Hey all, I have a quick question. When it comes to building up N/A engines it seems that the most that people get out of an extened port 13B is 200hp at the wheels. But people have been saying that 350rwhp should be doable with an extended port 20B. Why does it seem that 20B's can make more hp vs. displacement than 13B's.

Thanks
-Justin

 

If you do the math you would find that if you assume 350hp for a 20B is actually crank hp, and not rwhp, then the numbers match up pretty well.... Not saying that the answer, but its an awful coincidence in it isnt.


BC

 

ok. 13 b = 2 rotors.
now, 20b = 3 rotors.

if a 13b makes 200 crank hp, a similar 20b will make SOMEWHERE in the ballparck of 300. about 100 hp per rotor. it just makes more power but its also expensive, and that kind of power is obtainable via turo for a fraction of the price youll need to even get the 20b in and working in the car. not to mention , who makes a 20b na header?

 

I understand the plight for the 20b n/a, it makes approx 242hp at the crank in stock trim, minus the turbos. and about 251 horspower with turbos.

The reason people build all-motor 3 rotors is for the power band, which is more driver friendly then the 13bt. The biggest reason for this is where, when and how power comes on. Turbo rotaries (and piston engines) rely on the extra air the turbo provides to make more power. Typically power comes on when the boost builds. A stock 13bt makes a steady powerband the levels out at around 6500-7000 rpm. The mor boost you build the more power you will make here. However, when you put on a bigger turbine, to build even more boost, you are stuck with boost lag. On other words, the boost takes longer to build, so when the power comes on it hits like a jackhammer. This makes the car mostly un-circuitable. Mazda came up with a solution which came in several different forms over the years. The first was the 12aT "impact turbo" the turbo was tailor made for the exhuast signiture of the rotary. then came the s4 t2, which was a slightly modifed verson of the hitachi impact turbo, except fitted for use on the 13b 4-port. The next big leap came on the 13b-rew. The 13b-rew employed a squential twin turbo design, where one turbo builds low boost pressure, and the other builds high boost pressure. Effectively this allows a turbo engine to have powerband more similar to a naturally aspirated engine. Ta-da!!!!!!!!!

As great as a 13b-rew is, it's usually a very expensive transplant. The same can be said for modifying an non-"rew" 13b setup to accept the sequential twin setup. (there are a number of internal differences between a 13brew and a 13bt) Here too is the problem for circuit racers and serious track goers, displacement is limited and boosted motors are usually restricted. Hence the 20b's entrance into racing history.

Simply put, it takes less work (by a matter of physics) to get a 2 litre motor to make more power (do more work) then 1.3 litre motor.

funny things here though..... The power bands of "b" width motors and "a" are very different. "a" width motors tend to make more power up top, peaky unlevelling power (desirable for circuit use). "b" width motors tend to level power off before the redline. Some of this is port timing and compression, some fuel delivery, some is ignition differences.

In short, before deciding to drop 10k on a 20b, or 3-6k on a 13bt of similar power, consider what you want out of the motor. Why do want the power? how should it be delivered? is chassis tuning important? how should weight be distributed?
Total tuning is a very important aspect of a car construction, do you want a drag racer, or a race car?

 

Any competant exhaust shop.

My na 20B makes 295 rwhp using low comp turbo rotors and a short primary exhaust (both not ideal for top hp)

You obvousily have never driven a 400hp flywheel rotary if you need to ask why do it

There are plenty of na 20Bs around btw. from street ports through to PPs.

My engine has instant power all the way from idle through to 9000 rpm, more torque than you could ever hope with a 2 rotor and a much better sound.

I had 13b bridge ports for 8 years before the 20B, can't compare to a street ported na 20B.

*edit*

This was on a dyno where the street 13B Bridge Ports were running just under or just over 200hp. 206 was the best of the day.

A street port 13B on this dyno would see around 160hp-170hp at best.

 

20B PP Turbo in action:
http://videos.streetfire.net/video/E...892647E798.htm

20B PP at idle:
http://videos.streetfire.net/video/7...0E7C1A1CC8.htm

 

meant 400hp na rotary in my post

 

I get chills everytime I watch a vid with a 3 rotor in it.

If I ever have 20-25k laying around, I know what I'm going to buy.