98ws6blk

I just wondered what the pros and cons were of getting added valve lift from the cam profile vs rocker arm ratio?

bww3588

iTrader: (49)

Lift added to a given lobe can be controled better, it can be figured into the ramp rate etc...

All things being equal, if say you switched from a 1.7 to a 1.8, now your asking your valve and rocker arm to travel a longer distance in relatively the same time.

96capricemgr

iTrader: (1)

Valid points BUT there is also the argument that using the lobe means controlling the heavy lifter as it moves more. If you use rocker ratio the only additional mass moving is the tip of the rocker,spring/retainer/valve instead of all that plus the lifter and pushrod.

So IMO it all depends where you are starting from.

bww3588

iTrader: (49)

True. However, I still think all that can be controlled better using the lobe. Esp when using the rocker, the longer ratio puts more weight on The tip of the valve.

I could see investing money in a better set of lifters to handle the added stress, but I have seen more upper valve train failures resulting from instability because of cheap 1.8 rockers. The only rockers that seem to have good control as well as stability are over 1k bucks. I can pick up lifters that will handle that for about 3/4-1/2 that. And get away with using the stock rocker, or a standard 1.7 ratio equalivent.

DONAIMIAN

iTrader: (18)

Better to go with more ratio. My engine builder schooled me on this two days ago. Hence why cup and nationwide motors are running 2.1+ ratio rockers. Even the C5R heads require the 1.9 Jesel rockers.

96capricemgr

iTrader: (1)

Like I said I think it depends where you are starting from. A lobe designed to use ratio can help you move a lot less valvetrain mass and get the same lift but if you use an aggressive lobe and just throw ratio at it then you certainly could push things out of control and cause problems.

bww3588

iTrader: (49)

Did your engine builder tell you why they use 2.1 ratio rockers? And it has nothing to do with valve train control. Believe me, the lesser rocker ratio you can get away with, the better in most cases.

The reason they run such high ratio rockers is because they run such large cams. If they ground all the lift into the cam, And used a shorter rocker arm, the cam wouldn't fit in the engine. The lobe would be larger than the cam journal itself and wouldn't slide into the block.

The more weight you put over the valve tip the less stable your valve train will be without the proper springs...etc. but with higher spring rates comes increased wear on other components such as seats.

I do totally agree it all depends where your starting point is. A cam ground for 1.8 rockers would work great, and most mildish cams work great with either rocker. It all comes down to where your starting.

But imo, if your starting from scratch, it would be ideal to run with the lowest ratio you can. As I would rather put added stresses on the lifters and cam, rather than the large end of the rocker, tip of the valve and spring.

That's just my school of thought.

Very good question op.

DONAIMIAN

iTrader: (18)

Less movement on the cam side of the valvetrain, also with less lobe lift they can run a larger base circle, which means a stiffer cam.

postal14

iTrader: (8)

I pretty much agree with most of the stuff above. plus..

I think the lower the lift on a given base circle lobe the more the lifter gets ROLLED open instead of POPPED open. A heavy lifter being rolled up and down by a lower amount is going to be easier to control than one being popped open and close by a larger amount.

I think GM did a good damn job designing the LS motors and there are all kinds of race track influenced revisions in them compared to a gen 1 sbc. They went with 1.7 ratio compared to the old 1.5 sbc ratio, they did this because the advantages out weighed the disadvantages.

I am going to guess that your original question should be more like this:

should I get a 230/230 cam with .600" lift w/ 1.7's, or should I get a 230/230 with .567" lift but then use 1.8's to give me .600" lift.

I think when you are dealing with changing the rocker ratio by 0.1 that you wont get bit in the *** by any of the disadvantage stuff. Especially in the scenario above. Everything on the valve side will travel the same speed / distance, and everything on the lifter side will travel less /slower. I would imagine that you are more likely to run into trouble when you start going max lobes AND max ratios, especially on a budget.

96capricemgr

iTrader: (1)

I have never weighed any of it but it seems to me the lifter is the heaviest part of the valvetrain so asking it to move abruptly seems like it could get hard to control moreso then just the rocker tip/spring/valve.

In the January issue of HotRod there was an article "too small to be big" it is a gen 1 402 still 23degree turning 8500rpm on the street. Part of the equasion there is 1.85 rockers and I think the Dart block has a larger than standard cam tunnel. They used a fair amount of spintron time to get the cam sorted out though and the sick thing is he has less seat pressure with that solid roller than I have in my heads/cam LT1.
This is a friend's car so I know a bit more about it than the article states.

Thing is though when the engine was designed for aggressive street/strip use and done with custom parts not "making due" with standard aftermarket stuff they deviated greatly from standard gen 1 stuff. On a gen 1 like that a 1.7 rocker would have been considered wild because stock was 1.5 on all the gen 1 engines and all the gen 2 besides the LT4.

Lots of things get done a certain way just because that is the way they have always been done not because it is necessarily the best way. If Comp started offering cams specifically for 1.8 rockers you would have someone using them with stock rockers and not seeing the performance they should have and they would blame Comp. Sort of like shelf cams are usually wide LSA to tame them because they know guys are tempted to choose from a little too far down the page.

postal14

iTrader: (8)

Comp basically does do this. If you look into their master lobe catalog they have a series of lobes with lower lift.

XTREME RPM FOR LS1
Designed with Xtreme Energy™ technology to provide excellent power with the LS1’s enhanced cylinder head design and high rpm performance. These
provide outstanding low rpm torque with increased stability when coupled with the LS1’s larger base circle and 1.7:1+ rocker ratios. High lift versions
are excellent for use with improved heads and manifolds.

Dont let the high lift statement at the end of that fool you. The regular lift 230 lobe in this series is only .539" and the high lift 230 is only .573.

If you look up a comp Xtreme RPM series cam for an LS1 you will see it uses these series lobes. Every time you find one of these listed it will say that they work well with higher than stock ratio rocker arms.

Stability: from comps statement...
One last thing:
I have no doubt in my mind that if the LS motors came from the factory with 1.6's stock that people would then want to debate whether or not going to 1.7's is worth it /the draw backs/ would it be stable ect..
If you asked about going from the stock 1.7's to some jessel 2.1's you would be WAY out of my league.

Exidous

iTrader: (11)

If you were to compare to scenarios where you have two setups both with .600" gross lift:

A cam with .352" net lift plus a 1.70 rocker gives .600" lift

A cam with .324" net lift plus a 1.85 rocker gives .600" lift

The former would have more lifter travel and reduced pressure on the lifter.

The latter would have reduced lifter travel and increased pressure on the lifter.

The end result on the same given a proper lobe profile to make acceleration equal. The ratio is changed by moving the push rod closer to or further away from the pivot point. I've always thought about going to a lower lift/higher ratio to reduce lifter noise but the apparent sewing machine noise is the valves closing and slapping into the valve seat(usually the exhaust per master Mamo) so this wouldn't affect it much.

I could just be blowing smoke but this is the way I see it. :-)

It is a means of adding lift to a stock cam if you want a really easy install without having to remove the cam but the overall gains aren't going to be very big.

98ws6blk

This is a good debate. I wonder how pushrod weight plays into this as well....

96capricemgr

iTrader: (1)

pushrod weight would play in less than lifter weight since it weighs less.