M20 Stroker Build: Take 2

[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)

[digger]

If aim for 0.040”

[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]

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]

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]

Power is in the head, cam and intake manifold. Have you done a cranking compression test ? Might tell something about cam timing

[ForcedFirebird]

^^^agreed. Check your cranking compression. Something could be off, you don't want to see over 225psi.

The timing looks to be very low as we run more advance than stock in the 9.4:1 b27 race cars.Cranking compression is 190psi with the 272 and a touch under 200 with the stocker.

Here is a stock table from a 173.The vertical WOT map is basically your horizontal 100kpa in MS.

[digger]

the other questions i had were with old setup what was the timing map like? you sure the fuel is any good and not old and stale?

[Lugnuts]

Have you done a cranking compression test ?

I have now ;)

Pulled the fuel pump relay, all 6 spark plugs and zip-tied the TB open. Cranked the engine 10 times for each cylinder:

Cyl	PSI
1	200
2	200
3	200
4	205
5	205
6	200

The old timing map was very similar to the one above - max 18-19 degrees at WOT across the rev range. Similar issues with knock. Talking to the tuner after the dyno, I got the impression this engine knocks more than the previous. I suspect on account of the increase compression ratio.

Fuel was fresh in the tank a few days before the dyno run.

I should probably mention, I installed a Nuke adjustable cam gear since I figured I'd taken about 15-18 thou off between the head and block...wanted to account for the small amount of retard introduced.

Currently 1 degree advanced. I checked all the gear teeth line up with a stock one and the P2V clearance...she's good.

[digger]

I have now ;)

Pulled the fuel pump relay, all 6 spark plugs and zip-tied the TB open. Cranked the engine 10 times for each cylinder:

Cyl	PSI
1	200
2	200
3	200
4	205
5	205
6	200

The old timing map was very similar to the one above - max 18-19 degrees at WOT across the rev range. Similar issues with knock. Talking to the tuner after the dyno, I got the impression this engine knocks more than the previous. I suspect on account of the increase compression ratio.

Fuel was fresh in the tank a few days before the dyno run.

I should probably mention, I installed a Nuke adjustable cam gear since I figured I'd taken about 15-18 thou off between the head and block...wanted to account for the small amount of retard introduced.

Currently 1 degree advanced. I checked all the gear teeth line up with a stock one and the P2V clearance...she's good.

What I’d try is setting the cam retarded by about 4 degrees as that’s what others found to work with mild cams 284/272 on a 2.8/3.1 where it made more torque and power. That might help a little but there’s something odd going on. I presume you’re using something like a BPR6ES plug ?

[Lugnuts]

What I’d try is setting the cam retarded by about 4 degrees as that’s what others found to work with mild cams 284/272 on a 2.8/3.1 where it made more torque and power. That might help a little but there’s something odd going on. I presume you’re using something like a BPR6ES plug ?

Ok, I'll try adjusting the cam gear 4 degrees retarded. Wasted spark and no dizzy rotor makes changing it pretty straight forward.

Retarding should reduce dynamic compression on account of the later valve opening, correct?

Should probably coincide with another dyno session - see how (if) it gives us any extra headroom on the timing.

Yes, fresh set of NGK BPR6ES - all looked fine yesterday when I pulled them. Only done ~400Km since install.

I think previously you had suggested running a cooler plug, which I tried (unfortunately without any noticeable reduction in knock - purely butt/ear dyno though). They'll be in a box somewhere...might dig them out and see what condition they're in.

[digger]

others found it runs better, but the reduction in dynamic compression ( later inlet valve close )would help in your situation though I’m not sure it will be a drastic change as it would help more so with knock in the lower rpm area

You want to run the coldest plug that doesn’t foul, it might not help but it won’t hurt. The next step would be BPR7ES

[Lugnuts]

Set the timing 4 degrees retarded and went out for a drive.

Gotta say, factoring in butt dyno, I feel like I've lost some top end...bottom to mid feels like it has a bit more pep. Which doesn't make a whole lot of sense...

Maybe now 15 degrees isn't enough timing to make top end power???

99.995% sure I turned it the right way lol:

[digger]

It’s the right direction but 8 degrees at crank (4 at cam )

[Lugnuts]

So, I have a theory as to what's going on with my engine...the cam is an imposter. (supposed to be a 272/272 IE new grind, bought in 2013)

Why I think this is the case:

1. My car's idle seems very...stock; there's practically no lope. All (?) of the 272 M20 videos I can find sound noticeably different to mine (pronounced lope at idle),
2. Took the car for another dyno test, with 10 degrees cam retard (max permissible). The theory being to try and decrease DCR, and see if that allowed more timing.
   1. Tuner said he could "set any advance at cruise, and 4 more degrees at WOT" before knock. Engine also lost 10kw across the rev range.
3. Repeated a compression test with 0 degrees cam advance; cranked until the needle stopped. These seem on the high side for a 272 cam:

Cylinder	PSI
1	206
2	210
3	217
4	212
5	214
6	215

In short, I suspect my dynamic compression is too high.

My poor attempt at understanding & calculating DCRs for different cams (substitute the calculated stroke value (DST) in a static compression calculator). The biggest unknown in my calculations was the volume (CCs) of the Mahle stroker pistons.

https://www.enginebasics.com/Advance...20Dynamic.html
https://diamondracing.net/p-10-compr...alculator.html



























If my cam is eg, a stock grind 256, then the DCR is going to be in the region of 8.4 (maybe even 8.6?) :1, which as I understand it, is on the high side for a 10:1 stroker running pump gas. This would explain the knock at seemingly low timing, and why more timing (and less knock) is possible with the retarded cam.

I naively installed the cam, (which has been the one consistent factor in both of my stroker builds), and didn't confirm any duration/lift values. I understand the cam has a duration stamp on the rear end of it., which isn't particularly helpful...

Now I just need to get clear in my head how to confirm this with the engine in the car...