M20 B3x Naturally Aspirated Stroker build past, present and future

[Zambuzan]

digger, opinion on IE, if any??

[hasa]

Excellent engineering! One of the best postings I've ever seen in a car forum.

[digger]

Nice work, great post Digger!

Having weighed many +.5mm stock replacement pistons, I would have to gather they are closer to 502g on avg, I seem to recall having a few over, but not by much (only know this because they had to be matched to the lighter ones, and wasn't happy about it). You are extremely close, not that ~10gm will make/break your post. :)


Also, even though the 885 head is not a true "hemi" in the classical sense, it is hemi within a hemi, no? lol

Finally, when you decided that "11.5:1" was the upper limit of knock sensitivity with 290° cam, was that based on others' input? A bloke that works with me at the shop has a 3.1 m20 with a lesser cam and only knocks with <89 octane on a decent timing table with 11:1. Going off memory, he has a 280/274 bump stick.... a 290 single pattern would bleed off more pressure, bringing down the dynamic, correct? I have a strong feeling you will have a very viable combination!

i think its classified as a semi hemi ? there is a german name for it i've seen but cant find the name

its not a hard upper limit by any means, for sure you can run higher compression in ideal conditions but the pros (less than 5hp for 1/2 a point) vs cons don't make it worth pushing it too hard for a street car that at some point in its life will see quite adverse conditions. In theory it does bleed off more pressure, but the more VE the engine makes the more mass that is trapped and therefore the pressure and temperature are higher and you get closer to the limit. The dynamic CR is a ball park estimate.

[ForcedFirebird]

Nice work, great post Digger!

Having weighed many +.5mm stock replacement pistons, I would have to gather they are closer to 502g on avg, I seem to recall having a few over, but not by much (only know this because they had to be matched to the lighter ones, and wasn't happy about it). You are extremely close, not that ~10gm will make/break your post. :)


Also, even though the 885 head is not a true "hemi" in the classical sense, it is hemi within a hemi, no? lol

Finally, when you decided that "11.5:1" was the upper limit of knock sensitivity with 290° cam, was that based on others' input? A bloke that works with me at the shop has a 3.1 m20 with a lesser cam and only knocks with <89 octane on a decent timing table with 11:1. Going off memory, he has a 280/274 bump stick.... a 290 single pattern would bleed off more pressure, bringing down the dynamic, correct? I have a strong feeling you will have a very viable combination!

[digger]

An MLS gasket that has 86.5mm bore

[Zambuzan]

What head gasket do you use for an 86mm bore? Also, have you had any experiences with Ireland Engineering and their products? I'm planning to build a stroker kit and IE seems to have some solid stuff.

[Blake B]

You win. I don't know how I walk out to my car now knowing I will never be able to give it anything that.

I'm speechless

[digger]

It’s been quite long size I’ve posted anything of substance so here goes…

A rather important piece of the puzzle is the piston. When you want a custom set sometimes you have to design your own.

There are a few basic fundamental things needed to lock in before getting into details:

BASICS

1) Bore
2) Compression Height CH
3) CR (gives the Piston dome/dish cc)

To answer those I need:
- Block Deck Height – mine is 206.2 mm so allow say 206.0 after machining.
- Rod Length – 135 mm stock.
- Stroke – 93.8 mm (throw 46.9 mm) Alpina B3 crankshaft
- Deck Clearance – Target -0.9 mm (negative is above deck) gives nominally the recommended squish with gasket thickness below.
- Gasket Thickness – I’ve assumed nominal MLS thickness of 1.90 mm (0.075”) I can adjust the squish clearance as there is 1.52, 1.73, 1.78, 1.91, 2.03 and 2.18 mm thicknesses available.
- Gasket Bore – 0.5 mm larger than the bore is a safe bet so 86.5 mm.
- Combustion Chamber Volume (cc) – Mine is 46 cc.

1) Bore

This is easy my current pistons are 86 mm, makes sense to keep it the same especially with a big valve head. Others might like to use 85 mm to be able to use an OE composite gasket. There are 86 mm pistons that work with an OE gasket by having a large chamfer but there are a few things I’ve never really liked with the idea.

2) Compression Height

This is simple maths
CH = Block Deck Height – Rod Length – Crank Throw – Deck Clearance
CH = 206 - 135 - 46.9 - -0.9 = 25.00 mm

3) Compression Ratio

For a street engine running on 98RON with a cam around 290 duration 11 to 11.5:1 is about the upper limit that is “recommended”, some will say more is ok some will say less is better, in reality it just depends there are many variables. Something around that value with the cam in question gives a dynamic compression ratio of 8 to 8.4. So I split the difference and aimed for 11.25:1. I may look to setup knock control as an added safety measure.

Some more maths to work out the total combustion chamber size with

Swept Volume per cylinder = pi/4 * Bore ^2 * Stroke = 544.9 cc (3,269 cc)
Net Combustion Chamber Volume = Swept Volume / (CR-1) = 53.2 cc

Again some more maths to work the dome/dish cc, it is fairly simple maths

Piston cc = Net Combustion Chamber Volume – Cylinder Head cc – Gasket Volume – Deck Clearance Volume

Cylinder head cc = 46 cc

Gasket Volume = pi/4 * Gasket Bore ^2 * Gasket Thickness = 11.2 cc

Deck Clearance Volume = pi/4 * Bore ^2 * Deck Clearance = - 5.3 cc (this is negative because the clearance is negative i.e. above deck)

Therefore

Piston cc = 53.2 - 46 - 11.2 - -5.3 = + 1.2 cc (i.e a net Dish, a negative would be net dome)

Preliminary Piston Verification:

Will the long stroke work?

There are few issues with using the long stroke:

i) The short CH:

The required compression height is 25mm. This is quite short but not as short as some “off the shelf” pistons from MM.
To do this I decided on a 20mm pin, it allows a bit more room for the ring lands and gives more clearance between the valve pocket and the top ring as well as better clearance between the piston boss and crank. It means getting the rods rebushed and honed to 20mm so it’s not overly difficult you just end up paying a little extra.
Rings are:

1st: 1.0mm Barrel (thermal spray coating)

2nd: 1.2mm Napier. The napier ring is really good for added oil control as it scrapes the wall well.

3rd: 2.8mm 3 piece Standard tension (inc oil support rail)

https://www.youtube.com/watch?v=WCc0OUkTOEE

ii) Piston Boss to Crank Counterweight Clearance

The picture below shows the clearance, people would know that when doing the 2.8L budget stroker this is an issue due to the short rod and large counterweight crank. Fortunately the crank I’m going to use doesn’t have overly large counterweights. They are the same as the S52 and smaller than the M52b28 crank. Also the rods are longer so it’s actually less of an issue than on the budget stroker



Clearance = rod length – crank throw – crank counterweight radius – pin radius – piston boss thickness
= 135 – 46.9 – 70.5 – 10 – 5.5 = 2.1 mm which is plenty of clearance.

iii) Piston deck thickness

With the short compression height and a dish the little end of the rod limits how thick the crown can be



The maximum dish that can be run depends on a few things

The minimum piston crown (in the centre of piston) I’ve seen recommended is about 4.5mm for NA (opinion of people working in the piston manufacturing industry). I think allowing for 5 or 5.5mm is a better idea.

The minimum clearance I’ve seen recommended for the rod to underside of the piston is 1.25mm (JE data sheet)

https://www.jepistons.com/PDFs/TechCorner/SCPDrawings/piston_instrc4032.pdf

The rod has a radius of 16.25mm (drawing provided by the manufacturer)



So the maximum dish = Compression height – rod little end radius – underside clearance – crown thickness

25.00 – 16.25 – 1.25 – 5.5 = 2.00 mm

Piston Design


Dome Design


this is the data for the OE pistons



The OE design has the angled squish pad for generating turbulence in the combustion chamber which helps with the burn speed and reducing knock. It also has the dish offset towards the spark plug and exhaust valve.
With a donor 9.7:1 slug some simple radius gauges you can learn quite a bit







You can also see where the centre of the dish is by looking at the machining marks so you can measure the dish offset
So this is how the stock 9.7:1 m20 piston dome is constructed
Spherical dome (not a hemisphere) approx R100





Subtract a spherical dish approx R106mm
the centre of the sphere is offset in two directions from the bore axis (towards the spark plug) as well as being offset in the piston axial direction





section view showing dish



So in actual fact the squish is not a chamfer it’s slightly radiused. Same with the head the machined part is radiused (concave) to match the piston



Then there is adding in the valve reliefs



The reliefs need to be bigger to account for the bigger valves and deeper to account for the different cam. I ended up designing it for 45/37mm valves even though the exhaust is only 36mm, just in case. I also made the depth bigger to suit a higher overlap cam than I will actually be using first off again just in case. The cam I am using is pretty conservative so I can see something with more overlap being used later.
One issue with a smaller diameter piston such as 84.0, 84.5, 85.0 etc is the larger intake valve pocket get too close to the top ring groove so it needs to be lowered which then makes it harder to fit the rings in the short compression height




Dish Design

Given I need a 1.25cc net dish if I have a dome of -3.15cc then the dish needs to be 4.4cc which needs to be achieved while adhering to a limit of 2mm depth. This required having a modified spherical dish with a small flat section in the centre of the dish. Having a definitive flat section will help with setup of deck clearance when it’s at the machine shop

This is the final dome design done which is basically the same as OE with a few changes to get the CR where I wanted



So it was time to do a few mockups to check the dome to see if the squish pads were correct and that valve relief locations and depth were correct. So I had some 3D printed (with the bore 84mm) so it would fit into the spare block that has stock bore size.
This one was done with one the cheap machines by a mate but wasn’t quite accurate enough for my liking so I had a second one done





I also had valve relief checking device printed to check radial clearance and axial clearance.




Now I had proof on concept I contacted Mahle MS and gave them drawings and a 3D model step file so they can add on the slipper skirt details etc

There was a choice of 2618 (M-SP25), 4032 alloy (M124) & the proprietary Mahle M142 alloy. The latter was only possible with a billet piston due to the restriction of the M142 forging. The billet was $700 more expensive and the difference was judged to be not worth the extra cost as the mechanical property differences are pretty subtle as shown below. The added cost is presumably because of the added machining time.

I settled on 4032 as it’s a street engine that isn’t an extremely severe application and the better wear characteristics and being able to run a tighter piston to wall with a short piston height were the main deciding factors.





FINAL SPECS

Mahle MS pistons
Bore = 86.00 mm
Compression Ratio = 11.25
Pin = 20 mm with an offset towards the thrust side (for quieter running). Offsetting the opposite way does make a little more power.
Install clearance = 0.0020” (coated)

They come standard with
- slipper skirt with grafal coating,
- phosphate coating (reduces micro welding and is actually only needed on ring grooves and pin)
- accumulator groove between 1st and 2nd ring grooves and
- backcut 3rd ring land
- wire clips with tang

The piston mass is = 276 g
The pin is 50L x 20D x 5.0t mm and the mass is = 92 g
The mass of the rings is = 26g
The total mass is = 394 g
Stock piston mass is 375 g and pin 105 g = 480 g excluding rings (est 510g with rings)









So there is no excuse for someone using a flat top with the 885 head ever again

EOM

[LJ851]

with a bit of luck santa might be coming early and some parts might be arriving

Oh, baby !

[digger]

with a bit of luck santa might be coming early and some parts might be arriving

[318aye]

Glad you caught it!

[digger]

had a near miss the other day. coolant light came on, a quick check of temps and knew there was a real issue rather than a pesky little leak. shut it down pretty quick temps hadn't reached 1 pm but without coolant you never know whats happening in the head. hose had come off after atleast 5 years of not being touched and lost all the coolant really fast.

thankfully looks like i saved it in time, but need to keep an eye on it....

ironically i was on my way for the annual road worthiness check to get registration for another year so was close to my mechanic and had it towed there and he sorted it out

[digger]

Just emailed them and they said they are hardened chrome moly

$200Usd plus shipping

Not too bad really

well there you go, 4130 steel

[BUDNUNTA]

Just emailed them and they said they are hardened chrome moly

$200Usd plus shipping

Not too bad really

[digger]

When you made the crank spacer, did you use the 24v boss and machine it down?

Do you think that crank seal surface should be hardened?

crank spacer? do you mean the thick washer that is machined down or the spacer for the oil seal?

i would harden the oil seal surface as that's not something you want issues with. the recommendations I've seen are minimum 30-35 HRC and some say 45 HRC or even 60HRC for high surface speeds etc here are some links

http://eriks.nl/documentatie/afdicht...n/oilseals.pdf pg 16

https://www.parker.com/literature/En...stems/5350.pdf pg 21

http://www.skf.com/au/products/seals/industrial-seals/power-transmission-seals/radial-shaft-seals/shaft-requirements/harness-and-surface-treatment/index.html

http://www.dichta.com/dms/site-dicht...ft%20Seals.pdf pg 10

https://www.timken.com/pdf/10175_%20...TechManual.pdf pg 6

you'll notice that there are also some quite specific surface finish requirements

last time i use a MM piece, i don't know if they harden theirs ( i would guess they do) but its definitely a quality piece, but spendy