Brettus

iTrader: (2)

After a discussion with Mazdamaniac a month ago I decided to try running my FI rx8 without the aux ports operating . Basically turning it into a 4 port .
Some time ago I started this thread https://www.rx8club.com/series-i-major-horsepower-upgrades-93/greddy-turbo-owners-make-sure-you-fit-your-blocking-plate-164382/ about why the blocking plate that comes with later kits should always be used with the stock Greddy turbo , however I didn't consider not using the ports with larger turbos a good idea .

Edit : here are some discussion points on the subject - skip to page three for an outcome to the discussion.

*No dip in power as the ports open
*Need to run higher boost to get same power output BUT this means turbo is in a higher efficiency range and therefore inlet air temps are reduced .
*Easier /safer tuning due to removal of the lean spike when the ports open


here is a turbo efficiency chart showing what the difference was when I turned off the aux ports and increased the boost pressure to compensate .

Raw numbers :
Aux. ports closed
At Turbo 14.5 psi
At UIM 10 psi
flow 310g/s

Aux ports open
At Turbo 12psi
At UIM 7.5psi
Flow 310g/s




This is a later test ,this time no increase WG tension (and plotted on the correct chart) , just back to back runs .
NB this test was done after fitting an upgraded intercooler - so interesting to see the effect that has had .

Raw numbers :
Aux. ports closed
At Turbo 14.5 psi
At UIM 11.8 psi
flow 325g/s

Aux ports open
At Turbo 13psi
At UIM 9.5psi
Flow 347g/s

ZumnRx8

Nice Now I know what you meant in my Turbo thread


*Need to run higher boost to get same power output BUT this means turbo is in a higher efficiency range and therefore inlet air temps are reduced .

*Easier /safer tuning due to removal of the lean spike when the ports open

freaklinkmusic

interesting. so in theory, if you mated a 4port engine a 6spd trans, you would see the same results? (provided that engine management was available)

WOW.

thats should give the 4port guys confidence.

arghx7

So you still have your SSV opening right? At what RPM?

FYI, the normal practice on turbo converted 6 port 2nd gen Rx-7's is to leave the aux ports all the way open at all times (sleeves are often removed). Now it's a little different on that car because of the throttlebody design.


(that's an FD TB, but it's the same basic design that was used on all EFI rotaries including 6 port motors)

Instead of a single butterfly TB and a SSV, the throttlebody is a two-stage design. As you put the gas pedal down, the larger butterfly for the two primary ports opens first (mechanical linkage with throttle cable, remember). Then as the linkage continues to open, a separate set of two butterflies opens which supplies air to the secondary and aux ports. But the aux sleeves still need to rotate open, and they are moved by air pressure (not an actual motor). On the factory turbo Rx-7 engines (all 4 port), the other set of throttle butterflies just supplied air to the two secondary ports. It's been that way since the EFI 6 port 12A in the Cosmo and the EFI 4 port turbo 12A in the 1st gen Rx-7.

Have you considered removing the aux port sleeves altogether and making it functionally a large 4 port like the old 4 port turbo engines? Because right now you are like the smaller Rx-8 4 ports. But there's always a tradeoff between maintaining velocity and having a capability for top-end. It would be an interesting thing to try. But let me make sure I'm clear on this. With the SSV closed, there is no way air could be fed to the Aux ports?

Mawnee

Go easy on me....since this is into the rotoray stuff that I"m fuzzy on....

But isnt what you are doing, gimping the engine? You are making it a lower output engine so you can compensate with more boost to reach the same numbers? I know it would need more boost, but would that greater boost= greater airflow? Or are you just creating backpressure to raise the guage?

What about going the opposite way? Always open. Wouldnt that avoid the lean spike as well since the intake volume would be the same throughout the entire RPM range. This would make the engine higher flowing at all RPM ranges provided you have a turbo capable of supplying the needed air.

I always thought the idea was to remove as many restrictions as possible so the engine can consume as much air and fuel as possible.

Talic

having the 6 ports always open will cause a problem as the rotors wont be spinning fast enough to prevent the air from escaping through the other ports during the "compression" cycle.
at 6200+ the rotor is moving fast enough to keep the air contained inside.
as that was my experience with my procharged rx8 at idle i would be at neutral pressure or 0 bar or 0 psi (if wastegate was closed)
with all 6 ports fixed open. but i never noticed any high end difference with them operating except i lost a little low down.

correct me if im wrong.

but converting it to an all time open 6 port would only benifit supercharged versions as the pathways will be full at all times.
on the turbo appplication below 4k would feel like a moped.

Mawnee

Ahh ok that makes sense

What about setting them up to only open under positive manifold pressure? They are operated by a diaphram arent they? couldnt we just setup a system so that they open as soon as the intake is above say 2psi? That way it would still function for no/low load driving. But always open when there is enough pressure to ensure there is no backflow.

arghx7

I'm not sure I'm understanding this point. During the compression cycle, all the ports are closed and gases only escape through the various clearances/gaps in the rotor seals.


Anyway, at some point having only those 4 ports open becomes a restriction. Now I won't claim to know when it becomes limiting. But having the extra two ports are like adding more lift... the 6 port induction was VTEC almost 10 years before VTEC (about 1982 vs 1990). What would a Honda be without VTEC?

From personal experience, I used to have my aux ports permanently open on my non turbo Rx-7. That's because the actuators/sleeves were controlled by exhaust backpressure up until 89 models and that made the system unreliable when you start modifying hte car. Having them open all the time definitely hurt low end, and having them closed all the time hurt top end. But on an n/a engine that's a no-brainer.


Now here's diagrams of the intake system operation. This is straight out of the Series II service highlights.




Now here's another thing to consider. WHAT IF... you opened the SSV and the APV at the same time? The engine would be functionally somewhat like the FD then, or like those 2nd gen Rx-7's with the aux port sleeves removed. It would have only the primary ports open for low throttle/load, to maintain some really low end, but then you'd full port volume at the time that the SSV opens and you wouldn't have to deal with lean spikes up top. VDI would be shut at all times and I'm not sure how much the VFAD would even matter.

Mawnee

Wow fantastic post! Now I can get my head around it. Pictures FTW!

Brettus

iTrader: (2)

Yes ssv still opens at stock rpm 3750 ish.


Opening the aux ports all the time yields a 25 odd whp LOSS right up until 6500rpm at which point they start becoming efficient .
Yes I am effectively making the engine smaller by not opening the ports at all BUT it seems to me to be all about the turbo not the engine . Sure if you fit a big *** turbo on there you definately want the aux. ports to open if you want to make 400whp+ .
BUT - most turbos fitted to the 8 are not designed to do that so I figure we need to get the best out of the system we actually have . The fact is the turbos we are running like to operate at higher pressures than what we run them at . By shutting off the aux. ports we are optimising the turbo and not the engine .
So if you want to make more power - fit a larger turbo and open your aux. ports . If you are happy with 300 odd whp and the fast spooling turbo you have - leave the ports closed and up the boost ...............

Mawnee

Ahhh, so I open mine..cause I haz big turbo.

I think I understand what Brettus is getting at now though.

I actually just lowered my APV to 4800 for my run to grab lunch. Its wet out so I was only at a couple psi, but the car was like "LETS GO!" when they opened. Cant wait to try it with full boost. I had actually not touched those settings at all till today.

Brettus

iTrader: (2)

Yes I am "gimping" the engine but optimising the turbo .
And yes higher boost means higher airflow . With aux. ports open and EBC operating my UIM boost profile looked like this :

up to approx 6000 10psi
from 6000 - 8000 tapering down to 7.5psi

Without aux ports open it looks like this:

3500 - 8000 10psi

mass airflow and therefore WHP at peak is identical but I don't have the dip associated with the ports opening.


I think getting the best result is more about the turbo than the engine - do you want a fast spooling turbo giving you great drivability or a big *** one makeing **** loads of power with compromised drivability . Which you choose determines if it is better to open the aux ports or not ....

05rex8

iTrader: (7)

interesting discussion..subscribed

Mawnee

^ Yea I love stuff like this. I know more about our intake manifold now than I did this morning

TeamRX8

iTrader: (25)

I think Brettus is making a disguised twin turbo thread ...

think about it ....



.

05rex8

iTrader: (7)

meh, it's better than the 'which chip should I get?!' or 'who has lambo doors?!' threads...

arghx7

Fair enough. The 6 port engines are optimized for a broad n/a powerband (not a turbo) so you have to make choices like that.

Yeah it's pretty cool stuff. I've been assembling as much documentation as I can on rotary intake manifolds. The original 6 port engines were a little different in their intake manifold design because peripheral exhaust ports allow overlap between the intake and exhaust stroke. That overlap and resulting EGR effect actually pushes on the intake air charge as the intake port begins to open, which creates a whole other set of pressure waves to be tuned. But that effect isn't there on the Renesis, but it's all a tradeoff because the Renesis has so many other improvements from the side exhaust port design.



That's a cool explanation of why an engine properly designed for fuel injection is going to have a better powerband than a carbureted motor.



The exhaust obstruction effect applies to earlier rotaries but not the Renesis. It also applies to piston motors: if the exhaust valve is still open when the intake valve is open, you have overlap. As the intake valve first begins to open the pressure inside the combustion chamber actually pushes back on the intake air charge. That creates a wave which hits the plenum chamber, further compressing the air due to spring-like forces. That compressed air then flows into another cylinder just as that cylinder's intake valve is closing. OEM's take advantage of this effect when designing variable valve timing systems that advance the intake cam or retard the exhaust cam.



and that's the original VDI system used in series 5 Rx-7 engines.

TeamRX8

iTrader: (25)

I'm sure the ZOMG clueless fanbois will eat up those basic info repostings the way chicks go crazy when the mama bird regurgitates dead fish, but the rest of us informed adult birds are going to get bored and fly off if you can't be any more original than that

argh is a really good user name, it matches exactly what I'm thinking when you're posting this stuff like it's from some hidden secret vault only you know about

Brettus

iTrader: (2)

Brettus

iTrader: (2)

I would like to know what the more experienced tuners think is a more stable air/fuel charge
Note (temps are a guess)

say 310g/s at 10psi and 45deg C
or 310g/s at 7.5psi and 50degC


Also : to get more boost I have had to close off the wastegate more so probably have more backpressure before the turbo . Or do i ? If flow is the same am I just creating the same back pressure ?

05rex8

iTrader: (7)

I would say it's not worth the effort for 5 deg difference...especially if you have to keep messing with the W/G arm.

But I know nothing, so take it as you please, LOL

Brettus

iTrader: (2)

/\ I'm guessing the 5 degree difference - would be a good test to do though to find out what the real number was .......

05rex8

iTrader: (7)

yeah would be better to have more concrete temp numbers lol

Brettus

iTrader: (2)

How about if we took temp out of it all together - would there be any difference to charge stability at different pressure and the same airflow ?

Mawnee

Thanks for that constructive post.......
I found the imformation he posted both useful and relevant in trying to understand what Brettus is getting at. That makes me a clueless fanboi! And since you pointed it out you get some kind of uber epeen cookie?
That kind of elitist BS is exactly why search is nearly useless on this forum. You get searches full of garbage like this or countless threads of people telling others to "use search".