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Question for engine builders


Cameron
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I'm running numbers on a performance build I'm looking into, but I'm not sure what a good maximum piston speed a should be using in my calculations for the max RPM of the bottom end for a built street/strip engine.

I'm looking at using forged rods, pistons, crank, quality fasteners and bearings. No shortcuts here. This is also a turbo build, but I'm not sure if the extra pressure on the bearings from the boost will change the best piston speed to be using.

So far I've been calculating using 3800 or 4000 as my max piston speed, giving me a max RPM for an 80mm stroke at around 7250 and 7620, or for an 84mm stroke around 6900 and 7250, respectively. Any thoughts or opinions are appreciated.

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Mean piston speed

From Wikipedia, the free encyclopedia

The mean piston speed is the average speed of the piston in a reciprocating engine. It is a function of stroke and RPM. There is a factor of 2 in the equation to account for one stroke to occur in 1/2 of a crank revolution (or alternatively: two strokes per one crank revolution) and a '60' to convert seconds from minutes in the RPM term.

MPS = 2 * Stroke * RPM / 60

For example, a piston in an automobile engine which has a stroke of 90 mm will have a mean speed at 3000 rpm of

2 * (90 / 1000) * 3000 / 60 = 9 m/s.

It is a good indicator of the class and performance of an engine relative to its competitors. Honda S2000 has the highest piston speed for any production car (25.2 m/s).

Classes

low speed diesels

~8.5 m/s for marine and electric power generation applications

medium speed diesels

~11 m/s for trains or trucks

high speed diesel

~14 m/s for automobile engines

medium speed petrol

~16 m/s for automobile engines

high speed petrol

~20–25 m/s for sport automobile engines or motorcycles

competition

Some extreme examples are Nascar Sprint Cup Series and Formula one engines with ~25 m/s and Top Fuel engines ~30 m/s

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Thanks, but I'm looking for something more relavent to the M10 specifically. A book from the thirties is going to tell me somewhere around 2500fpm max mean piston speed, simply because that is what was acceptable at that time. I've read "Four Stroke Performance Tuning" By A. Graham Bell cover to cover, a couple times, maybe I need to review.

Also based on cd diesel's numbers, it would keep me right around the 3800-4000 mark.

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I think you are selling the old text a little short, you have the piston speed about nailed but the other issues with supercharging, inlet temp, detonation and the relationship of timing, PFS, CR, to BMEP are quite enlightening and worth the effort in finding a copy to read.

Here is a note from an article on a Rolls Merlin dyno session, Riccardo designed the Rolls Merlin.

"merlin 66--2050 HP @ 25 lb. boost(dry) or, 1.24 hp/cu. in

merlin RM17SM--2200 HP @ 30 lb. boost(dry)

--max. endurance tested @ 36 lb. boost (wet)--2640 HP

they also endurance tested a few for 100 hrs @ 18 lbs boost

Note: all tests done at 3000 RPM, which is 3000 FPM piston speed,

or the same piston speed as a 302 Ford turning 6000 RPM. To put

it into a context we can understand, this would be equivelant to

testing a 302 at about 380 HP @ 6000 RPM for 100 hours. All this

military engine stuff was limited in HP output by detonation, and

few engines were at WOT at maximum HP ratings. Some could get 30-40%

increase in power if suitable fuel was available, simply by opening

the throttle more."

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