drifter_xs

hi guys and ladies? lol i wanna go carb and change it to a distributor set up. what are my options? pros and cons? how much is it "used"? where can i find it? and if you have done it, what were your impressions of it?

S4 6 port

thanks guys

rx7freak13v

hi there,
you can pull a dizzy from any 81-85 12a or 13b with electronic ignition. it is a direct swap and you do not need the hole front cover as a lot of people misinform.

then you will need to figure out what to do about coils. some people use the trailing coil and just re wire it for the dizzy from the wiring harness. easy simple. others (like myself) use a pair of msd blaster 2 coils. simple task. next is which carb are you thinking about using? if your thinking 4 barrel a mechanical secondary 600 cfm with center hung float bowls and needle extensions is a must. (my buddy has a vacuum secondary holley and even after rebuilding it, the 4 barrel begins opening at 6k and at 8k is only about halfway open. he is playing with making it mechanical lol. unfortunately though it is side hung floats and breaks up under cornering like crazy. holley makes a 600 cfm with center hung floats and mechanical secondary but are difficult to find used for a decent price and kind of pricey new. edelbrock only makes mechanical secondary carbs with center hung float bowls and can be had rather cheap. otherwise tuning isn't too difficult. also, you will have to find a intake manifold and use a 1 inch spacer between the carb and intake manifold. if not they won't work properly.

weber carbs are the preferred method of carbing a rotary engine. you can get the side draft adapter from sites such as mazdatrix or racingbeat and can adapt a side draft style weber. something 44mm or bigger is fine and bigger usually means better but depends on what your wanting. most people use either 45mm or 48mm side drafts. the preferred weber however is an ida style weber down draft carb. these also require special intake manifolds to suite them but end up with intake runners of roughly 14 inches which is most optimal. they typically lose the choke however so winter starting could be difficult (i had no trouble starting my carbed 13b in the winter with temps around 18* f. it started first try every time, wish my efi set up started that easily). ida' are typically 48mm or larger and flow great. all webers are very easy to tune with any tuning experience at all. parts are readily available and you will love the response you get with them. webers work great on dd and race cars and have been used on race cars for many many years. they are simple and parts are easy to find. they are also weber clones available such as dell'orto, mikuni, etc...

as far as fuel pumps go, most people say to downsize to a lower psi pump. this is exactly what i did not do. instead i bought a mallory 4309 fpr and regulated fuel to roughly 4 psi with a quality fuel pressure gauge. later adjusted fuel as necessary. this works great, i had no problems and neither does my friends who use these regulators. mallory makes great products. i tried using holley fpr's from off the shelf at the local part stores but they kept breaking. unsure why. but never had an issue with the mallory.

you will need to hard wire your fuel pump using a relay. do not run it to a toggle switch!

otherwise there are write ups on how to do all these procedures and is quite simple. you will not use vacuum advance with a weber carb and personally haven't used it on holley set ups either.

most people will tell you not to waste your time changing to a carb and that it is a downgrade but in my opinion i don't think so. simply because tuning a carb is simple. every aspect of a carb is simple. yes you will need to spend more money on tuning kits and in the end will end up costing as much as adding a stand alone ecu to you factory intake but you also get shorter runners which ups your hp and response. however you sacrifice lower rpm street ability for top end power or vice versa. you cannot have both 100%, but you can get pretty close. others will also say that if you use a 4 barrel get it from racingbeat, but i have tuned a several that are not racingbeat items and work fine as long as they are mechanical secondary. the racingbeat carb has been worked through a lot to prevent the need of a spacer between it and the manifold. (the spacer creates a plenum effect which allows the vacuum off the engine work the idle and off idle transition). others might flame me for explaining this but it is what i have come to understand. that makes it opinion and not fact but as said before i haven't had an issue getting off the shelf carbs to work properly and most holley powered fc's you see are off the shelf holleys.

lastly, if you plan to turbo in the future then i would suggest a stand alone ecu as well. it is more tunable and i don't want to tell you otherwise. they are all out arguments stating that a carb can make the same peak hp as efi, but efi people state otherwise. i have no idea. i do know that i prefer simple carbs over efi and needing a laptop to tune my car when im on the side of the road and something broke. but on the other hand, efi can easily correct for altitude changes and blah blah blah. also, carbs are very sensitive to contaminants in the fuel, so cleaning you tank prior to install is ideal. again most people will say that carbs are only racing classes where efi itb's are not allowed but again, as said before i love carburetors and if you want to change to one i will not flame you for doing so.

i think i covered all topics, if i missed anything please let me know and i will share more info.

disclaimer: i am not a professional, just another enthusiast for these beloved rx7's and another fan of ancient technology (carbs). anything i have shared is not to be taken as fact but instead personal experience that i have learned. so if anyone has anything to add, thanks but do not flame me for sharing what i have learned. thanks a lot guys!

drifter x, btw, if your planning to drift your car, efi is recommended because of fuel slosh in the float bowls

K-Tune

iTrader: (26)

I present this question: "Why are you even considering it?"

wthdidusay82

iTrader: (15)

If he wants to make more power keeping it na after doing all the other normal bolt on mods its his only option other than a custom intake manifold.

Though dollar per HP, its really not worth it.


Rotary >Pistons

rx7freak13v

cheap performance would be my guess, but isn't really all that cheap unless your getting a good deal, especially with a weber.

i am sorry i stated somewhat false info in my previous post. so far my 4 barrel experience had been with holley. i was told by a lot of people that edelbrock carbs were ALL mechanical secondary with center hung floats. this is false and i just did homework on it to be sure i was giving correct info when i learned that i was wrong. all edelbrocks are vacuum secondary but do not use a diaphragm system like holley does but uses a counterweight system wich works basically the same way as a holley vacuum secondary carb. which means my plan for a cheap edelbrock is a no go and i will be looking for a price on a 600 cfm holley double pumper with mechanical secondaries and center hung floats. wish me luck. again i am very sorry i had spread bad information without maing sure 100% first. i never do that, no matter how sure i am and the one time i do, it's wrong info. forgive me.

k-tune, sorry i know the question wasn't aimed at me. im just sharing the same plan i am currently pursuing.

what did inspire you to want a carb? the look?the simple tuning? a huge performance increase? (<------that one isn't really happening. the increase in hp comes from shorter intake runners and enough intake flow to match a high flow exhaust) or the simplicity of tuning?

85fb

I had a p port with a distributor, msd 6as, and proform 750dp. It was fun, easy to set up, but had absolutely the worst reliability.

gerald m

When the intake and exhaust start to overlap you would get a lesser performance with the bigger carb set up because there is the tiny bit of time when the intake system really doesn't have much vacuum on it causing the fuel vapor to come out of suspension and drop in the intake runners causing pooling and almost a flood like condition . Don't get me wrong I love carburated engines ( maybe because I don't know enough about the injection systems on the little engines ) but they do have there drawbacks . the 750 is a big carb for rotary use and I would think would need to be jetted down considerable . to get it to work proper on the full range RPM's . Not on a rotary but we use to mount some of the larger center hung carbs backwards to help stop the float problems . I actually built a air box with plexiglass ends so I could see what was happening inside the intake .and I really could see the fuel vapor pulsing at most RPM ranges instead of a even flow . Just my 2 cents . Gerald m.

Coopmiami

Worst idea ever. I would rather walk 30 miles to school than have a carbd rx7

tuscanidream

iTrader: (6)

Every 12a I see usually has crazy miles and still runs great. Your silly.

rx7freak13v

i love the simplicity of a carb if set up properly. but it isn't really an upgrade unless you are doing it as a system.if you change to shorter runner i.e. carb and not have free flow exhaust you will lose power everywhere. also once set up properly you are going to notice a little loss of low range tq due to the shorter runners but an increase on top end hp. it is a give and take. carbs are nice because you can tune it on the side of the road with out a laptop, on the other hand you are probably going to have to tune it on the side of the road lol at least until you get it perfect

AGreen

iTrader: (11)

I really won't deny the fact that you can make some good power on a carburetor, but there really is a reason that carburetors got phased out. Fuel injection is just a better system. I installed a new standalone with new sensors on my car for less than the cost of a new carburetor. It's more reliable than a carburetor, and albeit slight, you make more power.

I'll make it an analogy. Putting a carburetor on a car that came with fuel injection is like putting windows 3.1 on a brand new laptop. Yes, it can be done; and yes, there are probably good arguments for doing so. But you really have to stand back and take a look at it and think to yourself "damn... this is just not practical".

j9fd3s

iTrader: (3)

i've got a Weber fed P port and it runs like a stock car, its surprisingly docile.

that being said, there is a hidden cost to the carb, you have to buy jets, and venturi's and stuff, it does add up, and by the time you're done buying the carb, jets, regulator and fuel pump, you could have bought a really nice standalone.

so its not really "cheap"

NCross

iTrader: (18)

The only reason I can see is eliminating every electrical bit in the chasis need for EFI. Weight savings. Simplicity.

It can be reliable, but it will take you awhile to get the tune dialed in. I loved my sterling Nikki on my SA22, but I don't know how many times I wished it was an EFI system.

gerald m

I love the carbed engine but for sure it has drawbacks ,I think most of us realize that . Same reason I went carb on my bike is because I just didn't need the problems of all electrical but have also been tempted to change back over to EFI . The Carb wont give much economy compared to EFI but if set up proper will make the little engine go hard . During rebuild i installed apex seals that can take a little extra pressure and next year I would like to try 100 hp nitrous kit instead of EFI . There is good and bad about both I guess it's what ever the individual person wants and is COMFORTABLE with , comfortable being a key word .

lostFC

Good info.

The aux ports are well in the compression stroke. That causes a reversion pulse back up the runner and the wrong way through the carb. It makes tuning impossible. The spacer you mention is a good way to divert that pressure wave back down the other rotors runner which is starving for air (has vacuum).

I had success with a modified 4 hole phenolic spacer with only the secondary runners sharing 'plenum' volume. With that setup i wan a Holley 600 with vac secondaries. It operated flawlessly and drove great. After lots and lots of tuning...


I would suspect a port matched TII manifold and a good EMS could make better power though. Dunno if thats been done and tested.

j9fd3s

iTrader: (3)

yep, the NA manifold is better, the turbo comes out of the block making a really tight bend. better combo is an NA intake on a turbo engine, filled to match.

rx7freak13v

I keep explaining to people about the turbo intake pinching off the the runners just before the block but for some reason when you are staring at people in the eye giving great advice they dont believe you lol.

Lostfc, thanks for pointing out the reversion effect from the intake as the reason why you need a plenum under the carb. I failed to explain that part, more or less i forgot that was the reason why lol. Long posts get tiring.

lostFC

Yeah makes sense, t is a tight 90*at the motor.

Interesting the 4 port makes more power... Makes me wonder what a ported '4 port' Renesis will do.

I need to start searching the boards. Ive got catching up to do.

j9fd3s

iTrader: (3)

the 6 port is designed to enhance low RPM power, while keeping the top end, and it works great for that. when you try to get more hp three things become a problem.

the first is that the ports close really late, late intake close = higher RPM power peak, and you loose low end to get it. the 80degree intake closing time is the same as the P port engines, so peak power will be around 8500rpm, which is ok

except the ports do not flow very well, certainly not enough to make hp at 8500.

the third problem is that the intake port are is already big, so making them bigger doesn't fix the fundamental problem.

so you end up with a characteristic flat powerband that peaks really late.

the 4 port has a better balance between port timing and port flow, so it makes power at a lower rpm, and then the runners flow a bit better, but the ports are small so it responds to porting

the renesis is a whole different can of worms, the intakes are better, they made the irons thicker, so the bend is less sharp, the port runners are bigger, the intake runners are tuned in pairs to the ports (port 1-2, 3-4,5-6 all have separate runners, and different lengths) and works very well.

the problem is that the exhaust port went from one big fairly ideal shape, to two small ports with tight bends, and they left no room to port it, so you're a little stuck with what the factory gave you.

the shop that has the highest hp renesis spent 6 months going between the flowbench and the dyno, so there are gains to be had with the renesis, but they aren't easy

RotaryEvolution

iTrader: (30)

a carb setup CAN make more power than a stock n/a fuel injected setup even with a standalone but not many people are experienced enough to do it or make it reliable.

fuel mileage and power out of the hole will suffer.

lostFC

Since we are way off original topic.

Of course this begs the question, what will the peripheral exhaust rotor housings do with 4 port Ren irons? The Ren rotor housings look like they are cast identical... In fact the Ren housings look almost like there is a block-off where the exhaust port should be. It the Ren apex seal robust enough to handle the peripheral exhaust? In theory, you could probably use standard S5 rotors and counterweights at that point since the extra rotor seals are now unnessesary.

Fack off... now Im gonna have to get a 4 port ren to open...

RotaryEvolution

iTrader: (30)

or don't bother, run a full peripheral intake and exhaust engine and notice it is now simpler to make more power.

you could go even further with a semi pp intake and exhaust combo with tuned adjustable length runners/auxiliary ports but then you're getting into aerospace timeframe testing R+D. that is how i would envision the final version of the 16x to be, if it ever sees the light of day, however it would run dirty and get rather poor fuel mileage but the engine would at least be competetive with modern piston engines.

j9fd3s

iTrader: (3)

yeah the P port is superior in every way except emissions, and low rpm low throttle openings...

it even gets better gas mileage! remember jacks FC? the NA with the E6k? we did two track days i with the carbed P port, and his FC, and it took him 4 tanks of gas, and i'm still on the 1st one... he's faster, but still its a pretty big difference. i'll pull him in a straight line too

we're both running the same useless borla, and they both get flagged for sound at the same DB level...

RotaryEvolution

iTrader: (30)

being that was my first haltech tune i doubt it was that great, it was basically a power tune and had a bit of room for improvement.

i'm sure i could do a bit better now.

did he ever get it dynoed on another dyno? the calibration for that particular roller was obviously not right for these engines.

j9fd3s

iTrader: (3)

it did run really well, he set his fastest lap times with that map. so he didn't redyno, he ran it for a season. he just scored a sprint RE, and he transferred the E6K map to the E8 software and then the sprint software, which of course doesn't work.

so we just street tuned it at his work. the new haltech software is WAY better, i don't know how we ever had cars running on the E6K, its worse than a carb...

we didn't do anything fancy, we just made sure it was like 12.8-13.2 and that it did part throttle, and he could run thru like 2-3-4 without a hiccup.

step two we're gonna just go old school, and try a couple different AFR's and run a 4th gear datalog, and see which is faster.