[jamaca85]

is it true that when you run a hyd cam you lose abt .40 on your lift due to the lifters. i was told running a hyd cam with a 500 lift will be more like a 460 lift cam due to the lifters.also you lose 6-10 degrees of duration too .... is this true. if its true then wouldnt it be better to run 1.65 rockers to keep it a 500 lift cam. any thoughts?

[steve25]

Yes! Whatever amount of preload you add to any type of Hydro lifter you loose in lift , other then Rhoades type ( and others ) lifters that are design to pump up from being bled down at low cycling rates.

[slowbird]

Originally Posted by steve25

Yes! Whatever amount of preload you add to any type of Hydro lifter you loose in lift , other then Rhoades type ( and others ) lifters that are design to pump up from being bled down at low cycling rates.

Please explain. Im not seeing how preload has anything to do with it.

[65CatCoupe]

I believe the biggest loss is in the rocker arms; if factory they can lose .040" due to lower than advertised lift multiplication (say 1.42 vs 1.5) plus flex. Also lose a bit on pushrod and lifter compression but I thinks that's smaller say .010".

[steve25]

Are you not changing the effective lenght of the push rod when adding preload?

[PAUL K]

Originally Posted by steve25

Are you not changing the effective lenght of the push rod when adding preload?

Regardless of pre-load on the lifter, everything North of the lifter is going to be moved by the height of the cam lobe. A mechanical cam will loose lift, due to lash because the lobe has to make up the missing space before it starts to move the valvetrain.

[PAUL K]

Originally Posted by jamaca85

is it true that when you run a hyd cam you lose abt .40 on your lift due to the lifters. i was told running a hyd cam with a 500 lift will be more like a 460 lift cam due to the lifters.also you lose 6-10 degrees of duration too .... is this true. if its true then wouldnt it be better to run 1.65 rockers to keep it a 500 lift cam. any thoughts?

Without splitting hairs the information you were given is not accurate.

[pastry_chef]

I agree with Paul K.

Higher rocker ratio is excellent.
A bit more or less lift over the nose of the cam will make little difference.
More important is the area / valve duration at .100, .200, .300 etc. This is increased with higher rocker ratio.

[Steve C.]

At one time there was information from a very good tech article that used to be on Comp's website that approached a similar subject. In regards to a hydraulic lifter It mentioned like a shock absorber, the lifter "gives" just a little as you load it under running conditions.

From Spintron research results, the engineers at Comp Cams actually see running valve durations decrease as rpm increases due to this increased inertial loading. The faster the cam spins, the harder it hits the "shock absorber", and the lifter loses some valuable duration as a result. The article showed results from the testing that indicated a "real world" dimension for a running engine with hydraulic lifter tappets. The actual loss in duration on a running engine could be quite a bit as suggested in this comment from another similar article:

"As rpm increase on an overhead-valve engine, this can decrease running valve duration by as much as 10 degrees."


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[pastry_chef]

https://www.youtube.com/watch?v=8UaXhfbAMF0

I'm a fan of solids, either flat or roller.

[Steve C.]

All hydraulic lifters can absorb a small portion of the cam’s lift profile in running, through fluid bleeding past the lifter’s plunger piston during the lift cycle. In stock or mild street applications, absorption is likely negligible. Highly aggressive cam profiles and spring loads in a radical street or racing application can strain the hydraulic lifter’s mechanism to the point where some performance potential is lost through absorption. Lifters with tight internal clearances and valving most accurately follow the cam’s profile, and are termed stiff.

http://www.hotrod.com/articles/mopp-...aulic-lifters/

In a production lifter, high rpm spring pressure plus inertia loading depresses the piston in the lifter resulting in less valve lift. Depending on the amount of preload and the rocker ratio this can result in a valve lift loss of .050-.075-inch, depriving the engine of its design spec airflow window and making the cam perform poorly.

http://hotrodenginetech.com/why-john...n-your-engine/



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[slowbird]

So to recap, some amount of lift/duration is lost in running conditions and increases as rpm increases. And zero lift/duration is lost due to preload.

[Steve C.]

Of interest...

Question: Installing high-ratio rockers obviously changes (increases) valve lift. But does it also change the cam’s effective duration. If so, how much (is there a simple formula or equation for this as there is for figuring the total valve lift)?
Answer: There is no real formula, but you would be safe to assume that you increase the duration about two degrees for each point of rocker ratio. However, with the increase in area, it may act as if the cam grows four degrees in duration per point of rocker.
JOE MONDELLO RACING ENGINES

( other articles suggest similar but amounts can vary )

.

[Blued and Painted]

Working on a very cold morning, I went out to cold start a customer vehicle. The vehicle started, went to fast idle, then died. When I attempted to restart, the engine spun over fast with no compression. As a test I turned the engine backwards several revolutions to suck oil out of the hydraulic lifters. Compression returned and the vehicle started.
So, stock lifters in good condition don't leak much, and don't run 20-50 in the winter.

[Steve C.]

Subject here relates to comments made in post #4...

Rocker-Arm Comparo

Curiosity led us to bench-test 20 different 3⁄8-inch stud-mount rocker arms on a small-block Chevy. All of them were advertised as having a 1.6:1 ratio—and exactly one of them multiplied the lobe lift of the cam to the valve accordingly.

http://www.hotrod.com/articles/hrdp-...er-arm-comparo


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[slowbird]

Originally Posted by Steve C.

Of interest...

Question: Installing high-ratio rockers obviously changes (increases) valve lift. But does it also change the cam’s effective duration. If so, how much (is there a simple formula or equation for this as there is for figuring the total valve lift)?
Answer: There is no real formula, but you would be safe to assume that you increase the duration about two degrees for each point of rocker ratio. However, with the increase in area, it may act as if the cam grows four degrees in duration per point of rocker.
JOE MONDELLO RACING ENGINES

( other articles suggest similar but amounts can vary )

.

Fwiw Seat duration doesn't change with rocker ratio but every point after that does.

[ponchjoe]

I would assume any stack up tolerances would be considered by the cam designer. The advertisers lift would include these.

[jamaca85]

Originally Posted by PAUL K

Without splitting hairs the information you were given is not accurate.

thanks paul...

[Steve C.]

"Is it true that when you run a hyd cam you lose abt .40 on your lift due to the lifters. '

If not .40 then how much ?


.

[PAUL K]

Originally Posted by jamaca85

thanks paul...

Glad I could help Jaime. Good luck with your project.