Factory Piston Pin height

[engineermike]

@K4fxd …the conrod is 8”!

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[GregO]

I’ve been away from average piston speed for a while. Without doing a deep dive what are current reliable piston speed numbers ?
Materials continue to improve over time so that number should continue to rise.

 

[engineermike]

I dunno, I used the 7000 rpm LS7 as a go-by.

 

[Decible]

In my heart with good hd rods and 3.84 stroke you could run this combo with a max of 25 to 28 lbs of boost up to 7800 rpm and it live a long time. You would need l19 or 625 main bolts and rod cap bolts and I’d do steel main caps…..but I think for a street strip build it would rip!!!

 

[GregO]

Just for fun or serious.
https://lmengines.com/pages/piston-velocity-calculator

 

[engineermike]

Piston velocity at tdc is 0! Acceleration is quite high though…

 

[GregO]

Piston velocity at tdc is 0! Acceleration is quite high though…

Right on Mike.
I’ll correct that

 

[K4fxd]

…the conrod is 8”!

No we were running these in production bikes. No room to lengthen the cylinders. So shorter than stock rods.

 

[Decible]

Just for fun or serious.
https://lmengines.com/pages/piston-velocity-calculator

At 74* the 5.2 hits 8034 at the same rpm (8000) the 5.4 hits 8467….st 7800 rpm 8200. It’s not terrible….

boosted ls7 at 7600 is 8375 with same rod ratio essentially

 

[GregO]

No we were running these in production bikes. No room to lengthen the cylinders. So shorter than stock rods.

So are we talking 7.xx”
What is the stock length you’re referring to.

 

[K4fxd]

7.6

 

[Decible]

7.6

That’s still a good rod ratio. In modern times the idea of 1.8 has been pushed aside due to material science and it’s been proven in most cases a 1.6ish ratio makes better torque. This combo and an LS7 are in the 1.5x range which is really on the edge of my comfort zone.

 

[K4fxd]

That’s still a good rod ratio.

Yes, but piston speed is off the charts. All my engine programs say not to spin higher than 6000. These live in the 8000 range. Piston tech has come a long way.

 

[GregO]

That’s still a good rod ratio. In modern times the idea of 1.8 has been pushed aside due to material science and it’s been proven in most cases a 1.6ish ratio makes better torque. This combo and an LS7 are in the 1.5x range which is really on the edge of my comfort zone.

The days of Smokey’s “Use the longest damn rod as possible” have changed a lot.

 

[Decible]

The days of Smokey’s “Use the longest damn rod as possible” have changed a lot.

This is very true! 496 bbc by all accounts has a horrible rod ratio (1.37) but we spin them to 6800k and 7xx hp in big compression (14.1 to 15.1) race trim back in the day

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