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Pro-touring72 said:
Found this while seaching some info on rods..


• Reciprocating weight accelerates to a stop then changes direction. Rotating weight moves in a single direction. Loosely speaking, removing one ounce of reciprocating weight is more beneficial to longevity and efficiency than removing a pound of rotating weight. In other words, don’t waste time grinding the crank counterweights. Concentrate first on the rods, pistons, and pins.
This is why lightweight valvetrain parts are important. Not just the valves but the lifters and pushrods.
 

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Discussion Starter #42 (Edited)
NovaResource said:
This is why lightweight valvetrain parts are important. Not just the valves but the lifters and pushrods.
That sounds logical and common sense but consider this:
The camshaft rotates at 50% of crank speed so even at a whopping 10,000 rpm the cam lifters and pushrods are moving at only 5,000 rpm. A pushrod weighs much less than a connecting rod and reciprocates slower.
Lightening up the pushrods is counter productive if it reduces stiffness. Large diameter steel pushrods are better than lightweight pushrods. Ever wonder why you see aluminum rods but you'll never see aluminum pushrods?

Lifters are another curious thing. Which will rev higher? a lightweight solid lifter or a roller lifter. Again it's not the weight that allows a roller lifter to track a lobe at high rpm. It's it's ability to follow the contour. Remember that the cam is going half the crank speed.

The lifters and pushrods are essentially motion transfer devices. They are trapped between the cam and the rocker shaft.

I worked on a valvetrain instrumentation program for about a year. I'm the resident strain gauge expert and I was asked if it was possible to measure valve train forces at very high rpm. They said in all the SAE papers on the subject, no one had had ever succeed in collecting data higher than 4,500 rpm. I thought it might be possble so I engineered some innovative measurement techniques. I instrumented every thing in the valve train. We ran Spintron tests and collected data. It was very interesting and some of it defies common sense until you explore the physics.
I can't reveal much of what I know because of confidentiality and I certainly can't post pictures, but the bottom line is don't skimp on pushrod stiffness to save a few grams.


(I merged all these questions together into one thread since they are all related.)
 

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I agree with everything you said Paul and that was the point I was trying to make in the other thread about what differences there are between pushrods and overhead cams:
http://stevesnovasite.com/forums/showthread.php?t=27702
I never said making the lightest pushrods were most important, I just said the weight was one of the cons to a pushrod valvetrain system. Overhead cams have less parts and reciprocating weight. That's one of their pluses. Here's my quote from that thread:
The main problem with pushrods is weight and strength. The equipment used to open the valves with a cam in the center of the block requires lifters, pushrods and rocker arms, all that have weight. Plus, they have to move in an up and down motion, constantly changing direction. Controling that motion requires stiff springs. To work against stiff springs, the pushrods have to be strong (where the weight comes from) so they don't bend. If they bend, the true lift and duration of the cam is not always reproduced at the valve.

On the plus side, it's not a complicated system. Overhead cams require very complicated control systems (long chains or belts) but have fewer parts controling the valve motion.
I still stand buy my statement that weight saving in the valvetrain will yield benefits as long as there is no loss of strength. Once you make the parts so light that they can hold their shape you are losing power.
 

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Discussion Starter #44 (Edited)
My point is the weight of the pushrod is not as important as you think.

For example:
A car handles better if the unsprung weight is very low and the chassis is stiff. Removing stiffness from the frame and rollcage saves weight but sacrifices handling.
By the same token a pushrod engine revs better if the valves are light and the pushrod is stiff. A higher revving engine makes more power than one with a lightweight but unstable valvetrain.

Here's a quote from
Inside Racing Technology said:
Instead of the jewel-like intake and exhaust valves in the CART heads, NASCAR valves are twice the diameter as the CART valves and are more than twice the weight. This weight difference is critical because valve train mass is the single most important factor in getting the engine to live at elevated rpm. NASCAR does allow titanium valves, retainers, and keepers but instead of four small valves per cylinder controlled by a direct-acting camshaft in the CART engine, the NASCAR "motor" has to use the same configuration as in the Ford vehicles sold for street use-two valves per cylinder operated by rocker arms and long pushrods. This engine is very different from the purpose-designed race engines in the surrounding work stalls. This is the engine Cosworth has begun to develop for the NASCAR Busch Grand National series.
Notice that in this statement all the titanium is on the valve side?The long pushrods are made from steel because of flex, certainly not because it's light weight. Shortening the pushrod reduces flex but you have to move the cam up which effects geometry. Adding diameter to a long pushrod adds weight but increases stiffness. The stiffness benefit outweighs (figuratively) the weight gain. Basically removing weight from the valve side and adding it to the pushrod is a good thing.

Also:
SOHC engines have more parts than a typical pushrod engine even more still if it's DOHC with 4 valves.

Despite what magazines and the internet experts perpetuate, having pushrods doesn't make an engine low tech.
Didn't Penske run a Mercedes pushrod engine that dominated Indy so well that it was later legislated out of the running?

I've discussed these points with my friends in the magazine world and they admit that some GM insiders claim that Ford did a PR number on them to try and discredit the pushrod design and tout the Mod motor. They had a lot riding on it and they wanted it to succeed. GM stuck with pushrods because the data showed it to be superior in cost and performance. The magazine writer's lapped up Ford's "pushrods are ancient" propaganda. GM still gets grief over it even though a 3.8 Series III pushrod V-6 gets 30 mpg and makes over 200 hp.

Ford 2 valve OHC V-8's trail GM 2 valve push rod V-8's in both fuel economy and normally aspirated engine power. So once again advertising has influenced popular opinion despite the facts to the contrary.

To be fair, I might add that Ford has made gains with the 3 valve heads on the mod motors. This narrowed the gap with GM but they had to license the technology from Feuling. The additional parts added to the excessive cost and complexity of the engine. Costs more and more expensive to fix. To me this is not value added.

Just because an opinion is popular doesn't necessarily make it the truth, but that's only my opinion!
 

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Very interesting Paul. What is your take on I beam vs. H beam rods? My engine builder prefers I beam rods (though I don't know why). He builds primarily race engines.
 

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Paul Wright said:
My point is the weight of the pushrod is not as important as you think.

For example:
A car handles better if the unsprung weight is very low and the chassis is stiff. Removing stiffness from the frame and rollcage saves weight but sacrifices handling.
By the same token a pushrod engine revs better if the valves are light and the pushrod is stiff. A higher revving engine makes more power than one with a lightweight but unstable valvetrain.
Paul, you are only reading half of what I wrote:
I still stand buy my statement that weight saving in the valvetrain will yield benefits as long as there is no loss of strength. Once you make the parts so light that they can hold their shape you are losing power.
 
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