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rocker ratios

3K views 18 replies 13 participants last post by  vinniekq2 
#1 ·
I understand the ratio of a rocker, but I see a lot of people running both 1.5 and 1.6 at the same time , I am assuming that the 1.6 is on the intake side to add more lift.??????

why not just run a 1.6 ratio on both Intake and exhaust sides???
 
#2 ·
I don't know the ins and outs on running split ratio rocker arms but for some info I know is that running a higher ratio on the intake allows somewhat of getting a little more out of the cam on the top end of things and that is most of what I do know which is not much. I have read that some cams actually don't work as good with running split ratio's and are at there best at just the 1.5 ratio and see no gains. It comes down to the cam and other factors as to if you would benefit or not with changing over. I am sure someone else will post more info.

Roller-Rocker Ratio Test - Car Craft
 
#4 ·
I use split ratio rockers. I use a moderate cam(some say big for the street) when you get up into .600 lift cams the duration also goes up. If you are building a mild engine 500 hp or less then just buy the correct cam for your application. If you decide to bump up the power a split may be a good thhing,,,so lets start with your intentions? and are you considering say 600 hp? carb or fi?rpm range? shaft rockers?etc,,,,,what heads? what are the flow numbers?

of course the intakes get the extra lift with the exhaust maybe getting the extra duration
 
#5 ·
People run split ratios for lots of different reasons. In some cases it is just about getting more lift without changing the cam.

In short track 2bbl racing applications where you're not allowed to run a roller cam it's about maximizing the area under the curve on the intake valve. A higher ratio gets the valve off the seat faster, lifts the valve higher, all without increasing the duration to numbers that aren't desirable.

Sometimes it's the case where a class has a lift rule. As in the above example you can make the engine act as if it has a more aggressive cam profile than it really has without going above the max lift rule.

In some cases putting the 1.6 on the exhaust side can be used as a crutch to help an under performing exhaust port.

Other times high ratio rockers are used to get lift numbers that aren't attainable with standard size cam tunnels.
 
#9 ·
Dyno runs show the 1.6 ratio is more effective over more of the RPM range when used only on the intake. The proviso is that these runs were with split duration cams where the exhaust already has more duration and lift than the intake. I haven't run nor seen dyno runs with an even duration and lift cam so the result may be a bit different that what I'll describe.

With the 1.6 ratio rocker on the intake only the engine does lose some torque and power under 2000 RPM but it picks up an increasing amount of torque and power as the revs go up from 2000. This typically peaks at a 15-20 horsepower gain at peak and shows a much slower degrading of power over the peak power RPM (flat holding of peak power).

With a 1.6 ratio rocker on the exhuast only the power curve shows a degredation of 2 to 7 pound feet of torque and a like amount of horsepower untill about 5000 RPM. From there up the engine gains about 2-7 torque and horsepower units with a fast power drop after the peak reading.

With the 1.6 ratio rocker on both intake and exhaust the torque and power loss seen below 5000 RPM with the 1.6 on the exhaust remains with the turn around seen about the same 5000 RPM point as with the exhaust only tests but this gains very quickly into the 15-20 pounds of torque and units of horespower up on the top end with a faster power drop off than the 1.6 intake only but not as fast as the 1.6 exhaust only.

This shows that for daily driver situation running split ratio 1.6 on the intake and 1.5 on the exhaust beefs up the power band mostly used.

For a competition engine running into stiff gears through a loose converter or slipper clutch using 1.6's in both valves is sufficient since your typically not using the lower RPMs.

Remember these are dyno tests using a split duration/lift cam and dyno room exhuasts which short of running headers and a high flow muffler bolted to the collector, this system looks nothing like what's on a street vehicle.

Bogie
 
#10 ·
I have a book by Chevrolet called POWER, it´s quite an old copy 1994.
Concerning rocker ratios and placement, they did test and found the best power was made with this set up.



Tests were carried out on a NASCAR 358ci small block equipped with 18 degree cylinder heads and a Bow Tie single four barrel inlet manifold.
First tests were run using 1.65:1 on intakes and 1.6:1 on the exhausts.
Apparently the different ratios compensate for the difference in intake runner
lengths.
With today´s heads and inlet manifolds I wonder if this still applies.

I would have scanned the page but did´nt want to infringe copyright.
 
#13 ·
This was intended to compensate for the porting on the end cylinders which because of the flow inertia tends to want to roll the flow into the cylinder wall adjacent to the valve rather than want to sweep that flow across the port. While in the end the flow on the outboard ports does sweep in the needed direction (toward the spark plug and exhaust valve) energy thus flow is lost in the translation of naturally preferred direction to the functional direction so the larger rocker ratio buys some space and time for the transition. This condition is a lot worse when a two plane intake is used.

The inertial flow on the inboard ports wants to roll across the bore so is more efficient at filling the cylinder. The use of the slightly higher ratio rockers evened the cylinder filling between the inboards and outboards. The SB2 mirror port heads were also an attempt to even mass distribution between the outboard and inboard cylinders.

On the exhaust side you see a difference between the outboard and inboard ratios as well but this is a cooling issue where the inboard exhausts run hotter than the outboard so they get some extra seat time.

Bogie
 
#14 · (Edited)
Another thing to think about is how your exhaust is set up. I'm using 1.6 rockers on intakes & exhaust on Vortex heads. I did some major exhaust pipe improvements by adding about 16 ins. of 3 in. pipe to the collectors then welded on 3" to 2 1/2" tapered pipe to the 2 1/2 exhaust. Then replaced my 2" crossover to a 2 1/2". I did this mainly to improve intake scavenging, another route to go to improve intake flow. I lost some of my low end torque, probably more that just some of it more like a lot of it. So the 1.6 rockers need to be taken off the exhaust. What I lost on the low end I did gain on the top end, above 4500 rpms, but I don't go there much. But all in all I'd rather add 1.6 rockers than spend all day cutting & welding pipe.
 
#15 ·
. Another reason for 1.6/1.5 lifters is that a particular head may need special machining to accommodate the increase lift if 1.6 used on exhaust of a particular split duration cam...


. Some stock engines have a known max lift before machining and/or special valve springs needed, so cams for those engines may have less lift than expected for the duration used...


. This is also seen in the GM Hot Cam... 218/228 split durations, but .525"/.525" lifts, because of concerns of the roller lifters hitting the OEM dogbones and the lift limitations of stock LT1/LT4 heads/springs...


. Comp Cams did tests and claimed reducing exhaust lifts by 2/3 had almost no effect on power output... but available cam lobes used to be really standardized, so if used a lobe with more duration, it just came with more lift whether needed or not...


. In N/A engines, exhaust valves operate at 14.7 psi max (unless down in Death Valley or counting 'ram' effects)... in comparison exhaust valves operate with enough 'extreme' pressure to drive turbochargers...
.
 
#18 ·
The LT4 Hot cam was designed as a stock replacement cam originally for the LT4,which used 1.6 rockers,therefore,it was listed with the specs for 1.6's & offered this way in kit form.With 1.5's it only has .495 lift,nowhere near the lift needed to reduce base circle of the cam,so,i doubt hitting the dogbones was of any concern to GM in this case.
 
#16 ·
alright, using a Howards Cam 180885-10 (tech inspect recommended to me in another thread) and using the OEM spider and dog bones with Howards 91113 lifters, would I benefit from using the 1.6 ratio on heads , to gain a little lift and not effecting the dogbones.
(idealy using AFR 195 heads, OEM roller block 383 stroker)
 
#17 ·
You should be fine as for the dogbone issue.It may help power,it may not.The issue with the dogbones is not really hitting the dogbones.LOL.The problem is, after .530 lift,the base circle of cams starts to get smaller,so that the cam is able to pass thru the cam tunnel.When the base circle becomes smaller,the lifter will travel down low enough to allow it to drop below the dogbones & rotate from it's axis.When the lobe comes up again against the misaligned roller,stuff happens.Hitting the dog bone is a moot point at this time. LOL.Using a 1.6 rocker will not effect this because the added lift from the ratio is only seen at the valve.Not at the cam.Lift is not the problem.If you could use a cam with .600 lift on a standard base circle,there would be no problem.The smaller base circle on cams above .530 lift is the problem.
 
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