Maximum Piston Speed

[elcamino73]

> Maybe someone can help.  I'm looking at turbocharging a small block
> Chevrolet.  The problem is which block do I use?  I considered using a 400
> cu. in. small block, but am having second thoughts.  The 400 cu. in. has a
> 3.75" stroke and the 350 has a 3.5" (more or less) stroke.  I had planed
to
> limit RPM to about 6500 RPM.  But now I'm wondering if that number isn't
> very much too high for the 400 small block with a turbo.

Might I ask why you would ever see 6500 RPM pulling a boat?
 I'm currently in the middle of destroking my 400 with a 3.48 crank and 6
inch rods.
And then supercharging the set up.
Minimizing cylinder side loading is what I'm after, The extra bore and air
flow appeals to me as well.


> I've tried to find some material on piston ring sealing, as I think that
is
> the primary problem with high piston speed on US designed engines.  As it
> turns out the maximum piston speed is at the same point that maximum
torque
> is being transferred to the crankshaft.

Can you tell me how you figured that out?
Isn't max average piston speed always at max RPM?

  Since I'm interested in stroker engines that will have 100,000 mile
lifetimes.

 I think that piston speed is a particular problem.  For example, a 3.5"
stroke engine that has been
> stroked to 3.75" has a maximum piston speed increase of 7%.  All other
> things being equal that is a HUGE increase.


> However the moment of inertia for the engine has been increased by the
> square of the ratios (3.75^/3.5^2) which is 13%.  Actually, there should
be
> more metal in the larger crankshaft since there is less overlap from main
> journal to rod journal which exacerbates the moment of inertia effect.  It
> seems that with a high ratio rearend (so that the primary load at high
> transmission ratios is engine inertia) will be adversely affected.  In my
> case, I'm more interested in a stump puller (boat actually) and would
> consider the high ratio first gear a pain in the !@#$ most of the time,
but
> need it when pulling up the boat ramp.  Since I'm not a racer, I don't
> consider this important, but included it in the discussion for the sake of
> completeness.
>
> Here's the question.  Under high loads (turbocharged) what are the maximum
> piston speeds that are associated with long life?
>

Stock bottom end motors should not exceed 3000 ft per min (average)
according to Performance trends computer dyno.

> Is there a need to provide extra fuel when under high load to 'save' the
> rings?  Maybe the real question is whether detonation is the only effect
of
> high power that kills the rings?
>
> Does a long rod help with piston skirt wear?
Yes
Do they hurt?
No!

 Is the offset different with a long rod piston or does all pistons assume
the 5.7" rod and
> ignore the long term effects?
>
> I've heard that all pistons for 6" rods are built with NO offset because
> most folks don't want the performance hit, and don't care about longevity.
> I'm considering using a long rod and a 400 cu. in. piston with big pocket
> heads (and a thick head gasket)
Worse thing you can do is to trash the squish that is built into this motor.

to attain the piston offset.  The target
> compression ratio is 8.0:1
>
> Are there bigger fish to fry?  That is - ring speed the least of my
trouble?
> I once heard that because of gas action behind the ring, the ring actually
> seals better under higher load.  Does anyone know with certainty?
>
> Can I save the rings on a stroker by giving more fuel?
>
> dh
> I'll say you have lofty goals!
Many of them seem to conflict in what they would like to see in engine
geometry.
>
>
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