Hello.
(I have searched and read a lot before making this post, but there are so much talk about these turbos that it is hard to find what i am looking for)

Due to new laws where i live, tuning and single turbo conversions are now legal. Thus i need to start planning a proper build.
For legal reasons, i can have a maximum of 360hp on the crank, and we are given a 15% drive train loss, putting me at 414 rwhp.
Now, when it comes to turbo choice, the rules are that they base themselves of the manufacurers numbers which for BW EFR is 650 and 750 HP respectively for the 7670 and 8374.
I am allowed a 40% larger max power rating than my actual power goal. I will have to convince them regarding the lower volumetric efficiency of the wankel, when choosing a turbo.

Other than that my goal is a fun, everyday driver with "moderate" power goals, stock ports (maybe small street when i rebuild) and a V-mount intercooler. I will run the new Haltech Nexus S3 ECU, planning on water injection and in general build a "no cost spared" car with reliability >> hp.

But regardless of that, what i was hoping to get is if some of you with experience with either the 7670, 8374 or both, can give me some feedback on how the car is to drive after the conversion, what realistic power numbers i can achieve, when it starts spooling and when it falls of.
I want to keep it in the spirit of Mazda and their vision for the car. Im worried that the 7670 will be "to small" and not keep up in the top end, and also worried that the 8374 will be to big and very hard to legally argue for. yet the 8374 seems to be everyone's favorite.

So what would you recommend?

 

8374 spooled very quickly for me..
I had 30psi by 3200 or so RPM. The runners on the manifold were 50mm as opposed to others that are larger. a little more response with that manifold vs others but due to this restriction i had elevated EGTs

I have driven a few cars with 8374 and a few with 7670..
7670 is nice and very responsive but i do prefer the 8374 between the two.. The 8374 was also very responsive. I wanted to introduce some lag to my setup because it was a constant fight with the steering wheel on tight turns that i didnt ease in with the gas so i switched to the 9280 but that is a different story altogether.

8374 1.05 EWG gets my vote

 

I'd go for the 8374, even with a mild motor the spool is amazing and you'll make better power. I'm not sure they'll look different enough to make a difference between sizes for "argument" purposes, unless they're going to look at spec sheets.

 

You said for legal reasons you can have a max of 360hp. %15 drivetrain loss would be 54hp and put you at 360 to the wheels.
Are they only looking at the numbers on paper and Borg Warners advertised peak hp?

 

Does the law/regs specify what boost the 360hp needs to be generated at? turn the boost way down, put a restrictor in, are they going to dyno it?

 

I don't have any experience with those singles but is anybody else shocked that there are laws allowing modifications restricted by Horsepower? This is the first I've heard. It would be a step in the right direction for California. Maybe they should take some notes from Norway if they want to restrict but still allow some car culture for happier residents?

 

Crank and wheel seem to be **** about in the first post. Seems a little weird too, when vehicles were properly engineered over here (lots didn't, and ran the police and EPA gauntlet), ECUs were locked, so you couldn't diddle with boost or fuelling.

 

@zepticon
If your "engine" hp limit is 360, that would put you at 306 hp at the wheels (due to 15% drivetrain losses or 0.85 x 360 = 306). In that case, you'll choose the 7670 EFR unit. And in doing so, you'd essentially have a responsive, modern day equivalent to the stock/twin turbo FD, but w about 40%+ power over the original '93-95 models at similar boost levels.

If for whatever reason you are allowed whp levels at or above 360, you'll want to choose the 8374 unit since at 360+ hp, you'd be pushing the "comfort" levels of the slightly smaller 7670 model; all the while requiring add'l boost to get you there. Conversely, the 8374 unit, combined w your planned mods & WMI, will happily do 360+ hp at around 15-ish psi wout breaking much of a sweat; all day long!

 

Thanks for the replies. As some of you said, i got some of the math crooked. Ill redo it and see where i went wrong. I should probably have went to bed and not stayed up late to work on the numbers x)

Crank HP at 360 is what i have to work with, and i think i added 15% instead of substracting it. Also boost can be what ever, but they use the BW spec sheets to determine max legal turbo size, and thats the issue im facing there.

As for the california/emission thing, here in norway we have driven gasoline cars to the brunk of extinction. And they are so few as to not matter from an emission pov. We have mostly diesels and EVs, and 99% of new cars are EVs. We have had a lot of changes here, mainly through the hard work of the national AmCar organization, that have made modifications simpler.

 

So new numbers, and this is what i got to work with:



So i will have to argue that the rotary have a lower VE than piston engines and thus the numbers given by turbo manufacturers should be calculated at a 75% VE compared to pistons.
If i am not successful in this, i am forced to go down to a 7064 and that would not work well.

But looking at the BW specs, 7670 and 8374 are rated at 650 and 750 so if i manage to convince them to accept 75% then both are "legal".
There is no ECU lock after the initial approval, and thus i am arguing more about what a good turbo for a stock port/small streetport is, and less about the actual approval process.
The only other issue is that the dyno should "flat out" when its getting close to the rev limiter. I am not sure if this is possible on a the 8374 at 360 crank

 

The 7670 sounds like it would satisfy what you need - both legal and fun-to-drive. I've been considering a single and have been more interested in the smaller turbo since I'm not chasing big numbers and prefer quicker spool for street driving.

 

I really wish America would have embraced Diesel more. So much potential but it never really caught on. People really look at you funny when you need to plug your block heater in so your truck will start in the morning. EPA hasn't helped and the "modern" DPF systems are a nightmare and turn a reliable diesel vehicle into something that lives at the shop. Luckily I live in a state with no emissions inspections and I don't have to worry about these issues with my Diesel or any of my other cars.

Looking at your updated sheet, I would personally go with the 7670 as that is even overkill for your power goals but will provide you with the best driving experience with limited lag IMO.

 

a well setup nearly stock FD can happily produce 306 whp with not much effort and good reliability, if you want to simplify you can convert to non-sequential (adds lag).
Check this thread: https://www.rx7club.com/3rd-generati...added-1104322/

Also, dynos read different so you need to find out what type of dyno they're using or which dynos they allow if you have to provide the readings. A Mustang brake dyno reads lower than a Dynojet roller dyno so if you get a 306whp Mustang reading it will be higher on the Dynojet.

 

I'm assuming the power figures are DIN based on "as delivered" in your market, 265 JIS? Do you have to argue this to a government entity (no hope probably!) or some independent third party? For the latter, a brochure showing 280 in the late cars, might prove convincing, which would give a touch more wiggle room ~253 DIN?

I don't follow the turbo sizing based on your VE and max power figure. The max power in your table above, that nominal 66% increase doesn't seem to jive with anything obvious to me - I can't say I've paid much attention to turbo hp figures, I'd imagine they might be based on SAE, given the market where they are produced and give you a little more room to present your case in DIN.

 

It is a government entity, but they are actually pretty decent as long as you bring a convincing argument to the table.
I can try to explain how its set up here (i should probably have done a better job in the beginning).

Any tuning or upgrades are based on a max power to weight ratio of 20kW per 100kg. Where this number comes from i have no idea but i think its a bit low, tho better than nothing.
If the manufacturer have made cars with higher power to weight ratio than this, that number will be what they use.

In my case the fat euro spec FD is 1325kg, which converts to 265kW, or 360hp. I have used my current title to do the kW -> HP conversion since we primarily use kW but still havent phased out "hp" in eveyday life.. I dont know it its DIN, SAE or JIS or something else. But i will have to research that as it might give me more wiggleroom.

When it comes to selecting a turbo, they have some limitations put in place so that i wont add a turbo with the obvious intention to turn up the boost as soon as leave after the approval.
Those limits are only allowed a 40% larger max power limit on the turbo than what i am approving it with, and the power curve should be somewhat flat at the rev limiter.

Given these limitations, there are not much to choose from that would make sense for the FD. I feel that the 7670 would be the minimum to bother with this.
So then i need to bring up the issue that the rotary have lower efficiency that the piston, so at a given turbo size, the rotary will be 75% less max power.

So based on the feedback here, my own reading and such, i think the 7670 would be the best overall option, i can also present the 8374 as a "candidate" but then argue that the 7670 is the reasonable candidate to choose. That will let me set the premisse for the appplication, and be ahead of their doubts

 

Probably best to note it isn't volumetric efficiency that is regarded as 75% of modern piston engines but brake specific air power or air efficiency.

s8 spec factory turbos, apexi intake, u type or larger intercooler or some description and orientation with some form of ducting if not front mount and a 3+ inch exhaust with high flow catalyst (fuel pump and larger secondaries or a set if you wish, definitely run a hydramat or some form of surge tank) will make you target power, but I would rather have an efr7670 as you don't get the crossover lull.

 

Back in the day (early 2000's) I had an M2 chipped stock ECU and a boost controller set at 13psi, M2 Airbox and Intercooler, 3" downpipe and RB dual catback with stock catalytic converter, stock injectors with Walbro 255 pump and HKS twin power ignition booster, stock engine and twins (1993).
Made 250whp at 9 psi with boost controller off and 315whp at 13psi on a Dynapack hub dyno. The 250whp 9psi run had a beautiful torque curve with no transition dip, the 315whp run had a small dip with a dramatic (about 40 ft-lbs iirc) jump in torque when the secondary turbo came online.
Reliable as any other car, never had solenoid or vacuum hose issues. Still on the same motor and turbos but with many changes since then. I believe a properly sorted out stockish setup can be reliable.

 

It won't be, not on stock ports with those small turbos.
Expect the power to take a BIG dive after 7000rpm. You get that even with an extended street port and 8476 size turbos. They fall off around 7000rpm ish.

If you want nice fat power to redline of 8000rpm you will need much more flow, ports and turbo wise. (and it will put you out of your turbo size range)

 

A dive is also good.
I think the case is that a still climbing curve is indicating an artificially limited turbo setup, and that they suspect my goal is to remove the limitation once approved.

 

For those numbers stay stock turbos.

If you really want to go single, 7670 is the right choice.

Because you are limited by max HP at 306whp, your whole goal should be to make 306whp at every RPM aka massive toque like a rally car. 7670 has done 420whp pushed hard, so you’re 25% below max power that’s a good spot to aim for.

Raise the boost as quickly as possible as hard as possible, hit 306whp, then taper the boost off to maintain 306whp until you hit redline. They kinda gave you “race rules” to follow, might as well build to the maximum allowed in your “class”

 

Hey Zepticon, apologies if this has already addressed, but have you considered submitting brake specific fuel consumption data as part of your explanation? I would imagine that it might help you in your explanation at least by a little bit. I think there were some folks here that had some Mazda publications on it. Might be worth asking and looking around for?

I think (and please take this with a grain of salt as I have not driven vehicles with either turbocharger so this is probably marginally better than conjecture at best) that both compressors will do the job, one will do it with better low end response and a ton of torque but tapers off up top and the other picks up slightly later but will hold on longer if I understand correctly. One data point you might take into consideration was that I was traction limited in certain instances on my stock twins setup at approx 300 whp and a 3.9 final drive so I had even less torque multiplication than folks with manual ring and pinions and that setup more than likely made less torque than what you can expect a 7670 to make.

Another thing you might consider is the whole heat management thing. For all intents and purposes it probably won't make any appreciable differences in your approach, and is definitely worlds better than the OE cast iron setup with respect to heat retention, but I would give some consideration to what you want to prioritize. Same can be said with the driving dynamics I wrote of above.

Hope that helps.

 

I know there exist a SAE paper on BSFC but i have noe found it yet. I have added a few lines in my application to address this but i lack the documentation. I was not aware that the SAE paper was available, but if that is the case ill look around. Thanks for the tip!

That sounds pretty good i rarely get tu use the top register anyway, without being waaay out of legal limits.

Horse powers are not the main reason i do this. Its more "i want to modify ****" and "lets make it reliable" and "**** its so hot". Also, having a single turbo opens up possible shady avenues for tuning in the future.

 

On gate pressure with the obligatory creep up to 11ish psi from most IWG 8374s I've fiddled with you'll be over your mark to the tune of ~320 to the wheels. Obviously this can be reduced by choking up the exhaust and intake or reducing timing to an extent but I figured you should know.

 

How do they regulate and verify the boost level?

 

This is what you should be chasing. Flat HP curve, right up to that 306hp limit. 7670 will give you that early torque, and will carry power to redline @306hp peak without any trouble. You’ll need good boost control to make this happen.