twint789

Hey guys

Not a newb. Used to post here a long long time ago and now am back for some convo once in awhile. Still major busy

Onto business......Those that have NA 20b conversions what kind of mileage are you getting with the hp you are modified too.

Do you expect 12a reliability. Meaning it could go on for 100's of thousands of miles before a rebuild?

Thanks

-xlr8planet-

iTrader: (3)

not a 20b owner, but let me chime in

Considering the apex seals and housings on a 20b a far superior to those in a 12a, I would say one of those in NA form should last a long time

t-von

iTrader: (8)

As with any rotary ( NA or Turbo ) tuning and maintenance will determine the engines life span. Rebuild the engine with Mazda's newer 2 piece apex seals. They will last much longer than the shitty 3 piece apex seals that come with the 20b. 3 piece seals are nothing but a liability on high mileage rotary's. Also mileage wont be as good as it could be if you use any stock Rx7 tranny. The gearing will have the engine rpm's too high for normal daily commutes. Larger engines need less rpm and are more torquey so they get the job done easier. For this reason I'm strongly considering the Pontiac GTO 6 speed tranny conversion.

twint789

Are we talking 25 hwy mpg? city? 20? 30? haha. yeah right on that.

Anyone?

Falken

I think I heard that unmodified deturboed 20Bs crank out 250ish

300 is probably easy with porting.

Mileage should be stock or better until you go over stock turbo power levels N/A.

12arotary.

I got about 18-20 hwy city driving stock 20b running NA

t-von

iTrader: (8)

Your not going to get 25mpg with the stock tranny. The engine needs taller hwy gearing to get better econmy.

Example: look at the LS series V8's frm Camaros and Corvetts. Those transmissions have .50 6th gears ratios and low axel ratios. I think at 70mph the engine rpm is at 2k or below. That's why they can get 28mpg on the highway. If they had the .74 5th gear of the fd and taller 4.10 axel ratio, they hwy econmy would prabably be in the teens.

peejay

Automatic models do not have super tall top gears and they also get good fuel economy.

I was getting about 30mpg with 3.91 gears and my 12A. When I went to 4.78 gears, fuel economy stayed about the same. (Went UP slightly, but this may be due to a change to synthetic fluids in the diff)

j9fd3s

iTrader: (3)

mine was turbo, but highway cruise was a best of 19.9 mpg, mixed city highway was about 14-16.

NA would do a little better, although not much.

T-von has a point too, the stock 20b is setup for low rpms, so it could be geared to take advantage of that

twint789

Thanks to all for the replies

t-von

iTrader: (8)

But if you had 20b in that same vehicle, your economy wouldn't be anywhere near that. All other things being equal. Smaller displacement engines can spin a higher rpm and not be effected as much economy wise. Larger displacement engines are a different story. Best economy will always be generated at the lowest possible rpm while having enough torque to keep the vehicle moving. If the engine's porting is modified with too much overlap (which causes a loss of torque in the low range) low range economy will also suffer because the engine is making less power.

peejay

There are too many other variables to distill it down to that. You need to look at the BSFC curve for the RPM/load you will see on the highway.

For example, the 12A/Nikki/distributor combo has a quirk to it that decreases BSFC at higher RPM for the same engine loading, within the RPM range seen on the highway. Notice that the 4 speed cars got the same highway fuel economy as the 5 speed cars...

No, it doesn't. My 12A P-port definitely made more low end torque than any of my 13Bs and it definitely had a bit of overlap to it.

Funny, it also got about 25mpg on the highway, too, even though it would NOT run lean of stoich, or cruise at 60mph on flat ground with no wind. (Add wind or a slight grade and it'd smooth out) Cruise at 60mph was around 70KPA absolute.

My 13Bs both cruise at the same speed at about 40KPA, same as the idle. I think I got as much as 21mpg with the latest 13B...

To a degree, higher throttle openings make for better fuel economy, if you don't need to go into power enrichment. (This is how EGR boosts fuel economy)

That's how I drive my VW. It is a complete dog below about 3000rpm. So I keep my foot to the floor and upshift if it starts to accelerate...

t-von

iTrader: (8)

Yes it does however, I guess we both need to clarify what our definition of low end is. When I said low end, I'm talking about 3k and below which is highway range for my fc/fd at 70mph based on gearing. What rpm range are you speaking of for your pp because they don't cruise well below 3k. I've never seen any dyno graph showing a pp making more torque than a side port below 3k low end range. Anything above 3k is considered mid-range not low which is exactly where your rpm's are if you have the 12a tranny. At 3k and below, the average pp has too much of the fuel/air mixture being sent out the exhaust as waisted power potential with all the overlap. Less fuel/air being burnt = less explosion which also = less power being generated at that range. This is the main reason PP's aren't very street able down that low. If your running a 12a tranny then your gearing is a little higher and you are at a much higher rpm at 60 than my fc or fd at 70. That higher engine rpm negates the negative effects of overlap within your engine so I can see how you are getting good economy.

What rpm are you at 60mph? Average highway interstate cruising is at 70mph not 60. There is alot more friction and air resistance at 70mph vs 60mph.

peejay

Stock 12A FB gearing, ~2800rpm at 60.

Speed limits here are 60mph and it's enough of a ticket magnet to make it unpleasant to go around speeding all the time. I never took the car outside of the city, just basically driving it from where I live/work to AMS (low impedance's place) about 40 minutes across town.

Drivable powerband started at about 2000rpm. Idle was in the 1000-1200 range and shifting in the city was done at about 2500. The key to smooth driving was to load it and short shift, *always*. The EFI helped this tremendously as compared to the carburetor, but you could not drive it with a light foot. (Pulling away from a light in traffic, for instance, required waiting for the car ahead to move out 2 or 3 lengths and then giving it a quick burst and rapid clutch engagement... can't just lazily sliiide the clutch and hold light throttle)

The problem is not air and fuel getting dumped out of the exhaust - if it was overscavenging like that, there wouldn't BE any drivability problems, just emissions problems. The issue at hand is exhaust dilution at low loads, because there isn't enough exhaust energy to get a proper blowdown started. The proof of this is that reducing exhaust restriction makes the car more drivable.


This is why overlap is only an issue at low LOADS not low RPM. Mazda went to zero overlap why? Better combustion (and therefore emissions) at low loads.

Incidentally, with a Nikki/12A and 4.78 gearing, I did achieve over 27mpg at about an 80mph *average* speed on a several hour trip, measuring fuel from topoff to topoff. A foru member was with me and we were both shocked that the car would only take a little over 6 gallons before it started puking through the filler neck. Now that my vidcap set is sorted I can grab the video of us cruising down I-71 at a sedate 5500ish RPM in the rain...

t-von

iTrader: (8)

Zero overlap does also improve low end torque as well because you still loose less fuel air mixture out the exhaust which doesn't get burnt in the combustion cycle. In the simplest terms to create more torque, you have to explode more fuel/air to create a bigger push against the e-shaft. I find the concept of creating torque to be that simple. And to prove that, I did an experiment on my 6port 13b. I used some devcon to close the opening edges of both primary and secondary ports when I rebuilt my engine 3yrs ago. I used the rock hard RA apex seals in this one and reused a scared housing from when the engine blew. Start up compression was at 90psi when I fired it up. When I fired the engine up and went on a test drive, I could immediately notice an increase in low end torque. The engine seemed to want to pull the car car all by itself. Keep in mind this engine was in my 91 vert (which is by far the heaviest Rx7 at close to 3,000lbs) which makes if even more amazing. I found that I could cruise around town in 5th gear more often without the engine feeling too underpowered. On the highway at 70mph and with cruise on. I could feel the engine working less to pull the car up slight grades. The power was there. Doing this mod did effect the idle since I'm on the stock ecu. Apparently the programing doesn't know how to compensate for the extra fuel being burnt during combusted at idle. It also stinks of gas out the exhaust pipe and my idle is also lumpy. It could definetly use a re-tune to get the best out of it. Also my highway fuel economy has gone up. One time was able to go 430 miles on a single tank fill all highway. Now 26mpg may not seem outstanding but remember, it's pulling a heavier car and the engine needs to be re-tuned.

One last thing. My car has zero overlap just like the Renesis Rx8 engine. Both cars weigh about the same too however, no Rx8 is getting the kind of highway economy like my s5 vert. The difference is the rpm to which boths are at on the highway. At 70mph my rpm's are around 2,800 while the Renesis is almost at 4,000. LOL an they wonder why they can barely get 24mpg highway.

j9fd3s

iTrader: (3)

NODE and i have done the 1000 mile sevenstock round trip in my now his 79 SA22C, its an unmolested 90,000 mile car, factory engine, paint, etc.

it gets its best mileage between about 3500-4000rpms, which is 70 something mph, speed limit in the 5 is 70, people go 80.

if you slow down mileage actually drops, but not much.

it does about 23mpg, keep in mind its got the thermal reactor on it, and it does pass the california emissions test.

diabolical1

iTrader: (3)

yeah, i've always found that to be curious. virtually all the Rx-8s i've seen with engine swaps to older-type rotaries seem to report better gas mileage.

diabolical1

iTrader: (3)

yeah, i've always found that to be curious. virtually all the Rx-8s i've seen with engine swaps to older-type rotaries seem to report better gas mileage.

t-von

iTrader: (8)

Your best mileage between the 3500-4000 rpm range is a direct result of the 12A's factory intake setups short intake runners. That design moves the torque peak higher in the power band. So you need those rpms to put the engine more in it's efficiency range based on it's intake design. With a longer runner intake set-up, you could greatly improve that engines economy on the highway down low and benefit from the increased torque.

t-von

iTrader: (8)

Are these swaps retaining the stock rev happy 6 speed tranny? I think the Rx8's problems have more to do with the ecu injector staging programing and rev happy gearing. I know that the Renesis secondary injectors come on at 3,750 rpms. What I'm not sure of is if those secondary injectors are load dependant. The primary injectors are small so the secondaries probably come on line above 3750 regardless of load. If this is the case, that would explain why the Renesis isn't that fuel efficient. People who drive these cars generally keep the rpms in the mid-range. At this range you have 4 injectors flowing all the time. It's hard to be fuel efficient under these conditions. Those of you that haven't driven one, you will quickly notice that the engine will quickly rev past 4k with ease. If you don't watch it and shift before 3750 rpms, the 8 most definitely will drink some gas.

dbragg

iTrader: (7)

holy thread jacking batman!!

jk fellas

lots of good information here, but i would like to see some more about N/A 20b setup. There has got to be more people out there with N/A 20b that can chime in.

peejay

You do not lose fresh air and fuel out of the exhaust. It doesn't happen with any kind of streetable exhaust.

If that WAS happening, you'd still be getting 110-120% VE and making excellent torque, since you'd have a fully scavenged combustion chamber.

We don't want to explode ANY fuel/air. That leads to a tow home.

Incidentally, have you ever driven a stock 12A? They don't have a noticable powerband. It's flat from idle to redline.

j9fd3s

iTrader: (3)

i asked this question too, the rx8 does NOT go out of closed loop @3750. the mixture does get richer the higher you rev it though, it goes from 14.7@3500 to something like 14.2 around 4000rpms.

it seems that fuel economy goes down a little, but its really hard to tell

t-von

iTrader: (8)

Technically explode is the correct term because that's exactly what happens in any combustible engine. The only major difference is it's in a controlled environment. You know what I meant and you don't have to be so literal about it.

I've owned an 81 and still have my 84, 91, and 94 that I'm currently doing a NA 20b conversion on. Also I have been a certified rotary nut since 1991.

EpitrochoidalPower!

Actually technically the fuel "exploding" would be detonation and is not what you want in your engine, the fuel-air mixture is meant to burn not detonate.