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#1
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Piston Speed?
My new MPN states that a race prepped motor should have a max piston speed of approximately 4000fpm. This means that a 340 has a max rpm of 7250. A 360 has a max rpm of 6700. One of those engines with a 4" stroke has a max rpm of 6000.
My question is, why is this a limitation? Is it do to the oil film breaking down on the cylinder walls? Based on what I have read, I would have said that these are good limits for RPM of these motors but I really don't know why. What are the deciding factors that you use when determining how fast to spin your motors? |
#2
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Mass, momentum and loading
The computer programs that I use for dyno simulation have piston speed listed. They also give notice to the limitations or requirements of different speed ranges. Typically it has to do with the type of components used and their associated strength to weight ratios.
For instance, a 340 that is built with lightweight pistons, pins and high strength rods can run over the 7,250 rpm figure you quoted. The same can be said for the 360. This is because we are reducing the stresses on the reciprocating components while increasing their strength. Each component in an engine has an rpm limit/stress limit. Exceeding that limit causes catastrophic damage to one or more components. One of the best deciding factors is where the power band is. You as an engine designer/builder must determine where in the rpm band the engine is going to produce the best/usable power. Then you must select the proper components that enable you to run that rpm and survive. Your budget may provide for a set of W-9 heads, but if you don't have the money set aside for the rest of what is required to get to the 8,500+ rpm required to get the most out of the heads, then they are a bad purchase. Bearing speed is/can be a limiting factor on any engine. Generally, it is the main bearings that are the cause. Valvetrain dynamics often are the deciding factor on rpm limit. It just depends on the rules, money and technology that is available to you. |
#3
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The 4000FPM quoted is probably a fairly accurate limit for race prepped stock components. That is a good limit for a stock type oiling system as well.
On the other end of the spectrum, serious race engines regularly run piston speeds of 5200FPM for sustained periods of time. For example, those piston speeds would be typical of Nascar engines. To get that extra 1200FPM reliably requires many thousands of dollars in components. 340king is absolutely right. Determine what your requirements(HP, Torque, RPM range) for your racing are first, then determine what you must do(and spend) to have a reliable engine under those conditions before you get the heads, cams, etc. The thing you don't want to do is buy all the go-fast parts, bolt it on a stock short block and expect it to live at 8000 RPM. It won't do it, it will lay down on you every time. And after it blows it will probably destroy all the go-fast parts you bought. |
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