Dynamic Compression Ratio

We are all told to allow .060 to .080 for aluminium rod stretch when building an engine, but did you consider the increase in compression ratio caused by the rod stretch? This can result in more than a point or more in compression! Most people ask what is your static compression ratio not your dynamic compression. What is your thougths.
Thanks, Joe

 

It's a moving target. Don't forget that the block is expanding also.

I also think stretch is commonly spoken but used incorrectly.
Stretch would indicate that the length has permanently changed.
Expansion would be a better choice.

Dave Koehler
http://www.koehlerinjection.com

 

Good point Dave; are you saying they cancel each other out?
Joe

 

Here is an experiment, put a rod in mom's oven around 200 degree, then measure. You might be surprised.

 

aj481x,
Are you saying expansion due to temperature is the major factor and the dynamic forces on rod are negligible?
Joe

 

Nope, most people are just surprised how little they expand.

 

Lets not forget Aluminium Rods come .010 short from desired rod length spec,. unless specifically requested when ordered to not be .010 for stretch.

 

Part of that spec accounts for piston rock in the bore, and clearance to account for same. Dynamic compression is the net result of a bunch of factors, including the cam/valve timing, geometric clearances, and boost.

 

I quess what I'm asking is does anyone know how much a normal rod stretches, at ~8500 rpm. Engine builders say that you should have between 0.060 and 0.080 head clearance. In a 4.600 bore motor, if the rod increase in length by 0.070 this would result in an increase in compression of 1.63. I would think that much of an increase should be considered in the design of the motor. What do you think?
Thanks, Joe