a9x_hatch

i have just bought a new cam .571 .573 lift and im wondering what the largest rocker size is i can put on it without having to worry about cylinder valve clearence any ideas?

smask04C5

iTrader: (1)

depends on cam duration, degreeing cam and claying a piston will provide you with the info you are looking for. If you are using stock intake valves, stock heads w/o milling, you should be fine

XTrooper

With that valve lift, I'd personally stick with the stock 1.7:1 ratio and get a set of the Crane 1.70 Quick-Lift roller rockers (actually 1.72). They'll give you a max. valve lift of .577/.579 (well within the margin of safety) and, more importantly, will have a ratio of 1.82 when the valve leaves the seat up to ~.300" of valve lift as opposed to a ratio of 1.52 for the stockers. This translates into more power.

If you went with the Crane 1.80 rockers (1.82), your valve lift would be .611/.613. That's higher than I would be interested in going, but it's an option you could look into, but you'd definitely have to be sure of PTV clearance.

BrentB@TEA

I prefer the stock rockers or maybe the Jesel Y2K's
If your cam was designed for a 1.7 rocker then going to more of a ratio is not always a good thing.
I do not think the valve train can control it as well as a stock rocker.

XTrooper

You may not think so, but the fact is there's absolute no problem with going to a higher ratio rocker arm ratio and it's been proven not hundreds, not thousands, but tens of thousands of times over the course of the last 4+ decades. If you use the proper valvetrain components, the only thing you'll get from a higher ratio rocker arm is more power. BTW, cams aren't "designed" for any particular rocker arm ratio beyond grinding the lobes to give a specific valve lift with the standard ratio rockers. This does not mean they can't be effectively used with other ratios.

PREDATOR-Z

Whether it is 1.70 or Crane 1.72's (advertised as 1.7) The max lift will not affect PTV clearance, if cam fit at standard ratio/lifts.
The cranes will start at 1.79> at .200/.300 go to 1.72 till max lift> back down to .300/.200 where they go back up to 1.79. (variable lifts)
So the low lift ranges benefit from extra lifts (and a minimal increase in duration), which translates into more power.
All of that while keeping proper arc geometry (if installed at specs of course, and rocker tip is well centered on the valve stem end)

But as a rule of common sense, higher ratio rockers are used mainly to obtain more power on low lift cams. the same can be achieved just by increasing the size of the cam a bit, without adding stress on the valvetrain.

XTrooper

I don't know who started the myth above, but that's just what it is, a myth. Higher ratio rocker arms, when correctly designed and installed, do not add stress to your valvetrain. If anyone has proof to the contrary, I'd like to see it.

PREDATOR-Z

This is my understanding of it:

Higher ratio rocker arms open the valve faster, higher, and hold it open for a longer period of time as compared to lower ratio units.
There will be more pressure on the cam lobes due to the friction and pressure caused by the higher lift and result in a greater spring load.
If a cam lobe is very aggressive that would need heavier springs to keep the lifter from lifting off the lobe.
Radical lobes will also add more side stress on the lifters and the bores and could possibly cause lifter and/or bore failure.
Also the added pressure on the studs from either higher ratio rockers, or more radical lobe has to have proper studs speced for that particular purpose.

So basically, while high ratio rockers do increase power benefits on smaller lift cams, these are somewhat limited to a certain ramp rate and amount of cam lift, before the forces and pressures induced overcome the parameters for which they are designed for.

mrr23

iTrader: (3)

the valves aren't open for any longer period of time. that is controlled by the duration of the cam. the open and closing points aren't changed.

Adrenaline_Z

^ that's not totally true. Predator_Z is correct.

The open and closing points are the same, but the effective duration at
each lift point increases as the rocker ratio increases.

Therefore, the valve is held open longer on the nose of the lobe to take
advantage of the head flow.

IE:

Take 0.150" lift on the lobe and multiply by 1.7
Take 0.150" lift on the lobe and multiply by 1.8

You can see the effects of rocker ratio in this diagram:


You can see the extended durations where the blue, red and purple lines intersect at 0.400" lift,
yet the open and close points remain the same.

X-Trooper,

When using a higher ratio rocker, due to the increased valve action, the
rocker studs tend to flex more. It is good practice to install stud girdles on
high RPM engines which use higher ratio rockers.

In addition, the valve stem will flex more under higher lift and faster
acceleration which will wear down the valve guides.

a9x_hatch

the cam is a comp cam 228-230 dur .571-.573 lift 112 lobe seperation and the heads are patriot stage 2 ls6 with 2.055'' in 1.6'' ex please help me im tryin to get power with drivability so in my case the larger the lift the better

PREDATOR-Z

I do not know the .006 advertised lift of this cam but i believe it is an XE lobe.

If it was me, I would stick with the 1.7 ratio. If you are desperate (and have $$$) you could go with CRANE 1.7 Gold rockers. Those will not affect your PTV and still make more power down low and a little more at peak.
I wouldn't put 1.8/1.85 rockers on this lobe profile for those lifts.

mrr23

iTrader: (3)

adrenaline z - no predator z is not correct. his exact words are
the period of time the valves are open is still the same no matter what rocker you use. what you are describing is lift of the valve off the seat at certain durations of hte total event. that's two different things. so, again, they are not open for a longer period of time. duration = time. lift does not = time. the only thing changed was how far off the seat the valve is at a given duration (time) of the cam.

Adrenaline_Z

If we're talking about valve opening to valve close, then yes...you are correct.

If we're talking about effective duration at any lift point above 0.000",
then Pred_Z is correct.

So, you're both correct in your own respect.

PREDATOR-Z

Yes that is what I mean<

a9x_hatch

so could i put a set of 1.75's on it?

PREDATOR-Z

Yeap....

mrr23

iTrader: (3)

technically, the duration anywhere hasn't changed on the cam itself. all cam manufacturers rate the duration vs lift at the lobe. not at the valve. so, technically, the duration of the cam still hasn't changed. the event at the valve has changed though. just to clear that up some as well.

XTrooper

Thanks for the info. This is why Crane sells their rocker arms in kits that include heavy-duty studs, guide plates, and hardened pushrods.

Also, keep in mind, that everything you mentioned above can occur when using a cam with high lift and/or aggressive ramp rates and is not exclusive to higher ratio rocker arms.

Adrenaline_Z

^ For sure.

Lobes with fast ramps are not good to pair with high ratio rockers.

The valve velocity increase from the combined rocker ratio and
aggressive lobe ramp is a recipe for disaster. The valve train parts
wont like that very much.

Easier to float valves too.