Reyn Speed Shop, a small outfit in Southern California, has decided that BMW’s classic 8 Series needed something a little... louder. But by “a little,” I actually mean they’ve taken a Nineties grand tourer and injected it with a V10-fuelled dose of rage. Forget gentle upgrades — this is a full-blown personality transplant.

Welcome to the ‘858 CSL’, a name that actually tells you what’s going on under the bonnet. Yes, there’s a massive 5.8-litre V10 crammed in there, a heart transplant from the E60 M5 that’s been stretched and tuned by Carbahn, none other than Steve Dinan’s tuning house. Why a V10? Because nothing else sounds as gloriously manic or delivers power so ferociously. “To keep with an exotic and technically advanced drivetrain, the E60 S85 was the ticket for us,” explains Rey Rivera, the man behind Reyn Speed Shop.

With the help of Carbahn, the M5’s original 5.0-litre V10 has been bored out to 5.8 litres, resulting in an estimated 470 kW (630 bhp) and 663 Nm of torque. That's a significant leap over the tired V12 of an old 850i. And yes, your arms will get a workout — Rey chose to stick with the six-speed manual that was optional in the US-spec E60 M5, so all that power is channelled through your right hand and left foot. The performance? Not officially tested yet, but it’s safe to assume: ballistic.

And the sound? Unmatched. "You can’t beat the sound and the flexibility of that power band," Rey adds. 

So what’s the suspension setup for this V10 brute? Surprisingly, it sits on standard Electronic Damper Control (EDC) shocks and struts for now, but a collaboration with Moton is in the works to develop a bespoke four-way system to handle the 313 kg weight loss over the stock 850i. Yes, you read that right — this thing has shed more than a quarter of a ton, bringing it down to a trim 1,500 kg.

How? Enter Jon Sibal, a renowned designer who’s teamed up with Rey to reimagine the E31’s silhouette, with hints of the classic BMW M styling ethos. Flared arches nod to the E30 M3, a front valance that whispers E39 M5, and a rear spoiler evoking the E46 M3 CSL. The weight loss program was radical — all body panels are carbon fiber, and those pop-up headlights are gone, replaced by cooling vents that help keep that angry V10 from melting.

“The E31’s design is timeless, so we kept its essence while blending in elements from the E60 M5 where necessary,” Rey explains. Thanks to this cross-platform wizardry, maintenance should be straightforward — it’s a full integration of E60 mechanicals into the E31 shell.

Why did Reyn Speed Shop pick the E31 and infuse it with M5 DNA? “We wanted to create something special, something modern that pushed our abilities,” Rey says. And there’s logic to it: both cars share similarities that allowed for this unholy mashup, all while retaining OEM diagnostic and servicing capabilities.

The result? A stunning amalgamation of BMW’s past and present, four years in the making and countless hours in the garage. But Rey isn’t stopping there. When asked what’s next, he hints at another light ‘refresh’ involving a 5.4-litre V12, individual throttle bodies, a six-speed manual, and an old E9. Yep, looks like there’s another bold project on the horizon. 

Stay tuned, folks — it seems like Reyn Speed Shop has plenty more wizardry up its sleeve.