buck35

Dumb questions? ?? Or no real definitive answer.

getrdunn

iTrader: (7)

Kinda funny you mention that buck. I think you will get to many different answers that will be debatable yet not an easy question to answer as build components etc will vary especially the cam itself. Some will say it will throw off the geometry and create problems. Others might suggest it's like a band solution for a poor cam choice. Others will say it just simply works great but chances are haven't even done back to back testing. In regards to trying to achieve a short duration high lift cam this is my opinion is or can be a better solution and easier on the lobes. I've personally done it in several different engines with some noticeable gains and some not. By going to 1.8's you also change duration slightly as well. So again I think every application will vary.

Assuming the heads flow well at net lift and if I was wanting a short duration high lift cam I would certainly go this route. I've personally seen engines run hard and get thrashed on with 1.8's with good valvetrain geometry run for hundreds of hours with no lobe wear or issues what so ever. Example custom cam from comp with 236/242- 667 lift with 1.8's nets at like 707. Now as we know with a bob cam that would never last.

getrdunn

iTrader: (7)

So in a nutshell with example of cam lift and duration above I highly doubt a 236/242-707 endurance cam would last however with the 1.8's achieving the lift it definitely takes some wear issues off the lobe. Provided you have a good stable well aligned valvetrain I'm all for it. Again much what may work one cam certainly doesn't mean it will work with another. Lots of variables but nothing ventured nothing gained.

buck35

Thanks for the reply, im certainly no engine builder but I try to read and absorb as much as possible on here. Theres a lot of math for sure in the answer, was just wondering if adding to the fulcrum instead of the cam lobe to achieve the same result on a build would be easier on the valve train or harder.

Black Baja

iTrader: (2)

There is no change in geometry with different ratio rocker arms. The distance from the trunnion to the valve stays the same. This distance is what influences geometry (longer arm better geometry). Longer arm sweeps less (think about it).The way to change the ratio is from the pushrods cup to the trunnion. The closer the cup is to the trunnion the higher the ratio (does not affect Geometry) But nothing changes from the trunnion to the valve. Because the cup moves closer to the trunnion the pushrod/lifter/cam lobe loose there mechanical advantage and put more force on everything from the trunnion to the cam lobe. If you look closely at the rocker arms being made there is no change in design to help compensate for the added load on the rocker. Higher ratio rockers DO NOT add duration. They DO however open the valve faster. Which is a ++. The faster you get the valve off the seat the more power the motor makes. Doesn't matter if it's 400hp or 4,000hp. So, it does have some advantages. you have to be careful when changing ratio that you don't run out of room for the pushrod. Higher ratio pushes the pushrod towards the exhaust. For the little bit of added lift you get stepping up to a 1.8 I don't think it's worth the added stress on the cam/lifters and pushrods. If you want that little extra bump in lift just put it on the cam lobe. Now if we're talking 2.0 ratio now we're doing something and at that point I think the +'s might out weigh the minuses.

getrdunn

iTrader: (7)

NEVER said it would change the geometry. RE: duration increase... MEANT to say it will change duration once off the seat. I've personally seen it and had a degree wheel on an engine a time or two also. Should have clarified the duration.

MILD THUNDER

iTrader: (3)

Here's what my cam software shows, showing a 5500rpm, 540ci with AFR 325 heads. For the intake lobe. using a hyd tappet

1.7 rocker 1.8 rocker Degree of change
296.9 .006 296.9 no change
268.5 .020 267.5 -1*
242.0 .050 240.5 -1.5*
212.0 .100 209.0 -3*
161.0 .200 155.5 -5.5*
103.0 .300 90.0 -13*
.372 lobe lift .352
.622 valve lift .622
.158 lift at tdc .158

Thats the cam being designed around the rocker ratio that will be used. I can only speculate as to why the program does what it does, but my guess, is that there is a given valve movement rate that one may think is ideal. Lets say you had a cam designed for a 1.6 Rocker. And you go and put on a 1.9 ratio rocker. On a symetrical lobe. There's probably a good chance, you're gonna slam the valve off the seat on the closing ramp. That leads to valve bounce, tons of spring psi needed, and , prob beat the fuk out of the valve seats. As well as put more stress on the lifter and lobe. What certainly would change, is the timing events from a rocker change I would think. Not at the seat measurment so much, but at the .050, .100, .200, .300 etc. Plus lifter wheel diameter plays a role too.

I think thats why a cam grinder wants , or should want, to know what lifter wheel you are using, as well as what rocker ratio, when he is designing your camshaft.

mike tkach

iTrader: (3)

very well said bb.as the rocker ratio increases so does the ware on the parts.it,s just the law of physic,s.

dunnitagain

Anytime you increase ratios the biggest issue you have is the closing cycle , the increased valve lift acceleration also brings a parallel acceleration in closing . This most always induces seat bounce for the valve.

buck35

[QUOTE=dunnitagain;4512307]Anytime you increase ratios the biggest issue you have is the closing cycle , the increased valve lift acceleration also brings a parallel acceleration in closing . This most always induces seat bounce for the valve.
This was the focus of my question, that if you ruduce lift while increasing ratio to achieve basically the same thing, whats the tradeoff. Slamming the seat was my thought but wasn't sure. 1.7 seems the magic number on ratio, just curious what happened when you switch things up a bit.
Mt , thanks for spending your time to run that. Like I said previously, I'm here to read and learn, and occasionally ask a not too stupid questions.