Inertia and Engineering question... - Team Camaro Tech
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post #1 of 10 (permalink) Old Mar 15th, 19, 01:18 PM Thread Starter
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Inertia and Engineering question...

If controlling valve train becomes more of an issue in OHV engines at higher rpm, why not run a higher ratio rocker arm and a lower lift cam? Wouldn't this allow the heaviest part of the valve train from having change direction at such a high speed?

Also, I was trying to find who makes the lightest hydro roller lifters for a gen 1 or 3 with no avail. Any ideas?

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post #2 of 10 (permalink) Old Mar 15th, 19, 01:51 PM
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Re: Inertia and Engineering question...

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If controlling valve train becomes more of an issue in OHV engines at higher rpm, why not run a higher ratio rocker arm and a lower lift cam? Wouldn't this allow the heaviest part of the valve train from having change direction at such a high speed?
Yes that is a viable idea: but the rate of acceleration increases with rocker ratio and the spring's applied force is linear. So you will have increased issues with harmoinics and accelerated wear as parts load and unload.

Weight of valve lifters are usually advertised (example Crower lists their weights ranging from 112 g to 133 g) A similar solid roller lifter weight is 100 g. Check for each on line catalog for various vendors and you can compare bore size (Chevy or Chrysler), and whether their lifter is a tall or standard lifter height (GEN V and Gen VI are tall lifters) to compare the weights (another difference is full needle bearing roller or a solid brass bushing).

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post #3 of 10 (permalink) Old Mar 15th, 19, 01:57 PM
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Re: Inertia and Engineering question...

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(another difference is full needle bearing roller or a solid brass bushing).
Big, which would be the best longest lasting?

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post #4 of 10 (permalink) Old Mar 15th, 19, 02:15 PM
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Re: Inertia and Engineering question...

OP

Google David Vizard, valve train. His articles get very deep about which components and why


Conical springs, Titanium retainers & keepers. High $ stuff but if your building "that kind" of motor...good stuff to read
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post #5 of 10 (permalink) Old Mar 15th, 19, 02:52 PM Thread Starter
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Originally Posted by SoCal805 View Post
OP

Google David Vizard, valve train. His articles get very deep about which components and why


Conical springs, Titanium retainers & keepers. High $ stuff but if your building "that kind" of motor...good stuff to read
Will do. Thank you!

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post #6 of 10 (permalink) Old Mar 15th, 19, 02:55 PM Thread Starter
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Originally Posted by Larger Dave View Post
Quote:
If controlling valve train becomes more of an issue in OHV engines at higher rpm, why not run a higher ratio rocker arm and a lower lift cam? Wouldn't this allow the heaviest part of the valve train from having change direction at such a high speed?
Yes that is a viable idea: but the rate of acceleration increases with rocker ratio and the spring's applied force is linear. So you will have increased issues with harmoinics and accelerated wear as parts load and unload.

Big Dave
Hmm... I need to read into that because I don't understand. I'm not arguing, just ignorant. Nonetheless, thank you!

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post #7 of 10 (permalink) Old Mar 15th, 19, 03:40 PM
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Re: Inertia and Engineering question...

What peak rpm are you designing for?

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post #8 of 10 (permalink) Old Mar 16th, 19, 07:41 AM Thread Starter
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Originally Posted by Vega$69 View Post
What peak rpm are you designing for?
Lol! I'm just trying to learn. I have the 435 roller BP 383 and am leaving it alone since it ain't broke. Now, the 6.0 lq4 in my truck does need some love soon.

Apparently the Cadillac racing engines have hydraulic roller lifters that are good to 8k in their 24 hrcars. I'd imagine those weight less than the ls7 replacements.

The main purpose of my question is that if a normal gear head was building a 6500 rpm Chevy small block of any year, wouldn't using a lower lift cam with higher ratio arms put the same amount of stress on the rockers and springs while moving the push rods and lifters less far and fast? That set up seems like it would necessitate a less aggressive spring set up since the lifters aren't moving as fast.

IDK... Just spit balling ideas.

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post #9 of 10 (permalink) Old Mar 16th, 19, 09:09 AM
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Re: Inertia and Engineering question...

Valve springs are used to control the valves (in a race application you put a "Rev Kit", or a second set of springs on top of the lifters to keep them in their bores). With a higher rocker arm ratio you are accelerating the valve at a faster rate of speed. As such you need a stiffer spring to retain control of the valve.

Where the changing of the rocker ratio enters the world of engineering your valve train is in regard to Newton's second law. As you increase the rocker ratio the rate of acceleration increases proportionally. As the quantity of "a" increases in F=ma the amount of force increases so you must make the parts (such as the rocker arm stud) stronger which is why a high horse SBC head will have BBC rocker studs installed installed in place of the smaller SBC rocker arm studs. Additionally by reducing the mass of the valve (the "m" in F=ma) you reduce the stress on parts which is why titanium valves are popular on the track. But because Titanium work hardens you have to replace the valves every season of racing which is why you can find them abused, and for sale on flea-Bay for nearly the same price as the racer pays for a new set.

The limitation on hydraulic rollers is due to two factors. As mentioned the faster the valves move the stiffer the spring has to be to control valve float. Problem arises with hydraulic lifters collapsing under stiff spring loads. The upper spring load limit on a hydraulic lifter is about 300 inch pounds of pressure. This limits your upper RPM.

The second issue is what you addressed in your first post; the weight of the lifter. A solid roller is just an empty hydraulic lifter. With no inner parts (piddle valve, spring and plunger) it is inherently lighter. The lower the mass of the lifter, the faster the rate of acceleration you can maintain in your valve train. This is why real race cars all have solid lifters. You can add another two grand or more to your engine by going to a solid lifter. The higher the RPM the more power strokes you make per minute which makes more power overall.

Two years ago the NHRA limited RPM in Pro classes to only 10,500 RPM to help the smaller teams compete because at those RPM ranges (they were pushing 12,000 RPM with a 500 cube BBC) the price of titanium valves, stud mounted rockers in 2:1 ratios, 5/8th inch diameter push rods mounted in offset Chrysler diameter roller lifters where driving up the price of racing. But you had to have these parts to bring a car that you wanted to be competitive to the line. (in fact most Pro Stock races are won or lost in the first sixty feet with their chassis set up as most have the same motor making with in 13 to 17 horsepower difference between the fastest and the slowest car.

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post #10 of 10 (permalink) Old Mar 18th, 19, 02:33 AM Thread Starter
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Big Dave,
Interesting. I knew that higher ratios would necessitate stronger arms and studs but didn't see the forces being increased on the push rods diue to the theoretical lower lift cam. I suppose I should have considered the overhead cam engines. Their rocker arms run real low ratios! 😉
-Rob

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