Stroking/boring M20B25/B27: Sectioned blocks show critical dimensions. PICS!

[digger]

That's why I said, have we even found the limit of the head yet, because it seemed like Tony wasn't really sure just how much power it would actually develop vs it's potential. You are just assuming the head is now the restriction.

Yeah, the problem in development of a huge stroke is fitting the ring pack and crown in the same space, which the usual piston suppliers have a bit of a cry about. I wasn't saying one couldn't be made, but i don't see it happening without some concessions, one of which is cost, another is reducing the pin diameter which means rods too.

Yeah, I wasn't talking about the combination fouling on the crank counterweights, more that increased stroke changes the rod angle in relation to the block and the bottom of the bore and side of the block. What did you end up with before to the block and rod? about 5mm? Was Wondering if there would be enough room for the rod to clear as the added stroke is going to push the rod closer to the block.

You can see why bmw made the M50 block taller when they replaced the M20. Even just a few mm is enough to make a difference with much longer strokes.

in fairness i think TK said 360bhp ish which is what it probably would show on pump fuel on the engine dyno (if it could run with that CR) on 98RON. add e85 in the mix it might be 20-30+hp from the fuel/CR. older versions of pipemax do not have adjustments for fuel iirc and guestimates from TK are somewhat based on pipemax (headflow and VE essentially) so he might have not expected as much benefit from the fuel but i am speculating about this. EAPro shows that E85 and high CR would be about the numbers i wrote above but i dont know how realistic it is.

nevertheless getting much more CFM from the head in a way that translate into more power (burns properly, correct velocity etc) is limited and will ultimately limit the peak power possible. once the head has an efficient Cd (shape) all you can do is get the size bigger to a limit of the casting, layout and size of the valves, chamber shape etc to turn more rpm. TK wasnt that far from this on the race engine.

there is a limit to how much power you can make with a given CFM and head characteristics and fuel, changing the stroke will for the most part only affect the rpm where it happens. there is more tunability in higher rpm so a smaller higher rpm engine has a bit of an advantage with wave tuning etc

if i run a sim on my engine with more cubic inches all i do is get more torque no more peak power but it peaks a few hundred rpm earlier.....its not until i start influencing the head flow and cam profile do i start getting much higher peak hp

[Madhatter]

That's why I said, have we even found the limit of the head yet, because it seemed like Tony wasn't really sure just how much power it would actually develop vs it's potential. You are just assuming the head is now the restriction.

Yeah, the problem in development of a huge stroke is fitting the ring pack and crown in the same space, which the usual piston suppliers have a bit of a cry about. I wasn't saying one couldn't be made, but i don't see it happening without some concessions, one of which is cost, another is reducing the pin diameter which means rods too.

Yeah, I wasn't talking about the combination fouling on the crank counterweights, more that increased stroke changes the rod angle in relation to the block and the bottom of the bore and side of the block. What did you end up with before to the block and rod? about 5mm? Was Wondering if there would be enough room for the rod to clear as the added stroke is going to push the rod closer to the block.

You can see why bmw made the M50 block taller when they replaced the M20. Even just a few mm is enough to make a difference with much longer strokes.

[LJ851]

Build more torque (and not just shift where you are making it), you are going to make more horsepower. Can't build one without the other, so increasing the stroke and capacity is going to result in more power.

Increasing the stroke will result in more torque which at a given rpm will produce more hp, this does not apply equally across the rpm band though. Generally the peak tourque rpm is lowered and the max hp number at high rpms is not always increased.



Edit: Damn i'm a slow typer.

[digger]

Build more torque (and not just shift where you are making it), you are going to make more horsepower. Can't build one without the other, so increasing the stroke and capacity is going to result in more power. Has TK even found the absolute limit yet, in terms of flow potential I mean? It seemed more like he had the flow figures and potential, but the result surprised even him. Would be a different way to go about it reaching the maximum potential though, wouldn't need the revs and probably such an aggressive camshaft with the added capacity.

I don't even think you would be able to run a stroke that big within the confines of the block. Rod angle would be pretty extreme, a 140mm rod would leave you about 18mm compression height which isn't going to work. Lowest I've ever seen is about 20mm I think, and im not sure if you would be able to get a 22mm pin in there at that thickness. Even if you went to a 135mm rod, I reckon you might struggle to get any of the usual piston companies to produce you a piston to suit, and that's if the rod angle doesn't cause problems at bdc. If you have to start sleeving the block because of the added stroke, you'll reduce the clearance even further.

i meant it won’t make more peak power, that is governed more by the head and induction. if you make more torque at 4k then yes you are making more hp at 4k to.
i think the focus should be more on correct induction and exhaust (sizing an length) and quality of burn than getting a few more CFM.
if you look at MM engines they use a 139.75 mm rod with 86mm stroke. that is a nominal compression height of.
206.0-139.75-43=23.25mm
Take the extreme case of HIOP 95.8mm stroke with 135mm rod using 23.0 mm comp height
206.0-23.0- 47.9=135.1mm, so use a stock length rod
Check clearance bottom of piston boss to crank CW
-47.9+135=87.1
Assume R70.5mm counterweight same as S52
You have 87.1-70.5 = 16.6 to fit the following
-Clearance between counterweight and piston boss say 2mm.
-Piston Boss wall thicknesses say 5mm
-Piston pin radius.
Leaves a piston pin with radius
16.6-5-2=R9.6mm
or D19mm
Theoretically possible with a heavy wall thickness
This is assuming the crank CW is only 71mm radius if not you need to machine it and rebalance or you are screwed.
You can go even smaller on the comp height of the piston but you need to consult a piston guy to look at options. I wouldn’t ever use a 2 ring piston except on drag engine that does a few passes and is torn down regularly. A compromised ring pack will hurt you more by virtue of poorer ring seal than you gain by a few more cubic inches.
I think the intermediate shaft would need to be about D12-13mm which is quite a bit thinner than the 18mm stock. The clearance to the bore would need a decent chamfer.
I think you could get it to work but don’t know how good it would be. BMW/Alpina used a 135mm rod with the 93.8 crank and a 21mm pin. This shows what BMW thought about rod ratio couldn’t care less it seems still came with warranty. The VAC S54 strokers are even worse for rod ratio……..They have extra deck height to play with and ended up with 305bhp and 270lbft in the B3S which is well above the M50/52/S52 engines.

[Madhatter]

Build more torque (and not just shift where you are making it), you are going to make more horsepower. Can't build one without the other, so increasing the stroke and capacity is going to result in more power. Has TK even found the absolute limit yet, in terms of flow potential I mean? It seemed more like he had the flow figures and potential, but the result surprised even him. Would be a different way to go about it reaching the maximum potential though, wouldn't need the revs and probably such an aggressive camshaft with the added capacity.

I don't even think you would be able to run a stroke that big within the confines of the block. Rod angle would be pretty extreme, a 140mm rod would leave you about 18mm compression height which isn't going to work. Lowest I've ever seen is about 20mm I think, and im not sure if you would be able to get a 22mm pin in there at that thickness. Even if you went to a 135mm rod, I reckon you might struggle to get any of the usual piston companies to produce you a piston to suit, and that's if the rod angle doesn't cause problems at bdc. If you have to start sleeving the block because of the added stroke, you'll reduce the clearance even further.

[digger]

Only if you manage to get one of those cranks

I'm all about more torque and making the motor have more low end than high end power. To me it is the key to a fun street motor.

i agree and my next build will be inline with this philosophy. there is of course a reason for doing all this given the recipe for a S52 crank M20 has been known for years ;)

[CrazyJew89]

Time to eliminate it and dry sump it then! 8^)

Only if you manage to get one of those cranks

a bigger crank wont bring anymore hp anyway just lower the rpm where the peak is made (where you choke the head). this of course means more torque which would be good on the street though no need to ring its neck to make it go.

I'm all about more torque and making the motor have more low end than high end power. To me it is the key to a fun street motor.

[digger]

Time to eliminate it and dry sump it then! 8^)

yeah not keen on the that kind of ordeal. you can still fit a huge crank without needing to go that far

a bigger crank wont bring anymore hp anyway just lower the rpm where the peak is made (where you choke the head). this of course means more torque which would be good on the street though no need to ring its neck to make it go.

[whodwho]

...the intermediate shaft would be very thin...

Time to eliminate it and dry sump it then! 8^)

[digger]

HIOP's site still has the 91mm and 95.8mm crank listed with their stroker kits. So in theory I could get one, but from what I have seen it seems unlikely I could get either of them to fit in an M20.

So if I have read everything right, the largest M20 you can get with purely adding stroke is 3L with the 89.6mm (s52/m54) crank, right?

i tried emailing but never got a response

91mm would fit subject to counterweight size, 95.8mm would be very very questionable the intermediate shaft would be very thin and you would need very slim shouldered rods

[CrazyJew89]

HIOP's site still has the 91mm and 95.8mm crank listed with their stroker kits. So in theory I could get one, but from what I have seen it seems unlikely I could get either of them to fit in an M20.

So if I have read everything right, the largest M20 you can get with purely adding stroke is 3L with the 89.6mm (s52/m54) crank, right?

[digger]

hiop used to do a 91mm and 95.8mm for the M50/M52 stokers
BMW made a 93.8mm stroke for the S52 block that went into an ALpina B3 3.3 and B3S
MM do an offset ground S54 crank (s54 has bigger rod bearing so can be offset down the M50 size while increasing the stroke) for M50 based engines with 93mm stroke but adapting the nose is an issue

[tinkerputzer]

Biggest i'm aware of (in regards to stroke) for use in a m20 is 89.6mm (s52/m54).

[CrazyJew89]

What is the biggest crank someone can get their hands on?

[digger]

Thanks Alex. Diggers findings pretty much confirm there are no differences in oil gallery locations between the b25 and b27 blocks. A light chamfer on the bottom of the bore is really all that is needed to clear the rod shoulder with the bigger cranks.

both blocks will accommodate the biggest crank you can get your hands on (there are some rare pieces out there though) and the bore limit is pretty much the same.

quite rapidly as the stroke is increased the following all quickly escalate to be super critical
1)- low deck height (limiting the rod length/piston compression height which affects counterweight to piston clearance)
2)- intermediate shaft clearance to the side of the rod
3)- bottom of bore clearance near the rod shoulder
4)- counterweight to block internal clearance
fortunately these have all have been/ are able to be solved