Cams & Rockers

im thinking about getting the S1X

I've got a question maybe someone on here with the correct formula can help me.

When using a Modified Rocker Arm we know that they increase lift and can easily calculate that increase by multiplying your actual cam lobe lift by the rocker arm ratio and it will give the actual valve lift.
An Example:
Lift of .3594 times the Ratio of 1.5 equals .591 Lift at the valve. (I used numbers from a motorcycle because I'm familiar with the numbers off the top of my head. Don't know the exact Stock Riviera Ratio and cam lift at .050").

My question is; How do you calculate the additional Duration that occurs when using say a 1.9 ratio rocker arm. Actually how would you measure it mathematically, if possible.???

We've been told and it is just plain intuitive to believe that the longer, modified, rocker arm opens the valve sooner and closes it later as well as effecting the lift, as measured above.
I need to find out if the duration can be calculated mathematically. I could measure it mechanically with a degree wheel and dial indicator, yet I would not come out with a repeatable number. Each time you took a measurement with the stock rocker arm for reference and then remove your indicator so you can bolt on the new modified rocker and then try and set up the dial indicator for another measurement, it would be skewed data because you will not get the dial indicator mounted precisely the way it was set up for the previous measurement. Therefore rendering the measurement or height of valve irrelevant.

I hope someone knows the answer to this.

Thanks for any input.

Rick

I could be way wrong here, since I'm not super familiar with the ins and outs for rocker ratios/cam stuff but...

I always thought duration was the same with modified rocker arms. Only lift was changed. The push-rods will always start moving(up) at the same point on the cam lobe, and hit the top of the curve at the same point vs rotation of the crank. Since high ratio rockers are "rocking" at a slightly higher ratio, you get more valve movement for the same push-rod movement. The result being more valve lift, with the same amount of push-rod movement. I suppose the high ratio rocker must move slightly faster too, since its moving farther at one end of the arm, for the same movement on the push-rod end. .. but the effect of a lever takes care of that. But duration should be the same.

I believe Andrew is right on this. Only by replacing the camshaft could you increase duration. Its been almost 30 years since I went to school for this, but after some thought, I believe this to be the case. Or not? The manufacturer of the rockers would probably have the correct answer.

Andrew is right on the money. Duration is ground into the cam by the manufacturer and cannot be changed without regrinding or swapping the cam.

Actually, this might surprise most of you, but using higher ratio rocker arms does increase the effective duration of the valves being open, at least where it counts. It's hard to think about, and difficult to calculate how much extra duration you get with a given ratio, but the best answer is this:

Realize that when a valve initially starts to open, it's not really letting much air pass through at all, so the initial duration isn't really that critical in the grand scheme of things. What's important is how much duration you have at say, .25" or .50" lift. That's what matters. And because a higher lift ratio means a quicker lift motion, the 1.9:1 rocker will rise to the .25" and .50" lift heights quicker than the stock 1.6:1 ratio, so it effectively gives longer lift. In fact, the more valve lift you have, the more added duration you will have by using a high ratio rocker.

Agreed but that's not what he asked lol. Fact is high lift rockers do not change the duration (the valve will open and close at exactly the same point with both) tho it will increase the amount of time you're above a certain lift point (or the effective duration).

I'm pretty sure AA is right on. The duration, as we express it, has to increase if the valve is reaching .050" faster and also dropping below .050" later on the fall, which it is on higher ratio rockers. Since It's not due to the lope actually being moved, I would think it's gotta be a miniscule difference though.

Quote :

Agreed but that's not what he asked lol. Fact is high lift rockers do not change the duration (the valve will open and close at exactly the same point with both) tho it will increase the amount of time you're above a certain lift point (or the effective duration).

Well technically yes, if you're talking about initial duration, the time will not change, but at any point after the valves start to open, the duration is longer for a high-lift rocker than for a low-lift rocker. I think this might be why the standard spec for measuring lobe duration is with .050" lift. It's a more realistic way to measure duration.

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The duration, as we express it, has to increase if the valve is reaching .050" faster and also dropping below .050" later on the fall, which it is on higher ratio rockers. Since It's not due to the lope actually being moved, I would think it's gotta be a miniscule difference though.

It's surprising how much added duration we get with high-lift rockers. Here's a visual to help show what I mean. Note how the duration ramps up as the lift increases. We could say rockers give extra duration exactly where it's needed most:

yay for AA and his info-graphics!

AA is right, but it all depends on at what lift does 'effective' lift, or airflow begin. Although yes, it does come sooner, I think in this situation the difference will be inconsequential, especially as compared to the gains from the overall ratio increase.

I am unable to view the visual that AA has posted, but after much thought I respectfully disagree that the high lift rockers would change duration. Duration is the measurement in degrees that the valves are open in relation to crankshaft rotation. The valves will travel quicker to reach their wide open position, and back to closed but the amount of time (degree of crankshaft rotation) they are opened does not change. For a street car that is driven daily this is actually a good mod over a cam replacement. Higher lift cams with longer durations create a valve overlap which degrades normal drivability. I have never installed rockers with an increased lift ratio, but I would assume idle and lo speed performance would remain smooth. This is a mod I may consider. I dont think a Riv with a big cam and a rolling idle is a good fit.

Any one that has done this, has it effected normal drivability?

No ill effect on drive-ability or idle for me. Although I just went with the 1.8 Yella Terra, due to having a different non-daily driver car that gets more money thrown at it.

I think AA is meaning Effective Lift to be just what we want to accomplish by going up in RA ratio. To get more lift over the factory camshaft lobe duration. = potentially more flow during that lobe duration.

Dave - if you think in terms of when the valve initially starts to lift from its seat until exactly when it closes again, you're correct the duration won't change from a 0" checking height. But that's not very useful when comparing actual durations because of things like ramp rate/height, which vary between different cams. So there is a standard checking height to measure durations more consistently - .050" lift. This is the effective duration.

Using the standard .050" check height, a cam pushing high-ratio rockers will have a longer effective duration (valve time being open) than if pushing a lower ratio rocker. This is because the higher ratio rocker lifts the .050" check height faster (earlier) than a stock rocker ratio would. It also closes later, adding to the duration.

Another way to increase the effective duration is to use a roller cam profile, which keeps the valves open longer at a given check height, even though the initial duration doesn't changes. Here's a diagram showing how a roller lobe has longer effective duration than a flat tappet:



If you can't see that image, try visiting this page and scroll down: http://www.thirdgen.org/sbc-camshafts-primer

In a very similar way that roller cams increase effective duration, a high-lift rocker will do the same thing. For a given cam profile, switching to a higher lift rocker will give more and more duration at higher valve lift check heights. After some searching, I found an article written by Jim Hand, which may better explain my point. Give it a read and maybe it will change your perspective:

http://www.pontiacstreetperformance.com/psp/RockerArms.html

Rick, AA I'm sorry for injecting so much confusion into this thread. While I may have technically been right I didn't answer the question you had asked and instead got hung up on the technical definition of duration. Unfortunately I don't have a mathmatical formula that could help you determine what you're looking for either so I'm gonna bow out of this discussion before I cause more confusion.

Derek, I think it's good to look at things from both angles, nothing to be sorry about. Often there's more than one correct answer depending on perspective, so your info was relevant. I feel my points are confusing as well, and it's fair to say there's a good reason every cam manufacturer lists their product with two specs: duration, and duration @ .050" lift.

This type of multi-spec comparison is what I like to think of as "lab vs. real world", and can be used to evaluate many types of products:

• Static vs. effective cylinder compression: supercharged 3800 V-6 static ratio is 8.5:1, but effective can be 12:1 or more.

• Gross HP vs. SAE net: gross HP for L67 engine is probably close to 290, SAE net BHP is 240, and WHP on dyno is about 200.

• Max Watts vs. RMS Watts in audio amplifiers: Max or "peak" watts is usually about double the RMS standard used to do fair comparisons between amps.

There are lots more, but it shows there is no single correct way to ultimately evaluate technical specs.

After further review, my ruling on the field has been overturned. AA, the article by Jim Hand cleared it up beautifully. My narrow mindedness had me focusing on the duration at the cam and lifters and not at the valves. Sounds like a nice mod for a daily driver. I look forward to doing this along with other recommended mods to compliment it.

In the future, I would appreciate if you would just agree with me though.Out of respect, of coarse. I am your elder.

Certainly, Dave. You win the next one!

Cams are the ultimate for midband and upper RPM power, but high-lift roller rockers have worked very well for me in the 5+ years I've been running them. If you can get past the price, and are okay installing yourself, I think you will be pleased with the results.

The install would be no problem. If you could recommend a good supplier I am interested in checking them out. Also what ratio would you recommend? I may do this at same time as coupler.

For the Series I SC engine, I think the best choice are Yella Terra 1.8:1 rollers:

http://intense-racing.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=IR&Product_Code=3800-YTRR-6659&Category_Code=HCV

It's a shame there's not a 1.9:1 ratio available, but at least you can keep the stock pushrods and valvesprings. Install should be a snap.

Thanks AA. Price was not as shocking as I had thought. Unfortunately, not much else available for the '95 Riv. I might start shopping for a newer one for myself. Maybe '98 or '99.

What kind of performance difference is there between 1.8 ratio and 1.9 ratio rockers? Anywhere I can find some data contrasting the two?

charlieRobinson wrote:

What kind of performance difference is there between 1.8 ratio and 1.9 ratio rockers? Anywhere I can find some data contrasting the two?

Get 1.9's obviously.. same price and more powerz..

98riviera98 wrote:

charlieRobinson wrote:

What kind of performance difference is there between 1.8 ratio and 1.9 ratio rockers? Anywhere I can find some data contrasting the two?

Get 1.9's obviously.. same price and more powerz..

I'm not really sure why they make 1.8s. Stock is 1.6, if people are increasing lift on a big cam they typically use a 1.7, and 1.9s are for people who don't want to replace the cam. The 1.8 doesn't fit in there...

They make 1.8s (and modded stock 1.9s) for those who don't want to replace their springs or pushrods.

The 1.8 and 1.9 rockers themselves are the same price, but true 1.9s require higher # valve springs and probably should use OR push rods. There's an extra $150-200 on top of the rockers + install time.

And not all 1.9s are created equal. Modified stock 1.9s will deflect more, so they are more like a 1.8. You can install these for cheap and say you have 1.9s, but if you go with aftermarket aluminum 1.9s like HS or YT, these will give a true 1.9:1 lift ratio, and so require the springs and rods.

But I partly agree with Travis: go with 1.9s because they do give more power. But realize they cost more and take a bit longer to install.