How are you guys doing static compression math with alu rods , are using total clearance numbers or do you add rod stretch or elongation from heat or what? The extra .020 clearance is a 1/2 compression point on my engine

I would like to hear too as I'm doing mine with no stretch in math?

Umm....technically...the rod doesn't stretch when it's "Static"!!!
I never worried about all that "stretch" or growth" stuff with my BME aluminum rods, until I had it apart after 350 passes and saw shiney spots on my heads, and the pistons...figured .054 clearance was enuff, and it almost was...

With my GRP rods I run .035" clearance. You shouldn't have stretch during compression stroke. But, you probably do when the valves are open.

Leon

Wow .035 is tight for a alu rod , you must have a light piston and a short stroke, I run my BMEs @ .058 with a 430 gram piston and a 4 inch stroke

I believe the original question was "should rod stretch be calculated as additional deck height when figuring out the mechanical compression ratio"?
Doug

Ummm...technically Doug,
Rod stretch would be calculated as less deck height...right? My head is starting to hurt.

The BME spec for piston to head is .060 , around .025 more than a steel rod engine, when doin engine math if you use the .060 figure you are shorting the compression ratio quite a bit, So what I was asking was what was a good figure to use for engine math that was closer to actual ratio after heating the rod ,stretch or what ever you wan to call it , since Alu grows, swelles stretches or what ever more than steel if you had a real number you can figure the ratio closer, Take Bills engine , he had piston to head interference at .054 so we now know in a running engine with a alu rod the .054 clearance is no longer there , thats why you got the shiny spots so if most engine math is done using .035 or .040 piston to head math a Alu rod engine should figure a lower than .060 number, now my head hurts also

for te sake of figuring your actual static compression ratio,(on a warm engine) just use .015 shorter deck height in your equation.

Thanks Dram, I was hoping some of the guys with more experience would kick in ,I cant be the only crazy one thinkin of crap like this .015 is a good number I was going to be greedy and use .019 but .015 is good

Aluminum rods grow during shut down when in gear. That is not stretch. That is why many racers shut down in neutral and avoid piston to head or valve contact.

Grow is a good word too They also grow a lot at high revs pulling down form tdc on the intake stroke