hey guys im rebuilding my first motor by myself and i have a few questions.. first ill tell you about my motor, parts and what im looking to accomplish.. s5 turbo motor, megasquirted, trust fmic, s5 srock turbo, walbro fuel pump, i might end up with 1260 secondary injectors, umm.. ok.. thats about it.. im doing the porting myself and have decided to do a midsized street port and may be do the exhaust port also.. im looking for something that is very streetable but still pretty stout..

so here are my ideas and i would like to know what you guys think about the designs.. please tell me your experiences thanks..

exhaust ports:

1. i think im keeping them pretty much stock width but may smooth out the sides just a little. now should i open the ports sooner and close them stock or do i open them stock and close them late... or do i do booth?

2. how much is a lot or a little.. maybe like a couple milimeters? down or up?

3. im thinking of opening sooner with a strait edge and then closing just a little later with a D shape.. however i have heard that the port shape can shift your power band considerably. if so has anyone had any experiance with exhaust port shapes? i would like to maintain as much low end as possible.. and i believe the square port (not a D port) might be a better choice..

i believe i will open with a flat edge to the port because it seems like you get a stronger exhaust pulse to spin the turbo... ummm turbo..

so over all this is what im thinking.. open a little bit sooner then stock with a strait edge, but not much wider at all. then close just a bit later with a strait edge or maybe a D shape. well tell me what you guys think and what your experiances have been.. thanks alot!

 

Recent discussion about that same question. Consensus? depends on who you ask.
https://www.rx7club.com/showthread.p...t%3B%26quot%3B

 

which template are you using to do the intakes? why don't you use the exhaust template from the same source? if the creator of the template is worth a ****, then you should have a pretty balanced port setup.

personally, i don't agree with porting the intakes and not doing the exhaust with a turbo. if anything, the exhaust is where i'd focus my efforts - especially since you're keeping the stock Hitachi. however, that's just my opinion. i'm not looking to mess up your thread with a ****-fight.

the best thing is to read up and maybe consult with someone that knows something like Rotarygod, Judge Ito, Peejay or RXBeetle (there are others, too - thise names just came to my head first).

good luck.

 

thanks for screwing my thread! now the ***** about to hit the hypothetical fan!

just kidding.. thanks alot for your comments! i was expecting "SEARCH!" and then there would have to be an old fashion stick ... and i would win..

im actually not using templates.. i know .. a bit crazy.. but its kinda one of those things where one size does not fit all.. ill probably make my own template after i decide my design so i can understand the effects of different port shapes.

right now im about 99.4% sure im going to port the exhaust also. what im trying to understand is what effects different port shapes and differnt timings have on power output and the power band. so your information is actually usefull!

"the exhaust is where i'd focus my efforts - especially since you're keeping the stock Hitachi" why is it that you say this? im curious to understand your logic.

 

my logic? wow ... my logic ...

well, my logic is this: i figure improved exhaust flow is good for all engines, but particularly so for turbocharged engines. i haven't read anything that would lead me to believe that the stockports won't be able to keep up with the stock turbo. so porting much larger seems to be a waste of effort unless you're open to getting a larger turbo at some point.

what i would do is clean up the stock intake port runners and increase the venturi-shape on them. i'd open the ports a little earlier and smooth out the airflow. it may not be a HUGE volume, but your turbo is still going to provide positive pressure. by the way, i commend you for making your own templates. it's not hard, but i learned that it's not as easy as it seems if you want to do a good job.

now, for your actual concern. the exhaust. i would open earlier and go wider. i wouldn't bother with a "D"-shape at all. i suspect that there may be spiking problems on the horizon, so i'd also probably weld the wastegate shut and use an external one. maybe have a manifold made??? money ... i know. also, a quality boost controller would be in order.

by the way, form your original post it sounds like you understand quite a bit more than you think you do. just an observation. anyway, having said all of that now, my advice is read as much as you can on this and draw your own conclusions. after all, this is your "real"car not my "hypothetical" one.

 

Having experienced a few engines that my cousin, Jeff20B, has built, a curved closing edge doesn't seem to produce very good results. His brother's RX-4 with the upside-down D-shaped ports was more gutless than a stock automatic Cosmo! But after changing to GSL-SE housings with a curved opening and straight closing (at the recommendation of Judge Ito, if I remember correctly), the difference was amazing. It had FAR more power.

We tried a curved opening, straight closing on my own 12A, and it made fantastic power with only a very mild port on the intake.

 

^^ thanks.. so how was the powerband with the curved opening and strait closing? was it weak in the mid rpm ranges and then peaked at high rpm? or did it make good power thru the mid range but started to flaten out at higher rpms??

after more thinking about it.. it would seem that the curved opening might be a better solution.. pressure would be high and exhaust port area would be low (at port opening).. causing the initial exhaust gas velocity to be higher then with a strait edge port opening..

it also seems that the volume of air needed to spin the turbo at different rpms would be non-linear.. that is it would require substantially more volume the faster it spinning.. therefore curving the port closing would seem to be a waste.

 

Since I've never had a turbo motor to experiment with, I'm not positive how it would affect the spooling, but I would imagine that your theory is correct.

As far as my 12A went, the primaries were totally gutless, likely due to inadequate primary jets. So I'm not sure how the idle to ~3500 RPM power should have felt. But once I would open the secondaries it would really fly. Jeff20B mentioned that it felt like it gained an extra rotor.

In typical (mild-ported) rotary fashion the powerband was very linear. The midrange surprised you (especially after the weak low-end), but it didn't ever really drop off, and continued to pull hard past redline. At least, if you ever did leave the powerband you couldn't feel it. That tells me that the exhaust ports did not become overly restrictive at the top-end.

I wish I had gotten some dyno sheets for that engine to see what kind of power it was really making, and where the powerband actually was.

 

After I took possession of said 12A, I almost imediately swapped out the stock flywheel and exhaust for a Racing Beat light steel and their complete dual pipe "street port" exhaust system. I've got to tell you this thing flies!

I'll try to describe it as accurately as my subjective position will allow.

Very mild intake port job (came stock on '74-'75 engines).
Exhaust ported a couple mm wider and taller.
mechanical secondaries and increased accel pump shot and duration
94 primary jets upped from stock 92. Have plans to try RB's recommended 105 jets for '74 spec ports (for 13B, not sure whether they'll work right on a 12A)
Old school intake manifold possibly from an RX-3 which outflows a 1st gen manifold.

Before the flywheel and exhaust swap, the car was gutless from idle up to 1500-1800. Above 1800, it had fair midrange up to about 3000-3500. From 3500-7000, it was a very different engine with the secondaries open, which causes me to believe the primary jets at 94 are still too small. Gotta try those 105s when I have some time to tear into the carb...

After the flywheel and exhaust change, the car is actually a lot smoother and easier to drive! I posted about this in the 1st gen section. Who'd have thought a light steel flywheel would be easier to drive than a stocker? Anyway from idle to 1500 it is still gutless but it's easier to drive, which probably means there is a little more useable grunt down there, since the flywheel doesn't require as much power to move it, and therefore the car. With so little exhaust flow near idle, the RB exhaust isn't actually doing anything yet.

From say 1800 to 3000, the midrange is more pleasant. Again we're still on primaries and it still feels sluggish. However, it is quieter and takes less time to react to throttle inputs, which I feel should be mandatory on cars like this. Forget stock, lol.

From 3000-7000, with the secondaries open, it practically feels like it's got a supercharger or magical secondaries or something. The "extra rotor kicking in" just about sums it up. This thing has up high what my 20B had down low in the GLC; power that catches you off guard.

If I could change anything about this setup, I'd take a little power away from the upper end and place it down low where this engine needs it (in my opinion).

Maybe it's my background in 13Bs talking, but I really miss the good low end. Well, the 12A isn't staying in the car forever, so I just have to put up with it for a little longer until the NF01 can go in. Hehe.

 

i would definately do some work ot the exhaust...even if it just a mild match port job.........if you mess with intake port you should bring your exhaust up to spec too..just my opinion

 

yeah thats the plan.. im in the process of trying to understand the effects of different port shapes. i think right now i will probably go wider by a millimeter in each direction. go down with a curved opening in 1-2 mm and maybe up with a strait edge 1 mm.

i might try and estimate the area of the intake ports and exhaust ports and try to maintain that ratio..once all porting is complete.. i havent thought alot about this idea yet.. but its something im going to play with.

 

The earlier you open an exhaust port, the shorter you make the power stroke.

B

 

but isn't a turbo setup more tolerable (doesn't suffer as great a loss as N/A) of a slightly shorter power stroke because of the better filling?

 

^ i believe that this is a common missconception.. there is a limited amount of time where the expanding gases are applying enough torque to the e shaft to keep them in the chamber.. for the amount of port opening advance i will be running i dont think this will become a problem in the least bit. you have to realize that when the gases are expanding the volume is also increasing causing pressure applied to the rotor to be dropping. the earlier they open the higher the pressure will be causing the exhaust gauses to exit on their own..without the rotor actually pumping the gases out.. i imagine that advancing port opening will help to spool the turbo a little quicker also.. ummm turbo

 

Well I know your belief of this being a common missconception is a missconception and that's not what I believe but what I have experienced.

 

so are you saying that you have modified the exhaust port and lost power? your statement is a little vague and has very little use to me.

what is your basis of knowing for sure that it is not benificial to open the ports a little bit early with a curved opening. all the other post so far have been beneficial to this topic, yours however is not. if you have some information you would like to share, then please feel free to explain why it is that you believe what you believe.

 

Yep, I think that's what he's saying. I've seen it before myself albeit not on my own vehicle. I learned about it about 6 years ago from another builder who'd experienced it. All made sense after I took a closer look at it. It seems that the natural (American) predilection is to tend towards the whole "bigger is better" thing, when in fact, it is not. That same rule applies to the exhaust port -- making it bigger might make it flow like a jet engine on a flowbench, but in a dynamic engine environment it will have decidedly negative effects. I am not sure if this is where you are coming from so I don't mean to project that onto you (assume you're thinking this and then argue against it), but I do know that opening the port earlier, especially dramatically earlier, will yield less overall torque and power.

Not to speak for Crispeed, but I honestly have no opinion on the whole straight or curved opening thing. That introduces some physics that's over my head.

B

 

thanks for the reply BDC..

your right bigger is not better.. but you cant say a degree or two of port advance will cause a loss of power due to a shorter power stroke.. there are many things going on at the same time with many different effects. now you can say that if you port the **** out of them that you would loose power.. how much port advance was the builder running when he lost power?.. it was probably very advanced... however i believe the key is to design your exhaust ports with a goal in mind.. an achieveable goal. not i want as much power as possible.. so im going to open the exhaust ports as large as i can!

my goal is to try to shift the power band towards the mid range as much as possible.. which i doubt will be very much .. i doubt advancing the opening will do what i want it to do...but at the same time i will be flowing more air on the intake stroke.. and should not use the stock exhaust ports.im thinking of opening just a bit earlier with a D shaped.. i havent figured out how much earlier it is yet. but im thinking of leaving the left and right edge of the exhaust port at the stock timing and curve the center down creating the D.. i doubt i will be running very much advance at all.. definatly not enough to cause a dramatic loss of power..

plus if it was really tru that you loose power when you port the exhaust then why does about every builder have a different exhaust template.. with different degrees of opening and closing.. there is plenty of power to be gained out of time of the exhaust.. but .. it is application specific.. thats kinda why im in search of good knowlege about effects of different port shapes and sizes.. i want to achieve a specific goal.

here are some port timings ive found.. there is a great degree of differance in EO and EC.. exhaust open and close..

Second and Third Generation Turbo 13B
IO 32° ATDC
IC 50° ABDC
EO 71° BBDC
EC 48° ATDC

Racing Beat "Street Port"
IO 25° ATDC
IC 60° ABDC
EO 84° BBDC
EC 48° ATDC

Racing Beat "J-Bridge Port"
IO 115° BTDC
IC 72° ABDC
EO 88° BBDC
EC 57° ATDC

Mazda Factory Peripheral Port
IO 86° BTDC
IC 75° ABDC
EO 73° BBDC
EC 65° ATDC

 

if you look at the factory timings, the difference between full race and street is 2 EO degrees. doesnt this make the case that the EO is about right stock?

 

Its also a P-Port!! You must look at the entire WHOLE picture and engine package. Its not just port open and close timmings, but its supporting pieces as use as well. You have to look at target RPM and OVER ALL PORT DURATION not only open / close timing. different engine configurations and power goals require differant port timming/supporting equiptment. These fundimentals cannot be summed up in one post, so continue reading SAE and forums, take it all in not forgetting all factors. You'll find the faster guys think alike and run simular set ups, then theres the odd ballers who are just as fast. Its a lot to do with supporting mods too.

~Mike.........

 

no it doesnt it does the opposite. mazda had their own design criteria when designing the ports.. mine is much diferent. 2 degrees may seem small but could really have a drastic effect.. look at area of a rectangular port if yo had a port that was 7mm wide by 5 mm tall and you increased the height by 1 mm to 6mm. you would increase port area by 20%.. this might not be the best comparison but it gives an idea how subtle changes can cause large effects.. thats probably why people have lost power while porting the exhaust.. the went to damn big!

RacerXtreme7 thanks for the post.. this is my point also! port design is specific to the application..! im trying to understand the effects of different port timings, widths, and durations. that way i can design my ports to fit my needs.

so please feel free to post about your exhaust ports. opening, closing, width, shape and their effect on the powerband and peak power. thanks again guys

 

Exhaust opening (EO) should be 75°BBDC for above, not 71°BBDC.

While the front rotor exhaust opening is happening @75°BBDC (195°ATDC), the rear rotor's chamber will be past TDC @ 15°ATDC begining it power stroke and reaching its peak torque transfer in the next 30 degrees @45°ATDC.
It's obvious the torque curve from each rotor face overlap each other with the 1st rotor face loosing steam and 2nd rotor face taking over.....like the pedaling action on a bicycle:-).<---1st example that came to mind !

Racing Beat and many other engine builders move their "street port" exhaust opening earlier.......Are they all wrong ?


Mainly with a performance turbo motor, I found porting the exhaust earlier a bit not only helps with engine breathing at higher rpms/boost but also help with spool up.

JD

 

thanks .. this has been my conclusion also. do yo have any experiance with curved opening and closing?

 

i aggree..........it is just as with a piston engine..valve overlap is there to make sure some ld gasses are leaving before fresh charge comes in...........also i have read that D shaped exhaust ports make most power at higher RPM.......i dont have all the info you guys do but it make ssense to me to do the exhaust too.....