Help Me Choose M20 Stroker Configuration (crank, pistons, rods)

[mikey.antonakakis]

Cutting to the chase, any advice on the following would be appreciated (M20 stroker turbo, maybe e85), see further below for background/context:

• Which version of 84mm crank to look for (given I'm in the US, although I suspect this may lead to eBay so possibly worldwide sourcing)? Or go with a different stroke, such as keep the eta crank or go bigger?
• Which pistons? IE 2.9L Mahle cast pistons, JE, CP-Carillo?
• What compression ratio? Open to running e85, although that may bump the power enough to need bigger injectors and pump (currently running Deatschwerks 700cc injectors and 340lph pump).
• Any insights/feedback on vendors and their lead times, especially experience with custom pistons, would be appreciated.

Background and context:
As the title states, I am planning to build an M20 stroker setup for my VGT turbo car, to replace the currently running "2.7i" I built in 2015 with the following:

• eta bottom end (block, crank, rods, pistons) with 885 head
• IE HD rockers and springs
• Enem Z45 turbo cam (280/280 with 112 lobe separation).
• HE351VE VGT turbo, 60mm/60mm, supposedly 26cm^2 with vanes fully open
• 2.93 LSD and Getrag 260

Still need to get it to a dyno, but it runs very strong above 4000rpm at ~12psi sea-level boost, i.e.180kPa MAP (I'm usually driving about 6000ft ASL, today's density altitude is 9500ft). Guesstimate maybe 350whp, based on traction/weight transfer, injector size and duty cycle. Not bad for 180kPa?

Starts making boost (slowly!) starting around 2000rpm, 10psi after a few seconds at 3000rpm, full boost pretty quickly at 4000rpm. I think this is not bad for the altitude and 60mm turbo. Primary goal for the car is to make really good power numbers for a given boost level, secondary goal is to retain good boost response at low-mid RPM via the VGT.

I think I have achieved both goals given that I'm using eta pistons with the 885 head. I don't think this is doing me any favors with low-rpm boost response, especially since I'm at altitude. I also did not get any machining done on the block, and suspect I don't have the best ring seal. Last compression test was fairly low, even given the altitude factor. So, I want to put together a new short block to improve the low-end, and digger's build thread and the great info posted over the years by ForcedFirebird have me inspired to do maybe 84mm crank, new pistons, and maybe new rods. In addition to the eta short block that's in the car, I have a spare b25 bottom end in my garage too.

[ForcedFirebird]

What's your budget?

Just by using what you have and minimal effort, just use the eta crank/rods, deck the block 2mm and use OE replacement pistons. This will give you approx 9.4:1, have the 81mm stroke (+torque), but also utilizes the piston shape for more power. All the "2.7i's" I've done with flat pistons only produce about 140whp (NA). The budget stroker will do 40whp more when using b25 pistons and shaving the block.

Stock rods are stout, but kinda heavy. Molnar makes a good m20 rod that won't break the bank.

Since you are at altitude, don't be afraid of more static compression.

EDIT: Read back your post and should mention there's LONG lead times on pistons right now. I'm getting 12-16wk estimates.

[mikey.antonakakis]

Thanks ForcedFirebird! Don’t have an exact budget, but I’m fine buying most sets of aftermarket/custom pistons, most rods (maybe other than the expensive Arrow rods from VAC). So let’s say $2-3k. The Bimmerheads kits seem like decent bang for the buck compared to the other vendors, for instance, especially since it saves me the minor headache of sourcing a crank, but there are a lot of configurations to choose from.

To add a very minor additional goal for the motor: very crisp revs would be nice, so the light weight of aftermarket pistons, rods, and a lightweight flywheel would be awesome. And their extra strength would be nice if the motor ends up being capable of some serious power.

[ForcedFirebird]

A stock m20b25 with head studs can obtain 500whp with a proper turbo and a good tune. I just got an email from a Stage Rally car that's been running my tune on his stock studded m20 for three years.

Are you looking to make crazy power? If not, just rebuild the engine with the b25 pistons and enjoy :) A lot of people dump crazy money, and many didn't need to, especially with a turbo.

EDIT: Took 15yr for 7900 posts lol.

[mikey.antonakakis]

Well, I'm already on a 280deg 11mm lift cam with adjustable cam gear, and I'm assuming both blocks I have available need overbore, so I will need pistons no matter what - and piston-valve clearance with the cam may be an issue, no? Again maybe necessitating different pistons.

As for crazy power, I'm committed to running this turbo for quite some time - it's hard to find super accurate data, but I believe it has a 7-blade HX40 compressor wheel, supposedly good for 69lb/min (>700bhp?). DSM guys running the 7-blade HX40 seem to be well over 600whp. Does that mean I want to run that kind of power? Well, if the motor can take it, yes, I'd like to make that kind of power. FWIW I plan to build a new intake manifold (not just for power, but I think I may be able to package a water-air core nicely into the plenum like some modern OEM turbo setups are using - or at least improve its packaging to be able to fit a larger core than I have now).

Another way I'm looking at it: if a stock B25 is good for 140whp N/A and 500whp turbo - what is a good stroker with a cam capable of?

[ForcedFirebird]

It's all relative. If you add 1 bar to 140whp, theoretically, you "should" make 280whp, and likewise, if you add 1 bar to 180whp, you "should" produce 360whp.

You are starting with a 140whp engine. Anything you do to make it more powerful NA, it will transfer to boost.

[ForcedFirebird]

7 blade hx40 on 16psi did just a slip over 400whp for me, 2.7i (flat pistons), 325td manifold, and external 38mm gate.

[mikey.antonakakis]

You are starting with a 140whp engine. Anything you do to make it more powerful NA, it will transfer to boost.

So, given 280deg cam, custom tubular exhaust manifold, and potentially custom intake manifold, and let's say 84mm crank, any specific piston recommendations for stroker setup? Looks like I could get my hands on the IE 2.9 Mahle pistons in a reasonable time frame, for example. Your feedback on the Molnar rods is helpful, they are a good price from Bimmerheads (although I guess out of stock right now).

How would that HX40 have done on max power with a 25cm^2 turbine housing? (commonly-given figure for fully-open vanes on the HE351VE).

[varg]

Very recent experience, M50 bottom end and M20 bottom end parts are close in price so I'll say that $3K budget will cover a used crank, (budget-ish) rods, forged pistons and machine work/block prep, and that's about it. I wish I could have gone with Molnar rods but they were out with a long lead time so I got IE rods for my 3L M50, they seem fine. The turbine wheel on an HE351 seems a little small for 600whp, but if people are out there doing it

[mikey.antonakakis]

The turbine wheel on an HE351 seems a little small for 600whp, but if people are out there doing it

Thanks!
That may be the case regarding the turbine wheel, I should clarify that I was talking about HX40 installs. That said, the HE351VE turbine wheel can be upgraded to at least 64mm if I ever find its limit (and 67mm compressor wheel is available too). FYI I am about an hour’s work away from having EGT and EMAP measured at the turbine inlet :) sensors are installed, just need to finish wiring!

[It's Soda Not Pop]

What's your budget?

Just by using what you have and minimal effort, just use the eta crank/rods, deck the block 2mm and use OE replacement pistons. This will give you approx 9.4:1, have the 81mm stroke (+torque), but also utilizes the piston shape for more power. All the "2.7i's" I've done with flat pistons only produce about 140whp (NA). The budget stroker will do 40whp more when using b25 pistons and shaving the block.

Stock rods are stout, but kinda heavy. Molnar makes a good m20 rod that won't break the bank.

Since you are at altitude, don't be afraid of more static compression.

EDIT: Read back your post and should mention there's LONG lead times on pistons right now. I'm getting 12-16wk estimates.

I've thought about doing this budget stroker as well. I mean I technically already have the 2.7i with a Bimmerhead's head. But I want more compression and N/A power. Will shaving the block lower it's longevity in the long run. As in once it's been used it can't be resurfaced because you've already skimmed it?

[ForcedFirebird]

Not quite "technically", you do have a 2.7i. If you started with an eta car, then adding the 885 makes sense. If you swap a 2.7 rotating assembly in your 325i, then it's a step "backwards".

The 2.5l has 75mm stroke and 135mm rods. The 2.7 has 130mm rods and 81mm stroke. BMW moved the wrist pins on the 2.7 to suit.

So, if you use the 130mm eta rods, 81mm eta crank, and b25 pistons, and bolt them in a block, the pistons are 2mm down in the bore. Have your machinist deck the block 2mm, and now you have your compression height back, and you can shave the head just as many times as stock. This is what we refer to as "budget stroker".

See post 43 here: https://www.r3vlimited.com/board/for...52#post7038352

for information about the "budget stroker", then see post # 11 about the 885 head on an eta bottom ("2.7i").

[mikey.antonakakis]

If it’s not already obvious, the other thing I will mention in addition to ForcedFirebird’s reply is that your cam timing will be affected by the 2mm shave, and any additional shaving of the block could potentially be made up for with head gasket thickness.

[ForcedFirebird]

If it’s not already obvious, the other thing I will mention in addition to ForcedFirebird’s reply is that your cam timing will be affected by the 2mm shave, and any additional shaving of the block could potentially be made up for with head gasket thickness.

Have you done the math on that? Yes, it does retard the timing a bit, but my examples put down 180whp without messing with the cam gear. That's about 35whp over a stock b25.

If you rotate a cam to advanced, the peak power won't move very much, but your power curve will move higher in the RPM range. If you retard the cam, peak power will have a slight LOSS, but the peak curve will move lower in the RPM range. That being said, my examples of the junkyard budget stroker builds would make MORE peak power if I was to advance the cam. ;)

[mikey.antonakakis]

Have you done the math on that? Yes, it does retard the timing a bit, but my examples put down 180whp without messing with the cam gear. That's about 35whp over a stock b25.

If you rotate a cam to advanced, the peak power won't move very much, but your power curve will move higher in the RPM range. If you retard the cam, peak power will have a slight LOSS, but the peak curve will move lower in the RPM range. That being said, my examples of the junkyard budget stroker builds would make MORE peak power if I was to advance the cam. ;)

Did not do the math on it, but just did - maybe about 2deg? (I'm making assumptions about cam sprocket diameter). So, pretty minor.
I trust your dyno results much more than any heuristics I've heard, but I always heard that advancing the cam moves the powerband lower, also seen in dyno tests in the video below - warning, GM content ;)
Although for a 2deg difference in this case, probably doesn't matter much lol.