The M20B26 thread

[flybynite]

Well the suggestion by Steve_k to do a B26 out of my B23 caught my interest but having read all I could still quite a few questions so thought i’d thrash it out here.

Why do it? Well the M20B23 was renowned for piston slap and the one in my 323i was no exception - it has to go. Putting those rods in a new 2.8 build was not going to happen. I could not see how that was going to reduce the side load.

Using the crank however is a different matter.

So basically a B25 with a B23 crank and cam. Questions I have left are

1. What to do with the extra 1.8mm of piston. The e30tech thread suggested thicker head gasket but working the chamber and custom pistons suggested too. As it was done in the US I guess they were using the Low Compression pistons anyway.

My preference would be work the head a touch and keep standard head gasket but who in he southern half of the UK has these skills anymore? By all accounts it is easier to cock up this head than to improve it

2. How to run it. Assuming we get Motronic 1.3 up and running do we tune the Motronic or do we go straight to chip it. Who can do that these days in the bottom half of the uk? Don’t really want to megasquirt it

It is very tempting to just stick the standard B25 in but this does sound interesting if it could be made to work and make some use of a dog of an engine.

[87mtech27]

The piston will only protrude up by .9mm. You can get thicker BMW gasket or you can get MLS gasket from the states.

Just to be safe maybe cut deeper valve relieves a¤”

[Speedtouch]

I can supply chips suitable for many M20B26/27 applications.

[flybynite]

The piston will only protrude up by .9mm. You can get thicker BMW gasket or you can get MLS gasket from the states.

Just to be safe maybe cut deeper valve relieves a¤”

Yes schoolboy error didn’t read it properly it’s crank stroke that will go up by 1.8mm. So as you say only 0.9mm to deal with

From what I read it is not the valve clearance that is the problem it is the piston crown hitting the top if the chamber.

I would possibly get a bit of work done on the head anyway so easing that section by a couple of thou should not be an issue.

I have no problem with a thicker +0.3 OEM gasket but to go much more would start to affect the CR which is one of the reasons of doing this.

Rather than raising the whole head up, I thought a bit of localised clearance would do the job and keep the CR. I’d be happy to run it on 97/99

[flybynite]

I can supply chips suitable for many M20B26/27 applications.

PM me the details when you can, should get it running with a 173 box They seem to recon 10:1

[flybynite]

I’m also curious why the use of the B23 cam. What is the difference between that and the B25 cam? Can’t find many specs for the 323i Don’t want to make it too lumpy

Other things going through my head are what else to do while it is all in bits? Again diminishing returns and I’m not trying to wring the last bit out of it but some things generally stand out as either a sensible or a must do.

Lighten the flywheel a bit? Would it make sense to use the B23 or B25 flywheel?

Ditch the pig iron manifold? Match it to the head. It will need a new exhaust anyway, the Ansa system it had on was never that good and is probably shot by now.

Any different rods that could be used and picked up cheapish?

[steve_k]

I’m also curious why the use of the B23 cam. What is the difference between that and the B25 cam? Can’t find many specs for the 323i Don’t want to make it too lumpy

Other things going through my head are what else to do while it is all in bits? Again diminishing returns and I’m not trying to wring the last bit out of it but some things generally stand out as either a sensible or a must do.

Lighten the flywheel a bit? Would it make sense to use the B23 or B25 flywheel?

Ditch the pig iron manifold? Match it to the head. It will need a new exhaust anyway, the Ansa system it had on was never that good and is probably shot by now.

Any different rods that could be used and picked up cheapish?

some say to use the B23 cam as it's a bit more "spikey" so to say, have a look in the wiki at the "build a 2.7" page, the guy who wrote that used the B23 cam as he says it gave a bit more lift & duration compared to the B25 cam,

or you could look at the likes of cat cams or kent cams.

while it's all in bits, check the bearings & main caps, port match the inlet manifold, big bore throttle body, clean the injectors,

maybe head work? port, polish & gas flow,

then the exhaust side, definatley get a 6 branch manifold (BTB, magnex etc) with an X-pipe.

flywheel?? yep get that machined down as well, also IIRC the B23 & B25 flywheels are the same, it will make a difference.

as for rods? the choice is your's, but i would go for either H section or I beam rods just for strength.

looking forward to the progress of this.

[steve_k]

forgot to add, for the bottom end i would add a crank scraper & a sump baffle extension plate, i might still have the .PDF template for the sump baffle somewhere.

[Speedtouch]

I can supply chips suitable for many M20B26/27 applications.

PM me the details when you can, should get it running with a 173 box They seem to recon 10:1

Let me know once you've decided on the engine build spec.

[flybynite]

It’s going to be a while as I’ve got a couple of other cars to do first to get some space! I’m just picking up bits I need as they come up and getting it straight in my head what I’m doing. Theory being when it comes time to attack it I’ve got everything sorted.

Mind you the 2860 with the long rods and M44B19 pistons would make use of the 731 head I’ve got and if I’ve got to bore out the B23 block might as well take it to 85

[reggid]

It's pretty much a waste of time with b23 crank imo. Such amount of effort for about 5hp . Get a healthy running b25 and let sleeping dogs lie. If you want a cam get a schrick cam forget about the b23 cam it's a step back in how modern the profile is compared to the b25

[flybynite]

It's pretty much a waste of time with b23 crank imo. Such amount of effort for about 5hp . Get a healthy running b25 and let sleeping dogs lie. If you want a cam get a schrick cam forget about the b23 cam it's a step back in how modern the profile is compared to the b25

I get what you mean, I really do and it has crossed my mind more than once. It is two-fold really.

I am doing quite a bit of work on it to get it back running right. It is worth it as the shell should stay original and un-welded. Whatever engine goes in will be stripped checked, and rebuilt even if it is only to satisfy myself it is good as it is not likely to be one I know or see running. I have all the B23 bits already, I know they are good, So why not just fit them when it is rebuilt? Not much extra work really.

Secondly I have had this car a long time, and it was high-spec even for a 323i it had a lot of Alpina bits fitted from new but never on the engine. After driving it so long with a piston-slapping dog of a B23 it would be good to give it a decent engine.

I am certainly not trying to make it something it is not but a home-baked B26 would be more in keeping with what it is which is why it caught my attention. It is why I was originally looking at the M30B35 I already have. Would Alpina/Hartge have done the B23 crank if it was such a waste of time? I am curious really.

@Reggid, two questions for you if I may?

What profile of Schrick cam would you use? And why?

I have read your build thread it is a lot of work you have done there but when you told someone else not to make the mistakes you made what did you mean?

OK that’s three questions

[reggid]

It's not much more to do a 2.8L and you'll get real torque gains. 284/272 is nice cam or 272

As for alpina they did a bunch of other stuff, not just a slightly larger crank the gain from the crank alone is not much its a 2.4% change in capacity. capacity alone never makes much in the way of gains to power, its breathing that makes power.

There was a guy who did one on e30tech It wasn't any better than a fresh rebuilt m20b25 bog standard . They (Alpina) also did a 2.7l where you start to notice the torque gains when it's done properly.

[flybynite]

Yes I did read that thread, I got the feeling they did not map it right when finished or really sort the engine

I know it would be a slight increase in cc (I’m not naive enough to think that alone will make a difference) but it’s also a slight bump in compression ratio. It would just add a bit to anything else that is done for not much effort, which i guess is what Alpina and Hartge thought.

My problem with the ”˜standard’ 2.8 stroker is it re-uses the those fxxxg 130mm rods. They are what caused the problems in the engine I have and I just don’t want to do all this and end up back where I started. When it is described as a ”ahandgrenade of an engine”a I do get what they mean. I have seen and heard what that side load can do in the B23 I have here.

I could build a B28 using 135mm rods and custom pistons but that is a slippery slope to do right and I would sooner put the money in other things.

Don’t get me wrong I like torquey engines but if that was everything I would simply fit the very nice M30B35 I’ve got here already. A standard M20B25 is still a likely option but I would still like to do something a little different and use the bits I have rather than hunting down more bits like a good M52B28 bottom end.

My reservation with the B26 is the piston/head clearance. If the standard 2.05 head gasket does not do the trick I’m not sure of the best way around it. A very thick MLS gasket is a cop-out IMHO and takes away the bump in CR over simply using a 5E engine. A touch more head work would mean standard head gaskets could be used, it could be done at the same time as other head work but at what cost to the chamber shape.

I’m still not convinced one way or another, I’m hoping that by the time I get to this a bit more info will come to light or someone else may have had a go first (@steve_k )

[steve_k]

i've got a 2.6 m20 but min was built a different way.

i used a m52b25 crank (bullnosed), H section rods, & +1mm pistons to give it 2554cc,
couple that with a copy of a griffin motorsport head, magnex 6 branch exhaust manifold, X-pipe on the end of the down pipes (next to the gearbox), 4 pintle (m50) injectors, big bore throttle body, port matched inlet manifold (soon to be replaced with an extrude honed manifold).

it's not in my car yet, it's in a mates e21 (he built the engine) while i sort a few bits on my sport. but when it does go in there will be a light weight flywheel in there as well.

[steve_k]

forgot to add, there's also a teflon lined crank scraper, sump baffle extention plate, & ARP studs top & bottom.

[flybynite]

No I was waiting for you to do something with that B23 crank you said you had

Why the M52B25 crank over the std M20 one? I thought the M52b25 had a cast crank where the m20b25/23 and B28 had forged

I take it still uses 135mm rods, whose rods did you use?

I’d be debating whether to go H/I beam or just get the std ones shot peened. I assume the standard ones are forged.

To go Arrow rods is quite a wedge and not sure I would trust the Chinese ones any more than forged std ones unless they had been checked balanced and sorted by someone first

[reggid]

m20b25 is still cast like the 81mm eta crank. the later model 75mm stroke engines are lighter from the look of them so knocks some rotating mass from the engine and probably forged, though cast is easily more than upto the task.

all rods from m20 and m50/52 are forged M52b28 or any 135mm 24V engine rods rods are probably the best BFYB. arrow rods are a total waste of money thats better spent elsewhere. power is in the topend inlet, head/cam and exhaust. if you want torque you want cubic inches, compression and conservative cam duration and proper exhaust.

[reggid]

[DanThe]

Im of the same opinion, 2.8 minimum or why bother

[flybynite]

To be honest there are a couple of posts on the net about a B25 developing piston slap after being stroked using short rods.

There are other ways to get to 2.8 but using custom pistons I will see how much I want to spend when I get there. A bit of that noise might not bother most people but it really gets to me and I don't want it in the next build.

5mm can make a difference in engine behaviour as the piston is only slapping the side by a few thou. There was 5mm (stroke) difference between the B20 and B23 and it seemed to make the difference between it slapping or not

It is looking more like getting a M20B25 running really well with a shrick cam and a bit of fettling is going to be the way forward, at least to start.

Any donor 325 is likely to be tired, The plan was to bore out the B23 block to a fresh 84mm standard and use the standard 84mm pistons from the donor. It would save getting new pistons.

Although the wiki says it can be done I now read that the early M20 do not take well to being bored out from 80mm to 84mm. I still cannot find out why and what age this problem ended

[reggid]

If the oe piston is slapping there is wear. You don't use parts worn out of spec for any build. There are lots of reasons for wear irrespective of side loading.When done properly there are zero issues.
You can get slapping on engines when you use certain forged Pistons that require excessive piston to bore clearance. There are Pistons that won't slap.

[flybynite]

No both times we took the engine down we checked every inch of it, Other than the witness marks for the piston slap, it wasn't just in limits it was bang-on both times, still had all the original honing marks on the bores.

The later 150bhp 323i was known for it, as I found out later, BMW acknowledged there was an issue and released a modified piston with a shorter skirt. It made a difference but did not cure the problem. There was no problem with the B20 at the time or the B25 that came straight after. They admitted there was no more that could be done unless I wanted to go B20 or B25

Having done it twice I am not going to build an engine that could have this problem, which was why I was looking at the B26. Same throw, longer rod has to be better. The 2.8 is a longer throw same rod.

When I do get round to it I may throw the B23 crank in to see what it looks like and how much clearance we have on the head, if I don't like it I will just carry on and build a good B25.

Only thing is it makes a difference to the donor engine I need. Unless I want to buy pistons B25 I need a 5E, B26 I need a 5K, B28 could be either but probably better 5K for me.

If I cannot bore out the B23 block I will need a better B25 donor or bore that out and go oversize but that means buying pistons again!

Like I say just building a good 325 is looking the best at the moment