OLDScience

This is assuming 39 Degree for both...

I'd imagine the lobes on the .921 are wider, but If I had to guess by looking at most cams I see, is that there is no difference,

Do the lobes have to be wider when specifying Chrysler lobe masters for early .921 lifter motors? What am, I missing here?

joe_padavano

Larger diameter lifters would slightly increase the effective duration of a given cam. This has nothing to do with the width of the lobe. A larger lifter diameter also allows the use of a more aggressive lobe profile without using rollers. This is why there are mushroom lifters.

https://www.motortrend.com/features/...982-1593-74-1/

66-3X2 442

You have probably opened a can of worms. I made this exact point a while back and got took to the woodshed. One even said why would you want to use a .921 diameter lifter because hydraulic rollers are better.

OLDScience

ok, thanks for the replies...makes sense to me now

I had a cam made up for me and repeatedly told the grinder to use .904 lobes and when looking at their master cam list, they sent me .842 profiles. So now I have to go back to square one with them again..

fleming442

Fixed that for ya.... 😁

OLDScience

More curiosity... Mushroom lifters would have to be taller (by the thickness of the mushroom height) and mounted from the bottom?

I guess they could be the same height just a change in pushrod height may be needed.

Koda

One would think the rest of the lifter would be shorter to match.

Where's the failure point happen for roller lifter pins in performance applications? I'm thinking it varies based on pressure fed or not.

joe_padavano

Yes, they have to go in from the bottom, but the height isn't any different. There's a length of lifter that hangs down below the lifter bore anyway.

Greg Rogers

Ford Y blacks have mushroom lifters. Big job to change cam and lifters.

CANADIANOLDS

No, the larger diameter lifter does not slightly increase the effective duration of the cam at any point.

the rest is correct.

CANADIANOLDS

if you agreed with the “it increases the duration slightly” you should have been taken to the woodshed, twice

to answer the original question..yes the cam cores are the same and will give the exact same durations with either lifter

CANADIANOLDS

If you actually have .921” lifters then you were right in wanting a .904” lobe if you wanted a more aggressive lobe ramp ..they make more power.

it’s not a width thing…. If that was your main reason for wanting a .904” lobe, then no, there isn’t a difference..stick with the .842”

Rallye469

This doesn’t make sense to me Canadian Olds(and I’ve never really thought about it before this thread).

But if the lifter is sitting on a cams base circle(valve completely closed),
and the lobe ramp is JUST about to make contact with the lifter,
why wouldn’t a larger diameter lifter feel the effect of the ramp first?

I would think the valve would open faster and would be open up wider, quicker with a fatter lifter, no?

If you think of it as a lever coming off of the dead center of a lifter, the tip of that lever would be out .4605 from the center on a .921 lifter.
It would be out .421 on an .842 lifter…
Thereby reaching the ramp .0395 sooner, right?

66-3X2 442

No I never said that. I said you could use a more aggressive cam with a .921/larger diameter lifter.

CANADIANOLDS

read what I said again about IF you agreed with the adding duration part?

and , you never used the words more aggressive at all…go have a look.

to be even more clear..do you agree with the part about a bigger diameter lifter adding duration to a lobe in that situation?

66-3X2 442

I was referring to my statement about the .921 lifters being larger in diameter allowed the use of a more aggressive cam was in another thread. In that thread I was arguing with some guy and you confirmed that I was correct. I never said that in this thread.
A larger lifter adds duration? why would it?

CANADIANOLDS

it doesn’t feel the effect of the ramp first because it starts at the centre of the lifter , where the base circle is in contact, then gradually moves outward as the cam rotates through the opening ramp, it continues to move outward as the cam rotates, and then starts to move back inward towards the lifter centre when its nearing the max lift point of the lobe , at that point , it’s back to the centre of lifter.

then it all goes in reverse on the closing ramp. centre, moves to outer edge of lifter (but never off the edge) then back to the center

the distance the lobe gets to the edge of the lifter dictates max ramp velocity. I’ve asked a few cam grinders how close to the edge can the lobe be? I’ve been told a few different numbers..anywhere between .020” and .050” seems to be it.

it depends on how risky you want to be..the closer to the edge, the greater the chance of a failure is even with the smallest amount of valve float.

I’ve enquired about having a custom .921” lobe ground which is even more aggressive than the .904” mopar. fifteen years ago it was $1,000 just for one master.

you can use lobes designed for bigger lifters on smaller ones IF , the lifter does not have any chamfer at all on its face…just sharp. a .875” can work with an .842”