Piston Sizing for Bored M90

[HairyRaisin]

Trying to ignore what’s going on with gas prices right now …

I have my M90 engine in at the machine shop being rebuilt. The cylinders are worn to about 93.9mm, and the head is cracked (original M90 head). I already have a B35 head that has tested well, so I am considering going to 94mm bore with the B35 head (if I can track down intake, TBs, crank balancer etc.).

I’ve seen plenty of folks talk about doing this – boring the M90 to 94mm, using VW forged pistons or JE customs. Slapping on the B35 head will lower compression if you don’t go custom pistons – I’ve read compression drops from 9.3:1 to anywhere between 9.0:1 – 7.5:1 ish . I’m fairly open but it does seem that using the B35 head, with the larger valves and better flow, is a good option granted I get custom pistons (I want to keep this NA so not interested in turbo).

Even though this has been done many times before, I can’t really find how folks are determining the shape and/or compression height of what customs pistons should be so that compression isn’t lowered too much. Paul Burke used to make pistons for this exact application, but I understand he’s long out of the game now.

Then again – would I even notice the compression drop, especially with the overbore?

Interested in people’s thoughts…

[Hit Man X]

B35 head flows better due to larger valves and intake ports are different, plus has a better chamber design. Also, the intake manifold is matched to the new style ports. Think of the B35 as an emission friendly B34 10:1 engine. Similar power and torque.

Custom pistons allow your static compression to be at whatever you want. IMO, starting with the best of the M30 bits and going from there makes the most sense...but the B35 head likes matched style pistons to take advantage of the combustion chamber. How to bump static comp from 9:1 to say, 10.5:1 area will greatly vary on your head chamber volume, pistons below deck, dome height, etc. Just far, far too many variables.

In the end, you are limited by the M30 head design.

[HairyRaisin]

Cheers Hit Man X.

I should be relying more on the machine shop to help here, but just a bit wary since they don't see M30s (let alone M90s) often. Have asked them to confirm deck height of the head so I can check if it has been skimmed before (I don't think it has). If it hasn't then that's a good start, but I guess I should really be asking the shop to CC a chamber to be certain.

Suppose I was hoping that, say the head has not been touched, one could say 'this swap will need domed pistons to bump up compression' or 'dish pistons with higher sides' etc. (working out exact measurements after the fact).

[Hit Man X]

Hopefully Lynn will pop in, I swear he built an M90 based block recently.

You may want to consider measuring everything yourself so you know exactly what you have versus simply paying someone. Unless of course you have more money than time which I understand.

Was the original M90 10:1 static? I am guessing it is a big bore, short stroke engine? Using a B35 top end will make it very pleasant to drive I would think due to the broad torque it makes, it can also be chipped easy running M1.3 Motronic for extra zip. Just know that B35 intake manifold is designed for easy driving probably 2500-4000rpm area and I doubt it will pull well past 6000 area if that is your goal. B35 exhaust manifolds also flow quite well.

I would assume it should make 220-230hp pretty easily with a high flow exhaust with a standard B35 camshaft. I could be underestimating it even.

[HairyRaisin]

Yeah fair point. I'm not exactly wheelbarrowing cash around though definitely more time poor... work has never been so busy, and the shop already have the engine in pieces.

I'm ashamed to admit I don't know the original compression (I have seen folks claim it's 9.3:1, and others say 10:1, but yes it's the big bore (93.4mm) short stroke (84mm) engine. If I use an online calculator with factory specs the static looks to be 9.23:1.

I should dig out some of these older forum posts I found - but have also read from one or two folks who have done this before (M90 bored to 94mm with B35 head and Motronic 1.3) claim the revvy-ness of the M90 makes up for the change in manifold characteristics. Hmmm..

220hp sounds good to me - not shooting for a power monster here, just a nice torquey cruiser!

[HairyRaisin]

Looking like I might need to find a B35 piston and send it off to JE or VAC to get the B35 head shape.

VAC Motorsports list an off-the-shelf 94mm piston to suit an M30B35, so kind of hoping they can just take that shape and change a few measurements around to form a custom M90 piston..

https://store.vacmotorsports.com/cp-...m30-p1608.aspx

[Hit Man X]

https://www.bimmerforums.com/forum/s...ecommendations

That is the Lynn I mentioned.

[lynnbilodeau]

Hairy and I have been in touch via PM.