full fat m20b25 spec

[steve_k]

time for a bit of a debate,

what would some of you class as a "full fat" m20b25 spec??

i'll be clear on a few things, NO FI, has to remain a 2.5 (no 2.7/2.8 strokers though a slight rebore/extreme hone is allowed)

so....................what would it consist of??

head work?
cam's?
6 branch?
induction?
ECU?
blueprinting?
lightning & balancing?
ETC.

so come on folk's lets hear what you would do to build the ultimate "full fat"
m20b25 screamer/hooligan car??

discuss.

[steve_k]

well i'll start then,

headwork, full port polish & gasflow, 5 angle valve seats, bigger valves (?), reshaping of the combustion chambers.

cams, schrick 282/274dg+ , adjustable cam gear pulley.

inlet/induction. Dbilas ITB's, dual throat carbs. or MAF conversion.

bottom end, HD/race bearings & shells, knife edge'd all steel crank, H section steel con rods, 0.5 - 1mm rebore & extreme hone, light weight high compression piston's, light weight flywheel, full blueprint & lighten & balance.

exhaust, BTB3 6 branch manifold, X-pipe, stainless steel exhaust (rear box only).

ECU, know here it get's tricky. for me it would have to be MS3 with COP, wasted spark ETC.

well thats how i'd go about building my ultimate "full fat" m20b25.

so come on folk's lets have your list of parts on what you do to build your own.

[sweep]

Build a 2.8 stroker steve

[steve_k]

Build a 2.8 stroker steve

been thinking about it pete

but with the above spec

now that would be fun

[sweep]

My blue car in my avatar is running a 2,8 stroker with sport gearbox and btb 6 branch and trick cam and all lightned and balanced bottom end and big bore t/b

[steve_k]

My blue car in my avatar is running a 2,8 stroker with sport gearbox and btb 6 branch and trick cam and all lightned and balanced bottom end and big bore t/b

sounds like fun lol.

next time i'm up sheffield way i'll have to pop round your yard for a nosey (& if poss a passenger ride?? )

[daimlerman]

There's little cheap improvement for an M20,bang for buck, an increase in capacity wins every time.

Take my 2.7 as an example;

2litre giving 129bhp on a good day,add a 84mm bore plus stock 325i bits to get 175bhp...

[steve_k]

There's little cheap improvement for an M20,bang for buck, an increase in capacity wins every time.

Take my 2.7 as an example;

2litre giving 129bhp on a good day,add a 84mm bore plus stock 325i bits to get 175bhp...

i know what you mean malc but i just thought that it would be nice to see if folk could come up with a spec list to build a proper b25.

i mean not everyone wants a stroker lol.

[reggid]

it's a secret someone might steal my ideas?

[HairyScreech]

thing is if you are not limited by regulations to a set capacity then going up to a 2.8 is so easy its pretty much the first mod to consider in the hunt for power.
you only have to look at the leap from m52b25 to m52b28 to see the effect of the capacity increase.

if you want power then always start with the biggest version of the engine that is easily/cheaply available.
put simply don't tune a 287ci v8 when there is a 350ci for £100 more.

[hongkongfuey]

stock 2.5
cam
bbtb (home made)
hottuning manni (no x pipe)
m30 injectors and afm
home made intake elbow
dmcl chip
lightened flywheel

[steve_k]

@hairyscreech, not limited by any regulation r:e capacity just wanted to know what folk would do to build a full on M20B25 without going down the stroker route.

I'm looking at going for the 2.8 stroker build over winter when I've collected all the parts also been reading your 2.8 stroker build thread with interest.

[kimbo]

Our 'old' motor had:
Pretty much std cank + rods
Lightweight flywheel
Custom pistons
Modded combustion chambers (sort of, but not, Alpina style).
Oversize inlet valves
Ported
Hartge 6 branch
3 x 45dcoe's
Unknown cam
Made (iirc) 208bhp at the wheels.
Then it went bang (piston broke)

Our new motor will have / has got (half way through collecting parts, etc.)
Lightened, balanced + knife edged crank
Lightened flywheel (+ AP Racing twin plate clutch)
M52 rods lightened, balanced, polished + peened
Custom pistons, standard crown shape but (very) high compression
Head tidied up
Oversize inlets
Ported
BTB 6 branch
ITB's on custom manifold
Cam Schrick or Catcams (not researched it properly yet)
Strengthened rockers
Standalone engine management - possibly Emerald, possibly Haltec (got one sitting around).

Looking to see 220+bhp at the wheels (250 - 260 at the flywheel) at around 7500rpm, not expecting much action below 4500rpm.
It's for a circuit racer and some regs have a capacity break at 2.5l. I've still got the original block which I'm thinking of overboring and stroking to give about 3.1 litres for the 'ultimate' M20 - reckon I could get over 300 brake out of that, but haven't got the money at the moment to do both.

Kim

[steve_k]

@kimbo. Well the spec of both your engines is very impressive especially the "old" one lol thats definitely my kind of spec.

if you want to stroker the m20 out to 3.1 then have a search for reggid as his thread has all the details of how to go about it.

[reggid]

very easy to get carried away with these things.....The power potential is limited by the head which ends up costing power allover because you need to cam and tune exhaust /intake runners to make the torque at the topend to make good hp which makes the engine relatively peaky. I went with a tamish build because i use the car as a daily driver with A/C etc but there is a heap of hp available if you do your research and are prepared to give up some below 4k.

I'd like to build a 2L M20 big bore short stroke long rod i recon you could make 250+hp screamer without much problems and would be revving sky high but i dont have the cash so it'd be a 3.1L if i did it again so even when off cam there is a little bit of pull to it

[Kedge]

stock 2.5
cam
bbtb (home made)
hottuning manni (no x pipe)
m30 injectors and afm
home made intake elbow
dmcl chip
lightened flywheel

That's good result

Mine will just be running a BTB2, BBTB, unknown chip, lightened flywheel, refurbed standard injectors & Scorpian exhaust when it hits the road.

Seeing more mention of the M30 AFM. Does that need the M30 injectors to work ok? What benefits does it offer over the M30, higher volume of airflow? How's it compare to MAF?

[hongkongfuey]

The chip i have sorts out the m30 afm to run properly, the m30 is a bigger bore , and is basically plug and play, no wire changes. Dmcl can do the chip with or without m30 injectors. Its apparently easier to map???

[Mikey_Boy]

All good stuff... My 3 pence worth:

- Doing a stroker really is worthwhile - the extra torque can help with running bigger cams and helps drag the car out of corners nicely - torque wins races, not power!
- A MAF conversion is worth the pain if you are staying with injection.

I wouldn't bother with ITBs - a bit of controversy exists about these! DCOEs seem a better if you can live without a brake servo...

Depends on the application really...

[reggid]

All good stuff... My 3 pence worth:

- Doing a stroker really is worthwhile - the extra torque can help with running bigger cams and helps drag the car out of corners nicely - torque wins races, not power!
- A MAF conversion is worth the pain if you are staying with injection.

I wouldn't bother with ITBs - a bit of controversy exists about these! DCOEs seem a better if you can live without a brake servo...

Depends on the application really...

i agree re doing a stroker but power/weight is king. A 170bhp B27 is not going to be much faster than a 170bhp B25 when moving at any decent speed. The lowend and midrange is less useable because the engine doesn't operate much in this part of the power curve in an allout race. On the street torque is good no need to keep things on the boil to get moving

ITB add throttle response and help isolate nasty pressure waves from big cams at idle and low throttle openings. The throttles dont add WOT gains only the runner geomerty and plenum/airbox really influence this. Individual runners help even more with big cams and a big airbox/plenum does the same but keeps the inlet charge cooler

[basketweave]

I'd like to build a 2L M20 big bore short stroke long rod i recon you could make 250+hp screamer without much problems and would be revving sky high but i dont have the cash so it'd be a 3.1L if i did it again so even when off cam there is a little bit of pull to it

Why 2l? Some old school people I know recommend a 4-5mm over bore compared to stroke, and the longest rod you can fit in there. (for a revver)

I have the beginnings of this recipe for a revver:
2.7 crank (81mm stroke)
140mm rods (from a m50 325i)

What would be next in my recipe is a 731 head (so i can run flat top pistons)

Then it would be a dummy assembly to work out the pistons, say 85.5mm bore, maybe 86mm. Prob JE pistons. 12:1 comp.

That would give you your nice over-bore, and long rod combination.

Then ITB's, Autronic, and a nice big cam.

[reggid]

I'd like to build a 2L M20 big bore short stroke long rod i recon you could make 250+hp screamer without much problems and would be revving sky high but i dont have the cash so it'd be a 3.1L if i did it again so even when off cam there is a little bit of pull to it

Why 2l? Some old school people I know recommend a 4-5mm over bore compared to stroke, and the longest rod you can fit in there. (for a revver)

I have the beginnings of this recipe for a revver:
2.7 crank (81mm stroke)
140mm rods (from a m50 325i)

What would be next in my recipe is a 731 head (so i can run flat top pistons)

Then it would be a dummy assembly to work out the pistons, say 85.5mm bore, maybe 86mm. Prob JE pistons. 12:1 comp.

That would give you your nice over-bore, and long rod combination.

Then ITB's, Autronic, and a nice big cam.

ok maybe not 2L but 2L crank with biggest bore than can be done, only way to make peak power well past 8000rpm

[basketweave]

The problem with going past 8000 rpm's is the 2v head with the funky rocker design that has conniptions above 7000-7500rpms.

I would be happy with peak power in the low 7's, like 7300rpm or something. That's enough of a revver for me....

I have been recently told that the early cast cranks like the 2.7 doesn't like high revs either, as a friend of a friends cracked at 7500 rpm.

However Rama's 2.7 crank seems to be holding up well, as others.

[steve_k]

Well it looks like I've set the cat amongst the pigeons with this one lol.

nice to see some more leftfield combos on how to build a decent revving 2.5/2.7 lump.

keep them coming folk's as it's all good

[reggid]

The problem with going past 8000 rpm's is the 2v head with the funky rocker design that has conniptions above 7000-7500rpms.

I would be happy with peak power in the low 7's, like 7300rpm or something. That's enough of a revver for me....

I have been recently told that the early cast cranks like the 2.7 doesn't like high revs either, as a friend of a friends cracked at 7500 rpm.

However Rama's 2.7 crank seems to be holding up well, as others.

with the correct spring pressure, lighter valve train and non swiss cheese stock rockers it can and has been done

[basketweave]

OK fair enough, would like to hear an m20 at 8000 rpms!

I would be curious to know if they used the stock timing belt, I am not sure it would be suitable for high revving, high precision valve trains.

[reggid]

OK fair enough, would like to hear an m20 at 8000 rpms!

I would be curious to know if they used the stock timing belt, I am not sure it would be suitable for high revving, high precision valve trains.

unless you have a nice induction /exhaust noise they sound pretty mechanical (singer sewing machine).

i think if you changed it more frequently it would be fine but i have never heard about any upgrades. There are still lots of more modern engines that rev hard without problems on a timing belts

[HairyScreech]

Belt shouldn't be an issue until you start running the thing so fast it starts slapping the casing, a non-race m20 would never get near this point as i would expect it to be well over the 9k range with the length of these belts.

Rockers are a problem, the alu ones are stiff and light but like all alu components suffer fatigue and cant cope with a bit of valve bounce, worst thing is they don't fail safe and bend they fracture instead.

The bearing losses are huge on these things, i worked out the area of the bearings for the dyno sim and there is more overall bearing area than most v8 engines, not good.

[driftwood]

V interesting thread. I'd like to build a 2.5l too. Strokers have their advantages but I think I'd rather fit an engine from another BMW if I wanted more capacity. I don't want to go down the turbo route either, but I'd like to end up with something on the happy side of 200 bhp and that can rev. So, larger valves? An increase in cr? Lighter bottom end? Bigger cam?
I have some questions. How light can you go with the flywheel and which combination of stock BMW parts should I use for a lighter bottom end? I don't want to end up with a cr that needs super unleaded all the time and I can't afford a steel crank/rods and custom pistons. I have done some research but am getting confused. How aggressive can the camshaft be before it causes issues with the emissions for the mot? I'm gathering parts for an itb conversion and money is being put aside for an ecu. Can't decide which ecu though. Come on you experts, spill the beans!

[steve_k]

@driftwood, so far I've found that a cam in the 276*+ region to be a good compromise between a smoothish idle & a bit more power,

also, porting, polishing & gasflowing the head with multi angle valve seats helps the engine breath a bit better when combined with a BBTB, freeflow induction, bigger AFM (M30), maybe even a MAF conversion, & a 6 branch exhaust manifold.

as for an ECU, there's so many to chose from, emerald, MBE, DTA, megasquirt ETC, or you could go for an M5X ECU as hairyscreech is doing.

me personally I'm looking at going for megasquirt for the adjustability of it.

[HairyScreech]

M52b28 rods for a 135mm rod 2.5 as they are 100g lighter each and just as strong.
Short skirt piston like you would use on a 2.8 conversion with some anti friction coat, infact get the crowns and skirts hard anodized.
Get the crank bobbed and lighten the flywheel and you should be looking at 3-6kg less moving mass.

That is only going to get you to 10:1 create though so you are probably better off going for custom piston.

[driftwood]

Thank's for the advice. Wouldn't 10:1 be enough?

[HairyScreech]

10:1 is a hell of a lot better than 8.8:1 so there is that. Its just not anywhere near the max, these things will tolerate 12:1 at a push, however 10:1 will still be nice and safe on poor fuel or with a pretty basic ecu (like the m1.3).

You could look into the m50/52/54 b25 cranks, they may be lighter or have slimmer crank webs, never looked at them.

However for the trouble that requires a stroker crank could be used for little increase in cost and a pretty much direct % gain.

[mattrs]

This isn't the usual my dad is harder than your dad you illiterate galvi baby m3 loving homo sh!t!


Very interesting indeed.

[steve_k]

This isn't the usual my dad is harder than your dad you illiterate galvi baby m3 loving homo *********!


Very interesting indeed.

you been down the pub with spook??

[reggid]

Thank's for the advice. Wouldn't 10:1 be enough?

depends on the cam you use