86MonteSS

iTrader: (3)

is there any advantage to having 1.8 rockers on a smaller cam over having a bigger cam and staying with the 1.7 rockers?

Black99CamaroSS

iTrader: (3)

I am far from an expert on this but I know the 1.8 rockers will give you alittle more lift and duration but cost you a few hundred dollars(300-400 unless you get them used). Why not save that money on the rockers and get the bigger cam and spend the rest else where like on some good springs.

JPH

iTrader: (2)

what would be the cam profile you would be using if you decided to put 1.8's on?

86MonteSS

iTrader: (3)

224/224 around .560 lift with 1.7 rockers. .590 lift with 1.8 rockers.

any advantage over just buying something with a .590 lift? other than installation price?

1997bird

iTrader: (2)

It would help you out a little bit, but depending on your goals, it probably isn't going to be what you were expecting.

JPH

iTrader: (2)

it would help, but my advice is to run a bigger cam. the cost of rockers (rockers that work) compared to a new cam profile, you would be wiser to spend the money on a new cam. rockers will give you more valve duration, and about .030-.032 more net valve lift.

3.4camaro

iTrader: (1)

1.8 ratios are a waste of time. 1.8's put more stress on the valvetrain, and their only purpose is to do something the cam was supposed to do in the first place.

Get a bigger cam, and stick with 1.7s. I have aftermarket rockers with a roller tip, and they are 1.7s. Love my Harland Sharps

z28toz06

how do they put more stress on the valve train?

Old SStroker

Quote:
Originally Posted by 3.4camaro
1.8 ratios are a waste of time. 1.8's put more stress on the valvetrain, and their only purpose is to do something the cam was supposed to do in the first place.

Get a bigger cam, and stick with 1.7s. I have aftermarket rockers with a roller tip, and they are 1.7s. Love my Harland Sharps


3.4camaro, you are on. Explain, please.

nesikachad

I'll explain:

Take a cam and measure it with a plunger indicator between centers.

lets say the lobe's maximum lift is .350.

Now multiply .300 X the ratio of 1.7

We get .595 total lift at the valve.

Now multiply by 1.8. We now have .63 total lift at the valve.

What changes in a rocker arm to alter the ratio? It cannot be the distance from the valve roller to the rocker's polar moment (center of rotation) because that is determined by the heads.

It must be where the pushrod socket is located then.

Moving the pushrod socket towards the polar moment increases ratio at the expense of mechanical advantage.

More load is now being applied to the pushrods (deflection) the lifters, and the face of the camshaft.

The valve is going to see a little more wear now because the "wipe" across the tip of the stem is more extreme. Valve guides will wear out sooner.

The valve is going to be accelerated at a higher rate when coming off the seat, this will mean the valve springs need attention as well.

Nothing wrong with going to higher ratio rockers, just so long as other details are paid attention to.

You also have to watch for coil bind which is where the springs become bottomed out on each other. This is a sure fire way to break parts.

The poor mans way to check this is to take a paper clip and attempt to slide it between the coils of the springs while the valve is open as far as it can be. If it go's you are good, if it sticks, you need to do some work.

good luck

3.4camaro

iTrader: (1)

^ what he said. I'm drunk right now.

AdioSS

iTrader: (2)

I think have an idea of what Old SStroker was asking about, so how about we rephrase the question like this...

How can a combination of a mild cam with slow ramps using higher ratio rockers be more harder on the valvetrain than a more aggressive cam with faster ramps using lower ratio rockers?

Assume the motion of the valve is the same in each case.

I would say that higher ratio rockers would be EASIER on the rest of the valvetrain. The heavy lifters and pushrods don't have to move as far or as fast meaning less stress.

nesikachad

They don't move as far in linear distance, but they have more load (from the valve spring) applied to them.

Which is easier on your bicep when picking up a car?

A bottle jack with a long extension?

Or the same bottle jack with a short one?

the shorter extension requires more exertion on your part because you don't have as much mechanical advantage. You gain a shortened range of motion at the expense of additional load.

No different when fiddling with rocker ratios.

Don't get me wrong. This has been done for years and it's successful. You typically gain a bit of duration and quite a bit more lift.

You also accelerate the valve off the seat faster which is great for low end punch as the "stuff" starts moving through the ports sooner in relation to crank rotation and piston movement.

Just know that if you have high mileage parts, you may want to consider new pushrods and ALWAYS check your springs to ensure that they don't bind up.

RE AND CHERYL

iTrader: (31)

Just because a cam is mild you can't assume the ramps aren't aggressive. I was consittering this same thing and after some research and calls I found out that even most of the mild (around 224 dur) cams I was consittering recomemded stock ratio rockers for a street driven car. I am currently running a Stealth II. They recomended that I not run anything over 1.7 as spring life would be shortened significantly.

Look at a lot of built street and even race LSx motors, a large percentage of them are running stock rockers or at least stock ratio.

Re'

nesikachad

Whut he sed /\

silver01z06

not that you cannot get the same effect from 1.7 roller rockers but everyone is skipping the more accurate valve train and less deflection you would get. if you did the smaller cam and rockers you will get more out of the cam with better valve train motion.

and the word is not consittering, its considering. i am by no means a english major but it was just bugging the hell out of me sorry

nesikachad

Grammar ****! Fire the ovens!

Kidding.

Not quite understanding something though.

Ok, if I understand what you wrote correctly, you are stating that the valve train as a whole is improved when you run a smaller (base circle) camshaft and increase the ratio at the rocker arm to make up for the difference.

Is this an accurate interpretation?

If so, I think there may be some other things to consider.

It could be said that a smaller base circle cam is an improvement in terms of surface speed. (huh?)

Ok, spin a big circle at 1000 rpm and measure the velocity of the circle at the outside diameter.

Take a smaller circle and spin it at the same 1000 rpm and what happens? The wheel is spinning "slower." Not really, it just has a lower surface speed because its smaller diameter.

A smaller base circle means the roller tip isn't having to rotate as many times to travel all the way around the camshaft lobe.

Is this more efficient. I guess so, but does anyone really give a F___? I doubt it. (this is meant as humor)

What I really struggle with is stating that the valve train is more stable when you increase the rocker ratio to compensate for a smaller cam. It just seems in my head that when you remove mechanical advantage from a system, it now places more load on everything.

As I stated before, the benefit (in my mind anyways) is that the valve is accelerated off of the seat at a higher velocity than with the lower ratio. But, because of this, the springs now must manage the increased velocity and prevent things from coming apart and then banging into one another again. (valve float). The other potential issue being the amount of "wipe" over the tip of the valve as the rocker moves in an arc to open/close the valve.

If you increase the spring pressure to comp for the acceleration, you then increase the load delivered to the pushrods, lifters and cam lobe. Does it matter in the end? I admit I don't have the engineering education to sit and figure out loads, shear strength, and friction coefficients so I just don't know.

The big picture is all this is a mute point ultimately because we know this has been done for years. Build a SBC blower motor, stick bigger rockers on the exhaust side to get the heat out and clean the cylinder. It makes more power.

Want more torque? bump up both rocker arms and you have it.

Guess that's what makes this stuff so fun. Constant fussing like old ladies playing bingo on a Saturday.

Old SStroker

If you run less lobe lift and multiply it by a higher rocker ratio, you can run a LARGER base circle on the lobe, not a smaller one. The max base circle you can get into the engine is 2 x (Cam bearing diameter/2 - lobe lift). That puts the top of the lobe dead even with the bearing journal, so the cam manufacturer reduces the base circle a few thou so you can install the cam.

Example: 1.800 inch journal and a cam with .400 lobe lift. 2(1.800/2-.400) = 1.000 max base circle. With 1.5 rockers valve lift would be .600. If you went with 2.0 rockers, valve lift would be .800. Now if you wanted .800 with 1.5 rockers, your lobe lift would need to be .533 and the base circle would be .734. Ouch!

Now if you went to a 60 mm (2.362 in.) journal, the max base circle would be 1.562 with the .400 lift and 1.294 with the .533 lobe. Please check my math.

It isn't about surface speed of the roller lifter, it's about side loading. If you need to get a certain amount of lift per degree of cam rotation (Cam Velocity is the term for this and it is not related to engine or camshaft rpm), the larger the lobe (and therefore the base circle) the less side loading on the lifter.

Draw a cam lobe with a 1 inch diameter base circle and .400 lift over 70° of rotation. That's 140° of crank rotation for a 280° duration cam. Now draw a .750 dia roller follower moving up the flank. Look at the contact point compared to vertical (the lifter axis).

Now draw a 10 inch diameter lobe with .400 lift in 70° and look what the roller sees. Almost no side loading.

There's a lot more to this, but addressing one or two things at a time is easier.

We should be concerned about VALVE motion (lift, velocity, acceleration, jerk, and the next few derivatives), and work back mechanically thru the valvetrain to the cam lobe to get the valve motion we want. There will be limits at the lobe/lifter interface as pointed out above.

Lobes are designed for certain rocker ratios, or actually a range of ratios. You might use 1.7 to 2.0 rockers on a lobe designed for 1.85 rockers, but you'd get in trouble using 2.0s on a lobe designed for 1.5s. Well, it might be ok at low rpm, but you weren't increasing the ratio for low rpm, were you?

To be continued....perhaps.

Lickeyman

iTrader: (1)

Think of it like this.

You have a really big lever (pry bar) and your trying to pick up a huge rock. You lift up the rock with the short end and use the long end with your hands. You push down and the rock lifts up. But, you make a much larger movement with your hands to lift up the rock a little bit. The longer the bar the easier the lift but you get less movement on short end.

Now rocker arms are just like that lever only in reverse. The force is applied on the short end. And the longer the ratio (1.8 vs 1.7) the harder it is for the camshaft to push that short end up and push the long end down to open the valve.

Stick with the 1.7s and run a bigger cam.

Old SStroker

Did you ever notice that the highest-end engines are doing exactly the opposite of what you recommend? Even GM went from 1.5 to 1.7 and 1.8 for production engines. Hmmmmmm....