M50b25tu stroker Help

[DinkleDrups]

So I have a spare M54b30 (wreck't an e46). Will the forged crank fit into my vanos m50b25? I wanna stroke it 😎.. also going F/I so need forged crank

I know it will fit in a non-vanos m50b25, are the bottom ends on the m50s the same?

[pbonsalb]

You don’t need a different crank to turbo, but more displacement helps spool a turbo if you want to rebuild your motor. The M54 3.0L crank is the same as an S52 crank. Some do low budge builds, swapping M54 3.0L rotating assemblies into M50/52 blocks since the bore is the same for all 3 motors. Others build the block into an S52 using aftermarket rods and pistons. I did that. Stock 135mm rods are good for about 500 lbs tq and risky after that. A cheap but heavy alternative to aftermarket rods like Eagle or K1 (I use K1) are M50 nonvanos rods. No stock piston is a great choice but many have made 500-600 rwhp on them. Opening up the ring gaps a little helps. Best is aftermarket forged pistons, which you can get in lower compression for boost, from wiseco, Mahle, JE and others. I use JE 86.5 in 9.0:1 CR. With stock compression and pump gas, you must use a thicker headgasket to reduce compression. If you use only E85, stock compression is fine.

[DinkleDrups]

Parts i have so far..
Cometic .140 head gasket and Arp Studs
CP Pistons ( compression 8.5.1 )
550cc Bosch injectors
135mm Eagle rods
Have an entire m54 Motor
And a big truck turbo

So your saying the crank will work??
I want the longer stroke for the spoolie boi

[pbonsalb]

As long as the pistons are for the stroke of the crank. Are they CP pistons for an M50, M52, S50 or S52?

[DinkleDrups]

They're for the m50

[pbonsalb]

Then the piston pin height is for the stroke of an M50 crank using 140mm M50 rods. If you used an M54 3.0L crank or S52 crank, the pistons tops would be several mm above the deck and probably hit the valves and give too much compression. You could try to figure out if there is some combination of things you could do to use M50 pistons with an 89.6 crank, such as using 135mm rods and a thicker HG, but I don’t have the answer. I am a big proponent of keep it simple and buy the parts that fit.

Unless you have some reason you want to stay with 2.5L displacement, I would ditch the M50 pistons. Your M54 3.0L has the crank you want (and some have used the entire M54 3.0L reciprocating mass — crank and rods and piston — in an M50 or M52 to get to 3.0L. But if you are willing to go that far, you can go one step further and buy S52 spec pistons and bore the block to fit them and get closer to 3.2L.

[DinkleDrups]

Yeah i figured i'd have to get different set pistons for the m54 crank.
I like the idea to swap in rods and pistons from m54, I believe they're forged.
Now for the m54 crank i would need a s52 exhaust cam and swap the m50 exhaust cam in for intake cam correct?? Whats ur take?
I may get 3 series 2.5L and throw those new rods and cp pistons in it

[pbonsalb]

I don’t know the details on the cams. Some say minor work is required for the intake cam. There are some posts on the specifics. For boosted power not much over 500 rwhp the M54 rods and pistons will be ok. When you gap the new rings, go slightly wider for boost.

I would keep it simple and not build 2 motors. Build one. M54 3.0L crank that you have, Eagle rods that you have, and new aftermarket forged 86.5 or 86.6 mm pistons that you bore your block to fit. Sell your CP M50 pistons at a loss and move on.

[DinkleDrups]

I believe the m54 bore is 84mm same as m50 ?

[gdavid]

I believe the m54 bore is 84mm same as m50 ?

The bores are the same but you need to have the correct piston pin height, which is the distance from the center of the wrist pin to the top of the piston. With a longer stroke crank, the top of the piston sits higher up at top dead center. The pin height, connecting rod length and half of the crank stroke must be less than 210mm or the piston will be too high. That exact height, along with the volume of the cylinder head's compression chamber relative to the displacement of the piston will determine the compression ratio. Custom pistons that lower compression ratios have short pin heights and/or dished piston top to increase the volume of the combustion chamber at top dead center.

[pbonsalb]

I believe the m54 bore is 84mm same as m50 ?

Yes. However, after 20 years of use, some blocks may have cylinder bore wear and require oversized pistons and reboring to match. Others may work with used pistons from a different motor just fine with no more than honing and new rings. You won’t know until you have all the parts to inspect.

There is a member named spydergod who recently posted the details of his M50/M54 hybrid. I think he spent about $1000 at the machine shop but I don’t recall exactly what that was for. If it’s not too much more, bore for S52 spec aftermarket pistons and use aftermarket rods and you can have a bigger, better motor built for the boost you write you want and with lowered compression that you want for anything more than low boost when using pump gas.

[gdavid]

Yes. However, after 20 years of use, some blocks may have cylinder bore wear and require oversized pistons and reboring to match. Others may work with used pistons from a different motor just fine with no more than honing and new rings. You won’t know until you have all the parts to inspect.

There is a member named spydergod who recently posted the details of his M50/M54 hybrid. I think he spent about $1000 at the machine shop but I don’t recall exactly what that was for. If it’s not too much more, bore for S52 spec aftermarket pistons and use aftermarket rods and you can have a bigger, better motor built for the boost you write you want and with lowered compression that you want for anything more than low boost when using pump gas.

I built a m52 with the m54b30 internals, fresh bearings, rings and no machine shop work, I only honed the bored but you really need to inspect the block before you decide if you can use stock bore pistons.

[DinkleDrups]

The old m54 pistons got less that 100k on them, so im gonna use them. Breaking down the m50 engine this week and see how it looks. @Pbonsalb ill look into boring to s52 spec

[jakermac]

I haven't done the math, but since the CP pistons are for the M50 Vanos motor with 140mm rods, there's a chance they'd be pretty close to right for the M54/S52 crank stroke, 135mm rods and the .140 headgasket you've already got. You should check with CP directly to see what they say.

[pbonsalb]

Are the CP M50 pistons for stock rods or for the 135mm Eagle rods the OP has?

75mm M50 stroke to M54/S52 89.6mm stroke is +14.6mm. Divide by 2 so we count only the upwards motion and we have +7.3mm. Figure the M50 piston for 140mm rods has -5mm lower pin height. The +1mm gasket will add another mm of space, so we have accounted for 6.0 of the 7.3mm, and are still 1.3mm over. What do you do about that? Add a 1mm spacer plate to the +1mm gasket? People have done it. Not many, though. That would leave +0.3mm which would raise compression over stock. But since the pistons are 8.5:1, we would end up at not quite 9.0:1 after resurfacing the head, which would work.

Alternatively, go with either the +1mm spacer plate or the +1mm gasket and run E85 and you should be OK with just under 11.0:1 CR.

[DinkleDrups]

I haven't done the math, but since the CP pistons are for the M50 Vanos motor with 140mm rods, there's a chance they'd be pretty close to right for the M54/S52 crank stroke, 135mm rods and the .140 headgasket you've already got. You should check with CP directly to see what they say.

I thought of that. I will check with them

[DinkleDrups]

@Pbonsalb
That can work but would be terrible for boost.
Not an N/A build, leave that to my chevys. I want the Cr around 8.5.1 or 9.1. Will be running up to 2 bar.
Any ideas on compression drop with gasket spacer using m54 pistons?