[drelotter]

Hello there. :welcome:

I have a Chevy 350 bone stock with aftermarket aluminum heads. The are the Dart Pro 1 Aluminum Cylinder Heads. I don't have too much experience with builds I wanted to know what ratio rocker arms 1.5 or 1.6.
I don't know if there is an importance to which one I choose Please help me out thanks!

http://www.summitracing.com/parts/DRT-11321111P/

 

[454C10]

I would use the 1.5's

People use 1.6's to get more lift out of a low lift cam. However, too much lift can lead to quick lobe failure with flat tappet cams.

Just install the cam with the lift you want.

 

[ap72]

If possible you should run the highest ratio you can with the shortest cam lobe you can for maximum cam life. run 1.6's or higher.

 

[454C10]

no. 1.6's will make the cam lift rate more aggressive and will make more stress on the cam lobe. Lower lift flat tappet cams with softer ramps live longer, especially with todays crappy emissions engine oil.

the stock valve train is setup of 1.5's. going to 1.6's can cause troubles and often does. Using 1.6' is a band aid to extract more lift and duration from a cam that is "too small" for the intended use. So, use the right cam with 1.5's.

here is one example of problems with 1.6's.

http://www.vetteweb.com/tech/0204vet_small_block_rocker_arms_ratio/index.html

do a web search. plenty of write up on this subject.

 

[ap72]

How is something so damn simple so hard to get. Draw a picture. If I can't see something I find the best way to see it is by drawing a picture.

Shoot for.550" valve lift.

a lobe with .367" lift and 1.5 rockers will get you that, so will a lobe with .344" lift and 1.6 rockers. Guess which one will last longer.

Given the same duration the lower lift lobe will always last longer.

1.6 rockers are a no-brainer.


Secondly, rocker ratios do not change the cam lobe.

 

[woodysracing]

I have to agree with 1.6's being possible trouble.Yes they do change the cam,s intended lift,thats easy math...But...you also have the possibility of the valve getting smacked by the piston because of the extra lift.I've seen many bent valves because of the lessened clearance.I would use the stock ratio rockers and pick a cam that works with them not only for the clearinces but also keeping the valve train dynamics the way they are desinged.Rocker anlge,contact area on the top of the valve...etc.

 

[BBCMudbogger]

Have to go with woody on this one.Why go around your elbow to get to your arse?

 

[ap72]

I have to agree with 1.6's being possible trouble.Yes they do change the cam,s intended lift,thats easy math...But...you also have the possibility of the valve getting smacked by the piston because of the extra lift.I've seen many bent valves because of the lessened clearance.I would use the stock ratio rockers and pick a cam that works with them not only for the clearinces but also keeping the valve train dynamics the way they are desinged.Rocker anlge,contact area on the top of the valve...etc.

They do not change the cam. There's no math to it. They also do not dictate running a higher lift when your camshaft has yet to be selected.

There's no need to agree with anyone on this, there's no theroy involved here at all, it is what it is. Setting the contact patch between the valve tip and th erocker has to be done independantly of which ever ratio you shoose, 1.3's, 1.5's, 1.7's, etc will all require that adjustment.


Again, its not that hard to understand, just draw it out.

 

[BBCMudbogger]

Wonder why those dumb engineers used a 1.5 ratio anyways........

 

[ap72]

I wonder why those dumb engineers use ratios over 2:1 on racing engines...

Never assume anything from the factory was the best performing solution to a problem. As an engineer I deal with compromises in design on every project, systems are never designed for best performance, they are designed for adequate performance, liveable O&M costs, and the best value (low cost) to the client.

 

[BBCMudbogger]

Dont think the OP has a racing engine.

 

[ap72]

please don't make useless posts.

If there is some aspect of the merit of running higher ratio rockers that you do not understand then I will be happy to try to clarify it

as to the OP having a racing engine the choice of heads tells me he isn't building an engine to run the same as a stock 180hp chevy 350, also even those 180hp enignes are used at times for racing- any engine can be a racing engine.

Some people race with 49cc two stroke engines, some race RC cars with even smaller displacement and comparitively minute power levels- still "racing enignes".

 

[454C10]

How is something so damn simple so hard to get. Draw a picture. If I can't see something I find the best way to see it is by drawing a picture.

Shoot for.550" valve lift.

a lobe with .367" lift and 1.5 rockers will get you that, so will a lobe with .344" lift and 1.6 rockers. Guess which one will last longer.

Given the same duration the lower lift lobe will always last longer.

1.6 rockers are a no-brainer.


Secondly, rocker ratios do not change the cam lobe.

That makes no sense at all. Lifting a valve is lifting a valve. More lobe or more rocker ratio has the same effect on the stress on the valve train.

You are missing the fact that the 1.6's have more mechanical advantage on the cam lobe and therefore increases the stress on the lobe. Plus a higher lift will make the spring push back harder. There is "no free lunch" when it comes to lifting a valve.

Again, use the right ratio that the valve train was designed for.... then select the cam with the lift you want or dare to use.

 

[ap72]

wow you're WAY oversimplifying some very important issues.

A milder lobe has more contact area than an agressive one, which means the pressure the sping exerts on the surface of the lobe is less in the scenario I previously outlined.

Along with that the lifter and pushrod travel less, which means you can also run less spring pressure for a given lift with the 1.6 rocker. If designed correctly the 1.6 ratio can give you a more agressive valve opening AND better durability.

This may be one of those things were you're just going to have to trust me. If you don't trust me call up a cam grinder and talk to them about it. And the tech line at Comp is not even close to talking to a cam grinder. Its like calling the Jiffy Lube guy a F1 engine mechanic.

 

[454C10]

wow you're WAY oversimplifying some very important issues.

A milder lobe has more contact area than an agressive one, which means the pressure the sping exerts on the surface of the lobe is less in the scenario I previously outlined.

Along with that the lifter and pushrod travel less, which means you can also run less spring pressure for a given lift with the 1.6 rocker. If designed correctly the 1.6 ratio can give you a more agressive valve opening AND better durability.

This may be one of those things were you're just going to have to trust me. If you don't trust me call up a cam grinder and talk to them about it. And the tech line at Comp is not even close to talking to a cam grinder. Its like calling the Jiffy Lube guy a F1 engine mechanic.

Nope. Not gong to just trust you. Your logic is wrong. There is no free valve lift. higher lift cause more stress on the valve train, plain and simple. 1.6's lift the valve higher than 1.5 and the results is more pressure on the lobe due to the more mechanical advantage (simple mechanics). FYI, that is why lower ratio rockers are used at cam break in.

Furthermore, spring pressure required has nothing to do with the rocker arm ratio. The spring pressure is dictated by what it takes to control the valve. That is the amount of lift, rpm, ramp rate, valve weigh, (things like that). Using a higher ratio with a smaller lobe is the same thing to the valve as a bigger lobe with a smaller ratio. except the valve train is setup for 1.5.

sorry, but longer ratio rockers are poor substitutes to a proper cam profile used with the correct rocker arm ratio. 1.6's are good to use on engine when the current the cam is too small and a cam swap is not possible. 1.6's are just a quick band-aid fix to an already mismatched engine.

 

[cobalt327]

Hello there. :welcome:

I have a Chevy 350 bone stock with aftermarket aluminum heads. The are the Dart Pro 1 Aluminum Cylinder Heads. I don't have too much experience with builds I wanted to know what ratio rocker arms 1.5 or 1.6.
I don't know if there is an importance to which one I choose Please help me out thanks!

Go with a roller fulcrum rocker, not just a roller tip. The fulcrum is where you will see a benefit.

In the case of a stock cam, you will gain a little performance by using 1.6 rockers on your 350 with those heads.

But WHY run the stock cam? Those heads will benefit from a good cam, chosen for the application that you have for it. Even a mild cam- fully streetable and computer compatible- can get you 30-40 plus HP more than the stock cam.

The way I look at rocker ratios, is use a higher ratio when the base circle gets too small due to the lift required. If I were starting out w/a blank piece of paper to design a valve train, I would lean towards a 1.7:1 ratio from the get-go. Engines that have been designed w/a 1.5 ratio from the start will have less issues with clearances, etc. when staying with the 1.5 ratio.

For any given cam, a higher rocker ratio will provide more lift under the curve than the lesser ratio rocker. There will also be more open duration, without changing the opening and closing points.

If the valve train is going to be modified, a higher ratio should be considered- but be aware that there are current lobe designs out there that have the ramp speeds about maxed out. To use 1.6 rockers on these cams may invite trouble.

In any event, get a decent cam in that thing!!!

 

[ap72]

1.6's lift the valve higher than 1.5 and the results is more pressure on the lobe due to the more mechanical advantage

Furthermore, spring pressure required has nothing to do with the rocker arm ratio. The spring pressure is dictated by what it takes to control the valve. That is the amount of lift, rpm, ramp rate, valve weigh, (things like that). Using a higher ratio with a smaller lobe is the same thing to the valve as a bigger lobe with a smaller ratio. except the valve train is setup for 1.5.

on the same lobe 1.6 rockers will provide more lift, however IF you select the cam in the manner I described then 1.5 ratio and 1.6 ratio can both be used to provide the same lift.

Spring pressures required have a LOT to do with rocker arm ratio. The spring has to control the valvetrain, not just the valve. The rocker and everything on each side of it needs to be kept under control.

Using a higher ratio rocker with a smaller lobe IS NOT the same as a lower ratio with a smaller lobe. Again, you negate the effects of the lifter and pushrod, which actaully weigh more than the valve.

 

[BBCMudbogger]

Let me guess those RC engines and 49cc two stokes use 2:1 rocker ratios too......lol.....being racing engines and all

 

[ap72]

Let me guess those RC engines and 49cc two stokes use 2:1 rocker ratios too......lol.....being racing engines and all

two strokes don't run rocker arms...

 

[dawg]

gentlemen stop the bickering!
lol
on my 406 i run 1:6 on the intakes and 1:5 on exhaust.
seemed to produce the best power on the dyno.
1:6 = more lift and duration on the intake.