THE FORD SIERRA RS500- A LEGEND THAT WOULD'VE BEEN EVEN BETTER IF FORD DIDN'T OVERRULE COSWORTH'S PLAN...

I love Ford Cosworths, I really do, and a BIG part of the reason they're so good is they were designed from the outset to be Group A homologation specials. Group A rules at the time (ie mid 80s) dictated the standard block, head casting, crank, rods, exhaust manifold, inlet manifold, intercooler, and general turbo size, all have to be standard. And because of that, if you want to be successful in Group A, the parts fitted to the standard production car needed to be capable of pushing some serious power for full race and rally distances without issue, so Cosworth designed the engine for Ford to be VERY strong and tunable.
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ANYHOW, while I don't think it's ever been officially documented/admitted, being in this industry for so long means I've got to speak to quite a few people 'off the record', and there's one odd thing about the 2wd/RS500 Cosworth engine, that would've been a lot better if Ford would've let Cosworth have their own way, and here's the story...

Basically, Cosworth designed probably the best standard turbo manifold any production car has ever had- A strong, long runner, and fully twin scroll (2+3, 1+4) T3 flange exhaust manifold, proven capable of 600bhp+
But, bizarrely, the turbos fitted to it, the Garrett T3 on the Sierras and 2wd Saffs, and the Garrett T3/4 the RS500 had, are all single scroll, making the fact it's twin scroll, and the huge turbo spooling advantage that brings, totally and utterly pointless.

I'd always wondered why this was the case, it's bizarre. Twin scroll was pretty much unheard of on production cars back then, and would never deliberately had a single scroll turbo fitted, as the entire operation would be a huge waste of time and money.
While it's no issue with the little T3, the RS500 turbo, while good for 550bhp+, isn't exactly a responsive turbo, with good power starting from around 4500rpm upwards, and a twin scroll setup would've been a HUGE advantage- Even a much larger and more powerful turbo would've spooled up faster in twin scroll, making the RS500 even more dominant in motorsport than it was.

So why was a laggy single scroll turbo fitted then, despite a beautifully designed twin scroll manifold being on the engine? Well, company politics, that's why...

Basically, Cosworth had a VERY successful partnership with UK turbo legends Holset, who produced the turbochargers for the other turbocharged race engines Cosworth ran at the time- The 2.6ltr Indy Car lumps used by most the top Indy cars of the 1980s and 1990s- Including the 1987 Indy 500 winning car actually sponsored by Cummins and Holset (Cummins own Holset)...

The story goes that Cosworth fully intended, indeed full developed, as per their other race engines, to use top-spec Holset turbos. Unfortunately, Ford Europe had an agreement with Garrett to use their turbos on all the production cars, so overruled Cosworth and chucked on something 'suitable' from Garrett- The powerful, but unresponsive, single scroll 'RS500 T4'. This, below, is what it could/should have been...

The RS500 Garrett T4 is legendarily "Laggy", in fact you won't see a single article or film that doesn't mention it; even the racers all talk about anticipating the lag and planting their foot down early so the engine over-comes the lag just at the right moment, but thanks to people tuning Cosworth engines in the many years since, it's now well proven going from the twin scroll Holset plan to a single scroll Garrett was a MASSIVE MISTAKE...

A fair few people since have fitted Holset HX35s to the Cosworth 2wd twin scroll manifold, and the results say it all. 25psi+ boost by 3500rpm, despite being 600bhp capable turbos. The RS500 T4 is capable of ~550bhp but spools around 1000rpm later!
I don't know exactly what Holset Cosworth planned to fit, but I'd guess it was about HX35 size, and just imagine how dominant they'd have been with at least 50bhp more and the powerband starting 1000rpm sooner! Insane really. Politics eh...

Thankfully, Ford didn't get to meddle with Cosworth's exploits in the USA with Indy cars, so they carried on using Holsets on them for 10+ years later, such as this HX50...