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Author Topic:   rocker arm help
ready2race23
Dirt Full Roller

Total posts: 45
posted June 09, 2005 05:27 AM  
Putting together a new motor and was going to run 1.5 1.6 rocker arm. Witch one goes on int. or ex.? If you put them a certain way will it help top or bottom end? chevy 355 Thank you

[This message has been edited by ready2race23 (edited June 09, 2005).]

artie727
Dirt Full Roller

Total posts: 26
posted June 09, 2005 06:11 AM  
NORMALY, The 1.6 ratio will go on the intake and the 1.5 ratio goes on the exhaust.

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Artie Perilloux
A & M Motorsports

ddoti1
Dirt Full Roller

Total posts: 49
posted June 09, 2005 09:06 AM  
quote:
Originally posted by artie727:
NORMALY, The 1.6 ratio will go on the intake and the 1.5 ratio goes on the exhaust.


Is this for a 350 and if so, what is the advantage to it ? I m very interested !

leapinlizard
Dirt Freak

Total posts: 402
posted June 09, 2005 09:28 AM  
Sometimes on some heads, a power gain is realized by crutching a port with low flow potentiol with a higher lift. On SBC, the exhaust is usually the one with more duration and lift. This is why alot of cams are split patterned. Sometimes it's the intake valve with aftermarket heads or alky or lot's of other variables.

dirtbuster
Dirt Forum Champ
Total posts: 2007
posted June 09, 2005 10:16 AM  
putting the 1.6 on the intake will speed up the valve action. It will have the valve open farther and faster. Meaning you can get a better cylinder fill which means better torque and power. Kind of like the difference between a flat tappet and roller cam. With the same duration the roller will open faster which makes more power.

artie727
Dirt Full Roller

Total posts: 26
posted June 09, 2005 10:24 AM  
On a chevrolet engine the lift at the valve is determined by the actual lift of the lobe on the cam multiplied by 1.5, which is the leverage ratio of a standard rocker arm. If you go to a 1.6 ratio rocker arm the cam thinks it's actually bigger than it is because you now have a longer lever on your rocker arm. BEWARE of changing the ratio of the rocker arm to 1.6 you may run into problems. Depending on the head you may have to do a lot of grinding for push rod clearance and it will change the contact point where your rocker arm contacts the valve (valve train geometry) I hope this helps you understand.

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Artie Perilloux
A & M Motorsports

ready2race23
Dirt Full Roller

Total posts: 45
posted June 09, 2005 01:12 PM  
Artie,
Thank you It will have a set off double humps Ported and polished. When machine shop gets to look at them they are going to see if they've shaved or not.

sc1 racing
Dirt Freak

Total posts: 419
posted June 10, 2005 11:59 AM  
it never does any good to get more fuel in the engine than it cant pass throught the exhaust. you will never see a cam with more int lift than exaust and thats for a good reason..

always put 1.6 on exh, and 1.5 on intake...unless you already have a split duration cam. then i wouldnt mix them

Mell00
Dirt Forum Racer

Total posts: 88
posted June 10, 2005 12:45 PM  
Is that fact SC1, or just your opinion?

dirtbuster
Dirt Forum Champ
Total posts: 2007
posted June 10, 2005 01:19 PM  
SC1 racing,

I am going to have to disagree there. A lot of your cam choice will depend on heads and how they flow intake versus exhaust ports. There could be instances where the exhaust flows too much compared to the intake and in that case a larger intake smaller exzhaust lobe will help balance flow into and out of the engine. You can do somewhat the same with a different ratio rocker.

Just putting the bigger rocker on the intake doesnt mean it will fill the cylinders more than the exhaust can handle. Most likely you arent getting a complete fill anyway, and the exhaust isnt the restriction. Getting more air into the motor is the key to making power. The faster intake opening will have the valve open furthur throughout the cycle allowing the cylinder to fill more efficiently than before.

If you talk to many big time engine builders it is very common to run the higher ratio on the intake. In fact I would say it is most often done this way simply because it speeds up the valve motion. Now with stock heads it might not make a difference but there is a time for it.

Overall you need to match your cam and rocker/valvetrain setup to your heads. What works for one setup might not work for another.

Kromulous
Dirt Forum Champ
Total posts: 796
posted June 10, 2005 01:24 PM  
1.6's create more lift as a by product. Lift is generally not the key to making power, durations are where its at, timing events and the lift ramp of the cam.

A 1.6 rocker, and larger create a non linear lift ramp. Meaning (its hard for me to explain) if you graph out the valves movement on a piece of graph paper it will look like a bell, upside down sort of anyway. This bell will change shape once you begin tweaking rocker ratio and or durations and lift. It will become slightly wider thru out the designated lift.

When you change to a higher ratio the valve will acheive higher lift numbers faster, because of the ratio. Aka the lift ramp profile is altered.

This is where assymetrical cams are starting to come into the seen. On these the lift ramp on either side is different then the other. So you can have a steep lift ramp on uponing the valve and have it close more quickly. Its very interesting reading about them.

If you have the clearance between the piston and valve you can run 1.6's on either. Although it would be best served on the intake side, i believe anyway, as long as the head can flow about 80% of what the intake port can. If it cant you may actually make it run worse.

Its worth a try, just make sure you got the clearance.

Someone can correct me if i am wrong, but i think i understand it right so far.

Jeep83
Dirt Roller

Total posts: 9
posted June 10, 2005 02:32 PM  
Krom, actually lift is very critical for making power. Lift creates more area under the curve ( the amount of area the valve is open to fill the cylinder) The shorter duration cams need lift to create enough area to make power, while keeping torque #'s high and the power band at a lower rpm (for circle track use). There is a fine line between have enough lift to do the job and too much lift for the port do any good, thus the appropriate cam for the heads and intake you are using. Generally, one of the main restrictions on a small block Chevy head is the amount of valve lift the stock style spring is capable of.

leapinlizard
Dirt Freak

Total posts: 402
posted June 10, 2005 02:48 PM  
(1.6's create more lift as a by product. Lift is generally not the key to making power, durations are where its at, timing events and the lift ramp of the cam.)

Lift is power till the point where more lift doesn't increase flow through the port.

(A 1.6 rocker, and larger create a non linear lift ramp. Meaning (its hard for me to explain) if you graph out the valves movement on a piece of graph paper it will look like a bell, upside down sort of anyway. This bell will change shape once you begin tweaking rocker ratio and or durations and lift. It will become slightly wider thru out the designated lift.)

You could also say the valve is open further per degree of cam rotation without an increase in duration(time measured in degrees) Or more area under the curve(bell)

(This is where cams are starting to come into the seen. On these the lift ramp on either side is different then the other. So you can have a steep lift ramp on uponing the valve and have it close more quickly. Its very interesting reading about them)

Assymetrical Profiles are easier on the cam and lobe. By slowly closing the lash, you prevent these part's from slamming together and creating a shock wave that breaks part's. And slowing down the valve just before it seats slow recession and popping heads off valves is reduced. Especially wiht radical cam profiles. These are only a few advantages to using a roller cam/lifter.

Just my .02 and a half cent's

Kromulous
Dirt Forum Champ
Total posts: 796
posted June 10, 2005 03:05 PM  
"You could also say the valve is open further per degree of cam rotation without an increase in duration(time measured in degrees) Or more area under the curve(bell)"


Thats what i was trying to say. I said i cant explain it well.

I agree with Lizard on the lift as well.

Engine builder always tells me that the lift is un-important, long as its up there he says, duation figures is what is always looking at.

leapinlizard
Dirt Freak

Total posts: 402
posted June 13, 2005 10:16 AM  
(Engine builder always tells me that the lift is un-important, long as its up there he says, duation figures is what is always looking at.)


While I see the point. I tend to be more specific in nature.

sc1 racing
Dirt Freak

Total posts: 419
posted June 13, 2005 12:23 PM  
im just telling you what i know works for me.. i dont go by books or hear say..well i try hear say and if i feel the differance im sold..so no they are not proven facts, but i have tried every kind of senerio with mixing and 1.6 on exhuast, and 1.5 on intake . made me feel it in the seat...

Mikey14
Dirt Freak

Total posts: 265
posted June 14, 2005 11:19 AM  
would putting the 1.6 on the exhaust side create a greater vacume since it opens more quickly? This would help flow on a stock intake, stock head setup wouldn't it?

leapinlizard
Dirt Freak

Total posts: 402
posted June 14, 2005 01:14 PM  
There are no hard and fast rules in rocker geometry tech. There still are generalities though. Nothing wrong with whatever works for you.

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