camaroextra

iTrader: (10)

Ok so if you were putting a new cam in and you know you want maybe .938 net lift (or any random number). Would it make it difference if you got a cam with a lift of .521 X 1.8 rockers or a can with .551 X 1.7 rockers. they both equal pretty much the same net lift.

with both cams go ahead and assume the LSA, ground in advance and other variables are the same (unless you think that would come into play)

Im not sure that there is a diff but with the smaller cam and 1.8 rockers you would have less velocity (or range of movement) on the lifters and pushrod but more pressure (i think)
And then with the bigger cam and 1.7 rockers you would have more velocity velocity (or range of movement) but less pressure on them ( i think)

Please feel free to elaborate as much as you feel like on this idea and your opinion on it, or just tell me im an idiot and it doest make a differece. Or if i stumped you, let me know so i feel good about myself.

Joe Overton

total lift will be the same... rate of lift will be what changes .. How a rocker arm is made ( pivot point, bearing type) can affect rate of lift also ,not just the ratio itself...
To answer the question simply though ,total lift will be roughly same.

Robs98SS

iTrader: (1)

someone correct me if im wrong but if you are comparing two cams and both have the same specs except one has more lift than the other, wouldnt the cam with more lift have more aggressive lobes? As in the cam with more lift will open and close the valve faster than the smaller cam?

Thanks
Rob

LSONE

iTrader: (1)

from what I've read, a higher ratio is undesirable because it throws off your valve train geometry.

Joe Overton

Rob lobe shape determines pretty much how agressive it is...you can have two cams both the same at .050 and per cam card identical , but at .020 they are totally different animals... lift being the same ,how it got there being the difference.. so to answer just because its bigger dont necessarily mean its more aggressive..

Joe Overton

Actually most the time geometry can be corrected , and a higher ratio Actually can be a benefit..

Josh

iTrader: (9)

I don't think that's necessarily true, but I could be wrong. You could, theoretically, have a lobe shapped exactly like a "V" with .800" lift and a lobe shaped like a "U" with .400" lift, with the "U" lobe having more aggressive ramp rates despite having less lift.

But like I said, I could be wrong, as I'm certainly no expert.

Josh

Joe Overton

Josh I dont think anybody can claim they are a expert IMO..but you are correct..

Robs98SS

iTrader: (1)

Joe and Josh,

Thanks for the correction, now that I think about it you are definately correct. I was looking at it without taking into account a number of other factors. In the example Josh gave, of the cam with a V vs. the cam with the U lobes, what would these different characteristics be considered? Are these the ramp rates? and would the cam w/ the U lobe be considered to have more overall duration or would they both have the same duration?

I would think they would have the same duration at .05" lift but very different characteristics once the valves started opening more.

Thanks for the correction. I'm relatively new to this stuff and trying very hard to learn so bear with me please =).

I love this new forum, I learn new stuff everyday here.

Rob

camaroextra

iTrader: (10)

I guess what i really wanted to know is: is one set-up more efficient, or more durable, or have a better power characteristics than the other? if you think so please tell me why.

Joe Overton

Power,Yes if the heads flow well in the low lift ( off the seat) .200- .300 .400 a higher ratio rocker would show some gains over a stock ratio rocker arm using same lobe design most of the time, lift being only variable .....then again you may not see anything...efficiency would increase ..durability would be up to the valve train components used....

Joe Overton

Rob exactly cam card may say 2 cams are the same dur. lift at .050 but are not.. One being alot more agressive ...Ramp rates would be correct .. the more agressive lobe would be enhanced that much more with a higher than stock ratio rocker... trying to simplify things so its easier to understand....

LIL SS

iTrader: (9)

A lot of this is covered in these different Cam Threads:

https://ls1tech.com/forums/advanced-engineering-tech/396086-camshaft-discussion-part-ii.html
https://ls1tech.com/forums/advanced-engineering-tech/394934-camshaft-discussion-part-iii.html

All though there is a lot of text, It will go over a lot of details such as cam lobe shape. A quick brief explanation:

Lobe A) Comp Lobe # 3728
285* duration @ .006
236* duration @ .050
157* duration @ .200
.602 lift with a 1.7 rocker

Lobe B) Comp Lobe # 6580
261* duration @ .006
235* duration @ .050
153* duration @ .200
.595 lift with a 1.7 rocker

Lobe A has +24* duration at .006
Lobe A has +1* duration at .050
Lobe A has +4* duration at .200
Lobe A has .007 more total lift.

What lobe is more aggressive?

DAPSUPRSLO

iTrader: (14)

I believe at some point of small base circle/high lift lobes the lifter is moved two far down (while it follows the base circle) and the oil ring travels past the lifter bore and in some instances the lifter can come out of the plastic lifter guides. This will cause low oil pressure issue and/or turning of the lifter. One way to solve this, besides the correct lifter oiling ring location and oiling whole or using a lifter with a much longer body (ala morel), is to use a lower lobe lift/generally larger base circle cams with a higher ratio rocker. Ofcourse this is ls1 specific as traditional sbc has a much different provision of oil to the lifters.

I'm about 80% sure of this from what I've read on here, experts chime in to negate or reinforce

Old SStroker

Higher rocker arm ratios (RAR) can/are used to get the desired valve lift with limited lobe lifts. Flat lifters (NASCAR Cup engines) have a maximum amount of lift per degree of rotation (called Cam Velocity) based on lifter diameter. If you try to exceed this the edge of the lifter digs into the lobe flank and everything goes BOOM!

Roller lifters don't have the same cam velocity problem, but they create sideloads on the lifter bore that the flat lifters don't, so there is a practical limit to cam velocity with rollers. too. Max Cam Velocity limits the lift you can get with a certain duration, so if you need more lift you up the RAR. Some enignes use 2.0, 2.1, 2.2 and maybe more. Those aren't engines we drive around however.

If you are at the edge of the envelope, where almost all successful racers are, you need lobes designed for the RAR you are using. It's valve motion that we are concerned about, not just lifter motion.

There's no free lunch with RAR, because a higher RAR multiplies the valve spring loads into the pushrod and lifter. So with a 2:1 RAR and 600# over the nose, the pushrod sees 1200# static and much more dynamically. No wimpy 5/16 or 3/8 pushrods here if 8000++ is your goal.

Bonus question: If you are at max lobe lift you can get, and max RAR, and still need more valve lift, what do you do? Easy answer, but tough execution.

stevewix

i PERSONALLLY run the highest rocker arm ratio i can, taking into effect rocker arm stiffness and weight etc; for the simple reason being everything between the cam and rocker arm has to move less, and since f=ma; this is a good thing. velocity on lifters and pushrods, two very mass-critical valvetrain components is lessened greatly and thus is easier on the valvetrain components; placing less of an emphasis on how much these two components weigh. ramp rates are magnified along with lift and valve acceleration in a high-ratio valvetrain, so a kinder gentler cam lobe can be run allowing for theoretically less lifter side-loading; a totally worthless fact.

larger base circles can be utilized since the lobe lift is down; the cam will actually 'fit' in the block since there is less of a difference in base circle size vs. lobe size; thus lowering the ratio of lobe circle/base circle; and opening possibilities for more radical ramp rates and lift numbers.

more net lift is attained as well, since in many cases the valve lash number stays the same as the ratio increases; making the valve lash effectively less in the grander scheme.

that's it, i'm all out of bullshit.

Old SStroker

I guess we don't see exactly eye-to-eye, steve.

Good timing on our posts though.

stevewix

you must be referring to lifter side-loading. my fault, i didn't read your post; when it popped into my head it seemed trivial so i downplayed it. sorry about the confusion.

stevewix

make everything stiffer!

MadBill

Hey, SS!
Besides being a fine judge of character, per your kind words in the dynamic compression thread, you also gave the exact explanation I would have re the limits of lifter velocity and acceleration.
Re the bonus question: How do you get more lift? The same way as when you want to jump higher, you pole-vault! It's risky and tricky, but with the right stuff, at high RPM the pushrod goes 'sproing' as the acceleration turns negative approaching max lift and the valve soars 50 or 100 thous. higher than the lobe lift times RAR dictates. Then, if all goes well, the components comer back together and gently touch down on the closing flank, just like a ski jumper. (and, just like a skier that out-jumps the hill, if the landing is on the base circle, bad stuff ensues....)