Moparts

'Myth' of Mopar lifter diameter advantage?

Posted By: Diplomat360

'Myth' of Mopar lifter diameter advantage? - 07/17/19 12:11 PM

OK, so we have all heard of, read about, perhaps experienced the presumed advantage of the wider Mopar lifter diamater...however, I have to be honest and point out that for quite some time now as I have been looking at various cam listings I have been noticing the Mopar stuff (specific to the small block use - as that is my focus) with much less lobe lift then similar, if not the same, spec'ed cam grinds for the other engine brands.

Case in point, I am focusing on the retro-fit hydraulic roller cam grinds from Comp Cams, here are three examples:

1) Mopar Small Block - Xtreme Energy Retro-Fit XR292HR-10
Adv Dur => 292/300
Dur @ 0.050 => 242 / 248
Lift => .549 / .544
LSA => 110
RPM Range => 2800-6400

2) Chevy Small Bock - Xtreme Energy XR294HR Retro-Fit
Adv Dur => 294/300
Dur @ 0.050 => 242 / 248
Lift => .540 / .562
LSA => 110
RPM Range => 2800-6100

3) Ford Windsor - Xtreme Energy XR294RFHR Retro-Fit
Adv Dur => 294/300
Dur @ 0.050 => 242 / 248
Lift => .576 / .600
LSA => 110
RPM Range => 2500-6500

Alright...so the Ford grinds in particular are something that almost always lists a higher lobe lift. The Chevy...umm...here and there, but in the case of example we have here it is actually a tad higher, not by much, but it is.

So, is this because of the roller profile? Even then would you not expect the wider lifter to provide more room for a bigger diameter roller wheel and subsequently the ability to handle steeper lobe ramps?

OK, so this is pure theory (reasoning behind why these profiles are what they are), and I feel like too often we (the Mopar fans) like to say we get ignored by the aftermarket, but I find it hard to believe that given the amount of computerized manufacturing the makers like Comp Cams would not design a lobe profile that maxes out the lift.

Oh, and btw, if you say this is due to head flow restriction, while true for stock heads I would venture a guess that anyone running this level of cam has probably moved to heavily ported street heads or aftermarket stuff anyways.

So what gives here?
Posted By: jcc

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 12:56 PM

bump

I don't exactly know, but it seems there are many more factors involved here rather merely lift, such as area under under the curve, valve train accelerations, etc. Seems the larger lifter always is more sought if an option, even with the trade off of additional weight and cost.

Others will expound I am sure. Out. biggrin
Posted By: BradH

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 01:23 PM

Flat tappet: The wider foot allows for a higher rate of acceleration to be designed into the cam lobe.
Roller tappet: The larger OD allows for using a wheel with a larger radius that translates into a small increase in lifter acceleration.

Those are the potential advantages; whether the flat-tappet cam lobe or roller lifter are designed for them is something different.

Also, a brief PSA that faster is not always or necessarily better.
Posted By: EchoSixMike

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 02:10 PM

Any theoretical advantage is worthless if the cam isn't actually ground to take advantage of the lifter diameter.

We have a cam analyzer at the shop, and one of the outputs is minimum lifter diameter. The Lunati Voodoo cam I checked needed a min .88x diameter lifter, which allowing for design margin, etc indicates something that was designed and ground to take advantage of the lifter advantage.

I don't have much for comparison, almost everything we do where we bother to analyze everything to this level is .904/.937/1.00 roller tappet. Haven't seen any 1.062 yet, I think that'd be interesting. S/F....Ken M
Posted By: madscientist

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 02:20 PM

Originally Posted by Diplomat360
OK, so we have all heard of, read about, perhaps experienced the presumed advantage of the wider Mopar lifter diamater...however, I have to be honest and point out that for quite some time now as I have been looking at various cam listings I have been noticing the Mopar stuff (specific to the small block use - as that is my focus) with much less lobe lift then similar, if not the same, spec'ed cam grinds for the other engine brands.

Case in point, I am focusing on the retro-fit hydraulic roller cam grinds from Comp Cams, here are three examples:

1) Mopar Small Block - Xtreme Energy Retro-Fit XR292HR-10
Adv Dur => 292/300
Dur @ 0.050 => 242 / 248
Lift => .549 / .544
LSA => 110
RPM Range => 2800-6400

2) Chevy Small Bock - Xtreme Energy XR294HR Retro-Fit
Adv Dur => 294/300
Dur @ 0.050 => 242 / 248
Lift => .540 / .562
LSA => 110
RPM Range => 2800-6100

3) Ford Windsor - Xtreme Energy XR294RFHR Retro-Fit
Adv Dur => 294/300
Dur @ 0.050 => 242 / 248
Lift => .576 / .600
LSA => 110
RPM Range => 2500-6500

Alright...so the Ford grinds in particular are something that almost always lists a higher lobe lift. The Chevy...umm...here and there, but in the case of example we have here it is actually a tad higher, not by much, but it is.

So, is this because of the roller profile? Even then would you not expect the wider lifter to provide more room for a bigger diameter roller wheel and subsequently the ability to handle steeper lobe ramps?

OK, so this is pure theory (reasoning behind why these profiles are what they are), and I feel like too often we (the Mopar fans) like to say we get ignored by the aftermarket, but I find it hard to believe that given the amount of computerized manufacturing the makers like Comp Cams would not design a lobe profile that maxes out the lift.

Oh, and btw, if you say this is due to head flow restriction, while true for stock heads I would venture a guess that anyone running this level of cam has probably moved to heavily ported street heads or aftermarket stuff anyways.

So what gives here?



There are other cam companies than Comp. Many companies grind mopar specific lobes. Even Comp does. The MM series comes to mind. Unless you have an in at Comp, good luck getting them to grind a cam with the MM series lobe. The cam I run is very close to the MM lobe. But it's not a Comp because Comp wouldn't do it.
Posted By: krautrock

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 02:33 PM

you need to look at the shape of the lobe and without getting too deep into it, look at duartion @ .050 and also @.200, you have to find a lobe profile catalog usually.

here is an example from Howards.
https://www.howardscams.com/sites/default/files/lobe%20list%20email%201-20-2014.pdf

it lists regular profiles, fat rate profiles, ford profiles and mopar profiles.
just based on a quick glance it looks like the ford .875 profiles are just a tad slower than the mopar profiles...which seem to be close to some of the roller profiles. keep i mind that is still a pretty simple look at things. the nose of the roller profiles could be much different then the flat tappet cams which means more area under the curve.
Posted By: 6PakBee

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 02:38 PM

I have no scientific data to present, just to point out that Chrysler at one time offered the mushroom lifters that have a larger diameter at the cam then the stock versions.
Posted By: Diplomat360

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 03:40 PM

Originally Posted by krautrock
you need to look at the shape of the lobe and without getting too deep into it, look at duartion @ .050 and also @.200, you have to find a lobe profile catalog usually.

here is an example from Howards.
https://www.howardscams.com/sites/default/files/lobe%20list%20email%201-20-2014.pdf...

YES, perfect!

This is exactly the sort of information I'm curious about, especially the theory behind things like the nose duration, etc.

So given that I picked up a custom ground CompCams hydraulic roller (thus my focus on the CompCams parts, not meaning to ignore the other guys) I actually reached out to their tech folks and asked for the grind card, which they shared. The cam has the 13084 & 3039 lobes, these are the Xtreme Energy XFI rollers:

Lobe 13084:
.006 dur is 290
.050 dur is 240
.200 dur is 163

Lobe 3039:
.006 dur is 300
.050 dur is 248
.200 dur is 169

Now, if I had that same info available for the other grinds I would have happily posted those as well, alas, I do not.
Posted By: madscientist

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 04:18 PM

Originally Posted by Diplomat360
Originally Posted by krautrock
you need to look at the shape of the lobe and without getting too deep into it, look at duartion @ .050 and also @.200, you have to find a lobe profile catalog usually.

here is an example from Howards.
https://www.howardscams.com/sites/default/files/lobe%20list%20email%201-20-2014.pdf...

YES, perfect!

This is exactly the sort of information I'm curious about, especially the theory behind things like the nose duration, etc.

So given that I picked up a custom ground CompCams hydraulic roller (thus my focus on the CompCams parts, not meaning to ignore the other guys) I actually reached out to their tech folks and asked for the grind card, which they shared. The cam has the 13084 & 3039 lobes, these are the Xtreme Energy XFI rollers:

Lobe 13084:
.006 dur is 290
.050 dur is 240
.200 dur is 163

Lobe 3039:
.006 dur is 300
.050 dur is 248
.200 dur is 169

Now, if I had that same info available for the other grinds I would have happily posted those as well, alas, I do not.





A hydraulic roller (or a solid roller) is a bit different. But lifter diameter still matters, maybe even more so with a roller. With a lifter bore of .842 the biggest wheel you can get in that hole is .750 and that's pretty small. I can't remember if the .875 bore will let you get a bigger wheel than .750 but I don't think so. With a .904 bore you can get an .815 wheel and that's a big deal.

So no matter how I slice it, the bigger lifter bore is always better. Comp and Pro Stock don't use 1 inch and bigger lifters because they want to spend extra money.
Posted By: polyspheric

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 06:05 PM

The Chrysler mushroom: .970" foot. Why this size? Used on the Ford Model T - no, I'm not kidding, that's how old this is.
There's actual math to calculate the possible rate change: V = (tappet diameter - .040”) ÷ 114.6, where .040" is an acceptable safety margin to prevent edge contact.
Maximum values by tappet diameter in lift per degree of cam lobe rotation:
Engine Tappet OD Max velocity
Chev. Gen-3 L6, V8 .842” .00700”/deg.

Ford .875” .00729”/deg.

Chrysler, A.M.C. .904” .00754”/deg.

Oldsmobile V8 .921” .00769”/deg.

Model “T” mushroom .970” .00812”/deg.

Chev. Gen-2 235, 261 .990” .00829”/deg.

VW Type I• 31mm 1.220” .01030”/deg.
Posted By: madscientist

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 06:28 PM

Originally Posted by polyspheric
The Chrysler mushroom: .970" foot. Why this size? Used on the Ford Model T - no, I'm not kidding, that's how old this is.
There's actual math to calculate the possible rate change: V = (tappet diameter - .040”) ÷ 114.6, where .040" is an acceptable safety margin to prevent edge contact.
Maximum values by tappet diameter in lift per degree of cam lobe rotation:
Engine Tappet OD Max velocity
Chev. Gen-3 L6, V8 .842” .00700”/deg.

Ford .875” .00729”/deg.

Chrysler, A.M.C. .904” .00754”/deg.

Oldsmobile V8 .921” .00769”/deg.

Model “T” mushroom .970” .00812”/deg.

Chev. Gen-2 235, 261 .990” .00829”/deg.

VW Type I• 31mm 1.220” .01030”/deg.




Doesn't someone now offer a 1 inch mushroom lifter for the mopar stuff?
Posted By: Stanton

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 06:32 PM

Forget the lifter for a moment and install those cams on a fixture and track the lift with a dial indicator on the lobe. I think you'll find they are DRASTICALLY different with the exception of max lift.
Posted By: BSB67

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 06:35 PM

So, not to side track the conversation too much.....

But the simple geometry of a roller lifter suggest to me that a larger diameter roller (I.e. 0.815 verses 0.750) on the same lobe will move the valve faster. Correct?

If this is correct, when you order a Comp Cam roller cam with specific lobe numbers from their master lobe catalog, do they grind it differently for a Chrysler vs Chevy (0.815 verses a 0.750) lifter roller? Otherwise, their catalog specs would only be correct for one roller size.
Posted By: HotRodDave

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 08:13 PM

most generic shelf cams use lobes desighned for the smallest common lifter, the chevy, so even ford and chrysler with their larger lifters typically get stuck with the smaller lobes because it is easier to just make one small lobe and use it on everything than 3 specific lobes. Also just because a ferd, chuby, mopar use similar part numbers from a common manufacturer does not mean they were made the same, differnet head flow considerations and displacements require different lobes for optimal performance. Also ferds usually use higher rocker ratios so even if they do use the same lobe they will have more lift.
Posted By: AndyF

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 08:46 PM

You have to know what you're doing to find cam lobes that take advantage of the larger Mopar diameter. The generic cams use Chevy lobes. Even the Mopar Performance cams used Ford lobes rather than Mopar lobes. Comp has lobes ground for the larger Mopar lifter but most of their shelf cams use a generic lobe. Hughes Engines sells Mopar lobe cams. I've used the MM lobes from Comp before and they are a bit radical. I doubt I'd use a MM lobe cam in a street car but it could be a decent choice for a race car. Of course, as Brad pointed out, faster isn't always better. In fact, I've proven multiple times that you can lift the valve too fast. Lifting the valve too fast can cost you power and it will also wear out parts.
Posted By: polyspheric

Re: 'Myth' of Mopar lifter diameter advantage? - 07/17/19 11:43 PM

Only read your own posts.
Posted By: LA360

Re: 'Myth' of Mopar lifter diameter advantage? - 07/18/19 01:37 AM

Many have already explained the larger lifter bore is an advantage and not a myth, but shelf cams are never going to push the envelope as far as acceleration etc. They are generic, high volume production camshafts, they only thing they'll tweak is the lobe separations and how the lobes are paired.
Comp, Bullet, and Engle do high acceleration lobes for 0.904" as a custom for good reason. The average guy isn't going to correct lifter bore geometry with bushes, and often the bore orientation is out of phase, resulting in lifter or camshaft failure.

Jesel make 1.25" roller wheel lifters for Pro Stock and the like these days for good reason
Posted By: Diplomat360

Re: 'Myth' of Mopar lifter diameter advantage? - 07/18/19 02:54 AM

Originally Posted by LA360
Many have already explained the larger lifter bore is an advantage and not a myth, but shelf cams are never going to push the envelope as far as acceleration etc. They are generic, high volume production camshafts, they only thing they'll tweak is the lobe separations and how the lobes are paired...

OK, fair to an extent...strangely enough somehow the Chevy and Ford grinds end up with the higher lift lobes though...presumably out of the very same lobe catalog I have here, so why isn't CompCams putting the higher lift lobes on Mopar cams???

I mean like you said:
Originally Posted by LA360
"...They are generic, high volume production camshafts, they only thing they'll tweak is the lobe separations and how the lobes are paired..."

...so what is the reason for pairing the lower lift lobes for our small block engines? Is it poor head flow? Is it bad geometry??? That's the depth of an answer I'm looking for...

Bottom line being: the "shelf cam" argument here holds no water, as I just pointed out the other guys somehow end up running those lobes...so what gives? That's really what my question is about...would you not expect any manufacturer to want to optimize their sales regardless of what engine their product goes into? Makes no sense to me to want to sell less product...after all, if more lift was a bad thing we'd all be running our stock cams! LOL
Posted By: Cab_Burge

Re: 'Myth' of Mopar lifter diameter advantage? - 07/18/19 03:25 AM

I use to race NHRA stock class years ago when stock meant something. Back then the fastest SB Mopar racers knew that the stock OEM heads flow the maximum at or below .500 lift so they would have the cam lobes with the most duration with the max lift allowed by NHRA custom ground for there motors and use the LSA to run as fast as they could with that lift work
Back then Isky had the hot lobes for all the 340, 318 and 360 stocker motors, most ran the best with 104 to 106 LSA shruggy If I'm remembering correctly they all had less than .460 valve lift allowed at the retainers whiney shruggy
Posted By: madscientist

Re: 'Myth' of Mopar lifter diameter advantage? - 07/18/19 04:59 AM

Originally Posted by Diplomat360
Originally Posted by LA360
Many have already explained the larger lifter bore is an advantage and not a myth, but shelf cams are never going to push the envelope as far as acceleration etc. They are generic, high volume production camshafts, they only thing they'll tweak is the lobe separations and how the lobes are paired...

OK, fair to an extent...strangely enough somehow the Chevy and Ford grinds end up with the higher lift lobes though...presumably out of the very same lobe catalog I have here, so why isn't CompCams putting the higher lift lobes on Mopar cams???

I mean like you said:
Originally Posted by LA360
"...They are generic, high volume production camshafts, they only thing they'll tweak is the lobe separations and how the lobes are paired..."

...so what is the reason for pairing the lower lift lobes for our small block engines? Is it poor head flow? Is it bad geometry??? That's the depth of an answer I'm looking for...

Bottom line being: the "shelf cam" argument here holds no water, as I just pointed out the other guys somehow end up running those lobes...so what gives? That's really what my question is about...would you not expect any manufacturer to want to optimize their sales regardless of what engine their product goes into? Makes no sense to me to want to sell less product...after all, if more lift was a bad thing we'd all be running our stock cams! LOL





Don't confuse total lift with the lift rate of the lobe. Two seperate things. The reason all the cams for Chrysler's have low lift is the idea that the port breaks over at about .450 lift. I can make it break over much lower than that. I can also move it up, or at least flatten it out. Put an intake manifold on and it changes everything.

I run as much lift as I can get for the valve gear. I'm running (on the street) a lobe that is 281 on the seat at 255 at .050 so it's pretty close to a Comp MM lobe. With a 1.6 rocker I net about .606 at the valve. It isn't hard on parts. It will idle clean down to 700 but I don't idle it that slow. That's hard on parts. I let it idle at 1000 and it's not an issue.

As to why the manufacturer doesn't optimize lobes for a Chrysler is there is NO MONEY in it. Most guys to this day STILL buy a cam out of a catalog or worse yet, take a poll on a forum and Joe blow runs cam XXYD in his jalopy and it's the cats ass even though Joe has never done any testing. So they have to have a cam today and they buy that crap. Honestly, 98% of the guys out there won't ever know the difference.

It's economics. Simple money math. There are so many companies that will grind a custom cam with a .904 lobe on it for you there is no sense in every buying an off the shelf cam. But, you have to convince the guy on the phone why you need a faster lobe. I bought my cam from Jim at Racer Brown and before he used those lobes we had almost two hours on the phone and flow sheets plus a drawing of the cutter I used used for the valve job.
Posted By: Mopar Sam

Re: 'Myth' of Mopar lifter diameter advantage? - 07/18/19 09:37 AM

To the OP. The main reason the ford has more lift than the mopar is they have 1.6 rockers.
Posted By: LA360

Re: 'Myth' of Mopar lifter diameter advantage? - 07/18/19 11:11 AM

Originally Posted by madscientist
Originally Posted by Diplomat360
Originally Posted by LA360
Many have already explained the larger lifter bore is an advantage and not a myth, but shelf cams are never going to push the envelope as far as acceleration etc. They are generic, high volume production camshafts, they only thing they'll tweak is the lobe separations and how the lobes are paired...

OK, fair to an extent...strangely enough somehow the Chevy and Ford grinds end up with the higher lift lobes though...presumably out of the very same lobe catalog I have here, so why isn't CompCams putting the higher lift lobes on Mopar cams???

I mean like you said:
Originally Posted by LA360
"...They are generic, high volume production camshafts, they only thing they'll tweak is the lobe separations and how the lobes are paired..."

...so what is the reason for pairing the lower lift lobes for our small block engines? Is it poor head flow? Is it bad geometry??? That's the depth of an answer I'm looking for...

Bottom line being: the "shelf cam" argument here holds no water, as I just pointed out the other guys somehow end up running those lobes...so what gives? That's really what my question is about...would you not expect any manufacturer to want to optimize their sales regardless of what engine their product goes into? Makes no sense to me to want to sell less product...after all, if more lift was a bad thing we'd all be running our stock cams! LOL





Don't confuse total lift with the lift rate of the lobe. Two seperate things. The reason all the cams for Chrysler's have low lift is the idea that the port breaks over at about .450 lift. I can make it break over much lower than that. I can also move it up, or at least flatten it out. Put an intake manifold on and it changes everything.

I run as much lift as I can get for the valve gear. I'm running (on the street) a lobe that is 281 on the seat at 255 at .050 so it's pretty close to a Comp MM lobe. With a 1.6 rocker I net about .606 at the valve. It isn't hard on parts. It will idle clean down to 700 but I don't idle it that slow. That's hard on parts. I let it idle at 1000 and it's not an issue.

As to why the manufacturer doesn't optimize lobes for a Chrysler is there is NO MONEY in it. Most guys to this day STILL buy a cam out of a catalog or worse yet, take a poll on a forum and Joe blow runs cam XXYD in his jalopy and it's the cats ass even though Joe has never done any testing. So they have to have a cam today and they buy that crap. Honestly, 98% of the guys out there won't ever know the difference.

It's economics. Simple money math. There are so many companies that will grind a custom cam with a .904 lobe on it for you there is no sense in every buying an off the shelf cam. But, you have to convince the guy on the phone why you need a faster lobe. I bought my cam from Jim at Racer Brown and before he used those lobes we had almost two hours on the phone and flow sheets plus a drawing of the cutter I used used for the valve job.


Thank you, you saved me typing a response

Shelf cams are going to be generic, the cam company will specify some basics, but they're not going to know what the intake and exhaust system comprises of etc. Results will always vary.
Posted By: jcc

Re: 'Myth' of Mopar lifter diameter advantage? - 07/18/19 01:09 PM

Originally Posted by madscientist
Originally Posted by Diplomat360
Originally Posted by LA360
Many have already explained the larger lifter bore is an advantage and not a myth, but shelf cams are never going to push the envelope as far as acceleration etc. They are generic, high volume production camshafts, they only thing they'll tweak is the lobe separations and how the lobes are paired...

OK, fair to an extent...strangely enough somehow the Chevy and Ford grinds end up with the higher lift lobes though...presumably out of the very same lobe catalog I have here, so why isn't CompCams putting the higher lift lobes on Mopar cams???

I mean like you said:
Originally Posted by LA360
"...They are generic, high volume production camshafts, they only thing they'll tweak is the lobe separations and how the lobes are paired..."

...so what is the reason for pairing the lower lift lobes for our small block engines? Is it poor head flow? Is it bad geometry??? That's the depth of an answer I'm looking for...

Bottom line being: the "shelf cam" argument here holds no water, as I just pointed out the other guys somehow end up running those lobes...so what gives? That's really what my question is about...would you not expect any manufacturer to want to optimize their sales regardless of what engine their product goes into? Makes no sense to me to want to sell less product...after all, if more lift was a bad thing we'd all be running our stock cams! LOL





Don't confuse total lift with the lift rate of the lobe. Two seperate things...............


I didn't get it, that was addressed in the first sentence of the the first reply in this thread, and its still being discussed?

Maybe somebody should have just said forget any cam change and suggest OP get 2.5:1 rockers and be in bliss with ultimate lift. laugh2
Posted By: 451Mopar

Re: 'Myth' of Mopar lifter diameter advantage? - 07/18/19 05:13 PM

your comparing roller cams here, the cam lobes are likely nearly the same for each brand, with maybe a difference in roller tappet diameter and rocker ratio on the Ford.
The biggest advantage of the larger diameter lifter bore is with flat tappet lifter cams.
Posted By: fast68plymouth

Re: 'Myth' of Mopar lifter diameter advantage? - 07/18/19 05:49 PM

With regards to the 3 cams listed in the OP, there are two different families of lobes in play here, with the Mopar grind using the lobes with the most area under the curve.

The Chevy and Ford cams use the same lobes, with the extra lift for the Ford coming by way of the higher OE rocker ratio.

If you look at the more “entry level” hyd roller lifters, most have .700 or .750 wheels, regardless of the lifter bore diameter.

If someone wanted to use the lobes from the Mopar grind in their Chevy or Ford build...... its no problem.
Just order it up and they’ll grind it for you.

This is not the same as having a .904 specific flat tappet profile in your Mopar, and someone wanting to use it in their Chevy.
The only way that’s going to happen is if the lifter bores are enlarged to use .904 lifters, or a mushroom style lifter with a .904 or larger foot.
Posted By: polyspheric

Re: 'Myth' of Mopar lifter diameter advantage? - 07/18/19 06:25 PM

In my above list of lift per degree: this is not the limit for the engine, or the valve, the tappet, or the lobe.
It's the limit for the pushrod and rocker arm. Harley-Davidson cams made 60 years ago had FASTER rates than anything listed - a flathead with absolutely rigid valve gear: everything is a straight column load with no extraneous vectors. A 250 lb. spring controls a 1.94" valve @ 8,000 RPM.

W/r/t roller diameter: yes a larger roller* gives faster action and greater area (1928-84 H-D rollers: .855"), but there's a penalty.
When the valve closes and the pressure on the lobe is relaxed, the lash appears under the roller. When the opening ramp comes up again, the roller has to accelerate up to speed before it rotates on its axle. Until that point it's skidding across the lobe. A larger roller has far more inertia to overcome (much more than a simple comparison of diameters), which makes this worse. The obvious cure is thinner roller "wheels" (less metal toward the axle) but I'm sure that invites crushing with those 1,000 lb. springs X 1.9:1 rocker ratio = 1,900 lbs. on the lobe. Remember your math: the locus of points in common between 2 tangent convex shapes is a line with no thickness at all.

* be very careful here: a larger roller wheel will do bad things on an inverse (concave) flank designed for a specific smaller size, and even on a conventional (convex) lobe the speed increase is not linear but varies depending on where the roller makes contact.
Posted By: BSB67

Re: 'Myth' of Mopar lifter diameter advantage? - 07/19/19 01:41 AM

Originally Posted by polyspheric


W/r/t roller diameter: yes a larger roller* gives faster action and greater area (1928-84 H-D rollers: .855"), but there's a penalty.
When the valve closes and the pressure on the lobe is relaxed, the lash appears under the roller. When the opening ramp comes up again, the roller has to accelerate up to speed before it rotates on its axle. Until that point it's skidding across the lobe. A larger roller has far more inertia to overcome (much more than a simple comparison of diameters), which makes this worse. The obvious cure is thinner roller "wheels" (less metal toward the axle) but I'm sure that invites crushing with those 1,000 lb. springs X 1.9:1 rocker ratio = 1,900 lbs. on the lobe. Remember your math: the locus of points in common between 2 tangent convex shapes is a line with no thickness at all.

* be very careful here: a larger roller wheel will do bad things on an inverse (concave) flank designed for a specific smaller size, and even on a conventional (convex) lobe the speed increase is not linear but varies depending on where the roller makes contact.


Thanks for answering one of my questions.

Is it only inertia that figures into the skidding? I would think that angularity of the load and lower terminal roller rpm would favor the larger wheel.

Has this presented as an meaningful issue in the more typical hydraulic or street strip spring pressure applications?
Posted By: polyspheric

Re: 'Myth' of Mopar lifter diameter advantage? - 07/19/19 02:16 AM

The RPM difference is linear and varies directly with diameter (.750 to .800" is 12.5% slower), the inertia is closer to the cube of the difference (+21% and more due to concentration of mass at the rim).
Not a common problem, but IMHO it's one reason why mfg. favor hydraulic rollers: there is no lash take-up, the roller (and needle) speed never stops but varies throughout the range.
Posted By: HotRodDave

Re: 'Myth' of Mopar lifter diameter advantage? - 07/19/19 06:41 PM

Wouldn't the inertia of the spinning wheel plus friction from the oil between the wheel and base circle keep the roller spinning at or nearly the same as when it's on the lobe? I wouldn't think it stops and then starts in that very infinitesimally small fraction of time... if I'm wrong that would at least be the second time
Posted By: BSB67

Re: 'Myth' of Mopar lifter diameter advantage? - 07/19/19 09:33 PM

Originally Posted by HotRodDave
Wouldn't the inertia of the spinning wheel plus friction from the oil between the wheel and base circle keep the roller spinning at or nearly the same as when it's on the lobe? I wouldn't think it stops and then starts in that very infinitesimally small fraction of time... if I'm wrong that would at least be the second time


Kinda thought the same thing too. And oil? Also wonder if roller needles verse bushed would figure in. Inertia would go down with rollers. Probably splitting the hair of split hairs. Interesting, but so far from where I'm at with my street junk.
Posted By: Cab_Burge

Re: 'Myth' of Mopar lifter diameter advantage? - 07/19/19 10:39 PM

I've seen more than one solid roller cam that had skids marks on the back side of the lobe(shinier than the front or bottom of the lobes scope) from the lifter loosing contact with the lobe after max lift work
I suspect not enough spring pressure causes that for the intended upper RPM limits work shruggy I shoot for a little more pressure on the seats(15 to 40 Lbs) and over the nose and similar more pressure over the nose to allow for some pressure loss after engine run in time up twocents
I've seen from 10 to 40 Lbs. spring pressure loss after running the motor at the track and on the street for several years, that is on a motor that I had the rocker arms oiling full time and .039 restrictors in the oil feed circuit to the rocker arm shafts up
I sincerely believe you can do more damage to a solid roller cam and lifters by using to little pressure on the springs than by adding a bunch(40+ Lbs) more pressure with them twocents work
Posted By: Locomotion

Re: 'Myth' of Mopar lifter diameter advantage? - 07/20/19 01:35 PM

Having enough spring pressure is the same concern with Schubeck/Smith composite lifters. If contact is lost, there is the chance that slamming the lifter down back onto the lobe will create stresses that the lifter isn't designed for, leading to failure. Cams need pressure to control valve closure, not their opening.
Posted By: jcc

Re: 'Myth' of Mopar lifter diameter advantage? - 07/20/19 02:32 PM

Is this roller inertia issue why I have see lately pizza cutter width pro stock rollers?
Posted By: polyspheric

Re: 'Myth' of Mopar lifter diameter advantage? - 07/20/19 04:45 PM

Not sure, but Jerry Branch (Branch Flowmetrics) removed 2/3 of the stock H-D factory 3/8" wide roller tappet (1/3 from either side on an angle) 50 years ago. Yes, that triples the load per inch on the lobe, but evidently it was worth it because pretty common, and used (what we would regard today as) very light springs.
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