Observations - 20B vs 13B
 

I made a little spreadsheet to calculate area under the curve in the powerband for torque curves from actual dyno results.

I took 3 cars and did a little comparison of the area under the curve in the powerband. The three cars were:

XS Eng - T04S with 1.15 A/R at 22 PSI
My Car - T04S with 1.00 A/R at 15 PSI (tuned by me)
3 rotor FD - T72 with 1.15 A/R at 14 PSI

All 3 cars had similar mods like porting, open exhausts, etc

Keep in mind this is ONLY three cars and I'm not trying to say that X person or Y car can't do better or worse than what I'm showing here.

So anyhow, I put the torque curves into Excel and did a trendline on them and then integrated the area under the curve between maximum RPM and the new RPM after you shift. I used a 30% drop in revs between gears because this approximates the drop in the stock FD tranny from 2->3 and from 3->4 and this is where a lot of racing/speeding takes place.

On the two 13B cars, the theoretical optimum shift point was around 8200-8300 RPM but I didn't use that number for two reasons. 1 - My source dynos only went up to 8000 on those cars, so trying to estimate what may happen past that is sketchy. 2 - The stock redline is 8000 so I didn't want to go past that. For car 1 and car 2, I used 8000 as the shift point. (It doesn't cost them that much anyhow.) For car 3 (the 20B), The best shift point turns out to be about 7866 rpm so I used that.

And now the results -

CAR --------------- AREA
13B T04S 22 PSI - 907,344
13B T04S 15 PSI - 769,763
20B T-72 14 PSI - 822,119

Compared to the 20B, the 22 PSI 13B car had 10% MORE area under the curve. The 15 PSI 13B car had about 6% less area.

** Keep in mind that this is the area in the powerband as defined above. **

Obviously the 20B cars makes a little more power down low where the other cars don't. But if you're on a track or on the street where low rpm might not be that important, it's not helpful to you!

So what am I saying? Well maybe this is why you don't see more 20Bs... They cost a LOT and they just aren't worth it for most people. Maybe 20Bs just suck! Hahha no... We all know we want one :)

Anyhow this particular 20B that I chose to analyze probably would have done better on this test with another turbo on it.... or perhaps the boost gauge used was off by a few pounds because I think it should put out more power at 14 PSI than it appears here.

Either way it is still an interesting discussion... yeah the 13B might make more power, but you gotta pump about 50% more boost through it :) I'll take a 20B at 14 PSI over a 13B at 22 any day.

Brian

Good comparison... I know 20b's are probably capable of huge power, however I have yet see any real 20b monsters put down some really big numbers, maybe I need to search more or something, but there seems to be far more success with the 2 rotor at big numbers than the 3 rotor...Even in Japan, there wasn't really alot of 3 rotor craziness, alot of really high hp 2 rotors, but maybe its just not worth it for the fitment problems for most... I still want to try a 3 rotor,but I am not totally at the end of my rope with a 2 rotor yet..Max

I'd like to add 2c to this, just to see what people think.

I think at the end of the day its all in how you want to go about things - granted its a lot more expensive to go for a 20B with all the engineering typically required, however it does seem like the best way to go very very fast at the end of the day - looking at the results of drag rotaries around the world.

Keeping in mind the most important time to be able to supply sheer horse power is very early in the run of each gear - as all the rest of the acceleration is cumulative from that point....

For example - if you had a theoretical car and accelerated at .5g for 2seconds, and then accelerated at .7g for the next 2 seconds, you would in the drag race sense get fairly well left behind by a car which accelerated a .7g for 2seconds, and .5g for the next 2seconds. Of course you would be reaching the same speed in 4seconds, and would have experienced the same kind of g force (naturally) but the G-force counted for that much more - because the harder initial acceleration would have got you that much more speed, and the lesser GForce later would be accelerating you from that already gained lead.

Over simplified maybe, but with the higher low down/mid range torque of the 20B the earlier acceleration in each gear would potentially make greater use of the "area below the curve" giving it a head start through the otherwise still meaty power delivery.

Also keep in mind also that the above comparison between the 20B running 14psi and the 13B running 22psi means there is 28% more atmosphere available to the 13B to provide only 10% more total area beneath the curve but I'm sure thats obvious to all :)

I tried an 'area' program for shift points that gave wrong answers in some cases. Torque or hp curve intersections is the way to go.

It appears the 20B may have some VE disadvantages vs the FD 13B. Wonder why the curve dropped so much, even with a mega turbo for 15 psi.

Area under curve is general indicator, but units make no sense. If curves are close, the one that gives more torque at the upper 1/2 rpm range, where time-at -speed is higher due to drag, could be quicker in 1/4 than another curve with more area but low end bias.

really want torque vs time to get correct units ... work done, but easier said then done.

is a t72 even efficient at 14psi?

much more work has been done getting the 13B to high power than the 20B, i think.

based on linked curve, 450 rwhp at 7200 rpm and 14 psi.

I figured 48.5 lb/min at PR=2.16, puts it at an 70+%efficient (but low rpm) part of the T72 map.

Based on my calcs, I come up with a 1.95 pressure ratio and needing about 50 lbs/min airflow... (assuming 85% vol efficiency). That puts it in a good spot on the T72 map so I don't know why it didn't make bigger numbers.

20B information is hard to come by so maybe someone else would care to contribute more graphs with lists of mods etc?

Brian

I think the graph is more representative of turbo efficiency rather than engine performance. I think you would need to do some na testing or standardized turbo app testing to really find anything conclusive. Interesting thought none the less.

I have read of several 3rotor FDs in the 1000hp+ range so I'm not to convinced with the 2-rotor superiority statements above...

I agree! Now about those 1000hp+ Fd's. Were did you read about those? Please share! :D

Heh, I don't think the 2 rotor is superior... The 3 rotor is way more flexible (I've ridden in several and driven one too). I'm just pointing out that dollar for dollar, you can get a LOT of bang for the buck out of the 2 rotor.

Keep in mind that when you have a powerful (say over 600+ rwhp) 3 rotor, the stock trans, diff, and axles all become weak spots. (In a 2 rotor, the trans is generally pretty bulletproof even on high HP setups) A 600 hp 2 rotor and a 600 hp 3 rotor put very different loads on the trans because of the much-improved low to midrange torque of the 3 rotor. I know the owner of a major shop and he was telling me that they stripped 3rd gear completely bare on a 3 rotor RX-7 due to the torque of it.

I guess what I'm saying is that you can't just slap in a 3 rotor and make 750 rwhp. You need to spend a LOT of money on just about everything on the whole car and then MAYBE you'll get your power. It's just like when people come into the single turbo forum and think that for 2500 they can buy a single turbo kit and have a single turbo with 450 rwhp. That's not true; you need to upgrade many other components... so your 2500 kit ends up needed another couple thousand in mods to support it.


Let's see some more 20B dynos! I only have about maybe 4 of them; I want more!

Brian

Wargasm, thanks for elaborating. I agree with what you're saying. I think a 2rotor is more than enough for the majority of 7 owners.

T-von, I will have to do some digging around to find you the 1000+hp 7s. One of the cars was painted in plain yellow and I believe the other was in a redish purple paint scheme. They are running 7 sec quarters of my memory serves me correctly. I'm sure that someone will chime in with their names.

I couldn't find the info on that yellow FD, someone please fill the gap.

hey, wargasmn, you wouldnt happen to have the torque bands from the dyno's? if you do, could you post them please??

-Daniel

T-von, here ya go, as promised...
https://www.rx7club.com/showthread.p...hreadid=253103
http://www.cityperformancecentre.com...quis/specs.htm

The main difference I see here is that the 3 rotor is making that power on 14psi and pump gas, while the XS car is at 22psi on race gas.

That makes all the difference in the world to me because I drive my car daily, and frankly I don't care all that much if it makes the same power at 1.5 times the boost if it means i'm having to get 8mpg on $4.75 a gallon race fuel.

Also that huge increase in low end power changes the behavior of the car completely when driving around the street.

Plus....... This comparison is a complete apples and oranges situation. If you want an acurate comparason, you will need to do it NA. Or at the very least compare the cars on the same fuel, which would mean 15psi on pump gas.

But, this comparison is a good idea and its given me some great information... keep up the good work.

after reading the posts....

max hp seems useless if your going to shift at 700hp, is this correct thinking? maybe we should start braging about usefull hp :)


http://www.cityperformancecentre.com...quis/specs.htm

Too much theoretical talking and not much real world experience. Go take a drive in the cars and then make your conclusions. Numbers can only go so far...


-Ted

I dont get it. An extra rotor means more power period

geargrabber, I see what you're saying. But regardless, the 3 rotor makes 1000+hp; that was the point. I'm sure that the majority of that power is actually used after the last shift.

As an interesting comparison why not throw up some sheets with twins. How about using Boostn7's 402ish rwhp non seq sheet. That should make for a very interesting comparison.

Here is John's sheet......
https://www.rx7club.com/attachment.p...postid=1437472

STEPHEN

MVA Motorsports - 800 HP 20B

20B with 11 psi boost, unknown turbo - post-tuning. Next one with be pre-tuning. Slight increase in HP but not torque.

Next one...

20B vert dyno