12A to 13B Build Questions (4 port 13B)
12-20-17, 05:03 PM
#1
12A to 13B Build Questions (4 port 13B)
I have been searching a lot for information on building a 4 port 13B from a 12A. I am currently running a Streetport 12A in my 82 FB and I have my original/untouched 12A sitting around....
The 12A was original to the car and has 62K miles. No issues that I know of, and came from a good original car. I am counting on the original 12A irons being usable with no major wear. I know these aren't nitirded.
I am thinking of the following as a next step for it:
-Strip it down
-Source new or good GSL-SE housings.
-Source S4 NA, S5 NA or GSL-SE rotating assembly. This decision will be driven by cost/availability. S5 is most ideal for compression, weight, and availability of rear counterweight (for light weight flywheel).
-'Rotor kit', apex seals, side seals, oil control rings etc etc. This is a big expense
-Streetport my 12A irons and GSL-SE housings.
-Worth it to have the irons lapped and re-nitrided?
-New bearings, possibly stat gears
-Basic oil system mods, oil pump, increased pressure.
Does this sound like the ideal approach? The goal is to end up with a Streetport 13B to run with sidedraft carb as 4 port manifolds are easy to source.
I hope the difference in power and less weight of rotating assembly outweigh the costs. Streetport 12A perhaps 140whp. 4 Port 13B Streetport with S5 rotors maybe 170 whp?
This seems like the natural evolution of a 12A to get away from obsolete/unavailable parts.
Some reference threads:
https://www.rx7club.com/1st-generati...usings-864547/
https://www.rx7club.com/1st-generati...rs-12a-645778/
https://www.rx7club.com/2nd-generati...rences-988148/
https://www.rx7club.com/2nd-generati...gs-13b-371031/
The 12A was original to the car and has 62K miles. No issues that I know of, and came from a good original car. I am counting on the original 12A irons being usable with no major wear. I know these aren't nitirded.
I am thinking of the following as a next step for it:
-Strip it down
-Source new or good GSL-SE housings.
-Source S4 NA, S5 NA or GSL-SE rotating assembly. This decision will be driven by cost/availability. S5 is most ideal for compression, weight, and availability of rear counterweight (for light weight flywheel).
-'Rotor kit', apex seals, side seals, oil control rings etc etc. This is a big expense
-Streetport my 12A irons and GSL-SE housings.
-Worth it to have the irons lapped and re-nitrided?
-New bearings, possibly stat gears
-Basic oil system mods, oil pump, increased pressure.
Does this sound like the ideal approach? The goal is to end up with a Streetport 13B to run with sidedraft carb as 4 port manifolds are easy to source.
I hope the difference in power and less weight of rotating assembly outweigh the costs. Streetport 12A perhaps 140whp. 4 Port 13B Streetport with S5 rotors maybe 170 whp?
This seems like the natural evolution of a 12A to get away from obsolete/unavailable parts.
Some reference threads:
https://www.rx7club.com/1st-generati...usings-864547/
https://www.rx7club.com/1st-generati...rs-12a-645778/
https://www.rx7club.com/2nd-generati...rences-988148/
https://www.rx7club.com/2nd-generati...gs-13b-371031/
12-22-17, 07:59 AM
#3
Waffles - hmmm good
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If memory serves me right the 12A plates are nitrided and lapping them will ruin them. As long as they are good and flat to spec then they should be fine. Its the older 13B plates (pre 1980) that are not nitrided I think. Don't forget about dealing with the pellet on the 13B eshaft. Otherwise its a solid plan. What carb will you use?
12-23-17, 10:47 AM
#4
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Mazda started Nitriding the plates in conjunction with the start of Rx7 production, so ALL rx7 engines have nitrided irons. easy to spot, the nitrided irons are unpainted grey, regular ones are painted black.
12-25-17, 06:16 AM
#5
Old [Sch|F]ool
If they are nitrided and you have been a good little boy and ran an air filter on your engine, they should not need to be lapped.
I'd buy an entire FC engine core and a pair of GSL-SE rotor housings and put it all together, maybe with new Atkins apex seals and springs. I said I would do this, because this is exactly what I did, except I bought used Atkins seals on eBay from a low mile engine.
I did also use a GSL-SE eccentric shaft and GSL-SE intermediate. So it's more like a GSL-SE engine with S4 rotors and 12A end housings.
I mostly did this over a Turbo II engine because you can get a much larger bridge port on the '-85 coolant seal location than the '86- engines. For a street port the Turbo II engine is a far better starting point, you'd have to port a 12A to air all the way around to match the secondary ports in the T2 engine.
No need for new bearings unless the old ones are scored. If you use 2mm seal rotors, the 12 pin rotor gear seems to reduce loads on the stationary gears to where you don't need hardened ones. I've been running 8000+ for several seasons on a pair of GSL-SE stat gears.
Likewise, there's no need for anything biggerthan the 12A oil pump. If anything going larger is a detriment, not a benefit. With some light porting of the sharp edges on the inlet and outlet of the pump, and an RX-8 front cover gasket instead of the failure prone O-ring, I see 60psi by 3000rpm and 85psi shortly thereafter with 5W30 or 5W20 oil, and it will hold that pressure to 10k+ with no dropoff. The larger pumps flow more volume at low RPM, which you don't need, and the internal geometry makes them cavitate easier at high RPM, which you don't want. In '86 Mazda went to a 12A width pump (low cavitation) but a larger diameter. They went deeper again for the low RPM loadings that the turbo engines can do, but notice the RX-8 went back to the S4 N/A sized pump.
Incidentally, the most difficult thing for me to source 12A-wise is side housings. I have some good 12A rotor housings, I have some good 12A rotors, I have maybe one good side housing. All of the rest are either badly scored from people not running sufficient air filtration (removing the air pump and not covering the hole, or covering the hole with tape which is basically not covering the hole, seems to be a major offender) and the few I have that aren't scored or rusted from sitting have a bizarre form of failure I haven't seen before: The Nitriding is flaking off. There's a shop in Michigan that will grind and spray-harden your old side housings for something like $650 plus shipping, I may end up going that route. I'd like to keep my '81 12A powered.
I'd buy an entire FC engine core and a pair of GSL-SE rotor housings and put it all together, maybe with new Atkins apex seals and springs. I said I would do this, because this is exactly what I did, except I bought used Atkins seals on eBay from a low mile engine.
I did also use a GSL-SE eccentric shaft and GSL-SE intermediate. So it's more like a GSL-SE engine with S4 rotors and 12A end housings.
I mostly did this over a Turbo II engine because you can get a much larger bridge port on the '-85 coolant seal location than the '86- engines. For a street port the Turbo II engine is a far better starting point, you'd have to port a 12A to air all the way around to match the secondary ports in the T2 engine.
No need for new bearings unless the old ones are scored. If you use 2mm seal rotors, the 12 pin rotor gear seems to reduce loads on the stationary gears to where you don't need hardened ones. I've been running 8000+ for several seasons on a pair of GSL-SE stat gears.
Likewise, there's no need for anything biggerthan the 12A oil pump. If anything going larger is a detriment, not a benefit. With some light porting of the sharp edges on the inlet and outlet of the pump, and an RX-8 front cover gasket instead of the failure prone O-ring, I see 60psi by 3000rpm and 85psi shortly thereafter with 5W30 or 5W20 oil, and it will hold that pressure to 10k+ with no dropoff. The larger pumps flow more volume at low RPM, which you don't need, and the internal geometry makes them cavitate easier at high RPM, which you don't want. In '86 Mazda went to a 12A width pump (low cavitation) but a larger diameter. They went deeper again for the low RPM loadings that the turbo engines can do, but notice the RX-8 went back to the S4 N/A sized pump.
Incidentally, the most difficult thing for me to source 12A-wise is side housings. I have some good 12A rotor housings, I have some good 12A rotors, I have maybe one good side housing. All of the rest are either badly scored from people not running sufficient air filtration (removing the air pump and not covering the hole, or covering the hole with tape which is basically not covering the hole, seems to be a major offender) and the few I have that aren't scored or rusted from sitting have a bizarre form of failure I haven't seen before: The Nitriding is flaking off. There's a shop in Michigan that will grind and spray-harden your old side housings for something like $650 plus shipping, I may end up going that route. I'd like to keep my '81 12A powered.
Last edited by peejay; 12-25-17 at 06:50 AM.
12-28-17, 02:12 PM
#6
Thanks a lot for the information.
A big reason why I am not starting with a used 13B (GSL-SE, S4 or S5) is I have a very hard time finding complete engines in this area. The quantity/quality of Rx's in Canada (winter...salt...) means that good engine cores are hard to come across. The exception would be FD engines as these are imported from Japan frequently. If i did find one, it would most likely have major parts in pretty rough shape.
I will keep my eye out for a complete core 13B but still think piecing it together would net higher quality parts, maybe at a price penalty.
That is very good advice Peejay.
I want to get my 12A torn apart and start from there.
A big reason why I am not starting with a used 13B (GSL-SE, S4 or S5) is I have a very hard time finding complete engines in this area. The quantity/quality of Rx's in Canada (winter...salt...) means that good engine cores are hard to come across. The exception would be FD engines as these are imported from Japan frequently. If i did find one, it would most likely have major parts in pretty rough shape.
I will keep my eye out for a complete core 13B but still think piecing it together would net higher quality parts, maybe at a price penalty.
That is very good advice Peejay.
I want to get my 12A torn apart and start from there.
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12-28-17, 07:23 PM
#8
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Likewise, there's no need for anything biggerthan the 12A oil pump. If anything going larger is a detriment, not a benefit. With some light porting of the sharp edges on the inlet and outlet of the pump, and an RX-8 front cover gasket instead of the failure prone O-ring, I see 60psi by 3000rpm and 85psi shortly thereafter with 5W30 or 5W20 oil, and it will hold that pressure to 10k+ with no dropoff. The larger pumps flow more volume at low RPM, which you don't need, and the internal geometry makes them cavitate easier at high RPM, which you don't want. In '86 Mazda went to a 12A width pump (low cavitation) but a larger diameter. They went deeper again for the low RPM loadings that the turbo engines can do, but notice the RX-8 went back to the S4 N/A sized pump.
01-27-18, 08:50 AM
#9
Junior Member
Peejay! thanks for the good info on the 12a engine, I'm a newbie to the rotary engines I purchased a 12a from picnpull took it apart everything in great shape except of course the housings. So I found another engine and bingo it had good housings! I will not be porting it or anything has its going in a triumph spitfire the rear end can only take up to 150HP. My question is what transmission is best for this engine I got the engine out of a 86 GSL it had a tranny seemed ok a 5 speed I think. I heard this 86 was a good trans. could you give me some direction on this. thanks
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