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Old 05-27-2007, 06:29 AM
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Eggman Eggman is offline
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Default Rocker arm ratio question

I am getting ready to install my new engine and discovered that the comp cams 264 hr shaft I installed has 1.6 as it's rocker arm ratio.I am assuming that the factory rockers are 1.5 on a 1990 360???I have a set of Crane ductile iron adjustables on my present set up,but don't know if they are 1.5 or 1.6.(Hell of a time to find this huh?)
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Old 05-27-2007, 09:04 AM
daniel_depetro daniel_depetro is offline
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I didn't know camshafts has a rocker arm ratio?

Yes the 1990 engines have 1.5 ratio rockers.
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Old 05-27-2007, 10:15 AM
peg leg peg leg is offline
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Default Do the math

Do you know your max lift? If so, and you are below the max recommended lift with the pistons you chose, then use the 1.6 ratio. You'll just get more lift with the 1.6 rockers. If you are above the max lift, then switch to 1.5.
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Old 05-27-2007, 10:28 AM
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The pistons are dished and the valve springs are good to 500 lift.
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Old 05-27-2007, 04:09 PM
aarracer aarracer is offline
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If you do not run the reccomended Rocker arm Ratio you will have reduced your cam's duration timing (it will open later and close sooner) and reduced lift as stated in your cam specs. ie. It will be like a smaller cam
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Old 05-27-2007, 06:00 PM
daniel_depetro daniel_depetro is offline
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I understand that, I just figured all the SBM cams were for 1.5 ratio and the newer Magnum style cams were all 1.6 ratio.

I'll have to be careful when I order my cam in the very near future.
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Old 05-27-2007, 06:35 PM
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Quote:
Originally Posted by daniel_depetro View Post
I understand that, I just figured all the SBM cams were for 1.5 ratio and the newer Magnum style cams were all 1.6 ratio.

I'll have to be careful when I order my cam in the very near future.
Often the cams are rated in both 1.5 and 1.6 ratio, usually on the same line. Go with the 1.6's if your max lift will allow, its a faster lift rate.
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Old 05-28-2007, 01:30 PM
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OEM stamped rockers actually work out to well below 1.5. More like 1.45-1.47.
Cranes ductile are better but not usually a "true" 1.5 ratio either. Then with the different angles of the lifter and pushrods, you loose a little bit there too!

I did some measuring on one engine and it turned out that it needed a "true" rocker ratio of 1.515-1.52 to get the actual advertised cam lift!

When going to a higher ratio rocker, it wouldn't hurt to double-check the piston to valve clearance, and because the higher ratio increases the speed that the valve opens and closes, the optimum valve spring pressures may need to be a little higher.
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Old 05-29-2007, 01:26 AM
DartGT66 DartGT66 is offline
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Quote:
Originally Posted by aarracer View Post
If you do not run the reccomended Rocker arm Ratio you will have reduced your cam's duration timing (it will open later and close sooner) and reduced lift as stated in your cam specs. ie. It will be like a smaller cam
A change in rocker arm ratio does not change the overall duration, adn the duration at .050" lift is also measured from the cam, so that doesn't actually change either. Measured from valve, the .050" lift duration will change a coule of degrees, but nothing spectacular. The duration still is, what makes a cam "hot", not the lift. If the cam is designed for 1.6 lifters, it might give you a small benefit, btu there is a chance, that you will not notice the difference. I once spent a day at the track trying different rocker combos, I had stock stamped steel, 1.5 and 1.6 ratio crane gold rollers, in a 440 with a MP 292/.509" cam. The rocker changes didn't affect the performance at all.
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Old 05-29-2007, 02:37 AM
Tarrbabe Tarrbabe is offline
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Default I agree..............

rocker lift ratio does NOT change the duration of the cam. That is set when the cam is ground. The acceleration rate is increased but not the duration.
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Old 05-29-2007, 09:45 AM
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Quote:
Originally Posted by aarracer View Post
If you do not run the reccomended Rocker arm Ratio you will have reduced your cam's duration timing (it will open later and close sooner) and reduced lift as stated in your cam specs. ie. It will be like a smaller cam
This would be true for a Magnum head, not an "LA" head or a reduced rocker arm ratio if it is actually made. (I guess there could be a market for it ? Why I don't know.)
Going to a larger ratio rocker increases the intencity of the cam and the added lift would probably, more than likely need a valve spring change.
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Old 05-30-2007, 12:32 AM
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Quote:
Originally Posted by DartGT66 View Post
A change in rocker arm ratio does not change the overall duration, adn the duration at .050" lift is also measured from the cam, so that doesn't actually change either. Measured from valve, the .050" lift duration will change a coule of degrees, but nothing spectacular. The duration still is, what makes a cam "hot", not the lift. If the cam is designed for 1.6 lifters, it might give you a small benefit, btu there is a chance, that you will not notice the difference. I once spent a day at the track trying different rocker combos, I had stock stamped steel, 1.5 and 1.6 ratio crane gold rollers, in a 440 with a MP 292/.509" cam. The rocker changes didn't affect the performance at all.
I agree

Run it as is, and don't worry about it. Duration is what makes power.

There are gains in bigger rockers, but they are not massive.

FWIW I have seen reputatable engine builders say a 1.5 to 1.6 swap is worth about 5 hp on most cars.
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Old 05-30-2007, 01:58 PM
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I looked the cam up in Comp's catalog and the specs are for 1.5 for this part#.There are three different variations of this cam ranging from a long snout version for use with mechanical fuel pumps(my version),a short snout,and a 1.6 version.This is supposed to be a computer controlled cam but I stuffed it my 360 carbed motor.
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