RX7TR7

I was reading some posts about making 150hp with a 12A, they all seem to be upgrade the carb and intake to a Holley or better, and put on a header. That almost seems to easy! My car has 2 inch pipes with a split pipe to make duals in the back, and the cat and smog pump removed has anyone done that and dyno'd it to see what hp it makes? Im guessing maybe I have 110 instead of the factory 100?

Jeff20B

iTrader: (13)

Sounds like you're getting interested in making power.

See if you can hog out the Nikki and do other mods to it. Then see if you can build or buy a long primary. Then see about swapping from your stock flywheel to a light steel (or aluminum if you like putting up with a light flywheel in traffic). Then upgrade your ignition to direct fire. Then swap in a bigger fuel pump and a decent fuel pressure regulator. Not in that order.

t_g_farrell

iTrader: (1)

See my mods, last dyno I did put me at 134 HP at the wheels. Do the math and thats 150+ HP at
the flywheel.

That dyno run was with the Dellorto carb and the RE exhaust. I now have the RB SP exhaust
setup and a hogged out nikki and power is up all across the rpm range.

Link to my dynoe run results: https://www.rx7club.com/1st-gen-non-...2/#post9305785

RX7TR7

I'm not sure if I can get a header to fit. The rotary is in a Triumph TR7. It has the stock 82 manifold and down pipe. The rest is from an exhaust shop. Thats why I was thinking just see how much I can get out of the factory parts. Is port matching the intake and exaust manifolds? Also my timing is still set to stock for both the leading and trailing.

Jeff20B

iTrader: (13)

If you want real power, you're looking in the wrong place.

Port matching could hurt you. Rotaries have a strong reversion pulse. The stock manifold has an anti-reversion lip. Important to keep.

Look at doing a very compact long primary exhaust. Hey, if I can do one in an MG Midget, you can do one in your car. Then hog out the Nikki like I said. Adjusting timing won't get you very much more HP and cause the low RPM stuff to get less ideal - you'll lose power down low and could cause the exhaust to get too hot (glow).

RX7TR7

What is a long primary exhaust?

t_g_farrell

iTrader: (1)

Basically its the RB SP exhaust. You could get the build your own header parts from
RB and then use their center section and muffler. You would probably have to
have even those modified to hang under the TR7. Or like Jeff says just make your
own. The key is keeping the exhaust pipes dual from the engine as far back as the
you can to get the benefits.


Street Port Exhaust System for 79-85 12A - Racing Beat

j9fd3s

iTrader: (3)

the old Pro7 rule set was a stock 12A, with stock intake and exhaust manifolds. they ran a single 2.5" exhaust, and played with the carb a bit, and a really healthy one was about 116rwhp.

if you can find one, the 84-85 exhaust manifold is a little better, it has a shorter outlet. i'd think a little porting/smoothing here would help, as the stock outlet is too small (~44mm?).

next step is to play with the carb. if you look down one, open it, and think about airflow its pretty obvious what needs to change. there are a couple of nice threads too, i think its fat nikki?

feed the carb air from outside the engine bay, and give it enough fuel and you will be doing ok.

Jeff20B

iTrader: (13)

Look for my thread about faking a Sterling. I show pictures of an old racing carb that I refurbished and did some Sterling-inspired mods. Its venturis were hogged way out to 26mm primaries and 31mm secondaries. You won't need to go this extreme, but a little hogging out to 22mm to 24mm on the primaries alone will produce amazing results.

t_g_farrell

iTrader: (1)

+1 Listen to Jeff

RX7TR7

I drive the car everyday. I thought those carb mods kind of ruin the good street manners of the rotary

Jeff20B

iTrader: (13)

Nope, they improve it drastically. It's the aftermarket carbs that give a rotary with a big carb a bad rep. If you mod the Nikki incorrectly it can drive badly, but it's only temporary while you learn a little something about how to tune like tg_farrel and I went through last year. We modded our own Nikkis and after some head scratching and trying different things (because there was no info on the forum specifically about this at that time), the end result has been one of most rewarding and let's be honest, ego-stroking... nah, let's just say point-proving mods we've ever done. Wait, what point was I trying to prove? Oh yeah, that aftermarket Italian racing carbs or big American V8 carbs aren't the answer on a simple street driven 12A like yours. And neither is EFI on a 12A - no holes for injectors in the middle iron. And trying to add injectors to some kind of adaptor plate has been done, but isn't the elegant solution, and they tend to leak every time anyway. Or swap in a GSL-SE intermediate plate, but then you have small runners and less total port area than what a carbed 12A came stock with. There is also no intake manifold for an EFI 12A other than the 12A turbo, but it has small runners and was set up for boost, too, which you're not going to do and requires its odd three barrel TB. It's made by Nikki! But best to avoid all that if possible. Just mod your Nikki with info we and others have posted on the forum and after a steep learning curve, you'll end up with something amazing.

If all that sounds a bit frightening, then maybe it's time to step out of your comfort zone and do a lot of reading. All the info is in threads here on this forum. Start with Sterling's dating back to probably 2001 and just work your way up to present. There is also Sterling's now defunct but still readable backyard mechanics website where he has lots of info, most of it 100% useable if you're just starting out, right here: Tuning The Nikki

j9fd3s

iTrader: (3)

i agree! the best thing about carbs is that they are tunable, and the double edge of that is that you sometimes need to tune them.

i personally have a lovely italian racing carb, and it runs great. it took a while futzing with it, but the results were worth it, and it IS ego stroking.

the nikki is nice because instead of having to start from scratch with the jetting, you start from stock, and go richer and leaner and see what the engine wants.

wankel=awesome

And I have perfectly running Holley's in the northeast now lol. Just took years of playing with them and thousands of $$ down the drain to totally figure them out. In the end, most of the "rotary specific mods" for these are just solutions to faults of a poor carb design in the first place.

In fact, I have a version of the ford 289's 465 cfm 4bbl that runs perfectly in a wide range of scenarios. After the mods to make it run right on the 12a, it was a pleasant surprise that it actually ran better on the 289 v8 lol. The same mods to sharpen up and flatten the curve of the holley on the rotary does the same thing for a small v8. Its actually an awesome carb if you can bother fiddling with it.

I would never recommend buying one though. Especially not a "pretuned" version.

Jeff20B

iTrader: (13)

Wow! That kinda makes sense seeing how the RB Holley 600 I've had for a few years now ran about the same on a Camden, which has an open plenum much like a V8 intake manifold, as it ran on a typical separate runner rotary manifold from RB or even a little carb adaptor spacer thing I got from Hayes back in the 90s to fit on a stock intake manifold with the Hitachi and Nikki patterns drilled and machined into it. This Holley with the adaptor was the only carb that ever actually wowed me in the GLC, for example. However I still have yet to try one of my hogged out Nikkis in it.

This RB carb is still kinda glitchy and has all the usual Holley problems as it's never been modded by anyone other than RB. I hate to think what an off the shelf Holley would be like — read too many horror stories at this point — doesn't sound like I have much future messing with them. That's why I've dedicated my time and effort all of last year to the Nikki which turned out to be an amazing rewarding experience and now I'm looking at ways to streamline all the rotaries around here to run modified Nikkis exclusively. From NA to boost, 12A to 13B, the Nikki it seems, can do it all. But there's still that pesky Camden and a low hoodline preventing a tallish Nikki from fitting very well...

You know, back in the day Sterling was talking about how well one of his Sterling carbs would work on a Camden... I wonder...

wankel=awesome

Actually, the camden solves the mounting problems of the square bore carbs. It was built with a dual mounting pattern, so you can mount your carb correctly. Thats the major issue with the Holley's. Fuel control. They have almost none mounted sideways, but mounted with the primary bowl facing the nose of the car would make it pretty solid.


I have often thought of getting one just for that and the little boost in power.


Also, just in case you were wondering how I fixed the cornering issues with my Holley:
Valtech Precision Machining Inc. - How They Work

t_g_farrell

iTrader: (1)

I have to say that modding my nikki was a very rewarding project. It took me more than a year
to do it but I wasn't in a hurry. I wouldn't blink at doing another if I needed one.

Its a lot cheaper than an aftermarket carb:

used carb from pullapart $50
rebuild kit $30
time to do mod free

So for less than $100 I can produce a kick *** carb for my rotaries. I don't see a reason to use any other carbs for rotaries.

That jet block fix for the holleys is a nice solution, too bad holley doesn't do that in the first
place. I hear Edlebrocks are easy to get working on rotaries but haven't seen a detailed
writeup on whats involved.

wankel=awesome

Edelbrocks are nice, and they are arguably better than the Holley in every way. I just dont have the hood space to run one like I want...

What I learned about the Holleys was that in the case of the 12a, it is a little extra power. Nothing the Nikki cant be modded to do, but the real strength of the holley is its aftermarket support, availability on parts, and the even higher amounts of air they can support.

I can build a 600 cfm carb that runs perfect on the stock or street port 4 or 6 port 13B(mild differences) for less than 400$ that still has cold start mechanisms in place.

They are a wash for the 12A, in that the Nikki will always trump the holley, but nothing really comes close to them for a higher horsepower application for a daily driver.

And +1 on holley should have built a better fuel pickup system from the start. Its crazy how much people spend just fixing that aspect of them.

1st_gen7

iTrader: (2)

what this anti reversion lip? cant find anything on it, which manifold has it, intake or exhaust? im building an early 4 port 13b with a 48ida and a RB intake mani. still deciding if i should leave the RB manifold stock, port match it or port it out slightly smaller than the iron to leave a lip for a venturi effect. my irons are already ported so the intake ports are significantly larger than the RB manifolds, plus im using 12a irons so the middle iron has the tall ports.

1st_gen7

iTrader: (2)

no one?

Jeff20B

iTrader: (13)

Just leave the RB manifold alone. They made it that way for a reason. It's the anti-reversion lip I talked about.

The tall runners in the engine were a throw back to the 1976 reverse runner intake manifold available on CD or RX-5 Cosmos. The tall runners became the secondaries so they needed to be tall to flow worth anything. If you try to port match, you'll end up screwing with what took RB at least some enginuity and forethought back in the 80s when these manifolds were developed and tuned/raced. I'm sure they tested all kinds of designs and settled on what you have in front of you. I wouldn't mess with it.

Another reason the runners in the manifold aren't tall is because of the 74-75 irons which had short runners but 74 spec ports. These came stock in rotary trucks. They made 110HP at 6k and 117TQ at 3500. If you installed an intake manifold with tall runners on an engien with short runners, you'd have a bad mismatch. So maybe RB just did a one size fits all approach. Maybe it would be ok to port match? I still wouldn't. Too much work for probably not a lot of gain, and you lose the anti-reversion lip.

wankel=awesome

I was actually told by RB years ago never to touch the ports on the manifold. The story was that the gains wouldnt be noticeable, and that the only engines that would "want" a larger port from the manifold was actually the FB's 12A.

I had some numbers somewhere, but iirc the "ideal" primary port height was something like 1/16 of what the casting is from RB. Not worth cutting into, imo.

On the other hand, the FB's intake gasket from the factory is too small for the RB manifold. It covers up a bit of the lower primary ports. If you want, trim that gasket so its a match if youre worried about flow. Theres literally a bigger gain there than "matching" the ports to the engine.

j9fd3s

iTrader: (3)

the rotary is a bit odd, in that the intakes and exhausts are stepped from the factory. some speculation, this is due to the engine not having valves, so in order to stop gas flow reversing, they make a stepped lip.

there is actually an SAE paper on the non turbo FC exhaust sleeve/manifold, as it is designed to be free flowing, and noise canceling. not coincidentally on an FC the exhaust starts around 38mm, goes to 48mm, and then 50mm, and then a big open box (the manifold)

there is a second consideration that you want higher gas speeds, which is related to the port size, so smaller is better.

1st_gen7

iTrader: (2)

thats all the info i wanted and much more. thanks all of u!

im choosing to leave well enough alone.

j9fd3s

iTrader: (3)

you should play with the carb a little, or buy a good core, and play with that one. there are many things you can do to make a little more power without affecting drive-ability.

1. remove the choke plate. adds a little airflow, you still keep the fast idle when its cold.
2. Rx3 floats. apparently these do better around corners, no effect on fuel metering.
3. throttle plate screws. Nikki used some giant screws to hold the throttle plates in, they take up like half the venturi space, its kind of absurd.

just doing that is probably ~10hp, no real effect on fuel delivery. next step probably would be to try a leaner jet and a richer jet though, just to see if the engine wants something