"Also, Ive been curious about how rocker ratio changes things around in more detail, is there anywhere I can learn more indepth about this?"

What you want to know????

Bret

 

Well, you said it changes duration slightly @.050. How does it do this, and how does it change with different ratios and such. And also the opening/closing speed increases you say...more in depth?

How do you know what ratio rocker you want. I know we are limited on stud rockers just because of stability, but do you want to run the biggest ratio possible while maintaining stability?

 

IMO...it doesn't change the duration, as its rated as tappet lift, not valve lift. It will however change the duration of the valve being open .050 because it will take less lobe to lift the valve that far.

 

Good answer....

The opening and closing speed increases because the valve is moving a larger amount in the same amount of time. Remember duration is measured in degrees but the Webster's definition of duration is that it's a period of TIME!

Cup guys run stupid high ratios.... 2.0:1 etc... You are limited on rocker ratio mostly by the contraints of your valvesprings. If you run it in to coil bind you are screwed.

The other problem is that a spring with 400lbs open with a 1.5 rocker only puts 600lbs on the pushrod and lifter, and a 2.0:1 Rocker puts 800lbs on the same pushrod and lifter. You start really stressing things bad there. You have to worry about that side of the system as well.

Yeah stability is the key, but valve lift kills springs. The less valve lift the less you hurt the springs, more life you get out of them.

Bret

 

So one might pick a smaller cam, but jack up the lift by changing the ratio. Does this do something a different cam can't do?

Jason

 

no you could possibly get a differnt cam with the same specs as the prev cam using the higher ratio rocker with a lower ratio rocker it would just have a little more aggerssive profile and tad more lift and duration a .050 but same total duration.

 

So why would one buy rockers instead of a different cam? Don't the rockers cost more?

Thanks,
Jason

 

mostly it is to improve on an already exhisting combo. or better an off the shelf cam instead of haveing a custom one ground. also if you find one with the duration you want but would like a little more lift and are going to buy rockers any way. why not?

 

Bret, (we) both know it is not fair to compare lobes of a roller cam to the lobes of a flat tappet cam. Even having the two cams ground to provide identical valve action would require the roller lobe to appear far more aggresive due to the differences in the shape of the lifter, and how/where the lifter surface makes contact on the lobe of either cam. You threw that in there, just to see whom was paying attention, right? Did I pass?

Also, the link to the CC site, displaying the brief tutorial on the differences of the lobe shapes of the different liftered cams is without question, misleading for the reasons given above. CC basically compared apples to oranges. To bad they (CC) didn't display a flat tappet to roller lobe comparo for two cams of identical valve action. Then they would have taught someone something. All they accomplished was to over dramatize the issue.

 

True.....

The aggressiveness of the average flat tappet cam is not close to a roller, but something like a Cup cam would be right there.

I would actually like to see the physical lobes in cross section like that of a 270deg cup lobe and a solid roller comparison. The valve motion is one thing the lobe shape is another.... the really aggressive flat tappet cams have some huge bearing journals to fit all this in there.

Hey rocker arms always help get some nasty valve motion, which is what we are really worried about here.... just puts big loads on the camshaft and lifter interface on a flat tappet. There are some smart people who have worked that all out though..... Not getting into that! Just imagine.... a 200-260lbs seated pressure on a flat tappet and a 2.0:1 rocker arm! 400-500lbs on the lifter face when you are on the base circle! We aren't talking about Schubeck lifters doing this either. Super Secret $hit basically. Probably using a DLC and some trick cam cores.

Basically the average guy doesn't need to know this stuff, it's just going to confuse them because they will want to do it.

Bret

 

Jason

I'll steal a quote from Bret here, "look at the whole valvetrain as a system".

Lets say you are starting from scratch and designing for the same actual valve lift but considering doing it 2 different ways. a) with with a smaller cam and a higher ratio rocker. b) with a larger cam and standard ratio rocker. With the right lobes available you could achieve identical actual valve timing events (the comp XE lobes of different lifts allow for close to this)

a) will have the advantage that the lifter and pushrod will only will be undergoing the smaller displacements of the smaller cam and therefore have smaller inertial forces. I.E. the valvetrain will appear slightly lighter. If you use the same springs you will end up with the same spring forces because you have the same actual lift at the valve. Within the limits of whats available off the shelf, you could get away with marginally less spring pressure to control the 'lighter' valvetrain. The drawback is that the higher your ratio the poorer your geometry and you start to have losses there.

b) will have better geometry but the vavletrain will appear to have more mass because the lifter and pushrod are moving further and therefore have larger accellerations/forces. the opposite of a.

In both cases the valve, spring, and retainers undergo the same motion and therefore have the same resultant forces.

Like anything in mechanical design, your making a tradeoff between designs a and b. Remember, i was talking only about theoretical designs here. Sometimes your limited by whats available off the shelf, practical installation, longevity, ect. Either way the performance difference even with totally custom springs available is going to be very small between designs a and b and in practice, but it does explain the general evolutionary trend to higher ratio rockers.

-brent

 

since the rocker are is just a basic lever the A example will have a greater force on the cam lifters and pushrods because the pushrod is moved closer to the fulcrom point. so that extra force with less mass should be more damaging than a little more mass with a more effecient lever (lower ratio rocker) like Bret said opening force is multiplied with the higher ratio. now which would be harder to move or more damaging. remember just because they have the same opening pressure doesn't mean they have the same pressure on the pushrod lifters or cam. opening pressure is measured where the rocker arms meets the valve then multiplied by the rocker arm.

 

I didn't even have to say that stuff guys.... Thanks.

One of the biggest problems with larger ratio stuff is deflection.... You can also have more aggressive lobes with less lift and more rocker arm and you will have even more loads in terms of velocity, acceleration, jerk, V^4, V^5, V^6 due to the very aggressive traits of the lobe and the fast action and load mulitplication of the rocker arm.

It can get rather confusing here. The lobe determines a lot, but the rocker is also part of that system.

One of the reasons why the beehives work so well is the lower mass, as Brent said, less mass needs less force to control it. Less force means less deflection and more accurate valvetrain motion.

Bret

 

From your experience is there a lift point where the tradeoff leans one way or the other? I ask because I just ordered a rotating assembly that has 30cc dished pistons. If I could use 64cc heads and get 0.600" lift with the appropriate safety clearance cool. I haven't picked the heads yet.

So I am picking the cam last.

For flow I need the average for the head between 0.400" and 0.600". Highest average goes to the top of the list.

To get the velocity I have to call the head manufacturer and ask for the runner cross dimensional area? I plug the area into the velocity equation, and that gives me fps or MACH?
The highest velocity without going over .550 MACH goes to the top of the list.

I compare the list for the heads that have the overall best picture of flow and velocity.

Is this right so far?

 

So far I've figured out that I need 250-270 cfm and a minimcross sectional are of 2.16.

How does the runner length affect this cross sectional area. Do you have to go with a 200+cc runner to get the flow?