Need information regarding crankshaft choice compatibility

[Mitch Free]

I'll be rebuilding a rb25de neo into a  det in the future by using 26 rods and pistons. I have looked online and have found mixed answers so was wondering if anyone could give me a clear answer. My question is will I need to get a 26 crank to go with the rods and pistons or is the 25de neo crank compatible? Cheers 

Edited May 6, 2022 by Mitch Free

[GTSBoy]

You do not want to use 26 rods* and pistons. That is opening up a whole can of worms** that you do not want to open. The Neo combustion chambers are VERY SMALL compared to 26 (or vanilla 25) and so the Neo pistons have a different crown shape.

You want Neo turbo pistons.

*Interestingly....Neo turbo rods are 26 rods anyway. So, yes you can use the 26 rods, but you really don't want the pistons.

** Waaay too much compression.

[Mitch Free]

Thank you for clearing that up

[Mitch Free]

If I had the 26 crank that would correct the compression ratio?

[r32-25t]

No

[r32-25t]

You could machine the piston crown down to give the correct compression ratio though 

[GTSBoy]

On 5/9/2022 at 1:28 PM, r32-25t said:

You could machine the piston crown down to give the correct compression ratio though 

Assuming that there's enough meat there to take away. There might be, or there might not. You need to get rid of about 10cc worth of material, which averages to ~1.15mm across the whole crown, although you'd probably need/want to take more away in some areas and less in others, wrt valve pockets, quench pads, etc etc.

[r32-25t]

Depends what compression ratio you’re after, these days with e85 most are chasing around 10:1, also most rb engines have fairly decent sized domes to take material away from 

[GTSBoy]

Correction, I buggered my arithmetic. It's actually ~1.7mm, averaged across the crown, to be removed.

[GTSBoy]

On 5/9/2022 at 5:17 PM, r32-25t said:

Depends what compression ratio you’re after, these days with e85 most are chasing around 10:1, also most rb engines have fairly decent sized domes to take material away from 

True. Could thus get away with removing somewhat less than I calculated above. But probably only in the order of half. Doesn't take much to crank the comp right up.