The truth about 1.6 ratio rocker arms

Discussion in 'High Performance Modifications' started by Lowend, Sep 11, 2006.

  1. Lowend

    Lowend Administrator. .a car, a man, a maraca. Staff Member Lifetime Gold Member

    Mar 25, 1999
    San Jose, CA, USA
    Question: why do folks not add the 1.6 rockers to existing motors to increase cam lift?

    I read in the above mentioned thread that you have to "subtract" the "gain" from the exhaust side. So if this is the case, here are some hair brained thoughts on the idea.

    What would happen if you used the 1.5 rockers on the exhaust and the 1.6 on the intake?

    Answer Courtsey of our own moderator Marv Davis

    There a lot of things involved in intake and exhaust flow, but the magic number is around 78-80% (depending on which engine builder you takl to). You want the exhaust to flow 80% of the CFM number the intake will flow. The exhaust can be less because the exhaust is forced from the chamber by the piston rising in the cylinder. Just keep in mind the intake charge is drawn into the clinder by the pressure drop (vacuum) created as the piston decends the bore. There is a BIG difference between the vacuum intake side, and the forced exhaust stroke. That's why forced induction like blowers and turbos make such a DRAMATIC increase of power with only 5 and 6psi of boost.

    Just for simplistic numbers, let's say you have a intake port that flows 100cfm @ .500" lift. Good performance could be expected if the exhaust port flowed 80cfm. Let's say your exhaust port is lazy and only flows 65cfm at .500" lift. You can do one of two things to bandaid things. You can increase lift on the exhaust side in hopes to bring the exhaust flow up to 80cfm, or you can increase duration of the exhaust valve opening. If the exhaust port is a big restriction, opening the exhaust valve wider generally doesn't help much, some, but not much. So we increase the duration the valve is open, AND increase lift with the hopes it will flow the magic number.

    Then too you have what we call a 'knee' in the flow. The port, no matter how big of a valve, or how much it is open, can just lay over and simply won't flow more. Increasing lift on a very restrictive port like this can cause crazy pressure drops and pulses in the exhaust that can actually impeed flow even more. So the trick is to find a lift that takes advantage of the port design and flow capabilities, and match it with a duration that compliments the cylinder pressure and rpm range for the particular engine and it's use.

    This crap gets totally out of hand when you start theorizing scavanging effects of 'overlap' (being when the exhaust valve is open at the same time the intake valve starts to open). This will scavage the chamber of most all of the spent fuel, and even draw some of the fresh air/fuel mixture through the chamber into the exhaust stream. Cams with no duration and nearly no overlap will always leave some of the spent gasses in the chamber. You make power by burning fuel, if your not purging ALL of the spent gasses, and replacing it with fresh a/f mixture, your not making full potential of the motor. Cam designers have been looking for tricks to accomplish this efficiently since,, well since forever! There is also the vacuum peaks created as the pistom travels down the cylinder. It's not when the piston forst starts down the bore, because piston speed near TDC and BCD are at their slowest. And because the peak piston acceleration happens something like 60 or 70 degrees after TDC, you need to get the valve open the 'right' amount to take advantage of that greatest pressure drop during the stroke.
    Cam design turely IS rocket science. The cam companies do countless hours of R&D then live testing on the dyno, tweeking every grind to fit the 'best average' of what their customers are wanting to wo, and what equipment they 'typically' have.

    Sooooooo, in what you asked about switching from 1.5 to 1.6 rockers,,, are YOU smart enough to outthink all the design work that goes into the cams they offer for our cars???? Actually yes you are. The one thing you need to know is flow numbers for the heads your running. Not just peak cfm, but flow numbers at lifts from .200 to .600". Then you look at the specs for the cam you want to run and see if it looks like that extra .03" of lift of higher ratio rockers would really help that much,,, or not. Then when you think you have it all figured out,,, either go to the dyno or to the track and see if you really accomplished anything, or just wasted a bunch of time and $'s.

    Now I'm not saying 1.6 rockers are useless, and I'm not saying they will be this miricle power adder. For the most part, you will see very little gains, unless the added lift compliments the package you stuff them in to.
  2. flyguy

    flyguy Veteran Member

    Jun 22, 2005
    Los Banos, CA, USA
    My favorite bit of online data is the HP curve plotted in the (CHP?) article for a 440HP SB using a XE274 cam and GM vortecs. The test with 1.5's gives 420or so HP at about 5500 with a pronounced rolloff above that. They stuck on the 1.6's and the curve went up to 440 somewhere above 5500 and was amazingly flat to about 6000. It is a really interesting plot.
    I saw some other info that showed how the rising slope of the lift curve is steeper with the 1.6's making the cam look like it has a faster ramp up rate. (maybe like a poor man's roller cam) It isn't much at .050, but it adds quite a bit of area under the curve as it goes up.
    Don't believe much of the numbers I see in articles as far as absolute values go, but I suspect the compared data might at least point to a real trend.
  3. jakeshoe

    jakeshoe Veteran Member Lifetime Gold Member

    Dec 8, 2000
    Republic of Texas
  4. tom3

    tom3 Veteran Member

    Aug 1, 1999
    I'd think the 1.6 conversion would make more sense with a near stock cam, or lower lift RV cam. When you get into performance shafts they are pretty much designed to get the max power as ground. In SOME cases the rockers would make for pretty easy bolt on power, other cases, a waste of money.
  5. flyguy

    flyguy Veteran Member

    Jun 22, 2005
    Los Banos, CA, USA
    What can I say, it was interesting to me.
  6. jakeshoe

    jakeshoe Veteran Member Lifetime Gold Member

    Dec 8, 2000
    Republic of Texas
    Good point.
    Several cam grinders I have spoken with have agreed that the higher ratio rockers can partially substitute for the more aggressive valve action of a roller.
    Obviously there is a limit to what will work for a given combo and what a flat tappet cam can tolerate as far as load on the lifter, but anytime you have greater CONTROLLED acceleration of the valve, you have the potential to make more power.
    Higher ratio rockers don't just work in a stock or RV cam situation...
  7. pdq67

    pdq67 BANNED

    Jul 26, 2001
    Columbia, MO, USA
    Ah come on, Jake.

    They work with little-bitty cams too, but using them usually won't make enough extra power to notice is all.

  8. rebski

    rebski Veteran Member

    Ok I should proly keep my opinion to myself but here goes.

    In my experience splitting rocker ratio is not a good thing. The cams that gain the most from 1.6 ratio rockers are the old grinds mainly the OEM stock muscle car ones. The new grinds have so much R.& D. money in them that you are better off running what the grinder recomends.

    Now the old ones had to deal with stamped rockers and stock springs. The biggest benifit of bumping ratio is roller rockers and ramp speed. So If you upgrade to the lightest valve parts and best springs. You gain by poping that valve open and closed as fast as possible and overall valvetrain control. If you split the rocker ratio up 1.6 intake & 1.5 exhaust you distort the grinders original intent. I have always been of the opinion that you can gain more by exhaust efficiency than intake. The cleaner you leave the cylinder the better.

    Cam grinding is rocket science and they do have the money for it.

    Rollercams are rollers. Flat tappets are just that flat. Apples and oranges guys.

    This is just my humble opinion.
    Last edited: Oct 1, 2006
  9. HeadWizard

    HeadWizard Member

    Aug 8, 2006

    Marv Wrote a great article that is correct and very pertinent so don't just discount what he established in that post.

    Now then if you really-really want to know what happens when you change rocker arm ratios you have to do this on the engine stand while you're assembling the engine.

    Most of us degree the cam at the cam - but have you ever degreed the same cam at the valve? Tell you what try both - check the cam at .020 - .050 & total write that down. Then chack the same stuff at the valve with 1.5 rockers & write that down - then change to 1.6. 1.65, 1.7 and see what happens.

    The question here is what happens to the duration at each of these points when you change the rocker ratio?
    Is .020 & .050 the same at the cam as it is at the valve?
    The more you understand about this and it's corralation to and with head-flow at lift with regards to piston acceleration VS rod-ratio the better off you're going to be with every engine that you build.

    Denny@JDS Induction Products
  10. 74RAT

    74RAT Veteran Member

    Dec 28, 2003
    ft. stockton,tx,usa

    yep,, i agree,, you can even tune pushrod lengths/rocker geometry for a few extra thousandths lift during the beginning stages of opening to mid lift,, or the full lift point. shows up at the valve.

    i'm not brave enough to r&d with my own stuff though. then you're really playing with valve acceleration/decceleration rates like that. not really worth it in my opinion to go way outside the relhm of what the cam designer wanted. but if it has to be done to be competitive,, then i guess it gets done by some. but checking/degreeing at the valve with different pushrod lengths will really open your eyes too. small but it's there.

    although there are a few profiles out there that are designed to be run with high ratio rockers. are they worth it??

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