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Engine RPM hype

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Old 06-17-2004, 07:24 PM
  #1  
Alexey
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Default Engine RPM hype

I have heard many times people saying things like "my car/truck is faster because my engine top RPM is higher".
So I decided to check if all this engine RPM hype is based on facts by calculating what is engine RPM of my Savage 21 with .21 engine at top speed (I assumed top speed to be 30MPH).
Below is a diagram of the drive train:



So the final gear ratios are:
1st gear: 13.1 : 1
2nd gear: 9.1 : 1
Meaning that engine needs to turn 13.1 turns (in 1st gear) in order for the wheels to turn once.

Stock wheels are 6 inch in diameter. So their Circumference = 18.85 inch
1 mile = 63360 inch.
So my truck would need to do 30 * 63360 / 18.85 = 100838.2 revolutions to go 30 miles.
In 1 minute it would do 100838.2 / 60 = 1680.63 revolutions.

So 1st gear 1680.63 * 13.1 = 22016.3 RPM
2nd gear 1680.63 * 9.1 = 15293.8 RPM

As you can see engine never reaches its top RPM even if the truck was to go 30MPH in 1st gear.

A logical conclusion would be to say that the .21 engine is to weak to be able to reach top RPM but I have read a thread about a twin engine project Savage truck (which has 2 .26 engines) and it only goes about 33 - 35 MPH despite that it can wheelie at 2nd gear.
[link]http://www.savage-central.com/ftopic1093-0-asc-0.html[/link]

So is the engine RPM talk is just sheer uniformed stupidity?
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Old 06-18-2004, 05:26 PM
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Default RE: Engine RPM hype

Seems like nobody agrees with me
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Old 06-18-2004, 06:00 PM
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Default RE: Engine RPM hype

seems like your right.
with that kind of numbers and words,formats, steps in the equation guess your right
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Old 06-19-2004, 06:44 AM
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Default RE: Engine RPM hype

you have to have the truck geared right for it to reach max rpms, most vehicles are not geared this way as you end up with a hugely uneconomical vehicle that suffers from a very short engine life.
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Old 06-19-2004, 08:24 AM
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Default RE: Engine RPM hype

Sorry, I can not respond to your post becouse it contains to many numbers. Everytime I look at it my brain hurts and I get noxious. I am going to have to sit this one out.
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Old 06-19-2004, 09:03 AM
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Default RE: Engine RPM hype

Yea. Same.
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Old 06-19-2004, 11:45 AM
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Default RE: Engine RPM hype

Your final gear ratio is (first gear)27.22:1/(second gear)18.88:1 per http://www.rccaraction.com/rc/articles/hpi_savage.asp
Tires are 6.25 dia per http://www2.towerhobbies.com/cgi-bin...&I=LXFLP2&P=ML
6.25circ*pie=19.635 in. circumfrence
First gear
(35,000 rpm/27.22 final drive ratio)*19.635 in.=25247 in. per min
(25247 in. per min/12 in. per foot)/5280 ft per mile=.39847 mile per min*60 min per hour= 23.9 mph

second gear
(35,000rpm/18.88 final drive ratio)*19.635 in.=36400 in. per min
(36400 in. per min/12 in. per foot)/5280 ft per mile=.57449 mile per min*60 min per hour=34.47 mph

Keep in mind you are also going to have some tire growth at that speed which will increase your circumfrence which will boost your speed. You will probably spin a little higher then 35,000 rpm as well.
J~
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Old 06-19-2004, 12:45 PM
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Default RE: Engine RPM hype

OUch, stop it with all the numbers!
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Old 06-19-2004, 01:51 PM
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Default My mistake

Looks like I got to much carried away with idea and made a mistake with my numbers. I also neglected to double check my calculations before posting original message.

Now that I have redone my calculations I get the right numbers ((first gear)27.22:1; (second gear)18.88:1). I am totally puzzled as to how I managed to come up with the wrong numbers.

My apologies for posting wrong info and big thanks to joules for providing correct info.


Mathematically calculation of final gear ratio is simple ( [] ), once you figure it out of course:
Divide teeth numbers of gears the mesh with each other and multiply for gears that sit on the same shaft to get:

43 / 13 * 29 / 32 * 44 / 18 * 52 / 14 = 27.22 -- 1st gear

43 / 13 * 29 / 32 * 39 / 23 * 52 / 14 = 18.88 -- 2nd gear


Seems like I was wrong in my original assumption that truck engines never reach top RPM.
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Old 06-19-2004, 02:00 PM
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Default RE: Engine RPM hype

nice work on the calcs[sm=thumbup.gif], i would never take the time to do that

RPM creates horsepower

torque is the push
horsepower is the run

in other words torque is what makes you move, and horsepower is what makes you move fast
it all really comes down to what and where your running your motor, can you expect to see where its good or bad....
high RPMs = loose sirfaces
lower RPMs = hard surfaces/mud/grass... or vice versa i can't remember
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