how much lift for 1.6 rockers

[madvette74]

hello guys..
I have a comp cam XE-274H 230/236@0.50 487/490 lift
Ihave 1.5 roller rockers now, how much lift would the 1.6 make? and does it add any duration??

 

[Crimson Thunder]

Divide487 by 1.5 then multiply by 1.6487 / 1.5 =324.66 x 1.6 =519.46

 

[madvette74]

will this add duration as well??

 

[SG4206]

Duration stays the same ; but the opening rate at the valve is a little faster, because the total lift at the valve has been increased with the same cam lobe height.

 

[AdvancedAutoCC]

Don't forget to check piston to valve clearance (you usually only make this mistake once!)

 

[Jack]

Duration stays the same ; but the opening rate at the valve is a little faster, because the total lift at the valve has been increased with the same cam lobe height.

I've seen some cam grinders quotes stating going from 1.5 to 1.6 also increases duration about 1 more degree ... IMHO that would be insignificant in a street car.
JACK:gap

 

[SG4206]

I've seen some cam grinders quotes stating going from 1.5 to 1.6 also increases duration about 1 more degree ... IMHO that would be insignificant in a street car.
JACK:gap

On a hyd cam ; setting the pre-load correctly can not change the duration of the cam. The only way the duration can change ; is if the lifter is bottomed out, and that will wipe out the cam shaft. Think about push rod length, it doesn't change duration, nor does rocker arm ratio ; but an self adjusting lifter like(Rohdes) can change the duration because it changes the timing relationship between the rocker and valve stem.

On a solid lifter , changing the lash will change duration ; because you are changing the rocker to stem contact timing. Change rocker ratio , lash the same ; duration does not change.

Who ever you talked to at a cam grinder company , was incorrect. I can't believe you where told this. It is simple geometry.

 

[Glensgages]

SG4206:
I agree with your thinking, that increasing the rocker-arm ratio won't increase the advertised duration, but won't it, TECHNICALLY, increase the duration above .050" lift?

I'm thinking about checking these, once the cam is in the block, with a dial-indicator and a degree-wheel:
if you'd place the indicator on the lobe itself, and check how many degrees the indicator is off the base-circle by .034" ( .034" x 1.5 = .051" ), this would, theoretically, be 'duration at .050" valve lift' , correct?

But, if you planned-on running 1.6:1 ratio-rockers, I think, to find 'duration at .050" valve lift' , you'd check the duration at .031" off base circle ( .031" x 1.6 = .0496" ).

If my thinking is correct, you might actually add 1 or 2 degrees of duration at .050" valve lift.......
no?
:confused

 

[SG4206]

Glensgages-

You are correct in the way you have presented your senario. I don't think we're on the same page.


Consider this: On a hyd. cam, index an adjusting nut on a rocker arm stud , so it will go back on exactly in the same location. Change rocker ratio and index that nut. You now have increased duration at the valve. I believe this is what you are seeing.

Start the engine if it will ,and you could bend push rods or wipe out the cam lobe ; depending on how wild the cam.

To correct this problem , re-lash lifter pre-load ; which just changed the adjusting nut by raising the rocker and changing the valve timing. Duration is back to original spec.

What are your thoughts?

 

[Glensgages]

What are your thoughts?

I understand what you are saying, but the cam-grinder may-not have been 'technically' wrong when he said the duration could change a degree or two.

Years ago, a buddy changed rocker-ratios on his 454" BBC that ran 10-ohs @ 131, and he knew that I have some engineering back-ground, so he asked me if we could verify that duration at .050" increased.

He ran a true roller cam, and even figuring-in the lash (can't recall the exact number now), the lobe starts 'rising' a very short time earlier with the increased ratio, and closed a very short time later, too.

An easier way to envision this would be to have a SBC cam ground so that BOTH intake and exhaust of the same cylinder were ground on the SAME center-line (both valves opening and closing together), and having the same lobe-profile.

Put a 1.5:1 arm on one stud, and a 1.6 arm on the other, set the valve-lash identical (irregardless of 'where' the rocker-arm finally is on their respective studs), and slowly begin turning the motor over, by-hand.

When the lobe in-contact with the 1.6:1 ratio rocker has rotated enough that the contact-point between the lobe and lifter is at .031" off base-circle (and it's valve would be .050" off the seat), the adjacent valve (with the 1.5 rocker) would only be .047" (.031" x 1.5 = .0465") off the seat, correct?

Now, nudge the crank another 1/2* (30'), and quite possibly, the off-base-circle dimension is now .034", where the 1.5:1 ratio valve would be .050" off the seat.

A similar 1/2* (30') difference may appear when the valves close.

We're ONLY talking a very minute difference here, but it could amount to something worth mentioning, especially if you where running some 'one-off' valve-train pieces, such as Competition Eliminator.

I get 'this' honestly, as my dad was a draftsman/engineer, and he pounded the fact into my head that "when you change anything, no-matter how small, there is a difference!"

He would say:
" Extrapolate on this..... Envision a ratio of 1.5:1 , and a ratio of 3:1!!!!! You just KNOW that, at those ratios, something has changed..."

:crazy

 

[SG4206]

Glensgages -
Oh my god ! Hit me in the head with a two-by ; where is a hole I can crawl into?

I can't believe my brain was so focused in the wrong direction. After all the years of degreeing cams and re-lashing rockers on the dyno ; I flat missed this.

The cam grinder was right , I was wrong. I hope I can get all this egg off my face!

Now subdued----------

 

[Glensgages]

Doug:

there is no "egg" on anybodies face here, and maybe between the 2 of us, we've shed some light on an over-looked subject for other Forum-members.

Degreeing cams gets the camshaft 'in-phase' with the crank-train:
lashing rockers provides 'slack' on the base-circle.
I don't think either of these (less physically CHANGING the valve-lash) effects valve-timing at .050 lift".

Since increasing rocker-arm ratios increases valve-lift, but doesn't change the 'advertised' durtaion, the lifter MUST br traveling at a quicker speed (greater-distance/same amount of time).

At all points, once the lifter is off the base-circle, valve timing (in degrees) is increased AT THAT AMOUNT OF LIFT.

On a relatively 'small' cam such as this, I doubt the change in valve-timing at .050" is noticable, or worth mentioning, but on a big, blunt-nosed, soild-roller cam, it might be significant, 2*, 3* ???
:confused :gap

The next time you slide a cam into a SBC, and have the time, check over-all timing at .031" off the base-circle, and the timing at .034" lift, and please post those findings here;
might 'enlighten' us all.....

Good exchange of ideas!

 

[SG4206]

Madvette - Thanks for giving us the room to expound here. Hope your cam swap works good.

Glensgsges -
Called a couple racing partners last night ; hit them with the big question. They both failed also.

Gave them your example and now educated a couple more old far_s.

Isn't this fun............

 

[Glensgages]

Isn't this fun............

No;
it just shows us how little we REALLY know.....
:L :cry :gap :eek ;help

 

[Jack]

The amount of duration increase from increasing RA ratios will depend a lot on the ramp angles ... how steep the ramps are ... how fast the valves open or close ... and how wide the nose is. Although I struggled a bit with DiffEQ and MVCalc ... I did just fine in Geom, thank you. BTW the "advertized duration" is typically quoted using the SAE method ... that's at 0.006" lift ... so AD is not at a point where lifter has yet to rise ... it's 0.006" above base circle. If you've got really good gaging and even better setup skills, you can measure say a Comp 270 and you'll see a hair's more "total duration" than 270. Comp 270 simply a universal example ... recommend www.camcraftcams.com.
JACK:gap