rockers?

If you aren't making more power with a 1.6 rocker over a 1.5, then the cam is probably too big already, or you don't have a stable valvetrain to take advantage of the increased rpm potential. I rarely use a 1.5 ratio anymore, and always recommend a 1.6 or higher on a small block.
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I have stud mount rockers. This obviously makes it easier to make the adjustments I need. In regards to my cam selection, I'm probably pushing the envelope for my needs but I've worked diligently with lunati and and my machinist to meet my goals. Once I pick up my heads, I can start checking valve/piston clearance and pushrod length.
 
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