I know I posted about this before, but I just was reading some other posts about RAR and duration and said, geez, I have 1.75 RAR rockers on my BB vs the std 1.7.

My cam is a 287/291 @ .020 256/260 172/175 @ .200 @ .050 .643 lift (with 1.7RAR) lift 112 LSA.

I lash it at .016/.018 hot. I was told not to set it cold so I don't.
With my lash, the cam sees a bigger seat duration cam. When I realized RAR had more to do with duration that I thought, I had to ask this question again.

can anyone venture a guess as to what duration my cam is actually seeing at its lashed "seat" duration, .050 and .200?
I am also now suspect of my DCR calculation.. how can I use the advertised duration of my cam if the advertised duration is not where I set it?
I am currently running 10.14:1 CR With this cam.
I think that when I did the math if I could find a .015 shim gasket, it would bring my CR to 10.5:1.
I actually think with my duration (input welcomed here) I could run at 10.7:1 (aluminum heads).

Also, with my DCR being quite a bit below 8 with the "advertised" specs that my engine is not really seeing, do I possibly stand alot to gain from increasing compression, moreso than usual?

The car idles at 1000 in drive, 1300 in park, and has a 2600-6500 powerband.

Thanks..

matt

Is the .016/.018 lash the factory spec? If not, what is it? You can calculate the tappet lift point that the seat duration should be measured at with this formula "lash / rocker_ratio + .004". However without putting a degree wheel on there is no way know what this duration is, unless you have the designers specs. Going from 1.5 to 1.6 rockers on a SB increases the .050" dur by about 5º. I ran some computer simulations and got an 8º increase at .200" and 16º at .300". These may or may not be all that accurate. Seat time and DCR are uneffected by this.

I don't know what the change would be in your case. I suspect that a .05 ratio increase is not going to make much of a noticable difference. (The change from 1.5 to 1.6 is a bigger percentage change than 1.7 to 1.75.) Maybe a couple degrees at .050". The biggest advantage to aftermarket rockers is that they are usually more accurate that stock rockers.

Pat, the .016/.018 was on my cam card from comp cams, and that is how the cam guy told me to lash it. I called and asked if I could set it at something like .020/.020 or .020/.022 and he said it would be ok, but he does not like to go any further than .004 off of what the cam card says. He was also very anal about measuring rocker arm ratio, mentioning it to me a number of times. These are crower stainless steel roller rockers.
Using that formula, if I did it right, I got .01314, so .013 I guess for degrees of signigance sake. I wonder if comp cams could furnish this info upon request.
I am expecting a small mph gain because I think I have reduced my converter slip by about 3 to 3 1/3 %, but I think a compression boost that would still let it run on 93 octane no sweat would wake the engine up.

Matt

MadMarv,

Without getting into splitting hairs too fine , let's see what we can guess.....
Comp rates those cam lobes at .015", and with your tighter lash, you want to know what has changed. Your .013" is about .002" lower than their advertised .015", and the cam's velocity is about .002"/° around that lift, so you've got about 1 cam degree , or 2 crank degrees per side, added to their opening/closing numbers.
A good guess is that your cam is a 291/295 at .013".....

UDHarold

There you go, direct from the master's mouth (or keyboard, in this case smile.gif ).

If you want to keep the same duration with the higher ratio rockers, use this formula "(tappet_lift - .004) x rocker_ratio" to get the lash to use.

Thanks to your question, I have rethought my previous statement about seat timings being unaffected by rocker ratios. It appears I was mistaken. While it is true of hydraulic cams, lash on a solid causes the ratio to have an effect on seat duration. Since RR is a factor in calculating seat duration any change will affect the duration. To use 1.5 and 1.6 rockers again: given a lash of .024" the measuring tappet lift is .020" with 1.5 rockers. With 1.6 rockers the measuring lift is .019". Consequently, the duration is a bit longer with the higher ratio rockers. Again, without knowing the lift rate at this point on the cam's ramps, knowing what the duration increase is is unknowable without a degree wheel or a Cam Doctor type measuring device.

I gotta ask!!

If a solid cam is spec'ed 280 degrees advertised duration SAE, .006 degrees.

What is it's REAL "advertised" duration at a .030" lash???

Is it still 280 degrees OR is it quite a bit less b/c it is now up on the ramps higher??

pdq67

Sorry, NOT, SAE, .006 degrees, but SAE, .006"!!

pdq67

Thanks Harold and Pat.

I have my converter in now, and now I have the shift points somewhere near where I want them, I am going to try to make one more track appearance before I move.
If things still don't pan out, I am going to try lashing the cam at .020/.022 and see if the "smaller looking" to the engine helps out at all.
If not, and the seat duration is really like 291/295 @ .016/.018, I would venture to guess I could run something like 10.7:1 on 93 w/o trouble..
I felt from day one that this cam was too big, I was trying to sway the cam installer away from the cam I have now to Harold's 287/295 256/264, but it had like .670 or .680 lift, and I suspect the problem was the lift would be too extreme with my 1.75 RAR rockers for a mostly street driven vehicle..
Not sure though.

Matt

pdq67,

The way I understand the SAE checking method is this:
(ramp height X rocker ratio)-.006"

With a typical hydraulic cam in a SBC, ramp height is .004", X 1.5:1 Rocker Ratio, = .006" valve lift height, - .006", = 0 ZERO! This means that the .004" ramp height duration IS the correct valve lift duration.
Going to solid lifters, and the same equations:
My popular 276/284F and 288/296F use a ramp height of .01734", X 1.5:1 Rocker Ratio, = .02601" valve lash/valve lift height, - .006" SAE deflection height, = .0201" advertised cam duration. Where do I claim I check duration on those(and all SBC .026" valve lash cams)? That's right, at .020" cam lift.
This should work for all cams, depending on how other companies rate theirs. I use the same method, within a few 10/thousands, on all my cams since 1977.

UDHarold

Heck Matt, I thought the roller version of the 276/84 cam would have been a near perfect match to your combo. Curious to see what gives if you open it up some.

Harold,

In other words you are saying that starting at a point that is .006" up the ramp, they add .020" in height to the ramp to account for lash being .030", (for 1.5 ratio SB rockers lashed at .030"), at the valve but only .020" at the pushrod and at that point, that is the advertised duration number.

Right??

Or am I gonna have to measure and degree a cam to see what it is.

This stuff has flat got me CONFUSED!!!!!!!

BTW, my old copy of CC's cat. listed their HE, Magnum and race lobes measured at .015" and .020" and now their new e-cat. I just pulled up lists everything at .006".. So go figure??

pdq67

MY BAD!! STRIKE MY BTW above b/c I was looking at their hy- cams...

NOT their solid cams....

Sorry...

pdq67

Well, taking into account that the advertised duration of my cam is at 0.020" and the intake lash is 0.020, then it's safe to say that the advertised duration is the actual duration to be expected. Now since I'm running 1.75 instead of 1.70, I could expect another 2 degrees or so total duration at the lash point, correct?
Now the exhaust has 3 degrees more rated duration, but the lash is 0.026", so is it safe to assume that the actal duration at lash point will be a few degress less?

The cam is a solid roller Comp, RT series. Adv duration ( @ 0.020") is 278 intake, 281 exhaust, at 0.050" its 248 I, 250 E. I've already figured out net lift, just wanted to see if I'm on the program with the duration.

John

Originally posted by TD509EFI:
Well, taking into account that the advertised duration of my cam is at 0.020" and the intake lash is 0.020, then it's safe to say that the advertised duration is the actual duration to be expected. Now since I'm running 1.75 instead of 1.70, I could expect another 2 degrees or so total duration at the lash point, correct?
Now the exhaust has 3 degrees more rated duration, but the lash is 0.026", so is it safe to assume that the actal duration at lash point will be a few degress less?

The cam is a solid roller Comp, RT series. Adv duration ( @ 0.020") is 278 intake, 281 exhaust, at 0.050" its 248 I, 250 E. I've already figured out net lift, just wanted to see if I'm on the program with the duration.

John =
John,
Not exactly,,, I will try to get this right ;)
If it ain't Harold will come fix it :D

With your 1.75 rockers:
At .020 valve lash you are at .0174 lifter rise with the theoretical .006" deflection number (.0114 + .006)

Your advertised duration is at .020 LIFTER RISE
So, you would have to lash the intake at .029,
(.020 x 1.75 - .006 for theoretical deflections)to see the actual .020 "advertised" lifter rise duration numbers.
Your exhaust is much closer to advertised numbers at .026 but again would need to be at.029 to be correct.

Now,this is where things get real confusing when reading lobe lists.
If the lobe was designed to be used in a small block with a 1.5 ratio & you transpose it to a rat at 1.7 you will need to adjust the lash about 9% looser to end up with the same seat duration at the valve but,,, the engine will now see a more aggressive lobe as the .050, .100, .200 numbers etc are all going to be "larger" at the valve because of the increase in rocker ratio.
This can be good & bad. Good if the valve train can be controlled & bad if not.
You are also altering the effective overlap period slightly.

This is why we are so fortunate have people like Harold to ask questions of regarding cams,,,
It ain't near as simple as it first looks ;)

LOL, You've gave me something to think about, and it's 10:10pm...now I'll never sleep the night through!

I hadn't realized that we were taking into account theoretical deflection in the valve train, but it makes sense only if you know how it was arrived at. For instance, BBC with 3/8" 0.08 wall 1010 steel with stamped rockers or 7/16 0.120" wall 4130 tempered steel with shaft rockers?

I can understand (now) the SBC to BBC transposing with the 1.7 rockers, but I have to admit, I had to scratch my head a bit until the light bulb flickered on.

Thanks for the info Mike, food for thought smile.gif
I'll try to have a good nights sleep. graemlins/clonk.gif

John