People make a REALLY big deal about a lot of things when it comes to the specs of tuned cars, but personally, I think most people obsess over the wrong things. 
And turbo brand is one of them. Fact is, contrary to popular belief and sales talk, turbos are not magic, and if the general specs are the same, the general performance will be the same. Exact? Hell no. Similar enough that other factors make a far bigger difference? Hell yes. And this feature shows that above all else I think.

This comparison came about after a friend of mine got his FD RX7 mapped, and it made 600bhp on the dot, and to be fair performs bloody well. But I did wonder what it was like compared to other peoples, as spec wise it wasn't quite the same as what most do.
Luckily it turned out, the vast majority of the UKs powerful rotaries use this same mapper on this same dyno, meaning there's tons of comparable dyno graphs about, so while it's different days, that's no big deal in the UK as it's the same drab weather 247/365, so it's as fair a comparison as humanly possible.

Obviously you'll get people making excuses 'I had a misfire' 'My dog ate my homework' and so on, but the simple fact is ALL of these cars were mapped, and ALL of these results were publicly posted, so there clearly wasn't any massive issues with any of them or they wouldn't have been mapped in the first place. So no excuses!
The only car I know from the list is the HX50 one, and I know that was missing a fire ring in the wastegate so it was leaking like fuck, slowing spool and hindering it holding boost at high rpm, and compression test was showing under 5bar on all rotor faces (not bedded in fully and wide tolerances rather than fucked), but whatever, I expect a few of these cars had minor things like that so no excuses for anyone, it's as fair as a comparison as possible.

To make it comparable, I only bothered with cars that made over 500bhp, and as only one of the cars made serious power below 5000rpm, I only compared above that, as that's before the start of most of the cars performance powerbands begin anyhow. 

​And here's the graph...

That is a fairly confusing jumble of lines, but I'll do my best to explain it all to you...

Two cars, both with the biggest turbos and the biggest ports, made by far the least power at 5000rpm, barley over 200bhp.
​Most were in the middleground, and one was already making great power by then, one with small ports, a middle sized turbo, but MOST IMPORTANTLY, a lot of boost.
This boost thing is a funny one, and one you only see really with rotaries and stuff with real lairy cams etc- Basically anything with bugger all low down power when N/A- Basically, the powerband, at lower boost levels, isn't from whenever full boost is (like it is on most milder builds), it's from whenever the ports or cams allow the real powerband to begin, just like N/A.
BUT once you crank the boost above about 1.5bar/23psi, the positive effects of the boost MORE than counteracts this, and you once again end up with an engine that the powerband starts whenever full boost hits, and this car running 26psi shows it well, already over 400bhp at 5k.
The only anomaly here is the brown line, which I had as an engine with big ports, a big turbo, and fairly low boost. Yet seems to be doing well. It's possible I was given the wrong info on this cars spec...

By 6000rpm the big 76mm HKS T51R engine with the giant J ports has woken up, as has the car with the 67mm Holset HX50 25cm and a half bridge, and they're now matching the power of the two street ported cars running BorgWarners.
The EFR equipped car and the one with the big 72mm turbo are 40bhp down by this point.
The 'Anomaly' brown line car (which I'm sure I've been given the wrong spec for!) is about 40bhp up from that middle batch.
And as before, the big big boost small port car is absolutely flying, making over 550bhp already!

By 7000rpm, the situation is getting very different. The big boost small port car has already gone beyond peak power, and is now only in the middle of the pack.
The big port big 72mm turbo car has really woken up and has gone from last to making more power than a car that was making over 100bhp more than it just 2000rpm earlier.
All the BorgWarners are starting to tail off a little, probably due to the fairly small turbine housing choices? Just a guess knowing how they're often chosen, but rotaries do like a big turbine.
The 'anomaly' car is still doing well, but starting to flatten out, which again says to me the spec isn't what I thought it was.
And the two cars with the big turbine flow, the 25cm HX50, and the HKS T51R, are going up like a rocket.
Aside from the unknown 'anomaly' car, the HX50 and T51R cars are 50bhp+ ahead of the rest now.

Another 500rpm and aside from the three with the biggest turbine flow, and decent size ports, ie the 72mm turbo one, the T51, and the HX50 25cm, things are dropping down.
But the HX50 and T51R cars have now both gone 600bhp+ over 50bhp more than the next closest car.

By 8000rpm, while pretty much everyone is at peak or beyond, some cars are still FAST and still well worth revving. The T51R car has just about peaked and is still well over 600bhp, the Holset HX50 is still making 580bhp, and even the 72mm turbo one is doing ok, and would probably have really benefitted from a few more psi boost as it was running 17psi vs the HX50 and T51s 20psi.
The rest, well they're all still making 480bhp+, so FAST still, and defo worth revving out to this rpm, aside from the poor old big boost small port car, which has widly ran out of puff and the power has fell off a cliff, now making barely over 350bhp- In the real world the driver would've changed up at 7500rpm at the latest.

By 8500rpm, only the two 70mm+ inducer turbos on big port engines are still barely dropping, but the next two biggest, 66mm BW and 67mm Holset are still making over 500bhp so defo worth revving them this high.
By now the other four cars aren't even comparable.

By 9000rpm there's only two engines still in the game, albeit past their peaks, and lo and behold they're the two with the biggest turbos and biggest ports.

SO WHAT ENGINE DID BEST? Well, let's compare some averages...

Realistically, on something FD RX7 weight, 'FAST' begins at 400bhp.
So let's look how wide was the '400bhp or more' powerband...

Winner here was one of the ones with the least low down power, because it could hold it up top, the giant J port T51R SPL car! 3250rpm worth of 400bhp+ power band.

Joint second, was the car that was flying from low down, but tailed off big time up top, the smallest port car, and the car with fairly small ports but a big turbo that held on well up top, both with 3000rpm worth of 400bhp+

Joint third was the Holset HX50 25cm car, and the big 72mm turbo car, with 2750rpm of 400bhp+

Here's the full 'how long over 400bhp' results- 
1st- HKS T51R SPL J-Bridge 20psi- 3250rpm
2nd- BW S365SXE 26psi small street port- 3000rpm
3rd- BW S366 15psi Street Port- 3000rpm
4th- 67mm Holset HX50 25cm, 20psi half bridge- 2750rpm
5th- Full bridge, 72mm turbo, 17psi- 2750rpm
6th- BW S400 full bridge 17psi 2500rpm
7th- BW S363 street port 19psi 2250rpm
8th- BW EFR9174 19psi bridge 2250rpm

But 'over 400bhp' is a bit misleading when some made over 200bhp more than that!
So let's look at the average power they made above the magic 400bhp mark, which is probably the BEST comparison of what would be the fastest in the real world, as their powerbands are all fairly close in size...

Well first and last place are the same, but the rest, not so much!

Average power beyond 400bhp...
1st- HKS T51R SPL J-Bridge 20psi- 563bhp
2nd- 67mm Holset HX50 20psi half bridge- 523bhp
3rd- BW S365SXE 26psi small street port- 518bhp
4th- Full bridge, 72mm turbo, 17psi- 514bhp
5th- BW S400 full bridge 17psi- 511bhp
6th- BW S363 street port 19psi- 489bhp
7th- BW S366 15psi Street Port- 488bhp
8th- BW EFR9174 19psi bridge- 474bhp

What about their average power levels through the rev range when they're at 80% or more of their peak number? Well ok then...
1st- HKS T51R SPL, J-Bridge, 20psi- 599bhp
2nd- 67mm Holset HX50 25cm, 20psi, half bridge- 545bhp
3rd- BW S365SXE 26psi small street port- 528bhp
4th- Full bridge, 72mm turbo, 17psi- 525bhp
5th- BW S400 full bridge 17psi- 522bhp
6th- BW S366 15psi Street Port- 502bhp
7th- BW S363 street port 19psi- 498bhp
8th- BW EFR9174 19psi bridge- 474bhp

So how wide was the powerband when they were over 80% of their peak power? Let's see...
1st- HKS T51R SPL J-Bridge 20psi- 2750rpm
2nd- BW S365SXE 26psi small street port- 2750rpm
3rd- Full bridge, 72mm turbo, 17psi- 2500rpm
4th- 67mm Holset HX50 20psi half bridge- 2250rpm
5th- BW S400 full bridge 17psi- 2250rpm
6th- BW EFR9174 19psi bridge- 2250rpm
7th- BW S366 15psi Street Port- 2000rpm
8th- BW S363 street port 19psi- 2000rpm

SO what hell have we actually learnt from this? Well, that's a matter of opinion to some extent, but I can tell you what I think...

Does brand matter? NO.
The RX7 scene in the UK is allllll about the BorgWarner turbos. Well considering they were generally in the bottom of the lists, and the 'old fashioned' Holset, HKS, and the 72mm (think it was a Turbonetics?) were generally near the top, I think this assumption many have, is wrong.
TBF I've always known this, I said it countless times, but it's how it is, turbos aren't magic, and aside from a few exceptions, technology hasn't changed hugely, so like for like, the wheels, ie the important factor, from any of these brands are within a few % of each other, and the biggest names are often far from the best, just the most hyped.

Boost helps a LOT-
Is it a surprise the car running 6psi more than the rest did better than lower boost cars with similar turbos? No. Boost is king above EVERYTHING else. If you want your car to perform, no1 thing IMO is to enable to to be safe at big boost. Nothing else will gain you as much as big boost can. 

Good turbine flow gives MORE overall gains than faster spool will-
It's only an educated guess of course, but looking at the powerbands, I'd say most of these BorgWarners had pretty small turbine sides, IMO too small for a big power rotary, and it shows in the results.
Peppier low down, but then lose out massively overall, giving less overall performance.
If you want low rpm performance, I'm not sure a rotary is for you.
Perhaps the clearest example of this is the car with "The highest tech" turbo, the EFR9174. At £1500+ it was probably the most expensive, over three times the HX50 for sure, was a ball bearing, billet wheel, stainless housing, Indy Car developed, proper bit of kit. Yet was near the bottom of the list for everything, despite being 75bhp up from the big T51 at 5000rpm.
And why? The 74mm turbine wheel, which is one of the smallest if not the smallest in this test is going to be one factor. Possibly compounded with a small A/R housing, as that seems to be the done thing with a most UK rotaries.

You gain MORE overall performance building for a high rpm powerband than a low rpm one-
​Again, it's all about the flow. Big ports might kill low rpm performance, but what it loses low down, providing you have the rest of the spec to match, you will MORE than gain up top. Fuck, the T51R J Port car was TOP of every chart, all the averages, widest powerband, despite having the LEAST low rpm power...!

Your turbo spec needs to match your engine spec-
Big turbo and small ports, small turbo and big ports. It's a mismatch that doesn't really work. Sometimes can kill the performance at both ends of the graph, giving you less spool than you expected, but less top end too.
The EFR is probably the best example of this one again too. Decent amount of boost, big ports, but with a fairly small turbine side on that turbo, it stayed near the bottom of the pack from low to high rpm.

SO WHAT ONE WOULD I CHOOSE?
From these results alone? The T51R SPL car, without doubt. It kicked ass.
It might not come alive until after 5000rpm, but it's right up there with the rest of the pack by 5750rpm making over 400bhp, but makes over 500bhp 750rpm later, over 600bhp 750rpm after that, and holds over 650bhp to the 9000rpm limiter, and no doubt would've carried on making massive power to 10,000rpm if they turned the limiter up. And at only 20psi too. Biggest power, biggest powerband, biggest averages, biggest rpm, it just wins everywhere aside from low rpm power.
But if you want that because you don't understand what gears are for, go buy a diesel.

The BIG boost car would be massive fun on the road, a great laugh, super drivable and a tyre smoker, but the fact it's on it's ass beyond about 7.5k is hard for me to accept from a rotary.

Bang for your buck, it's the HX50 car. A turbo you can get for £500 brand new. That fits to an off the shelf old skool Greddy manifold (most BWs need different/newer manifolds otherwise the turbo hits the intake manifold), and so on.
Since this dyno a lot of things have improved on the car- Mostly a better wastegate setup that doesn't massively leak, and a better intercooler, so I'm keen to see how it goes next time it gets mapped on that dyno, and I can show you comparison data in a later post about that too...

Cheers.
Stav