Intake sizing is a subject worth looking into. I realize that the vast majority of owners are using the OEM throttlebody and, as such, don't need to put much thought into the size of their throttle plate(s) since Mazda already did the research.

However, as more and more people shift to advanced ECUs I think we'll see a lot more drive-by-wire conversions. These conversions open up a whole new world of control possibilities (traction control, driving/valet modes, throttle tuning, etc.) but they also offer pitfalls. Bigger isn't necessarily better. The larger your throttlebody, the less precisely you can control the airflow in the idle to low speed range. I think the best solution for this is to select the smallest throttlebody that will support your max flow requirements without restriction. This will allow your car to perform the best in all conditions.

This brings up my question: how much CFM is required for a given horsepower amount in a rotary?

Things like boost and porting level will obviously be different from one car to another, but the required CFM should remain relatively constant.Intake sizing is a subject worth looking into. I realize that the vast majority of owners are using the OEM throttlebody and, as such, don't need to put much thought into the size of their throttle plate(s) since Mazda already did the research. I watched a webinar from HP Academy a while ago that went into the balance between a throttle's max flow verses performance in the low range.

As more and more people shift to advanced ECUs I think we'll see a lot more drive-by-wire conversions. These conversions open up a whole new world of control possibilities (traction control, driving/valet modes, throttle tuning, etc.) but they also offer pitfalls. Bigger isn't necessarily better. The larger your throttlebody, the less precisely you can control the airflow in the idle to low speed range. I think the best solution for this is to select the smallest throttlebody that will support your max flow requirements without restriction. This will allow your car to perform the best in all conditions.

This brings up my question: how much CFM is required for a given horsepower amount in a rotary?

Things like boost and porting level will obviously be different from one car to another, but the required CFM should remain relatively constant for any given power level.

Accufab has a page that breaks down the CFM of common domestic TB sizes (https://accufabracing.com/cfm-air-flow).

I know there are far smarter folks than myself here. Has anybody figured out an accurate formula for CFM requirements based off a projected rotary whp? This would be assuming that all other factors are ideal (fueling, tuning, turbo efficiency, intake temps, etc).

 

Most/all drive by wire throttle bodies are extremely nonlinear in actuation. The bore is shaped to run close to the throttle blade in the first ten-twenty degrees of opening so that changes in throttle position make for fairly small amounts of airflow change.

I'm not sure if CFM is a relevant figure when talking about forced induction engines, comparing the actual standard-day airflow to the compressed air airflow might have you wanting things like throttle sizing physically larger than the pipes leading up to the throttle plate...

 

Yeah, that makes sense. My main concern falls along the lines that 5% open on a 105mm TB is drastically different then on a 90mm TB, or a 75mm one. I was hoping there would be a viable calculation of sorts for determining what the smallest effective size for any given power range is.

I was digging around a bit more and came across an old thread on ausrotary where Peter (rice racing) went on a diatribe that essentially said "it's too complicated, don't ask for a cookie cutter calculation." Say what you will about the man's personality, he was usually pretty spot on when it came to the mechanics.

I guess the CFM matching is really more for naturally aspirated engines. Would you agree that the main thing to be concerned about in a turbo engine is just ensuring the TB isn't the most narrow point in the intake? As long as it has more flow than the IC pipe, and more area than the UIM openings it should be fine? I suppose a more important factor would be smoothing the transition from TB/adapter to UIM.

I'd really like to use a bosch 82mm TB, I'm guessing that would require a custom adapter though.

 

According to the FIA homologation, those 450-500hp Ford RS200s had a single 40mm throttle plate.
Somehow I think that is a misprint

 

Yeah, the HPA thing I was watching indicated that after testing different TB sizes it was actually found that going bigger could actually be detrimental. Unfortunately I can't remember if it was through a publicly posted video/article or a members only one

Looking around, it seems like most of the DBW adapter plates for the rotary are designed around 90mm - 102mm throttle bodies. This seems somewhat misguided to me and means that, sadly, a custom adapter will be needed if folks don't want to go that big. Needing to expand out to a 3.5" to 4" IC pipe at the TB is a bit excessive imo.

 

I tried a 75mm throttle body on a Turbo II intake manifold and the car lost power. (Naturally aspirated)

I wonder if under high boost applications, there isn't a benefit to having an UNDERsized throttle body, you would overpressure the air, cool it off with the intercooler, and then it gets even colder due to the pressure drop through the throttle. Maybe? What is kind of odd is my Volvo has a fairly large throttle body for a 2.5l engine, but I'm measuring up to a 3psi pressure drop through the throttle plate.... Weird.

 

I'm guessing that the sweet spot will be by determining the total area and flow potential going into the 3 ports of the UIM, then going ever so slightly larger in the TB. Anything less than that can be a potential restriction, and significantly larger is just giving up levels of precision at low throttle opening %.

This is just me musing though... I'd love if some more of the heavy hitters could weigh in.

 

its a tricky one, because there are a couple ways to go about it. the simple way would be to just compare the area of the stock throttle body, and get one that is slightly bigger. i did this with my IC pipes, and you could see that i had one that was too small, and fixing it helped. there is a difference between area and flow, but they do correlate. i think off the top the area of an 80mm pipe and the throttle body were about the same, but we know that the stock setup uses a 70mm pipe.

the second way might be to just assume 100% ve, and calculate CFM that way, the catch is that you can calculate this two ways. one is regardless of pressure, so you get VE's over 100%, the other is with the pressure and this caps VE at 100%, but the gasses get more dense. this one makes sense to me, its a 654cc chamber and you can only put 654cc's in it, but you can certainly change the density of those 654cc's.

so its tough because we don't really have the info for the engine, unless its 100% stock then we do, and then there are a few ways to do the calculations, or you can just use whatever everyone else does

if you do some digging, Speed of Light did some flowbench testing of the intake/exhaust parts

 

biggest issue is they’re typically rated @ 28” vacuum for NA applications and so under boost are going to be higher, but here’s a list I saw from the past for the Accufab TBs:

https://accufabracing.com/cfm-air-flow

and Howard Coleman’s fuel injector sizing thread details out a basic rotary airflow calculation by whp

https://www.rx7club.com/single-turbo...e-e85-1056358/

that said, 13B-REWs with a 70mm RX8 DBW are making 550 whp (~1060 cfm) without too much issue and the FD manifold adapter is readily available. The RX8 TB mounting pattern is the same as a 70mm Ford TB. The inlet flange is an odd 3.125” OD, but there are 3” x 3.125” silicone adapters out there.

and I saw some commentary from the owner of a 900 whp 20B (~1750cfm) that a 90mm DBW made a huge streetability difference over a 105mm non-DBW

so hopefully this will give you some basic context …

 

Thanks for chiming in! Yeah, with forced induction it seems that bigger definitely isn't necessarily better when it comes to throttlebody diameter. I'm up in the air between going with an 82mm bosch unit or an aftermarket 90mm one. My main concern right now is going to boil down to gear/motor strength of the DBW unit. I expect that I'll be running north of 3.0 pressure ratio on whatever turbo I use and I've heard that some stock units can't really handle more than 20psi, which would be problematic to say the least.

In a nutshell, going larger in size transitions from diminishing returns into being detrimental unless the TB is the most restrictive point in the intake manifold/piping. Go as small as you can while still matching the effective area of your UIM/LIM piping and you're set.

 

what was your whp goal again?

 

That's complicated 🤣

The car is built for all the horsepowers (motec m150, pdm30, complete array of sensors, DBW, T/C, 4" exhaust, all new tefzel wiring for body/dash/engine, 680lph of fuel, vmic, WMI/AI, flex-fuel capable, etc., etc., etc.) however I'll limit it to approx 350-400whp for local backwoods cruising, which will account for probably 90% of my use. The remainder of the time I'll have the bigger turbo on and I'll send it for all the turbo can push.

I have both an EFR 7670 and 9174 on my shelf and plan on being able to change freely between the two. I'd probably only push 55lb/min of air through the 7670 and 82-85lb/min through the 9174.

The OEM setup is more than capable of handling anything the 7670 could want, however it would become a limiting factor when pushing the 9174 to its edge.

I plan on calling BBK today to see how their stuff handles high boost applications, I want to see if they're worth the extra costs.

 

how many CFM is 85lbs? you may have to account for the density 1cfm at 1 atmospheric pressure is going to weigh less than 1cfm at 2 atmospheric pressures (same size room, but more stuff in it)

 

I'm not sure to be honest, it looks like the flow for an 85mm-90mm TB is in the mid 1300 CFM range though. That should be enough just just about anything a 2-rotor will realistically be needing.

 

per Howard's linked thread above:

Honestly, that seems maybe slightly big to me and I think you're OK with an RX8 70mm DBW TB or something closer to that 3" range. Which I'm not aware of having any issue to upper 20ish psi boost level (13B-REW & BW S362/369-SXE). Obviously that comes more into play with the larger the TB blade is. So those larger V8 type 90mm - 120mm DBW TBs are going to be more susceptible to that issue and is typically where the stronger control motors are being marketed that I've seen.

because again, that rating you're looking at is at 28 psi vacuum; approx. 1 Bar Absolute.
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Assuming that Howard is correct, that 1230 requirement would fall in the 82mm to 85mm range. Per the accufab site, 80mm gets you 1142 CFM, and 85mm gets you 1322 CFM. If you split the difference at 82mm (Bosch's largest) it would be 1232 CFM, theoretically. Unfortunately the Bosch tech sheet doesn't provide CFM ratings for their larger blades.

I spoke with a sales rep at BBK this morning, he's going to talk to the R&D team about max controllable boost on their 85mm TB's and get back to me. Depending on that response I'll either get their 85mm unit for the '18-'21 Mustang GT to have a bit of head room, or the Bosch 82mm for exact needs and smaller post-IC piping.

 

again, the Accufab data is based at 28” vacuum

at 1.5+ bar boost it will be much higher, but hope it works out for you wrt drivability and such

 

Ah, I see what you're saying. As pressure goes up so does the cfm flow of any given passage. That's kinda what I was thinking when I figured that the TB just needed to be less restrictive than the next most restrictive pare of the intake track.

My LIM/UIM had previously been extrude honed and I don't have them in front of me, so I can't estimate what the effective area of that setup is. I think I'll end up getting a pro jay intake to just completely future-proof my build for e85 and any max power figure I could make. That intake comes with a cable-driven 90mm TB, so going to either an 82 or 85mm would be a reduction in size anyway.

 

If the bolt pattern is the same, might consider the LSA 87mm throttle body as a potential compromise between the known 82 & 90mm offerings. Aftermarket vendors can also port the 87mm model to effectively improve low-end throttle response; enlarging the opening for *better* air flow in the process.

 

28" of vacuum, assuming inches of mercury, is a nearly 14psi pressure drop across the throttle body!

 

yeah, I noted it as approx. 1 Bar in a previous post above

the PJ intake is pro drag oriented, so 90mm makes sense

but I still believe that if the boost is going to be running up near 2 bar atm. that you can use something in the 70mm-76mm range without suffering too much dP loss and have the drivability benefits, but that’s just my perspective from street driving a 13B BPT w/70mm 😆 and also noting that a 900 whp 20B (50% more displacement, 25%-30% more hp) found 90mm to be much more streetable than 105mm. So I agree that 82mm is going to be better than 90mm on a 13B, just that another step down is better still with no real disadvantage short of all out drag racing.

to each their own though

 

Carburetors are rated at 2" or 4".

I mean, the only way my N/A engine can make 28" of pressure drop is if I rev it up to 7000rpm and close the throttle.

 

yeah, but go watch the Rob Dahm video from a couple of weeks ago doing DBW on the 20B. He was eventually able to tune it by making it not doing much over quite a span of input, but only after a lot of hair pulling and cursing. Seemed obvious to me that the central issue was the TB being too large. That’s what you’ll be facing. I even have it slightly in the lower gears at low rpm with a 70mm, but imo that’s due do the BP pulsing, and not so severe as to be a problem.
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