Originally posted by F4UDOA:
JimDandy,
You mentioned the piston speed being 2800 FPM if the RPM was 2800. How is this possible? If the stroke is 6" and the RPM's are 2800 shouldn't I multiply those two numbers??
I am trying to go by your previous post to figure out what the calculation for piston speed is.
BTW, from the pilots manual of the F4U-1 PW-R2800-8W B series.
Combat RPM 2700
Military RPM 2700
Normal (max continuous) RPM 2550
Max cruise RPM 2150
Thanks
F4UDOA
Here's what it looks like:
(2,800 X 6)/6 = 2800. The 6's cancel out because you multiply and divide by 6.
Looking at the 109 again:
(2,700 X 6.3)/6 = 2835.
Just a coincidence the R2800 has a 6in stroke so the rpm will always be the piston speed in ft/min. Notice that the max continuous is kept almost right at 2500 ft/min for the R2800. It was a common rule of thumb in the engineering world at the time to limit the piston speed to 2500 ft/min. It was the same for automobile engines too. It was a limitation base mostly on the materials they were using for the connecting rod and crank which was generally cast iron. Cast iron is brittle and wont stand for a lot of cyclic loading. Modern castings are much more flexible and will handle 3500 ft/min easy. There were other things as I have mentioned that were also limiting factors to the rpm but the connecting rod and crank were the biggest. The figure of 2500ft/min and 3500ft/min are for continuous use. The casting technics and design and type of cast iron used are better today. For a forged steel crank and rods your piston speeds can be as high (continuous) as 4,000 ft/min. A really all out racing engines can hit piston speeds as high as 6,000 ft/min for a few seconds. Even those R2800 could have run piston speeds of 3500 ft/min for a few seconds I imagine.
Here's a comparison to a car engine:
Chevy 350cid - Stroke = 3.48 in
3.48 X 4,800/6 = 2,784 ft/min. 4,800 rpm was a conservative redline that was common on most Chevy's that reflected the peek horse power and hydraulic valves more than the engines ability to rev. The Z28 350 had a redline of 5,200 rpm which gives:
3.48 X 5,200/6 = 3016 ft/min. That's is for continuos use. That means you can get in your 74' Z28 and run it at 5,200 rpm all day long. I can vouch for a least 1 hour at 4,800-5,500 rpm in a stock 1979 Z28. The valves did start to float at 5,500 rpm. So I think those figures are correct. Of course you better have fresh oil and coolant, no leaks, etc. before you can do that. That's still conservative. Work backward from 3,500 ft/min:
3,500 = 3.48 X RPM/6, RPM = 6,034 rpm. That is the actual continuos use rpm of a 350 Chevy. I don't think your average factory engine with hydraulic lifters would do that all day long. I think with HIGH quality control and solid valve lifters the design would handle it. The limiting factor as I said above can reflect other things like the lifters. Hydraulic lifters tend to pump up and float the valves at high rpm. Now for an all out road racing 350 for a 24 hour race with solid or roller lifters:
4,000 = 3.48 X RPM/6, RPM = 6,890 rpm. That's not uncommon at all for a long distance racing engine to run that all day long. As a matter of fact that's a bit conservative.
An ALL OUT drag engine with roller lifters and aluminum rods:
6,000 = 3.48 X RPM/6 = 10,344 rpm! And you guessed it, that is a bit conservative too. That's not for continuous use though. That's a sprint for 5 seconds down the strip.
PS If you didn't already know what valve float is it's when the lifter that rides on the cam is thrown off the face of the cam holding the valve open into the combustion cycle causing back fire and even valve to piston contact. At high rpm the inertia of the valve train will throw the valve lifter off the cam. Stock automotive engines generally have hydraulic valve lifters that cause the effect to occur at lower rpm's. Hydraulic valve lifters are designed to take up the clearance in the valve train using the engines oil pressure. As the rpm's increase the hydraulic lifters over fill with oil and lift the valve off the seat. Racing engines run solid lifters that require periodic adjustment. Some stock engine are this way. Usually they are in older cars. Anyone that has had a old VW can tell you about adjusting valves.
Any way, high performance engines run solid lifters or solid/roller tipped lifters normally. They also run very high valve spring pressure to keep the lifter from flying off the cam at high rpm.
[This message has been edited by Jimdandy (edited 02-08-2001).]
