Can the VDI be wired open so that it is active all the time? Would the low RPM flow be any better? I assume the torque would drop slightly at lower RPMs. What about in a forced induction situation? Would it be more beneficial then? Also, should my dyno dip in power as it does when the VDI opens? What is causing the dip?

 

Yes it can be wired open. Benefits? none. Draw backs I guess the speed of the intake would not be optimal otherwise the mazda techs would not have created two positions for it. If the VDI running open was the best in all the rpm ranges it would have just been designed without the valve. Atleast thats mo.

 

Ok. What about in a forced induction situation? Any benefits then?

Also, what about the dip in my dyno chart? Is that normal?

 

On forced induction, among many things, i'm not an expert. I would think that if you are running a turbo, once it spools it should be open. If you are using a blower setup, just leave it open because if its doing anything towards positive pressure its moving faster than an N/a would be pulling the air.


I have seen that dip in other charts but I would think its because of the velocity change and fuel changes that occure there. Its probably not a very nice looking transition looking at it from those stand points, killing power.

 

Yeah, that's kinda what I was thinking. It is a centrifugal blower setup with a max of around 6 psi (for right now), so it will probably only push around 3-4 psi beneath 5k RPMs (when VDI would normally open).

I'll talk to my tuner/mechanic about it and see what he thinks. In the meantime, any other opinions?

 

It can be wired open, but then it will not be active. Since it can't change position, it would be inactive.

No, it would be worse.

Correct.

The VDI system is basically used to swap between two different torque curves, one that peaks later than the other. By being able to utilise both torque curves, average power (and hence overall performance) is increased.

Here's a dyno graph from Mazda's development of the VDI system, taken from the SAE paper on the S5. It shows the torque curve of the S4 13B compared to manifolds with different "communicating" lengths, which is the length the pressure waves travel from port to port. The chosen lengths for the S5 manifold were "d" (400mm longer) when the VDI valve is closed at low rpm and "a" (200mm shorter) when the valve is opened high rpm. The two curves cross at 5200rpm, which is when the ECU opens the valve. The resulting torque curve is higher than the S4's at nearly all points in the rev range.



The theory is the same, but the pressure waves in the runners that are fundamental to the VDI system are smaller percentage-wise compared to the increased manifold pressure, so the the effect is less noticable. Most manufacturers don't use variable manifold systems on forced-induction motors because the development costs don't justify the benefits. Your system is free though, so you might as well use it.

The VDI valve is opened at 5200rpm, which corresponds exactly to your graph, so it's a good bet that's what it is.

 

it may be good to give the VDI actuator an inspection, and to make sure it is getting the correct amount of pressure (or is that vacuum...) in the line. opening too slowly or incompletely may cause the hesitation.

you could also try hooking up the solenoid to an rpm switch, and then experimenting with different opening RPMs. i think 5200 will still be best, but there may be a different RPM that is better to open at because of the changes caused by the forced induced intake...

 

I think you're right about the VDI opening too slowly. I have a Racing Beat "Race Pipe," "Cat Replacement Pipe," and catback. I don't want to get a header because I'm afraid it will get too loud. I do still have an airpump. I read somewhere that modifying the exhaust creates problems with backpressure and doesn't allow the 6 ports and VDI to work properly. Is the "RPM switch" you're talking about electronic? I would think that would be much better than relying on a vacuum signal especially on a boosted car. I think it would be interesting to experiment with different opening RPMs as you mentioned. Is an RPM switch a cheap thing? Easy to hook up?

If you look at this guys dyno (also a 6 port with VDI) he has no dip in power:
Originally posted in this thread: https://www.rx7club.com/time-slips-dyno-128/na-6-port-177-rwhp-363329/

 

Bump for more input...

 

The benefit is almost non-existent on a forced induction car. It's pretty much the same as aux ports being used in an FI setup, you're flowing so much air by that time anyway that the best flow is generally the best bet.

 

Ok, so your suggestion is to just leave it alone and not worry about it, or....?

 

Basically in my turbo charged setups, with the aux ports it only helps the turbo spool a hair quicker, which isn't too big of a deal, and they hinder top-end by creating turbulance, so I prefer removing them. As for VDI, i just leave it in the open position. There was little to any change when playing with the VDI on the dyno. For an N/a setup however, these are all crucial, and I do believe changing the timing of the vdi after mods would prove to be a benefit.

 

So you would recommend just wiring open the VDI when running the supercharger? It is a centrifugal charger that should flow around 800 CFM. What if I were to use some kind of actuator to open the VDI sooner so that I could still keep the low-end benefits and then open it when I start to get more boost? I will be running the motor to 8k RPMs since I'm still making good power up there; therefore, I'm gearing the supercharger to max out at 8k RPMs. I'm not sure if I mentioned it before, but I no longer have 6 port actuation. All that was removed and my manifolds were port matched and polished.

 

What charger are you going to be using? And yeah, that should be an OK setup.

 

I would personally leave it working. It's there for a purpose and it still does it's job regardless of what the pressure in the intake manifold is. If this wasn't so then it might work better at sea level than at the top of Pike's Peak. It takes of the pressure waves that travel back and forth inside the manifold at the speed of sound from the ports opening and closing. It doesn't concern the amount of boost.

 

Theres only one positive outcome to wiring open ports on any NA, whether its the 6port system or VDI.
The only advantage is that you don't have to worry about the ports in the first place.

The negatives outwiegh that pro tho, heavily IMO.

 

There's a difference in the concept however, where-as it's benefits are for a drawn vacuum engine, instead of forced air, this is why you'll see almost no benefit on a dyno with a forced induction setup with it working or not.

 

It is one of the old Nelson kits. It was originally owned by Travis Shrey (sweet red vert), and it was most recently owned by Lars Young in Naples. Travis fixed a lot of the problems with the kit such as poor brackets. I bought the kit from him since his car is no longer running. I'll be running water injection to keep the IATs down and the detonation in check and tuning AFRs with an S-AFC II. I'm hoping for at least 225 whp, especially with the WI.

 

800cfm should do more than 225, guess it'll depend on the efficiency. Good luck though, always nice to see more SC vehicles, mine was getting lonely!

 

I say bump for archive, some excellent info that could be pointed to from NZconvertible that deserves somesort of applause..

 

I'm pretty sure a stock exhaust manifold will be a significant restriction with forced induction, but i guess it depends on how far you plan to go. If you're worried about noise then get a dual muffler system with decent-sized quality mufflers (not shiney little eBay cans). You should be seriously considering a new "turbo-sized" exhaust system anyway.

Exhaust backpressure has nothing to do with the S5's 6PI and VDI systems, they operate from air pressure switched in by the ECU. Exhaust changes have zero effects on them. You're thinking of the S4's 6PI system.

 

Yeah, you're probably right about the stock exhaust manifold. I have Racing Beat exhaust, which is some of the best as far as I know, so I'm not running crappy exhaust. I'd like to hear a full Racing Beat exhaust with a header to see if it is too loud for my tastes.

 

So, I've been doing some thinking about using a header. As I looked at the Racing Beat site, it seems that they offer two different headers. The first one is the "streetable" header that collects, and I think that would just replace my stock manifold and the Racing Beat "down pipe." Cost is $200. Below is a picture:



Racing Beat also offers a "Road Race" header that does not collect. I'm sure it flows much better since it is a "true dual" setup; however, I would have to replace several components of my exhaust and the cost would go up significantly (over $600). Below is a picture:



It seems that the "Road Race" setup would be considerably louder, and, of course, would flow much better. It would also be much more expensive. I think if I do go with a header, I would go with the collected "streetable" design. Would the performance gains be worth the $200? I've done a lot of searching and reading, and it seems that many don't get a very significant gain in an NA setup; however, I think in a Forced Induction setup such as mine, I would benefit more. Any more opinions?

 

I don't think I ever got a response about the VDI actuation. I saw that NZConvertible has said that backpressure did not influence the actuation; however, coldfire mentioned an "RPM switch" to activate the VDI. Is this a fairly simple device that can be adjusted on the fly (OK, let's try 4k, how about 4200?).

 

basically what is happening right now is that you have a solenoid that controls whether you get air pressure to the VDI actuator or not. at a certain RPM, (about 5200), the ECU sends a signal to the solenoid to open/redirect. basically with an RPM switch the signal to the solenoid no longer comes from the ECU, but the RPM switch. it is adjustable in something like 400 RPM increments or something.
check out summit racing, they have one that isn't too expensive.