So, as the thread states, Im looking for info on high compression 12a rotors. the joys of trying to make power without a turbo. i know well enough na s5 rotors into a 13b work, but what about a 12a? any company making a 10:1 dorito jr? haha. im assuming some of these new billet designs have a more advanced chamber to the rotor, which id guess would have the potential to hold a higher cr, but billet rotors are for gods and rich folks, and i left amex black card in the veyron, so thats a no go. im not sure if people really can machine a factory rotor to increse the cr, or what. anybody out there? i needs tha halpz!

 

and you would gain nothing from more compression. just a more picky fuel system and ignition.

want better results? work on a fuel injection system with electronically controlled ignition with adjustable timing curves.


or you could just drop in a later model engine and be done.
you're not going to find many companies investing into 12A development when the engines are already VERY difficult to find parts to even rebuild with. add in that most 1st gen and earlier rotary car owners are DIYers and you will find the market is very difficult to sell a pair of even $1k rotors to(most custom machined rotors start at $3k and go up from there).

you can only remove material from a factory rotor, which reduces its compression for turbo or altering your power curve. a higher compression rotor would require making it from scratch, which is hours and hours of work.

 

raising the compression ratio in a rotary doesn't have the same benefit as a piston engine, or i should say that its high enough already that going higher doesn't do much. you see this with piston engines too, but it usually occurs at a higher number, ie on a small block chevy going from 9.5 to 10 is probably a big gain, but going from 12 to 13 might loose.

in a 13B engine the cast recesses had a variance of +/-.3 compression, so the machined S5 and later recesses are just the same 9.4 rotors, at the high end of the tolerance, or 9.4 + .3 = 9.7.

the 12A rotors came with several different combustion recess shapes, but there were only 9.4:1 and 8.5:1.

 

SBCs gain right on up to about 17:1, they have nice flat chambers.

Now Hemis on the other hand start to lose out at over about 6 or 7:1!!! Basically the huge dome needed to get any compression hurts the ability to burn everything. The reason they worked at all in the 60s is because they could flow a lot more air than anybody else, and everybody's combustion chambers were ****.

I think of rotaries like flathead engines. There isn't any gain to be had in higher compression because of the internal airflow requirements.

 

Ok, so screw the high cr rotor idea. From what I've read here and elswere last night, the gains are too minimal and not worth the time of effort. Sounds like efi is my next route for power gains. Thing is, I'd like to stay carbed. I think ifni was doing a 13b build, I'd go efi in a second. I'd just like too see how well I get get a 12a powered car to go, running on nothing but exotic fuels (ie. Nitrous) I think with the IDA setup, I should run 93pump. But if anybody has any info on a better fuel for short 1/4mile bursts? E85? Or some sort of other fuel. I've been directed away from alcohol with my carb plans. Thanks to my fellow rotards on here.
Maybe someday this project will run into the 9's!

 

its not like a high hp 12A is a new idea.... they had a sports kit in 1971 to bridgeport the engine to go from 100hp to 180hp. in 77 the sports kit was peripheral port, for 250hp, in 79 the peripheral was revised for 260hp, in 81 it got dry sump and mechanical fuel injection for 280hp...

the recipes are there you just need to follow them

 

sheesh 280 out of a 12a would be wild. i havent really seen many break the 240-250 mark.

 

280hp is the ***** to the wall, no holds barred limit for an NA 12A. if your goin that far, you likely have lots of expensive custom bits, including rotors

 

Luckly I'm shooting for much more power then 280hp! At least on nitrous! I still gotta figure out how much spray I should run through the motor. If I can push anywhere near 500, I'll be golden. 2200lb chassis with the fluids. Then it's just gonna come down driver skill to push it to the times I'm hoping to hit. Practice practice practice!
The motor itself should be pretty extensively build for n/a power. Big porting, minimal back pressure through a couple of short pipes, big thirsty carb (I'm thinking efi will come from a future 13b nitrous build. I can sacrifice reliability with power now. I'm wise enough to know I won't be driving a 12app with next to no exhaust on the street ever. My hearing is already getting bad! Haha

 

Tbh, I would like to know how much power people have been able to push out of a 12a? If people have hit 280, where are the 3xx motors!?!
And WTF? We need to hear more about nitrous builds. I wanna see somebody in a big carbed 12a pushing a big hit through it!

 

Stock trans and diff will likely not stand up to that.

 

Yeah, trans and diff have been running through my mind since day 1. Mostly what trans I wanna use. I'm unsure about running a manual just due to the fact a misshift at 10k wouldn't be nice. Buy then I don't wanna lose too much power through a basic automatic, and all these high end race trannies are stupid expensive. I dunno. I wanna get this motor together first, then I'll work on power delivery. I mean, 9" with 4:10's behind a turbo 350 GM trans might be what I do. Cheap and sturdy.

 

Also, thanks for all the help, guys. I know a lot of my questions are regular topics on here, so I really appretiate everybody who has taken the time to throw in a little advice! Rotors for life!

 

you also have high standards if you think even nitrous will get you near 500 to the wheels n/a.

you're right about the engine needing a bit of mods to handle the power though.

 

maybe 500 is pushing it. but id see into the 400s at the flywheel. 12a parts are cheap, lets do a big hit. haha

 

12A parts are becoming almost non-existant. It would be cheaper if you used the
13B as your basis for this build.

 

But I just got a set of fresh 12a housings. Haha! We'll see.

 

I think if I do a 13b, it would be turbo. I just wanna see how far I can take a 12a just on exotic fuels. If you wanna call nitrous an exotic fuel, I consider it to be fuel. To be technical, is nitrous still n/a?

 

stock trends and differential handle about 260

 

You realize this thread is from 2012, right?

The axle housing cam barely handle 150hp before it starts bending and I used to go through two or three transmissions per year at stock power levels. It all depends on usage factors.

The main weakness in the transmission seems to be the bolts that hold the bearing plate backing out. This allows the mainshaft to shove back under load, until it is stopped by 1st gear grinding against the center bearing housing. Then the oil rapidly fills with aluminum shavings and the bearings degrade quickly, also wearing away the hardening on the gear teeth, which weakens them against shock loads. It doesn't take much before you have a gear that has summer teeth. Some'r there and some'r not there!

 

I think driving style and as you say Peejay, usage are a huge factor. I imagine with the rallying you do, its pretty hard usage compared to say someone occasionally pushing a street car to its limits. Thats good info on the transmission wear characteristics and I bet it was learned with a lot of hard work. Sometimes resurrecting old threads is a gift.

 

My favorite was shifting to 5th gear at about 70mph and shearing all of the teeth from the input shaft. After that I had 4th gear and lots of noise, that's it. That was an entertaining drive to work, fortunately there were no red lights after I got off the highway!

A friend did this with his turbo Miata, also, but it took him about ten years of rallycross abuse at whatever 10-12psi boost gets you with a 1.8. I think the Miata version of the trans is actually a lot stronger than the RX-7 because it has a different drop ratio to the countershaft, which changes the loadings on the gears. They also don't have as much grip because they have IRS so they can't put power down as well.

I actually haven't driven the monster for a couple years now because it shredded another transmission (2nf gear, this time) and I can't find transmissions and driveshafts anymore. Every transmission failure also destroys a driveshaft from 30 grit gear oil destroying the slip yoke. So I've only been driving my '81, which is as stock as I could make it in the 2020s, and its transmission (that I rebuilt!) has developed a very, very loud bearing that makes itself known in 2nd and 3rd gear. So distressing.