I have recently acquired a 1985 GSL with an ’86 13B 6-port recently rebuilt and a ’80 12A in need of a rebuild (neither are in the car).

My goal is a track car that I can drive to the track. It looks like I can register it in MD has a historical vehicle (20+ years and out of production), so emissions testing may not be an issue, but it still needs to be reasonable to drive on the street (to the track)

I’d like to get to > 200hp at the flywheel, but I’m not sure where the price/hp break point is on the RX-7 so that may not be reasonable. I lean toward NA for reliability on the track, but would like opinions.

From forum research to date, it sounds like the 12A seems more popular for modifying, but the 13B is like a better base for turbo charging.

Assuming header, intake and exhaust upgrades in any case, what are the recommendations on engine mods based on the 13B and 12A options I already own?

Here are some questions I’ve been trying to answer:
- Reasonable HP expectations for a 12A street-port (160-180 at flywheel?)
- Reasonable HP expectations for a 12A bridge-port
- Reasonable HP expectations for a 13B street-port
- Reasonable HP expectations for a 13B other porting options
- 12A Bridge port is rough at idle for street use, but sounds do-able for my limited street driving, is too loud for street use
- Is exhaust porting a must-do for the performance gains

The next step is suspension, but I need to figure out which engine to use 1st.

If you made it this fair I appreciate the attention!

What would YOU do?

Thanks,
Todd

 

13b will be the better choice if ur able to do the swap from 12a to 13b. the 13b will yield alittle more torque than the 12a with the same port configuration.

 

'74-'78 13B irons with a good sized streetport, -SE housings, and S5 N/A rotors.

4 ports respond awesome to porting and combined with the lightest and highest compression rotors, you've got yourself a real winner. Personally, I would cap it off with a good sidedraft, but for road racing, you might want to go down draft because they typically have better top end power.

Depending on port size, that motor could very likely get you close to 200 WHEEL hp.

As far as exhaust porting goes, why would you make your intake flow more if you are just going to bottle neck it at the exhaust? I'm not saying it won't work, but a good port on your exhaust will do you wonders.

 

I agree, with good porting and a Weber 48 IDA, 200 rwhp should be no problem for the 13b. 12a's can achieve this power output but someone who knows how to do a "race" port (read large streetport) would be needed. The only real issue I think is exhaust noise. To get this kind of power output out of either motor a non- restrictive exhaust is very important. It will be loud.

 

The 13B was running in the car before the prior owner pulled it to start the turbo, so to out it back in *shouldn't* be too bad.

Thanks,
Todd

 

Is this a rebuild of my 13B a build up of a different 13B (I don't know what the 'irons' are)? Can you dumb this down for the rotary noob?

My 13B is a 6-port, do I still have the same porting options?

Thanks,
Todd

 

Irons refers to the plates with which the ports are located. They are the pieces of metal that sandwich the housings.

Old 13Bs were 4 ports, like the 12As and like second gen TIIs. With your 6port, you still have the same porting options, but 6 ports tend to not get as good of results with porting as 4 ports do. Don't get me wrong, you can very well do a nice sized streetport on your 6 port and get 200hp, but the port may have to be more aggresive. Also, you'd have to figure out whether or not you want to keep your 5th and 6th ports.

The 5th and 6th ports are called auxiliary ports, and they are operated by exhaust pressure. A hose connected on one end to your exhaust is also connected to a pump on your lower intake manifold. When the pressure reaches a certain point, the pump is operated, which in turn rotates a rod that is connected to your auxiliary port sleeves. These sleeves are nothing more than a metal cylider with an opening cut on one side to match the port in your 5th and 6th ports. So, when pressures is acheived (typically 4-4500rpm), the sleeves are rotated, and your 5th and 6th ports are opened, allowing for more fuel/air to enter the engine.

If you changed your header, you would lose this hose hook up for your auxiliary port pump. However, Racing Beat sells a presilencer for the GSL-SE that has a pick up for hose. Most racing 6 ports, as far as I know, do away with the 5th and 6th ports completely (like th 13B I'm dropping in my car this weekend) because having all 6 ports open all the time means that you can get the maximum amount of air/fuel in.

Tell me this, what year is the 13B you have? If it is an S5 (89-91), just tear it down, streetport it, and remove the 5th and 6th port sleeves, slap a nice carb on it, and you are good to go. However, if it is a second gen motor, you can retain the fuel injection; but, some believe that it is a pain beyond all belief. If you do want to keep fuel injection, talk to Steve84GS TII. He is the man when it comes to second gen fuel injected swaps into 12A cars.

If you don't have a Mazda Factory Service Manual or a Hayne's Manual for your car, I suggest buying one ASAP. It will be the best $25 ever spent on your car.

If you have any other questions, there is plenty of knowledge in this section to help you get on your way.

 

Great explaination, thanks.

The 13B is an '86, so it's not an S5, what is it called? My plan had been to go with a carb since it sounded easier, but I'm looking for suggestions. Does the fact that it is '86 13B 6-port change the recommendation?

I have the rotoary aviation rebuid video, Mazda Rx-7 1979-1985 Shop Manual and Haynes Mazda Rx-7 Automotive Repair Manual on order and don't pick up the car until tomorrow. Assuming I go with the '86 motor, I'll start buying the '86 manuals also.

I appreciate the time educating me.

Thanks,
Todd

 

The '86 is from an S4. '86-'88 = S4 and '89-'91 = S5

As far as whether or not you should keep the recommendation or not, I'm sure you could still get 200hp out of an S4 block, but the S5 rotors really seem to be the best for a N/A power, imho. If you aren't looking to spend a whole hell of a lot, I would just suggest that you teardown your engine, generously streetport it, do away with the 5/6 ports, and seal it back up. If you can afford to spend ~$6-700 on top of the price of a streetport/rebuild, I would suggest the S5 NA rotors. Will you see a massive improvement over the S4 rotors? I doubt it will be night and day, but when building a motor that is going to run at higher RPM a lot, the lesser the rotational mass, the better.

As far as you having to buy '86 manuals, that's not necessary. The GSL-SE engine and the S4 motor are very common, with the exception of the intake manifolds and computer tuning. Your factory manual will cover the -SE engine; so, everything you need to know will be in there.

If you do plan to get rid of the 5/6 ports, completely remove them, don't just wire them open. Wiring them open is pretty dumb because you will just have a bunch of crap disturbing your fuel flow. Completely remove the sleeves, the rods, and block off the pump. You can seal the holes where the rods go with high quality silicone.

If you do decide that you'd rather keep them, do not keep the stock exhaust just to have the actuation tube. Get the Racing Beat exhaust and presilincer with the tube hook up. Street porting you motor and putting a nice carb on your motor won't do as much as it should if you are still running the stock, restrictive exhaust manifold.

I hope that I am helping with your understand of all of this stuff, and please, if any of you guys see something that I wrote that is wrong, please correct me. I am by no means a professional.

 

More great education! The S4 block is less effective to port then the S5, right? But the rotors are interchangeable? Since my rebuild is fairly recent I'll balance the rotors swap price/hp and decide to do it now vs. later.

Thanks again,
Todd

 

Well, the blocks between the S4 and S5 are the same. The differences lie within the intake manifolds and the rotors.

If you change out the intake manifolds on both engines with equal carb set ups, the only differences between the motors would be the rotors, as far as I know.

If you have had a recent rebuild, I suggest you do nothing internally. Unless you are super serious about wanting your 200hp motor, I would suggest that you just buy a good carb and exhaust, and rock your new rebuild until you are ready for the extra umph.

Maybe later on, if you have some cash lying around and are looking for some fun or (heaven forbid) something bad happens to your motor and you need a rebuild, you can street port the irons, toss in some S5 rotors and RAWK!

 

If you are set on rebuilding, and the 13B is your choice, go with the S5 rotors and TII irons. This will give you a 4-port with better choices for porting and better torque all around. The advantage of the 6-ports is better low end torque, when they are operating. If they are open all the time, you lose that feature.

Or you can buy the Jay-Tech 6-port Holley intake that allows the ports to work, for 800 bucks.

The 74-78 13B housings will work with the 12A irons, to build a 13B 4-port, good luck on finding a good pair of them.

 

Boo to the Holley.
Very good suggestion with the TII irons, though. I completely forgot about that.

BTW, I have a set of '74-'78 irons from my old 13B 4 port sitting in my garage right now.

Also, from the very few threads that covered the topic in the second gen section, I gathered that the "low end loss" synonymous (sp) with aux. port removal is neglible. Some think that it is only noticable when the ports are "wired open", and I personally believe that any negative performace would be due to the turbulence caused by the useless **** blocking fuel flow. However, I am by no means anywhere near an expert in fuel flow and how it is affected by different stuff.

 

Disabling the aux ports, whatever the manner, reduces overal perfomance totals, i.e, the area under the hp curve, period. It has been dyno proven in the 2nd gen section.

 

If you're looking for an affordable and fairly simple base with a ceiling at or around 225hp at the wheels which makes it a nicely driveable/raceable car go with the 12a. That's a port, new carb/intake, and exhaust.

 

And which magic port is this?

 

13B FI FTW but 12A is classic, so I cn't deside.

 

a PP 12A would make that much i would imagine. however the longevity of the motor would be sacrificed.