I've been researching a bunch of dynojet scans of 5 speed VQs with Y-pipes, etc. Nearly all the VQs exhibit a noticable "bump" or valley in torque (slight bump in hp too) between 3100-3700 rpms. Some VQs loose as much as 12 ftlbs in this valley. The bump seems to be directly related to the y-pipe because it gets much worse once it's added. What is causing this? Is it something with the programming conflicting with the Y-pipe's superior flow? My dynojet operator said it looks like cross-over from the standard programming to the WOT programming. Most likely it is fuel related, either lean or rich (most likely lean). When my motor is cold and I accelerate at a nice consistent pace, I don't feel a loss in acceleration from 2000-4000 rpms, but when my motor's warm there is a noticable "flat spot" between 2000-4000 rpms. This leads to believe it's a fuel issue. When the motor is cold, it is open loop (ie pays no attention is the sensors, runs rich). When it's warm it's in closed loop.

What's everyone's take on this? I think the S-AFC could possibly restore some of this loss.


Dave

 

I would guess fuel issue. Or maybe something about the natural resonance of the exhaust system that is exaggerated with the y-pipe.

 

The people who designed the engine (intake, exhaust, etc.) know a lot more about what's going on inside the engine, and making power, than the floks selling after market parts. Most of these after market "performance parts" do the equivalent of drilling a hole in the side of a trumpet "to let more sound out".
The intake, cam timing and exhaust are all tuned to provide a particular torque curve. When the valves open and close they produce pressure waves that travel through the pipes they are connected to. These pressure waves effect the gas (molecular state, not necessarly fuel) flow. The waves are reflected whenever they reach a change in impedance.
The stock Y pipe has a change in area. When the pressure wave reaches this point a reflected pressure wave travels back up the exhaust, and (probably) keeps the fresh charge from flowing out of the exhaust port. (This effect is over a smallish frequency, e.g. RPM range) Change the Y pipe and you loose this. Part of the fresh charge goes out the exhaust port, and you loose power.
It's very hard to get any significant power increases out of OBDII cars with simple mods; including reprogramed ECU's.

 

Yeah, that's what I meant...

 

I think the bump is due to intake resonance.

The torque peaks I see in every 4th gen dyno (well, for normally aspirated cars anyway) are at 2200, 3400, and 4600 rpms...exactly 1200 rpms between each peak. I think this may be due to the difference harmonic frequencies developed in the intake manifold that develop air "ramming" at certain frequencies/rpms. Perhaps with the restrictive factory ypipe, the engine is not able to take full advantage of those resonance peaks due to the corked up exhaust, hence the torque curve is "neutered" to a flat profile. I think those major peaks are just signatures of the motor. Look at a modified 2000 dyno curve. No major peaks. Totally different intake manifold. Coincidence? Probably not.

I'm sure that the ECU programming is also intended to neuter the torque curves somewhat. Witness the torque curve on the aftermarket ECUs that I've seen. Between ~3800 and 5000, there's a quite a bit of torque added, peaking at ~4600. The peak gain looks to be ~15 ftlbs. I think Nissan wanted to brag about the VQ having a relatively flat torque curve from 1800-5000 rpm, so they corked up the ypipe and neutered the ECU programming to achieve that.

I think the fact that you feel no torque loss between 3500-4000 when at part-throttle but you do when at full throttle is simply due to the ECU ignition timing maps. Once again, Nissan probably wanted a flat curve so they programmed part-throttle accordingly. When we modify our Maximas to unleash the full potential of the motor, all the torque peaks that were previously hidden are allowed to reach maximum potential.

Whether or not the ECU is in open or closed-loop mode is due to the throttle position and has nothing to do with the rpm. I've ran my car in diagnostic mode before to see the control mode of the ECU by watching the O2 sensor fluctuations, and the only thing that put the ECU into open-loop was when I went past ~35% throttle. Rpm, etc had no effect. The factory service manual's explanation of closed- vs open-loop mode support this statement also.

Now the little bumps all throughout the torque curve (superficial ones) I think may well be due to fuel delivery issues. I honestly think the stock fuel rail setup contributes largely to this...and I believe is the reason why Nissan went to a dual regulator, dual feed/return setup on the more powerful 2000+ Maximas. Look at a modified 2000 dyno curve. No little peaks. Dual fuel regulator/fuel system setup. Coincidence? Probably not.

 

This Keven97SE is really a Nissan engineer in disguise masquerading on the boards and has blown his cover by putting out too much knowledge

Man is this thread deep!

This would explain why my mods have taken away the linearity of my car when it was stock. That's the only thing I miss about the car being stock, though.


DW

 

What is this S-AFC thingy??

DW

 

so i dont understand... what is being said? that the aftermarket y-pipe does more damage performance wise than with original y-pipe? i am thinking of changing it in the very near future and was just wondering about this...

 

No, the ypipe does add HP and torque...your car will be faster. The point that Dave brought up is that the stock engine delivers pretty much the same amount of torque throughout most of the rpm range...ie the car "pulls" as hard at 2500 rpm as it does at 5000, etc. The ypipe uncorks the engine's potential but not the same amount at every rpm. The engine get stronger in certain rpm "windows" like between 4000-5000 rpm, but there's far less increase in torque at other rpms. You end up getting a little bit of a roller coaster torque curve. It's nothing terrible or even that noticeable, but the purposely-built flat torque curve is less so with a modified/ypipe'd Maxima. It's not a bad thing, just different...I'd just be interested in knowing exactly why it's so.

 

Apparently, the Gen 4 VQ had inherent peaks and bumps in its torque curve . . . given an un-restricted intake and exhaust. Nissan enigneers used the exhaust and intake to tune out all those peaks/bumps. Result: Linear torque curve, which makes for the best driver to car interaction. But, that did reduce(by reducing the peaks) and increase (by flattening the bumps) the total capability of the engine, overall the engine is not at it's full potential when stock, but it is quite linear. The mods free that up, and also free up the peaks/bumps. Result: the car is more powerful with mods, but not as linear, and therefore, not as predictable to the driver.

DW

 

No clue how/if this plays a factor.
Factory Y-pipe is equal length.
Stillen's is equal length.
ALL other Y-pipes are unequal with the rearmost tube being shorter than the front one. I've always wondered how this affects output?
anyone?????????????

 

The factory ypipe is NOT equal length. The rear pipe, although it angles toward the front of the car in an effort that appears to make it equal length, still is significantly shorter than the front pipe by over 6". Plus, I recall seeing dyno plots from the old Stillen ypipe and I don't recall seeing any significant difference to the torque curve as compared to the Cattman units.

 

Daym, this is some good stuff and ranks about a 9.5 on my Wife's "who gives a crap" meter.

I like the intake/exhaust resonance idea. That does make a lot of sense. How many of you guys with Y-pipes get a "turbo-like" feel from 3000-4000? It is kinda like the motor comes alive at 3000 rpms and rushes to 4000+rpms, even at part throttle. I've felt this numerous times and many of my passengers have noticed it too. That's the "valley" and "bump" I was referring to.


Dave

 

I love this thread. I wish they all got this technical. What do you techies do for a living? /How did you guys get your knowledge?

 

Sorry to interject, but can you all please do me a favor. Please rate this thread (bottom right corner of this web page). It will make it much easier to find it in the future. Please also rate other threads as you read them, as you find appropriate. The thread rating feature is one of the most underappreciated additions to the new forums.

(I'll delete this post in a few days)

 

Guys....you might want to check with cheston about this. I'm not sure if its the same as open or closed loop but at 3200 or whatever that rpm is the car LEAVES economy mode and enters a more aggresive mode. I think this only happens at WOT. It may be interesting to dyno at say 3/4 throttle.

 

The ECU programming amounts to fuel and timing changes made to the maps on the ECU... This is not the same as "leaving economy mode" and "entering a more agressive mode". Rather, it's just that when we sat down at UPRD and discussed what would be more suitable for the car's overall driveability and reliability, the engineer preferred not to advance the timing below 3000RPMs to minimize the "lugging" that some drivers like to do when trying to accelerate the car under a heavy load in a high gear at low RPMs, which is a recipe for valve pinging. Also, keeping things untouched below 3000 RPMs helps pass emissions testing with less hassles, etc...

--Nabil

 

whoops....well...I really didn't mean the UPRD ecu.

If you find a bone stock vq 5sp chart....at around 3200 it goes up....then dips....then comes back up.



I think what is happening is the ecu is changing at that point. I brought up chestons name because he has lots of general knowledge about the ecu.

 

indeed. this is one the most informative threads that i have seen in a while.

 

Wow...this is a new level of technical discussion on this board! Im going to check my dyno plots right now!

 

Stock driveability does get affected towards more peaky. Variety is the spice of life though

 

Ok guys, How many of you agree with me that the car feels faster when the motor is cold?

I mean, when I get up in the morining and go grab something to eat after the car has been sitting all night, it is significantly faster than when the motor is hot!

I mean, it is very noticeable, epecially in second gear.

When the motor has been running for a while, the car seems to lag.

LIke Dave said, I think it is the fuel. It is running rich when the motor is cold, but leans out as the engine gets hot.

How can we fix this? will the S-AFC fix it?

 

some on mentioned the fuel rails as being different on the 4th gen vq and the 5th gen vq. does anyone know the rest of the differences that jumped the horsepower from 190 to 222? i know that when nissan switched from the vg to the ve found in 92-94 se's, they utilized variable timing which led to the dreaded vtc sprocket failure. i'm sure everyone on this board has wither read about or experienced this problem for themselves. did nissan utilize something similar in the 5th gen vq and do us 5th gen owners have a problem like vtc sprocket failure looming in our future?

 

Nissan did surprisingly not that much to the 5th gen motor to get that 222/227 HP. The power-adding changes are:

1- New variable resonance intake manifold. Composite resin manifold stays cool, internal valve opens at ~5100 rpm to increase the resonance frequency and boost high rpm torque. Similar manifold to the VE30DE, actually, just plastic.

2- New fuel rail/regulator setup. One feed/return/regulator per rail, each rail serving 3 cylinders.

3- Slightly different camshafts. I've never seen specifics, but there was a rumor awhile back that the cams are actually slightly SHORTER duration (should mean less HP) but slightly higher lift. If the rumored specs I saw were true, there's probably little power added from the new cams. Nissan probably just wanted to fine tune the cams a little for the new motor.

4- Variable capacity muffler. Probably doesn't do a whole lot but add marketing hype.

5- ECU reprogrammed. Obviously Nissan could bring out even more HP with a reprogrammed ECU. Whether or not this contributed to the added HP or not I have no idea.

 

well, hopefully those changes prove to be reliable. it also seems that there is a lot more potential performance gains to be had out of the vq. where do you get your information? i'd really like to learn more.

 

I've noticed also how the VQ's torque curve is somewhat flat from 1000-3000rpm, then it rises around 3000-3500. Like you said before it peaks again around 4400-4600rpm before dropping off. Aside from an intake manifold swap, some cams would really help the toppend breathing. Its too bad the VQ30DE was designed in the OBD-II era. Otherwise if it was made before one could just simply swap in the older cams. This practice for the 94+ SR20DE's net around 5-8fwhp from using the 91-92 SR20DE intake cam. If someone with a service manual could post the valve timing chart that is in the S.D.S at the end of the Engine Mechanical chapter. We can see how the cam timing compares to other nissan engines.

 

If someone can get me some specs on the VQ's intake manifold and resonator+tubing, I'll look it up in one of my engine building books at work and see what kinda RPM that thing is tuned for.
Helmholtz resonators are used in MANY cars (including mine) to reduce intake noise and to provide a smoother power band, as it "detunes" the intake charge slightly at certain rpms where the valve opening and closings cause a resonance inside the tube.
This very well might explain the 1200,2400,3600rpm spacing that Keven mentioned earlier.

I wish I could rememebr all the variables involved in the calculations, but I know it involves intake tube length, diameter, resonator distance from the engine/end of tube, and the volume of the resonator.. also requires a few specs from the engine (displacement, rpm, & others), but I forget it all.

if I can get to a scanner, I'll scan a few pages of tech-jargon for you guys to throw around..


The exhaust resonators are very much the same thing.. they're tuned for a certain diameter/size/length past the manifolds in order to reduce resonances at certain RPM ranges on the car. blah blah blah.. it's really neato stuff when you start looking at it.

 

Matt, thanks for the offer to look this up! While I can't provide the exact dimensions, I'm pretty familiar with it so my estimate is:

Runner area: 2.75 in2
Runner length: 16 in (from plenum to cylinder head)
Plenum volume: 125 in3 (3"x3.5"area x 12" length)

These are my best estimates, but they're probably only accurate to within 25%. Still, maybe your calcs will give us an idea if the resonance frequencies are at least in the right ballpark to possibly cause the 2200/3400/4600 rpm torque spikes.