M20 turbo bottom end build

[Yuri]

I Have a spare M20b25 block that I’ve decided to build for fun. I want to do as much of the work myself as possible and thought I would start this thread for a few pointers. I am thinking about using the stock cast crankshaft because I haven’t really heard of anyone breaking on yet; however, I’d probably change it out if I found a deal. The main thing I’m looking for is pistons and creating a good compression ratio. I see IE has some that might be a good fit, but I’m in new territory here. I’ll be pairing these with max speed China rods, from what I hear they arnt half bad. Again, not trying to break records or my wallet. What are good forged piston options that would work with the m20b25? Ideally retaining the current compression ratio or mildly lowering it.

[Yuri]

Just picked up the rods! https://www.maxpeedingrods.com/produ...am-conrod.html Sent from my iPhone using Tapatalk

[Northern]

IMO If you're spending the money on upgraded rods and aftermarket pistons, it's not much more to grab a longer stroke crankshaft from an Eta, M52, S50us, or S52.

I don't know who I would buy pistons from, but with a non-standard crank/rods, I'd definitely buy from somewhere who has a lot of experience with M20s. I think IE would be an example of that, but I'm sure there are others too.

[digger]

i would get a set of JE pistons from top end they will be able to give you a replica of stock but forged in the stock 8.8, or 9.4 or 9.7:1 CR

[Yuri]

IMO If you're spending the money on upgraded rods and aftermarket pistons, it's not much more to grab a longer stroke crankshaft from an Eta, M52, S50us, or S52. I don't know who I would buy pistons from, but with a non-standard crank/rods, I'd definitely buy from somewhere who has a lot of experience with M20s. I think IE would be an example of that, but I'm sure there are others too.

The problem is that I impulse bought 135mm rods. Wouldn’t those options greatly increase my compression ratio? From a FI standpoint wouldn’t I want it around 8.8 if that’s the case? Found these specs online and thought I might as well attach them. Specs: M20b25 Bore: 84.0mm Stroke: 75mm Rod length: 135mm S50 Bore: 86mm Stroke: 85.8mm Rod Length: 135mm S52 Bore: 86.4mm Stroke: 89.6mm Rod Length: 135mm M52 Bore: 86.4 Stroke: 89.6 Rod length: 135mm?

[digger]

with a fixed dome cc (as in a replica of OEM) the additional capacity say with 84 stroke/85 bore/135 rod will increase the CR a bit say from 8.8 to 9.5 but thats not a deal breaker. it will probbay be a bit less as usually you get deeper valve pockets that kind of offsets it.

you can of course get random/ generic piston with whatever CR you want irrespective of the stroke or bore but its not usually the best option from a combustionchamber perspective but still works.

In short your rods do not cause any issues, nobody with any sense runs 130 mm rods with forged pistons so you have the correct length rods

[Northern]

The problem is that I impulse bought 135mm rods. Wouldn’t those options greatly increase my compression ratio? From a FI standpoint wouldn’t I want it around 8.8 if that’s the case?

Oh sorry, the link said 130mm and I just went with it. 135mm rods are better for rod ratio and there isn't any reason you wouldn't be able to order pistons to use with those and whichever crankshaft.

No idea what your power goal or fuel are, but stock CR m20 with 93AKI pump gas is easily fine for at least 400 whp.

I wouldn't lower CR too much from stock or it'll be a dog off boost, and I wouldn't go higher as it'll limit how much boost/timing you can throw at it (Unless you are planning to run E85/race gas only, with zero chance of needing to run a tank of pump gas)

[Yuri]

Wow I feel like an idiot 😂 so I just checked my order and that company only sells 130mm rods for the m20 😅. I am just throwing around ppp money like N ass over here not knowing what I’m talking about. I appreciate you guys putting up with me. Sent from my iPhone using Tapatalk

[Northern]

Any M20/M5x rod in 135mm from maxpp will be the same, so if they have a listing for an M52B28, M50NV, or S52 rod, it should be 135mm (just make sure they didn't fuck up the listing lol)

(and they definitely make them because there's a set installed in my 4dr lol)

[ForcedFirebird]

That ad clearly states those rods are for 325e, which would be 130mm. Maybe it's a mistake, but I see them both listed for eta and stating they are 130mm. I would have just stuck with the m20 rods, we actually use them (and m50nv rods) for budget 24v turbo builds. Recon OE rods are better than Chinesium any day. I have yet to see a set of clone rods that were withing tolerance out of the box (specially when you replace the fake ARP bolts/stud with real ones).

I'm not convinced 5mm rod length is a deal breaker on the baby sixes, you are talking about a 1:6 vs 1:1.55 ratio (130mm vs 135mm on an 84mm stroke) when the s52 is 1:1.50 and s54 is 1:1.52 - BUT - as digger says, if you are getting custom, might as well use the longer rods if possible.

Either way, if your goals are under/near 400whp, then the stock motor will hold, as long as you can keep the head down with quality studs and possibly o-ring block. At work we have built dozens of m20 turbo setups and only a few went "fully built" and they didn't make any more power (or were really more reliable) than they would have using stock components.

These stock bottom ends have shown many times to hold up to 500whp/TQ.

My $.02 :/

[Yuri]

Right now I’m running a stock motor with upgraded head studs oil pump and rockers. Works great! I’m at probably 300hp running 12 psi on e85. Lots of room for improvement(ran out of fuel on the dyno). The engine runs great. This spare block build is just to honestly learn, have fun, and at the same time build a tank of a bottom end for the future.

[Yuri]

If you guys were going to build a bottom end with a 500 whp goal, would you trust the oem cast crankshaft?

[ForcedFirebird]

If you guys were going to build a bottom end with a 500 whp goal, would you trust the oem cast crankshaft?

Absolutely. There's a member here making 520+ on 21psi and I built a local car that did just shy of that on 19psi, both running a Precision 6266. The local had a fully built forged m20 (he recently went to 1jz), the member here has a stock 2.7 with 885 head. Again, the m20 is wayyyy overbuilt and is a good candidate for boosting, but you gotta keep the head down (studs and/or o-rings), your tune had to be solid, and you MUST make sure you have a swirl pot/surge tank, the e30 stock tank likes to starve here and there.

[ForcedFirebird]

Right now I’m running a stock motor with upgraded head studs oil pump and rockers. Works great! I’m at probably 300hp running 12 psi on e85. Lots of room for improvement(ran out of fuel on the dyno). The engine runs great. This spare block build is just to honestly learn, have fun, and at the same time build a tank of a bottom end for the future.

Super 60 and t04e variants have consistently put down 280-300whp for me over the years @10-12psi. Probably have done a couple dozen + turbo builds, most of them use t04e/super60/hx35's and they are all similar in power output with ~12psi. If you want to learn, I would suggest investing in measuring tools more than parts. A well built stock motor with stock high QC will easily outperform your goals, IMO, you don't really "need" to invest in aftermarket parts aside from turbo-related pieces. Heck, there's two turbo m50's, and a supercharged m54 powered e30's in my shop ATM.