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Naturally Aspirated Performance ForumDiscussion of naturally-aspirated rotary performance. No Power Adders, only pure rotary power!
From the "12A" to the "RENESIS" and beyond.
I have a 20B PP that I bought built and assembled. It came with Racing beat super lightweight rotors that are the turbo 9.0 compression ratio. I'm debating getting a set of 9.7 NA rotors and having them lightened but I'm really wondering if its worth all the work and money. How much power am I leaving on the table using the 9.0 rotors instead of the 9.7? Is there any advantage to using the 9.0 like running more timing or more space for air/fuel inside?
there is more space for air/fuel mix, and possibly it will flow better inside the engine too. also you will be able to tune the timing and air fuel mixture to optimum.
and then the factory engines, up to the Rb26 only ran 9.4:1
you will be down on power, but it seems like maybe not a lot
there is more space for air/fuel mix, and possibly it will flow better inside the engine too. also you will be able to tune the timing and air fuel mixture to optimum.
and then the factory engines, up to the Rb26 only ran 9.4:1
you will be down on power, but it seems like maybe not a lot
I feel like its worth giving the 9.0's a shot, worst case I'll pick up some NA rotors later. I just wanted to ask, because if it was down 60-80hp I might go ahead and just get the NA rotors now and be done with it.
Sadly, because of the PDFs I didn't get good pictures of the charts- I will see how screen shots show up.
First this figure 9.11 from page 98 of the 1971 edition shows the curve of Brake Maximum Effective Pressure of the different Compression Ratios of the rotary between 6.5 to 12.6 CR.
BMEP is a theoretical tool to compare the torque a given engine can provide from a given displacement.
Fig 9.11 1971 ed pg 98
For a rotary engine BMEP is very flat from 8.5 to 11 Compression Ratio with just a slight increase.
Fig 4.37 1981 ed pg 49
This again shows BMEP for 9.2:1 CR compared to 10:1 CR. I found that looked like a peak of 8.6kg/cm2 and 8.8kg/cm2 or 122.3psi and 125.165psi.
Only a 2.3% peak increase BMEP from 9.2:1 CR to 10:1 CR which is down at 4,000-5,000rpm.
2.3% peak increase is definitely something on an NA build, if you are making something like 180rwhp at 5,000rpm a 2.3% increase in power would be 4hp for 184rwhp at 5,000rpm.
NA gains are hard won past a certain point.
I don't know when you will want that 4rwhp, but if you are racing competitively against someone with the same set-up you need it.
As j9fd3s said Mazda factory race engines use specially investment cast 9.4CR rotors in their NA race engines until late in their R26B Lemans race engine program where they went to 10:1 for broader power and most importantly better gas mileage (remember this is a 24hr endurance race motor where their winning strategy included coasting the 3 mile straight).
Mazda's own documentation as shown above shows there will be no increase in power past 6,000rpm- so likely no peak power difference in an NA race engine.
Thanks so much BlueTII for the epic fact filled post. That is exactly the info I was looking for. I will have to read the whole book by Yamamoto and see what awesome secrets are inside. I don't think my engine will make too much power under 5000rpm anyway so it should be fine if I lose my slight amount of power down there. Thanks again.
According to the chart above at 9.0 compression I could run Regular pump gas (87 Octane). Would running 87 still let me get the most out of my timing? I know knock would not be super easy but I would still want to be cautious.
If you are trying to make power and rules allow it; use Oxygenated race gas.
Fuel suppliers say oxygenated fuels can typically make up to 3 to 7% more power than ordinary fuels. The higher the oxygen content of the fuel, the greater the potential power gains it can deliver.
One fuel supplier said you can estimate the gain in power by dividing the oxygen content of the fuel by 3. For example, if a racing fuel is 12% oxygenated by weight, it should produce about a 4% increase in horsepower over a non-oxygenated fuel with a similar octane rating.
So, maybe you could get like 30rwhp from gas on 14% Oxygenated on a 450rwhp > 480rwhp NA 20B.
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
I think it is what NA import drag guys use (Hondas).
I guess i have never looked into different types of race gas, that stuff sounds great, i'm sure it comes with great cost too.
I am not really trying to go for every last drop kind of power with this setup, I just don't want to leave any reasonable gains on the table. I might defiantly have to play some some oxygenated gas at some point, sounds too easy.
So for the price of a balanced, lightened 9.7CR rotor set for a 20B you could afford about 375 gallons of it and make more power doing it.[/QUOTE]
This might be the best way to say that. Thanks. I am going to run the 9.0's and see how they do, I'm sure I wont notice too much of a difference either way since I'll just love it an be happy.
You mentioned 87 octane and timing. The last 3 rotor periph I tuned liked 26* advance at torque peak (8000 rpm) while the one before that liked 24* advance, same torque peak. This was on 91 octane. Just some food for thought on where to start.
You mentioned 87 octane and timing. The last 3 rotor periph I tuned liked 26* advance at torque peak (8000 rpm) while the one before that liked 24* advance, same torque peak. This was on 91 octane. Just some food for thought on where to start.
Thanks! I'll keep that in my notes. You care if i PM you and ask some basic questions about your tuning of P ports?
A lower compression ratio will have less pumping losses which should translate to more peak power at the cost of some low end torque. I think there are quite a few RX7s in Japan that removed the turbo and intake and swapped to ITBs that make over 250 hp bridge ports and up to 300 or so peripheral port while still using the turbo rotors. I would venture to say your intake/exhaust setup and your tune will have more of an effect on your final power figures than the compression ratio.
The highest power N/A dyno chart that I've heard of, had 8.5:1 rotors...
Foord for thought.
... I may have just had a brainstorm about why, thanks guys for bringing this up. Going to have to build a flowbench adapter sooner than I planned to test it.
Originally Posted by peejayThe highest power N/A dyno chart that I've heard of, had 8.5:1 rotors...
Foord for thought.
... I may have just had a brainstorm about why, thanks guys for bringing this up
Pumping losses.
The trailing side of the rotor has to push the whole compression charge through the slot in the rotor to get the compression/combustion charge across the minor axis (the pinch) in the rotor housing. The higher the compression ratio, the smaller the passage for the air moving through the engine and so the higher the pumping losses.
Yeah, I've heard that one more than a few times. Given the LDR vs MDR data and the fact that it's not showing up in terms of BMEP dropping off, I don't buy it.
I think the problem is the results don't show what is happening after 6,000rpm.
Here is the Leading Deep Recess versus Medium Deep Recess chart from Mazda I think you are referring to.
And here is the Compression Ratio chart from Mazda
Damn, neither one shows after 6,000rpm where I would expect the two lines to slowly diverge with the engine with lower pumping losses making more power.
Not coincidentally, it is the NA engine that retains it torque highest in the rpm range that will make the most horsepower.
The whole idea of Leading Deep Recess is to have the trailing side of the rotor shoot the compression charge through the rotor slot with higher velocity for increased tumble and increased flame front speed off the Leading spark plug. You get the increase in velocity by restricting the rotor slot on the trailing side. Pumping loss.
It takes energy from the rotor to accelerate the compression charge- there is no free lunch. Mazda engineers were trying to limit NOx emissions and get the most power at 2,000rpm when they designed the LDR rotors.
Here is the Mazda chart showing the rest of the story with RPMs over 6,000rpm. See how the MDR rotors take the lead in power in the high rpm?
Yeah, and it's on the order of a couple percent. Changing the recess shape is going to have a lot more quench/squish effect than deepening the rotor recess.
Look, I'll just knock out a flowbench adapter in the next week or two and test/demonstrate my point. It's too bad I don't have any dead turbo motors... be really interesting to see that data point instead of just GSL-SE vs RX-8 ones.
I think the difference, especially on ported engines, lies on the other side. The pinch point is almost entirely choking off overlap flow.
From what I remember (and what it looks like in the rotary animation videos) the rotor depression is past the rotor housing pinch and the rotor flank is up against the rotor face with a small gap between them at the point the exhaust port opens.
The exhaust port opening is when the real rotary overlap starts.
That is not counted into the official overlap timing figures because it is the upcoming rotor face exhaust timing compared to the previous rotor face instead of using the piston engine convention and comparing the exhaust and intake timing all on one rotor face.
The official overlap timing figures just show the overlap on the "dead" exhaust port (no flow) which contributes very little to rotary overlap power.
Briefly, I disagree with your last point; as you just pointed out the depression is past the pinch point when the next chamber opens the exhaust port, so flow is basically choked off. Also, if overlap effects were dominated by the exhaust exiting from the next chamber, exhaust tuning would do basically nothing, ala the Renesis engines. SAE900032 has some predicted and measured pressure traces and effects on volumetric efficiency - note what lengthening the exhaust primary to have the negative pressure pulse coincide with the intake port being open does, and how the intake pressure stops being as sharply negative around when the exhaust for the next rotor is opening.
No, the attachment you posted shows exactly what I am saying.
Intake pressure is lowest when exhaust port pressure is highest (when the exhaust port for the proceeding rotor face has just opened and the rotor face above is still in the overlap and now also in the *super rotary overlap tm* phase).
I think what is confusing you is that Mazda is adhering to the piston engine convention in that chart and shows exhaust closing and exhaust opening. Actually, a peripheral port never closes- it just switches flow to different rotor faces.
So, look at the chart again. EO (exhaust opens). Highest exhaust pressure as the combustion stroke blows down through the exhaust port. Lowest intake pressure as intake tract is entrained by the high velocity, high pressure exhaust gasses exiting the port on one side of the apex seal while on the other side of the apex seal the exhaust port is open to the intake stroke.
The confusing part is on the chart it next shows EC (exhaust closes). You have to remember that just after what they are calling exhaust closes is the exhaust port opening to the next combustion stroke and blow down happening-
right where the chart again shows the highest exhaust pressure and the lowest intake pressure.
11-19-17, 08:58 PM
The highest power N/A dyno chart that I've heard of, had 8.5:1 rotors...
