M20 Stroker Build: Take 2

[Lugnuts]

Those timing numbers seem low for a NA engine, the power numbers are around what I’d expect on a mainline dyno with mild cam, stock head and stock manifold

Well I guess if the power numbers are where they should be given the parts used, then that's something. Perhaps this is why people go straight to LS1/S52 swaps lol.

I just can't shake the feeling that something is wrong with the timing...pretty much every M20B25 timing map I've looked at has ~25-35 degrees up near 100kpa across the rev range. That and the M20 engine has been described as not knock-limited.

I read somewhere the stock timing map retards aggressively down to 15 degrees at 5000...which is pretty much what I have here...

The whole impetus behind the rebuild was to address the detonation with the previous engine that I thought was a consequence of flat-tops with the 885: no quench band meaning poor intake charge tumble/mixture. Seems that assumption was incorrect

Need to re-re-confirm the timing settings. I thought I'd discovered my #1 Tooth Angle offset was wrong, but the cranking timing light + fixed advance test showed it's bang-on 85 degrees...right where it should be.

Let's see what the MegaSquirt Tooth Logger can tell me...

Edit: looks fine - one long tooth, meaning the polarity is correct: Falling Edge.

[digger]

Those timing numbers seem low for a NA engine, the power numbers are around what I’d expect on a mainline dyno with mild cam, stock head and stock manifold

[Lugnuts]

Thanks Digger. Took your advice and got all cylinders as close to 1mm as possible.

Just picked up the car from the dyno. Blue is current 10:1 engine. Red is previous M20 engine that had flat-top pistons w/ 885. Using the same 272/272 IE cam in both engines. 98 octane.





Tuner said he can't add any more timing w/o the engine detonating. Said with 1 degree he picked up 5kw, suggesting there's more to be had, but it started to ping so he's dialed it back to keep the engine in one piece.

Do these numbers look wrong for a rebuilt 10:1 M20 engine?

Cranking timing was confirmed using a light: fixed 0 degrees advance and the timing marks lined up nicely. #1 Tooth Angle at 85 degrees BTDC (Megasquirt settings).

Tuner suggested maybe a longer duration cam with more overlap to try and bleed some dynamic compression - but I feel like 272/272 is pretty much the entry level/conservative option building an M20 stroker.

I'm kind of at a loss to understand what's going on. I'd imagined this M20 would make something closer to 180ft/lb / 190hp. Both this and the previous engine are, for some reason, severely knock limited and pretty much making the SAME POWER?!

[digger]

If aim for 0.040”

[Lugnuts]

Appreciate all the help with the main bearing clearance.

The pair of late model No 6. BMW yellows arrived. Current clearances are:

1	0.052 / 0.0020"
2	0.054 / 0.0021"
3	0.058 / 0.0022"
4	0.059 / 0.0023"
5	0.055 / 0.0021"
6	0.066 / 0.0025"
7	0.058 / 0.0022"

Thinking I'll go with a green + yellow for #6, dropping it to 0.056 / 0.0022"...a tad more consistent!

Spent a bit of time today measuring the piston to head clearances. Two strips of 2mm solder per cylinder, took the smallest measurement from each. Std 1.75mm Goetze gasket.

1	1.60 / 0.062"
2	1.50 / 0.059"
3	1.42 / 0.055"
4	1.37 / 0.053"
5	1.45 / 0.057"
6	1.50 / 0.059"

A bit surprised to see the "bowing" in the numbers...the deck and head are definitely flat....I only had 10 spare head bolt washers, so I just used the centre 6 and outer 4 bolts...wondering if that's what caused it.

Edit: looking back at my photos, it might also be from the strips of solder sitting at slightly different locations around the quench band...

My notes show a good target for P2H clearance is < .050" & > .035"....based on the above, I could take 0.010" off the block and still be safe.

Does that sound like a sensible plan? Unsure how "stretchy" the stock M52 rods are.

(ignore the crud on the piston...spark plug carbon...forgot to remove them before installing the head)

[ForcedFirebird]

As the link says Digger posted, you can add the shells wherever. Been a long time ST member, good stuff over there. I like what "Baprace" and "Wolfplace" had to say about it:

"I have used different size bearing shells from the same manufacturer and when I use a std on top and a .001 on the bottom there are no problems and on next rebuild I use the other 1/2 sets and no problem there either. Now if i am using one journal different I always try and put the smaller shell on the top so the "line bore" remains the same.
I have used different size bearing shells from the same manufacturer and when I use a std on top and a .001 on the bottom there are no problems and on next rebuild I use the other 1/2 sets and no problem there either. Now if i am using one journal different I always try and put the smaller shell on the top so the "line bore" remains the same."

Same here
My opinion it does not make a lick of difference which half you change.
The damn crank probably looks like a piece of spaghetti when running anyway

The only thing I do not do is put one in up & one down in the same engine
If I change one or more I always change the same, halves be it top or bottom

Consider you are moving the centerline by a whole 2 1/2 tenths,,,,
I seriously doubt the engine will know the difference

How do you actually add the spring shims to the relief and bypass? Are you basically just augmenting the force required for them to open?

Correct.

why would you even do that......... if there is nothing wrong with the bearing clearances or anything else mechanically that affects the oil flow path / leakage paths whatever the resulting oil pressure ends up it ends up, all you need to do is ensure the operating oil temp is appropriate for the oil viscosity.

the big thing that kills bearings is mostly interupted oil supply to the pickup and subsequent lack of oil flow rate to the bearings, aerated oil and things of that nature, its not lack of oil pressure in the sense of normal operation of the pump and pickup.

We only do it on the endurance cars since the oil supply does get interrupted at times, aerated, as well as loss of viscosity. Even with the increased pressure (and volume) the 10w60 oil is like runny water at room temp by the time we have been at or near WOT for 12-24hr straight. No need on a street car - as you said, those operating temps/abuse is not what it is in an endurance car. I was more aiming to the fact that .0001" isn't going to change the pressure significantly.

[digger]

https://www.speed-talk.com/forum/viewtopic.php?t=22401

[Lugnuts]

Don't be afraid to mix-and-match bearing half shells, either.

I actually tried this a week or so ago. With a green + yellow I get a good clearance.

Only issue is, as I understand, if you mix and match bearing shells for one main, you're supposed to do it for all mains - the thicker shell on the bottom half will raise the crank centre line.

I don't have enough experience to know if that tiny difference is something I can safely get away with?

Putting greens across the other 6 mains will tighten clearances by 0.01mm/0.0003"...which should be irrelevant for my DD use case...

[digger]

why would you even do that......... if there is nothing wrong with the bearing clearances or anything else mechanically that affects the oil flow path / leakage paths whatever the resulting oil pressure ends up it ends up, all you need to do is ensure the operating oil temp is appropriate for the oil viscosity.

the big thing that kills bearings is mostly interupted oil supply to the pickup and subsequent lack of oil flow rate to the bearings, aerated oil and things of that nature, its not lack of oil pressure in the sense of normal operation of the pump and pickup.

[tiger_e30]

According to my literature (blue text means it was converted from metric, and note the typo for m20 green bearings)...

Tighter clearance at the bearings means more oil pressure somewhat on m20's, but I have found modifying the relief and pump is the way to increase pressure. I even had one crank on the tight side of the factory tolerance and pressure didn't make a noticeable difference (~16psi hot, ~65psi max cold [factory relief]). By adding spring shims to the relief and bypass in the pump itself the race car sees ~90psi cold and ~25-ish psi hot.

How do you actually add the spring shims to the relief and bypass? Are you basically just augmenting the force required for them to open?

[tiger_e30]

Plus 1 to forced firebird. This saved me a world of hurt when I was measuring main clearances and was coming back high per Bentley

[ForcedFirebird]

According to my literature (blue text means it was converted from metric, and note the typo for m20 green bearings)...



Tighter clearance at the bearings means more oil pressure somewhat on m20's, but I have found modifying the relief and pump is the way to increase pressure. I even had one crank on the tight side of the factory tolerance and pressure didn't make a noticeable difference (~16psi hot, ~65psi max cold [factory relief]). By adding spring shims to the relief and bypass in the pump itself the race car sees ~90psi cold and ~25-ish psi hot.

Don't be afraid to mix-and-match bearing half shells, either. It's actually common practice to use a shell from each color to get where you need it. In fact, when I was a kid and the 383 Chevy stroker was becoming popular, many builders would use 2 sets of bearings stacked on top of each other to pad the difference when mixing Ford rods, 400 crank and 350 block (or turn some spacers to half and pad) - took a few years for real kits to come out, but a very popular budget hot rod engine that was quite reliable.

We’ve also found that measuring all eight rod bearings will create a range of clearances that might span 0.0005-inch. Swapping bearings between the loosest and tightest can sometimes narrow the range of clearances. This idea can also be used on main bearings. Of course, substituting half bearing shells is another common way to bring the clearances closer to the desired spec. For example, the builder can compensate for a slightly oversize standard crank journal with a 1X bearing that offers more clearance. By adding one shell rather than both, this will increase the clearance by 0.0005 inch.

http://www.superchevy.com/how-to/engines-drivetrain/1904-engine-blueprinting-tips-part-1

[digger]

M20 spec is 0.0012-0.0028
S50 spec is 0.0008-0.0018

honestly itll be fine its a fairly mild build, id more worried about not being too far tight on the big ends

i think its the other way, the tighter clearance creates more oil pressure

[Lugnuts]

Got 3 strips of FlexiGauge. Confirmed my measurements yesterday - yellow is loose, greens is tight.

I also have a 2nd bore gauge for smaller bores, but the stem and dial are compatible - same numbers.

Even took the block of the engine stand so I can get at #6 from the back side - no difference.

Does the S50 run at a higher oil pressure? making a tighter clearance possible vs an M20?

[digger]

Good catch. Haven't realized the bearing haven't been bolted down yet. Bearings change shape once clamped.

i meant as a 2nd opinion can get a slight difference between the method that might be favorable such that the tighter option because more palatable if it isnt already. he had the caps torqued down.

i'm pretty sure BMW recommendation for other engines such as S50 and the like are such that this would be in spec, so unless this a turbo engine or high rpm its probably ok at 0.0017"