Calculating Piston Speed?

vristang · Nov 2, 2005

I am looking for the equation to calculate piston speed at a given crank angle and rpm.

There are average speed equations out there, but I need to know the calculus equation.

I am doing this as part of the cam selection process.

I appreciate the help, as google has let me down on this one.
:Word:
jason

srothfuss · Nov 2, 2005

Hum... I'll need to dig up my Vehicle Dynamics books and see if it's in there. Only problem is that it's in a POD and I can't get to it until Monday night.

I guess I could work the formula out by hand.... It'll be a long night.

vristang · Nov 3, 2005

srothfuss said:
Hum... I'll need to dig up my Vehicle Dynamics books and see if it's in there. Only problem is that it's in a POD and I can't get to it until Monday night.

I guess I could work the formula out by hand.... It'll be a long night.

I would appreciate it.

I have been through all my textbooks with no luck.

maybe someone else can chime in and save you some effort?

ninety15.0 · Nov 3, 2005

try a simple dynamics or mechanics book. Your going to be using relative motion analysis...rotary of course. Try googling angular velocty/acceleration. your looking at a lot of variables man, i wouldnt expect super accurate answers without a specific formula.

jrichker · Nov 3, 2005

See http://users.erols.com/srweiss/ for a calculator. If you can't find the free version, email me and I will send you the zip file for the one I have. It has a function for piston speed, but I haven't used it.

tjm73 · Nov 3, 2005

Piston speed in feet per minute = (stroke in inches X rpm) / 6

ninety15.0 · Nov 3, 2005

tjm73 said:
Piston speed in feet per minute = (stroke in inches X rpm) / 6

that seems too simple...but ok

ForceFed70 · Nov 3, 2005

tjm73 said:
Piston speed in feet per minute = (stroke in inches X rpm) / 6

This will only give average piston speed...even then it doesn't look right.

vristang · Nov 3, 2005

tjm73 said:
Piston speed in feet per minute = (stroke in inches X rpm) / 6

I do appreciate this, but this is for either max velocity or avg velocity (I don't remember which). This is very useful for determining if your piston/crank/rod combo can handle a given rpm, as there are general guidelines published in some of the better books.

The formula I am looking for will tell me what the piston speed is at a given crank angle and rpm. Once you know the crank angle that max speed occurs at, you can decide where to open your valves. At max piston speed is where you want to have max flow, more or less. In this way you will be able to determine at what rpm you want the cam to provide the most efficient power. Of course intake and head flow numbers will play an important role, but it seems that piston velocity is where the cam selection process should begin.

My motivation is that I am getting tired of the lack of quantification that exists in cam selection. I do plan on posting what I find for the benefit of all.

Thanks again for the resoponses
:Word:

vristang · Nov 3, 2005

jrichker said:
See http://users.erols.com/srweiss/ for a calculator. If you can't find the free version, email me and I will send you the zip file for the one I have. It has a function for piston speed, but I haven't used it.

That site is crazy cool!

It will take me a day to go through it all. I will post if they have what I need.

Thanks

tjm73 · Nov 3, 2005

70_Nitrous_Eater said:
This will only give average piston speed...even then it doesn't look right.

It gives you the average piston speed at any given RPM point. It's from the Automotive Math Handbook.

Perhaps this will help with momentary piston speed questions. http://victorylibrary.com/mopar/cam-tech-c.htm

vristang · Nov 4, 2005

tjm73 said:
It gives you the average piston speed at any given RPM point. It's from the Automotive Math Handbook.

Perhaps this will help with momentary piston speed questions. http://victorylibrary.com/mopar/cam-tech-c.htm

You guys are bringing up some great sites. Tons of info there. But still not exactly what I am looking for.

I did find this guys site.

http://www.wfu.edu/~rollins/piston/

I used his formulas (slightly tweaked) with my engine specs, and came up with 6,300 f/m. That is a little different than the mean piston speed of 4,000 f/m that I get from the formula in The Automotive Math Handbook.

Anyway, I think I ended up with what I needed, but I would like to verify this formula is correct by finding a second source. If anyone finds one please let me know.
Thanks again for all the great info.
jason

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