RX8 XTC

I don’t know why i am asking this but it just keeps me thinking after watching the Rx8 Tech high lights.

My Question is what would happen if the APV (Auxiliary Port Valve) VDI (Variable Dynamic Intake SSV (Secondary Shutter Valve) are all opened OR just the (APV)
Wouldn’t we have more Air going in the Engine in Low rpm and +more air at the High Rpm??

for some reason i think But don’t know any thing ( I JUST THINK ) if the APV and the SSV where to be left open all the time wouldn’t we get better Gas Mileage + more Power since the Engine will take More air at LOW to High RPM ?? Maybe More torque too??

Just wondering what you guys might think about it or what you know about it

zoom44

Helmholtz or "resonance" tuning read this

http://www.mazda6tech.com/articles/d...echnology.html

actually not as good as i thought ill find another but read that anyway

zoom44

from paul yaw

zoom44

so in my own words-

its not just allowing more air in but changing the length of the intake track changes the time of the pressure waves. these presure waves if used correctly allow for a supercharging effect that you would not have by just leaving the ports open all the time. you actually get more air in an make more power all across the rpm range opening the different tracks at the right time then you would with them just open.

rotarygod

Those valves are thee to keep intake velocity high. More port area is NOT better until you actually need more. More open port area will make you slower at lower rpm's. Velocity of the gasses is #1 in terms of importance when it comes to making power. Volume is always 2nd to velocity. The reason velocity is important is because air has mass. Remember back to science class that an object in motion tends to stay in motion. This applies to air also. The air in the intake runners are constantly changing in speed and sometimes even reversing their direction. These reversals are seen as pressure waves. When the rotor is moving across the port opening the air accelerates towards the engine. When the rotor closes the port, the air decelerates and changes direction. However a nice side effect of velocity is that as the port starts to close off the port, the air wants to keep moving towards the port due to it's inertia. Quite simply it is moving fast and harder to decelerate. This faster moving air rams itself into the port longer and therefore feeds the engine more air than it would have if it were moving slower. This is why the Renesis brings additional ports online as the rpm's rise. Keep the intake velocity high to promote good ram charging. There is always a certain point though that you come to when the engine can't breathe enough air through this size port. Simple, just add another one. Mazda does this. Port timing also plays an important role in powerband. A port that is open later makes more power up top. A port that closes earlier does better down low. As the Renesis ports come online, the timing changes to reflect this powerband change and the engine gets retuned not only with intake velocity but also intake timing. This principle is the same concept as VTEC but Mazda first applied it to a rotary back in 1981 years ahead of anyone else.

The VDI valve retunes to reversing pressure waves that I mentioned earlier. The longer the intake runners, the lower the rpm to which the engine is tuned. The Yaw article above deals with this phenomenon. The shorter the runners, the higher the rpm tuning due not only to intake length but also proper timing of the return pressure wave from 1 closing rotor to the next. The VDI valve shortens the distance for this return wave to travel. If done properly it will time the pressure wave to interact with the high speed column of air that is already trying to cram itself into the engine due to it's inertia. This makes it possible to sometimes hit over 100% volumetric efficiency. This means you have put mroe air into the engine than it has in volume. We call this boost.

If you leave all of these valves open, you lose all of the benefits that they give you. Many RX-7 people wrongly leave all these ports open citing illogical justifications for doing so. What you will end up doing is hurting your gas mileage, low speed emissions, and you will make less power than stock all the way up to about 7250 rpm or so. You will have the same power above this point. There isn't much left above this point though. Keep them working like they are.

RX8 XTC

i see, i under stand why now Some help full info i read like they Say every lil thing has a Job to Do and serve Something.

thanks Zoom and rotary for making me Understand things like this YOu guys really do help alot :D

SO ill just have to Leave it alone now.

rotaryPilot

Well ok I have understood well what is and how it works the VDI. My question concerns the 4 port. Actually VDI exists in 4 port RX-8. So the VDI valve opens at 7250 till the rev limiter which is in 7800. So the VDI actually works in 4 ports for 550rpm only???

Is this true????

cajunrx8

RG on what model did they have Ram Charging in 1981? I don't think any US Mazdas had it until 84 with the GSL-SE RX-7.

TrochoidMagic

so vtec would also be affected in a similar way too.

i really don't understand why people want to get a vtec controller to shift vtec points to say like...under 4k rpm's?

its all noise, and the air flow into the chambers aren't achieving proper velocity.

rotaryPilot

well does anybody knows in which RPM the vdi is working in 4 port engine?

rotarygod

I was using that from a 6 port perspective based on the way air flows through the upper ports as they close. Mazda had a 6 port 12A engine in Japan in 1981.

nmarz77

Being and ex FI Integra owner I can say that changing the vtec point "slightly" up or down in a NA car can give you a little more horsepower in a very narrow range depending on your mods done. But for a vtec car that has forced induction it can definitely help out and give you quite bit more hp early on in the rpm range by lowering the vtec point.

muythaibxr

Honda also does odd things to make it so you "feel" the VTEC kicking in, like dropping the timing a bit just before VTEC kicks in, etc... get rid of that drop in timing, and move the VTEC point down until you can't feel it happen, and you end up with a slightly faster car due to a bit more mid-range torque.

I believe mazda already has all the on/off points for their valves happening at the right times to get mostly smooth torque curve, so there's really not a good reason to change them.

Ken

TrochoidMagic

yes. true. but tuning with VAFC with only a few "hundred" rpm's under and leaner in my prelude was somewhat slightly better. but i'm talking about people that drop 1k rpm on vtec timing...not FI'd.

in only ur case, yes it may make sense.having the vtec is like having a high cam profile for ur turbo. but that just doesn't make sense...having a lower cam profile at lower RPM's at all with turbo...it just seemed better to not even have vtec in turbo application, why go thru all the trouble? but this topic did not first include forced induction, anyways.

TrochoidMagic

i think i get what ur tryin to say here, and you must be refering to i-vtec in the newer honda's.

in my opinion, i loved the older VTEC style of just the low/high switchover. now it can somewhat feel "TOO" smooth as they have a "progressive" vtec system which u pointed out that alters the timing and advances/retards the cam timing for low/mid/high and also alters ignition together with the vtec cam profile.

so its smooth all across the rev range.

rotarygod

I can feel vtec kick in on my car and I wish I couldn't. It could be lowered so there would be more average power. Since I'm not a Honda performance person, I don't know how to make this happen. It's a daily driver but I'm all for a smoother powerband.

nmarz77

You need a VAFC. ;-) You'll be able to control vtec engagement and tune the fuel for a few more hp. I feel very strange talking about Hondas on this board.

rotarygod

That's OK. I feel strange asking about how to mod a Civic!

nmarz77

There's nothing wrong with modding a civic......it just makes you realize how gay a fwd car is to mod.

I feel the heat coming already for that one.

muythaibxr

Haha, I was talking about the older VTEC setup, like the b16a in the 1999 civic si, and whatever engine was in the late '90's preludes.

Scott Clark was comparing what he did with a megasquirt controlling the on-off point, ignition, and fuel to what the stock ECUs did...

They hooked up using an ODB-2 scantool to the cars, and put the car on a mustang dyno, and watched the power, and the timing, and the point at which VTEC came on, and saw that honda was actually reducing power (reducing timing) just before vtec kicked in, and then going back to a more "sane" timing just as VTEC came on in order to make the person driving actually FEEL it kick in. They were also engaging it at a significantly higher RPM than they needed to be doing.

Using the MS, he reduced the rpm at which the VTEC kicked in, and got rid of the timing dip, and did dyno runs until the torque curve (at full throttle) was close to flat through the VTEC engagement.

I believe they also made it engage at different points based on load as well.

They ended up with modest gains on the dyno, but those gains ended up translating to much faster lap times on the various tracks they were racing those cars on.

Ken