Lowering Cr On Vh41de Using Shorter Rods?

[benny bobo]

Hey,

Just say I wanted to turbocharge a VH41DE...

VH41DE has CR of 10.5:1, I am guessing 9.5:1 would be better suited to turbocharging...

Can you use shorter rods to drop the CR to 9.5?

Would this actually increase displacement to roughly 4531cc? ((4100cc x 10.5) / 9.5)

I am guessing I am wrong, but just want to know...

[2rismo]

What fuel do you intend to run the motor on?

[blind_elk]

It won't change the capacity, because the engine still has the same stroke (controlled by the crank, not the rod!) and bore.

A shorter rod will pull the piston down the bore, so it sits too low at TDC. If you are going to use a shorter rod, then you will need a piston with a lower pin height to push the piston back up to the top of the bore at TDC.

You could also investigate modified pistons (less crown), and possibly enlarging the combustion chamber a tad.

[benny bobo]

95 - 98 octane

[benny bobo]

Anyone know anything about using VQ30DET pistons in the VH engines?

Quote From http://forums.nicoclub.com/zerothread?id=263218#2780566

They have the same bore and wrist pin measurements, and very similar compression height, and have a dished design.

[Abo Bob]

It won't change the capacity, because the engine still has the same stroke (controlled by the crank, not the rod!) and bore.

A shorter rod will pull the piston down the bore, so it sits too low at TDC. If you are going to use a shorter rod, then you will need a piston with a lower pin height to push the piston back up to the top of the bore at TDC.

You could also investigate modified pistons (less crown), and possibly enlarging the combustion chamber a tad.

This doesn't make sense to me. If the rod is shorter and nothing else changes then the piston will be lower in the bore at TDC lowering the CR - but not a particularly good way to do it - and it will be the same amount lower in the bore at BDC thus a slight increase in capacity.

Why would you use a shorter rod then raise the piston with a lower pin height? You would end up right back where you started.

I think you should take the pistons and the heads to your machinist and tell him what CR you want. He can take some meat off the pistons and out of the combustion chambers. I would suggest 8.5-9:1 to make it easier to tune. What boost levels are you planning on?

[benny bobo]

not really sure... maybe about 15 pound.

[Abo Bob]

9:1 should work well then. Good response but safe enough to be easier to tune. This is what the standard RB25's are.