S5 NA Dyno (VDI vs non working VDI)
 

I took my car into Lucky7 Racing in Duarte, CA yesterday to get some things checked out with my FC. (I have another thread about it here) I just wanted to share my dyno pulls from a non functioning VDI (hard lines clogged) with a functioning VDI.

Only modifications done to the car are an Apexi cone filter, Racing beat downpipe and presilencer, and I think Atkins 6 port sleeves (PO gave me paperwork that he purchased these, but I have not checked if they're actually in the car). Otherwise stock header and exhaust.

The car seems to be running rich above 5K RPM. Is this normal for FCs? I know Automatic RX8's get stupid pig rich in the upper RPMS.

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yes, that is about right for stock ecu. thats the advantage of tuning the fuel curve with an aftermarket ecu.

Man..if that's about normal I wonder why the graph looks all funky past 7K and I'm getting weird hesitation.

Misfire most likely. Your AFR gets a little weird in the same area.

weird that you failed to mention that it was vdi after all in the other thread.

We only told you 90 times.

I mentioned it in my last two posts in regards to me taking the car to Lucky 7. I never checked it before because I was able to move the solenoid by blowing in the vacuum line leading to the VDI actuator.

My stock port race car with VDI wired open = 182 RWHP

With VDI wired closed 138 RWHP

I wouldn't of known that it was that big of a game.

Holy.

Wired closed as in, never opening?

Game = gain. I need to drink less.

I wouldn't of thought the VDI was THAT big of a deal.

I believe it. I can definately feel it when it kicks on in my S5 vert. I wish i could tweak it to come on just a little bit sooner (same with the 5th and 6th ports).

Yes wired closed as in does not open.

I was also surprised. I would have guessed maybe 10-15 hp.

Couldn't you with an Rtek 2.0?

Yeah, you could. I don't have one though.

You sure can, but to most people it isn't worth the price tag to get something like that for an N/A car. Only reason I did was for AutoX.

On my race car I use an electric solenoid to activate the VDI controlled by the MS3Pro ECU. It is set up spring loaded open so that if it has a failure it will fail in the open position. I have it programmed to be open below 2000 rpm so it doesn't heat up the solenoid if it is just sitting idling, it closes at 2000 and opens back up at 4500. When we were dyno testing we were starting pulls at about 3000, I tried it with the VDI disabled so it was always open and it was hard to see if there was any difference in the low end between 3000 and 4500, of course top end power was the same. I am still not sure if it is really any benefit for a race car which hardly ever sees rpm below 5000, but my reason to spend a lot of time to make it work was hoping that maybe it would help the throttle pickup when coming out of corners. If I ever see any problems with the system I will just wire it open and disable it in software.

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I was looking back at my dynos and wondering what could be causing the funky AFR's above 7k.

The car still runs great, but it does feel like I hit a fuel cut above 7K, although the car will keep on revving to redline.

I was planning on buying a S-AFC to try to tune it out, but I also want to figure out what the problem is before slapping a band aid on it.

My roommate decided to try to pinch the fuel return line (to increase fuel pressure) and it seemed to make it worse. Would it be possible that my secondary fuel rail/FPR is failing and consistently giving higher than normal fuel pressures? I have changed the fuel filter (oem) and fuel pump (Denso) since I purchased the car as well. I've also changed spark plugs and wires right when the car came into my hands.

How many miles since changing the spark plugs? The trailing plugs seem to last a good while but the leading plugs seem to get eaten away really fast on the stock ignition (fun fact, FD guys upgrade to FC ignition coils).

From my personal experience and other threads i've read, 15k miles is about max you will ever get out of the leading coils...add an MSD box, even less.

About 5k miles, and the car has had the high RPM fuel cut/bogging since I purchased the car, so I don't think it could be the plugs.

I was gonna test the MAF tonight, see if maybe it may be out of spec while fully open.

I cleaned the MAF/AFM by spraying a bunch of MAF cleaner into it while moving the plunger up and down. Testing with a voltmeter, it seemed that it was also within spec.

After going for a test drive, I noticed that the fuel cut/hesitation moved up the RPM range and only happens above 7500rpm now. Could it be that the MAF is somewhat faulty and my cheapo craftsman multimeter is junk?

Your craftsman multimeter isnt junk, its digital....well yeah its junk, but for the sake of this thread, even a Fluke $300 multimeter is no good. You need an old school analog multimeter to test it reliably. If you use a digital multimeter, you have to take it out of autorange and go really, painfully, slow. It's still not accurate, but can be done.

As far as hesitation above 7500...the redline is 8000, I think your doing pretty good with a 25+ year old car that is stock.

7500 is where your peak power is at on a stock setup, but you could have other airflow restrictions going on, like aftermarket non straight through mufflers or choppy welds in the exhaust pinching off airflow and choking the engine at high revs. a semi clogged cat will also exhibit those symptoms.

This does make sense. I have the racing beat pre-silencer and downpipe combo but still using a stock and pretty rusty exhaust.

This may not be helpful but my old motor did almost the same thing. Pulled good till about 7500 then it was like I hit a wall. Would rev to redline but the power was gone. Had a cone filter, RB header and pre-silencer and an old HKS sport cat back. During the rebuild I streetported the intake and exhaust. Pulls past redline now. Only changes were the streetport, lightweight flywheel and I did have my injectors cleaned while they were out. I've read on here the stock ports don't flow well that high and doesn't make power above 7500. Based on my limited experience with my motor I believe it. Long story short the stock port just may not make any power above 7500rpm.