How much RPM do I need to stretch my Lunati Pro Mod rod .025?
My pistons are 485 grams.

 

Exactly how do you plan to stretch the rod...........and why would you............lol

I guess what is throwing me is your asking about rpm and mention piston weight...

 

If you're talking about rod BOLT stretch........what the heck does that have to do with piston weight?????

You might need to seek out an engine builder.

 

The rods do flex and stretch in the motor,that is why we have quench but its multiple clerances and parts flexing/streaching that cotribute to it so there is no answer to this question just R&D

 

Thank you.
I am at zero deck, I have .026 head gaskets.
I need to spin 7000 rpm.
Would like to avoid a collision.
lol

 

.026" is a little on the tight side. Gotta have your **** right to run one that tight. Go with high .03x's at a minimum to be safe.

 

Yes, I agree have my **** right. That is why I need to find out the stretch equation. Modulus of elasticity of 4340 is???

 

.026 is too tight

Rule of thumb is .040",you can get away with .035 on aluminum block but piston rock would be a factor,get a .040 gasket and be done with it

 

Don't run it less than .038". There are way too many other factors to look at besides rod material. You have to figure out much your crank deflects, rod expansion, wrist pin deflection, piston deflection, piston rock, etc-etc-etc. You're not going to see it run any better if you run a .030" gasket or a .040" gasket if your compression stays the same. Even then, the little difference in compression equates to a minute gain if any.

 

So, if I'm understanding you correctly, your question is, "Will my pistons hit my valves at 7k RPM?"

The modulus of elasticity of 4340 is roughly the same as any other steel: ~29x10^6psi.

To get an ESTIMATE of your total elongation, it's going to be complicated. You'll need to take into account the cross-section of the rods at every point along its length, then add the stretch of the bolts, any clearance between the piston and rod and pin, then figure the stretch of the piston... There's a good reason why most folks operate based on rule-of-thumb here. Even if you take the time to do a finite element analysis of the entire assembly under max load (7000RPM), it's still just a guess. When you're playing with clearances in the 0.001" range, you don't have much room for error. Making decisions based on the general experience of quality engine builders is your best option.

Happy spinning!
-Mateo

 

.026 gasket is it a copper head gasket?

 

It is a Mr. Gasket steel gasket.(coated)

 

I got my rod stretched earlier. Sorry couldn't help myself, lol.

 

you simply cant calculate it exactly because the rod it self will also stretch due to its mass alone. also it will not stretch uniformly because it does not have a uniform cross section area.

stretch (strain) = stress x modulus of elasticity

stress = force / area

( the problem now is the rod don't have a uniform area cross section area ) anyway ....

force = mass x acceleration

max acceleration piston (ft/s^2) = ( RPM^2*stroke / 2189) *1.333

( stroke in inch )

if you really want to calculate the stretch you have to model the rod in some finite element program like ANSYS .

 

I made a mistake above stretch = rod length * strain

In my opinion a well designed rod will not stretch more than its elastic limit ( for steel that is about strain=0.002) so

stretch = 0.002 x 6.125 = 0.01225 inch

now there is stretch due expansion ( because of heat ) , I will write about it later.

 

You guys are killing me! I was halfway down the page before I realized you're just pull in the sausage