M20b23 crank In a stroker? Can I?

[Pdxseth]

It's literally just a b25 block with a b23 crank "alpina'a custom pistons" are the same as North American "I" pistons they ported the head with oversized valves and headers wich I already have. All I will have to do is copy that tune and I have a rep alpina motor. I don't want to put anymore money than I have to into this I already have all the main components. It's a daily driver I also don't need a ton of power.

Having a 2.6 is also a great conversation piece how many people can say they've done it?


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[Pdxseth]

Hartge used the 731 heads on H23's when they built 2.6L motors, but they modified the head a HUGE amount to fit the larger valves, not just the pistons. When you look at the closed chamber shape of the 731 head you can see it needed modification to fit the domed style pistons the 325i used too.

Modified chamber of the 731 looked a lot like this.





Compare it with the shape of the stock 731 head



Quite a bit different and a lot of material has been removed.

For each full point increase in compression ratio, the added power developed is roughly 3-4%, until you get above 12:1 then it's reduced even further. Obviously camshafts make a difference, but all things remaining the same, the increase in comp ratio doesn't result in massive gains in power by itself. As Digger said, you need to take advantage of a whole host of other things to take advantage of it. It kinda sounds like you are trying to do it on a budget, so you are better to stick with known combinations that work.

If you want high comp, best thing would be to use your eta bottom end and find a set of early eta pistons with the 10:1 comp ratio. Anything early, so prior to about 1985 from a 5 series should find a set of pistons with a domed crown in the centre.

I'm not going to use the 731 because of all the modification required(although I do have one) the 885 will work much better and it already matches my pistons.

I didn't know the early Eta's were higher compression is that why my girlfriends 83 528e pulls so much harder than my late 85 eta?

I was also considering using the 731 on her car with m 1.3 would that work well.


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[LJ851]

^ Did you just say "528e" and "pulls hard" in the same sentence ?

[Pdxseth]

^ Did you just say "528e" and "pulls hard" in the same sentence ?

No I said "pulls harder" I just never understood why hers having the same motor was faster than my lighter car, with the same gears.


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[LJ851]

There is one theoretical advantage to doing a 2.6 vs a 2.7 with an eta crank, The eta has a cast crank while the B23 crank is forged and much tougher.

Would you ever have a problem with a 2.7 crank? Probably not.

[Madhatter]

Alpina C1 were just hotted up 323i's. Port work, custom pistons (10:1 comp ratio) cam, headers, exhaust, etc, they were still 2.3L which is why they only produced about 170hp.

Hartge didn't just port the head, they cut out a huge amount of chamber shape to use the 731 head. The pistons of a 325i don't even go close to matching the 731 chamber shape, it's a closed hemi design with a big squish band in front of the valves which match the flat (or centre domed) shape of the pistons they used with the heads.

They also fitted oversized 325i valves, you cannot physically fit these valves in the 731 head without performing modifications to the chamber shape. The same squish areas get in the way and cause huge valve shrouding. Think a valve sitting inside a pocket with metal surrounding 2/3rds of it. You have to modify the head extensively to make it all fit. Especially if you want to use a domed style piston like the 325i ones, the chamber starts to resemble a 885 one.

The thing you have to remember too is, more aggressive cams with longer duration/overlap and increased valve lift create clearance problems too, so you can't just slap something together when trying to build this combination.

If you want the math to show you why it doesn't work with standard components;

206 - ( 38.4 + 135mm) = 32.6mm.

8.8:1 325i piston comp height = 34.7mm
9.7:1 325i piston comp height = 34.2mm
9.4:1 325i piston comp height = 34.9mm

And none of those comp height figures include the dish sticking out over the top which can be as much as another 4.43mm for the 9.71: comp piston. In short, every 325i piston is not going to work.

If you ran a shorter 130mm rod

206 - (38.4 + 130) = 37.6mm

So now the crown of the tallest 325i piston is sitting 2.7mm below the deck of the block. Even with it's dome on top sticking out 3.5mm, the majority of the piston is going to be still sitting way below the deck of the block, so you are going to have a massive reduction in compression ratio. I don't have time right now to sit down and work out how much it's reduced by, but I can guarantee you it is not going to result in added compression ratio, so you are only going backwards from 9.4:1.

[Madhatter]

Now I see the above is a waste of time because suddenly you want to use an 885 head. Make up your mind, because it seems like you have no idea what you want.

Even using an 885 head with a 323i crank won't work. Pistons will either be way out of the block or sitting down too far. You can't compensate for 2-3mm in just shaving the head or block. You need modified pistons, no way around it.

If you want to use stock 325i pistons and an 885 head, as digger already said you are better off building a 2.8L motor with an M52 crankshaft. There are good threads on this build already that will answer your questions. No need to go any further than this as it is your best option retaining largely stock parts (pistons, crank, rods, head) with minimal machining needed to fit it all in.

[Pdxseth]

From what I understood I just need to run a thicker head gasket and an adjustable cam gear to compensate.

But you obviously understand this stuff way better than I do. Maybe you can help.

Instead of running a thicker gasket and adjustable cam gear could I use seta pistons? I have those on the shelf too.


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[Pdxseth]

If you look at my OP I've wanted to use the 885 all along it's already built and ready to go.


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[Pdxseth]

Ok so I'm trying to learn this, what does the 206 number represent? It look like seta pistons are taller than I pistons at 36.2 and I piston are 34.2. So obviously I would have to use the 130mm rods with seta pistons so that would give 3mm more clearance? Isn't that what I need?


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[Pdxseth]

And what is 38.4 everything I've read says that U.S. "I" pistons are 34.2 unless 38.4 represents something else my original plan of I pistons and 135mm rods should still work right?


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[Pdxseth]

206 - ( 38.4 + 135mm) = 32.6mm.

8.8:1 325i piston comp height = 34.7mm
9.7:1 325i piston comp height = 34.2mm
9.4:1 325i piston comp height = 34.9mm



If you ran a shorter 130mm rod

206 - (38.4 + 130) = 37.6mm

.

With seta pistons 36.2

206 - ( 36.2 +135mm) = 34.8

I honestly still am clueless to this stuff but IF I'm correct this should put me closer to 9:1 comp

If 38.4 is not the piston height I'm lost I'm just asuming maybe aus had different pistons than us


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[Pdxseth]

M20b23 crank In a stroker? Can I?

All this math is too much for me. I've read the build thread on e30tech over and over again. I've learned a lot today.

It works
885 head
323 cam
M20b25 pistons
135mm rods
M20b23 crank
And a .098" mls head gasket will leave plenty of clearance

It's a little more powerful than a stock m20b25 with an untuned 173 dme so after a little tuning I think this combo has potential.

I also have a slight advantage because I've had port work done to the head,port matched intake, and headers. I also plan to run m50 injectors instead of m20b25 injectors.



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[kronus]

Neat thread. I considered this for a while, since I have a b23 sitting around as well. The conclusion is the same.. Dixon's thread trails off, the car is somewhere up near sac now I think, not sure if robert@peerless ever had a chance to get it fully tuned. Might be worthwhile to call and ask him.

[digger]

a 76.8 crank is 2.4% more than a 75mm crank is worth the effort? for me its not. people will do what they want regardless of advice which is a good learning experience.

a cheap build that is worthwhile for me is still a M52 84mm crank, 130mm rod, b25 piston and skim off the block and machine crank if it is the large CW version.

otherwise budget 1k for custom pistons and S52 style rods and have possibility for a 3L