M20 2.8 Stroker Build Proposal.

[djjerme]

That's an 885 head, the better flowing head, was on the B25 adds the super eta.. so you're golden.

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[djjerme]

If you want to do a 2.8 m20, sure. The 731 head is a boat anchor and should be sent to me immediately for proper disposal lol. Actually, let me know if you want to get rid of it, I would like one to play with on the flow bench.

I have thought about finding one as well. Curious to try and recreate the Alpina head and see what kind of flow can be had out of it.

Somewhere I have a picture of the Alpina modified chamber, and man, they really hogged a lot of material out of it!

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[digger]

I have thought about finding one as well. Curious to try and recreate the Alpina head and see what kind of flow can be had out of it.

Somewhere I have a picture of the Alpina modified chamber, and man, they really hogged a lot of material out of it!

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IMO don't follow what Alpina did as that was the 80's and it seems to be the opposite direction to what good modern combustion chambers look like. on the 885 they converted the semi hemi to a hemi and nowadays even the Chrysler dont do a true hemi because its not a good design. not sure what they did with the 731 heads back then but none of them really made any exciting numbers for what they were.

You would use the 731 head specifically because it has a small closed chamber (easier to get higher compression on a smallish engine) and can use a flat top piston to generate squish (hell of a lot easier than the 885 semi hemi dome and any piston manufacturer can make a flat top), changing from this doesn’t make much sense to me.

The 731 port starts off a smaller size than the 885 so it doesn’t flow well on a flow bench but its misleading to look at flow alone when the engine really only cares about size and may not even need as much flow as you think.

you can make a 885 or 731 flow the same numbers (731 just requires more material removal) if you just go making things big but unless you are building a big 3L stroker that you want to make peak hp close to 7000rpm with over 100% VE and all other parts such as cam, inlet and exhaust are tuned with the same goal you probably arent doing yourself a favour

[ForcedFirebird]

IMO don't follow what Alpina did as that was the 80's and it seems to be the opposite direction to what good modern combustion chambers look like. on the 885 they converted the semi hemi to a hemi and nowadays even the Chrysler dont do a true hemi because its not a good design. not sure what they did with the 731 heads back then but none of them really made any exciting numbers for what they were.

You would use the 731 head specifically because it has a small closed chamber (easier to get higher compression on a smallish engine) and can use a flat top piston to generate squish (hell of a lot easier than the 885 semi hemi dome and any piston manufacturer can make a flat top), changing from this doesn’t make much sense to me.

The 731 port starts off a smaller size than the 885 so it doesn’t flow well on a flow bench but its misleading to look at flow alone when the engine really only cares about size and may not even need as much flow as you think.

you can make a 885 or 731 flow the same numbers (731 just requires more material removal) if you just go making things big but unless you are building a big 3L stroker that you want to make peak hp close to 7000rpm with over 100% VE and all other parts such as cam, inlet and exhaust are tuned with the same goal you probably arent doing yourself a favour

I agree.

Velocity in the port is more common now a days. It has been proven over and over that a lightly ported head with smaller port volume will make more power than a port that was "hogged" for max flow. People have realized cylinder filling is just as important as flow. This is why you even see the "pros" using smaller ports with as much flow as possible. I know the LSx has been beat to death, but if you look at AFR heads (best in the business), their heads only have 5cc of material removed over stock - up until you are looking at forced induction. Then the cylinder filling becomes less of a factor because you are able to "force" it in.

Biggest thing holding the 731 head back is the valve size. It's amazing to me I have found on the bench that a bigger valve can only flow more if the valve job and throat work is up to par. Simply opening the seat for the larger valve has shown me losses every time - with the exception of the 400 24v castings. They showed a positive gain before any throat work, but we are talking about the m20 hemi head, not a pent head 24v :/

The 885 head really is great out of the box, and unless you know what you are doing, it is easy to ruin. I have flowed heads ported by others, and needless to say, they just wasted valuable port volume for nothing. Zero gain, or losses in most cases, but going back to velocity, I would say they are losing power - based on experience, never dyno tested another companies head, we spend enough time and money on our own research. :/

[zaq123]

and nowadays even the Chrysler dont do a true hemi because its not a good design.

as far as Chrysler goes... manufacturing cost and weight savings is what started the whole thing, from what I've read.

I like the concept of M20B25 engine for what it's worth and considering when it was designed, like it a lot. I've paraded 885 head in a few performance shops and typical first reaction is "Ah! oldie but goodie" comment.

I completely agree with all the above, do not mess with the head unless you 100% sure what you are doing ...unless it is an experiment of some sort.
Flow speed vs. flow volume on the performance scale..... bigger not better unless it's necessary

[digger]

as far as Chrysler goes... manufacturing cost and weight savings is what started the whole thing, from what I've read.

i meant they dont use the same combustion chamber as the "old" hemi, nowadays its more of a semi hemi at best

[TriniGT]

Hi there just to get some clarity, for a cheap stroker build I can the 2.8 crank from the 328i M52 E36, rods from a B27 engine and 325i pistons, using an 855 head from an M20B25 engine right?

All other accessories can be swapped from an M20B20 engine right? Of course just doing some slight cleanup in the exhaust and intake areas and refreshing with new main and rod bearings , ARP main and rods bolts and the recommended schrick 288?

[digger]

Hi there just to get some clarity, for a cheap stroker build I can the 2.8 crank from the 328i M52 E36, rods from a B27 engine and 325i pistons, using an 855 head from an M20B25 engine right?

All other accessories can be swapped from an M20B20 engine right? Of course just doing some slight cleanup in the exhaust and intake areas and refreshing with new main and rod bearings , ARP main and rods bolts and the recommended schrick 288?

885 head, but essentially yes, there are various different 84mm stroke cranks around that affect clearances between the crank counterweights and the pistons skirts. Depending which crank and piston skirts you have may involve some machining for clearance. Nominally the block should be decked about 0.5mm to set the squish up correctly but this needs to be done via mockup to get it spot on.

the late 320i have the same electrics/harness as late 325i AFAIK, but there might be a different fuel pressure regulator and injectors. You want the late m20 electrics/harness.

You don't need ARP anything tbh. A schrick 288 is only worth it with an RHD itb kit and standalone ECU IMO. On a stock intake and motronic you wont come anywhere near utilising the capability of the 288 cam but you will have a choppy idle. You will want to check the piston to valve clearance with a 288 irrespective as it will more than likely be a problem but its not hard to deepen the pockets a mm or two.

[dvallis]

Hi there just to get some clarity, for a cheap stroker build I can the 2.8 crank from the 328i M52 E36, rods from a B27 engine and 325i pistons, using an 855 head from an M20B25 engine right?

All other accessories can be swapped from an M20B20 engine right? Of course just doing some slight cleanup in the exhaust and intake areas and refreshing with new main and rod bearings , ARP main and rods bolts and the recommended schrick 288?

To a first order, yes. That is what we did for Das Beast first go around. Look at post #1 in my build for a parts list. Don't forget you will need a thicker head gasket to get proper valve clearance. But this makes the timing belt tight. Better to machine the valve pockets deeper but I don't know how stock pistons will like that.

We found out our 30 year old pistons developed micro cracks when pushed hard. Ended up using forged pistons and rods, shallower bowls and deeper valve pockets on the second build. 10:1 compression. Will have a dyno sheet soon to compare the difference.