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Gentlemen,

I have a 320/4 carburetor from 1975 and would like to know what the maximum power can be from an M10 with the standard pistons. I will get more power out of this engine, but I wonder how far I can go with the power with the standard pistons.

Who am I? I'm André and live in the Netherlands and I have a few old BMW: 3x E21 and 2x E9. My hobby is to restore them but not completely standard. I do all the work myself and that makes it challenging. And what I do not know, I ask a forum.

Thank you for your knowledge.

Regards, André

Welcome mate
Without being any expert at all, technically speaking, I would guess that highest C/R you can reach with the standard 2.0 pistons would be somewhere around or over 10.5:1 maybe a bit less than this
And with a properly ported head (bigger valves etc), and twin webers 45mm and choosing a hotter cam (say a 300 deg) you might see max 135-140 hp at about 7000 rpm ? It all depends on the cam you would choose and tuning the carbs though
You would need different pistons in order to get higher c/r though (other wise valves might hit the pistons), and you need c/r as cam gets hotter and carbs get larger
However, as I wrote, it is my guess, I think I am fairly close to the truth, but I might be up to +/- 10 hp mistaken (and 10 is an important number at this level) so someone with more experience might be more helpful here, and correct me if I am wrong.

Hi André! Keeping in mind what you did with the E9 I'm prettyu sure you are looking for more that this. When you want to stick to naturally aspirated I would recommend contacting Andy Wax at FW Motorsport in Germany. These guys are running a real reliable 320i in Group 2/H spec.

If you want to work on turbocharging, Tim Maatman at Tim's Turbo Centrum in Deventer, Netherlands. He is an expert in engine mods when it comes to turbocharged engines, perhaps you've already heard of his E30 with nearly 1.000 BHP.

Oops It seems I was speaking about something different than what you were asking actually

Thank Petroscf for your information.
Yes Jeroen, my idea is to go to 250+ with a twin scroll turbo. No high rpm engine. Next month I will start building the car and it will take a few years, I think???
I only wondered how far you could go with the standard pistons. Intuitive should 200 not be a problem, but could not find any information about it.
Have spoken to Tim once, I believe that his E30 engine also works with a VEMS ECU. I will ask him about his experience.

Thanks.

I'm not enough of an expert to judge that. Perhaps someone else can share his or her 2 cents but I guess there are multiple influencing factors like boost and compression ratio.

Your engine has a compression of 1:8, let us assume that you drive on petrol and therefore 1:12 effective compression is allowed. That means that you can use 0.4-0.6bar boost, that is not going to give you 250hp.

If you want that amount of power, you will need pistons with a way lower compression.