I am totally frustrated with my car. It seems that I've reached my modification limit with my exhaust alone. Everything else seems to take power and torque away. I had a custom exhaust designed for my car that gave me GREAT gains. I then installed the Stillen intake and did not gain any power, but to my surprise, I lost a fair bit of torque. I took it off, dynoed again, and I was back where I was before. Yesterday, I finally installed my long awaited Crawford plenum. I took the car to the dyno today, and another shock ! Lost HP and torque in the low end, gained a bit in the top end, but not nearly as much as others have seen. I spent a fair bit of time on the phone with Doug from Carwford Z and he has a possible explanation. He feels that the exhaust is doing such a good job that any additional intake mods cause the car to run lean. The ECU then tries to compensate by retarding the timing etc. causing the loss of torque and HP. This sounds like a valid explanation but I'd like some input from the experts on the board as well. In both cases (Stillen and Crawford) I dynoed the car the very next day after the installation. Doug mentioned that sometimes if you drive the car for a week or so, the ECU may change to a different air/fuel map and I will realize the full potential of these mods. Another option, I suppose, is to get a custom Technosquare program made to accommodate these mods. Attached are the dyno graphs of by base car with my exhaust vs the Stillen and the Crawford plenum. Any advice or suggestions will be greatly appreciated.

Dyno of Exhaust vs Stillen intake:



Dyno of Exhaust vs Crawford Plenum:

 

Well first, I know dyno time is expensive, but I would have been curious to see a dyno of you car shortly before installing the plenum (since it appears your baseline dyno for comparison was done over a month ago). I know if it's the same dyno it should be consistent, but we also don't know if some unknown variable changed on your car in the last month.

But lets assume the dyno is correct. I know of at least one Z owner that lost power in the low rpms with a crawford plenum, but that seems to be the exception and not the rule. I don't buy the fact that the exhaust is working 'too good' and I don't understand why it would make the car run lean. For one thing I'm assuming you just have a catback - there are Z's out there with the plenum running much more 'open' exhausts (test pipes, headers, etc). and the plenum still added good power. Whatever increase in airflow an intake modification provides is instantaneously read by the mass airflow sensor and the ECU will add fuel accordingly, so I don't see why it would run lean in that scenario.

2004 Black 6MT

 

i have seen an exhaust make the car run too lean, mine with headers on a VQ35 engine. It is possible

Also was right at the 240 ft/lb torque mark. go invest in a 450 dollar Apexi SAFCII. then have the car dyno tuned with the plenum on. You should see a 5 hp or more increase just with the FMU

About the ECU taking a week to adjust, that is true as well

This ECU is freaking weird. Go read up on it.

Better Life thru Chemistry
Black on Black 03.5 Sedan 6MT - Custom Catback, Custom Intake Tube, HyperGround Kit

 

Yeah that's strange, I'd like to hear a technical explanation as to why mods like this make it run lean. This reminds me of the more primitive days of speed density fuel injection systems (like over a decade ago) where doing mods would lean it out, because fueling tables assumed a preset airflow (stock). Once cars converted to a mass air flow system this issue went away, and ECUs adapted well to mods that improve airflow through engine (either by opening intake or exhaust)... I'd be pretty disappointed if nissan can't get this right for whatever reason.

2004 Black 6MT

 

Its possible that you are running lean, i.e., not enough fuel. When you dyno, they don't give you Air/Fuel ratios? I always thought they did - then you could see for yourself what the effects were.

On Honda's, you can get SAFC's or VAFC's (air/fuel controllers). Can you get something like that for the G35 Coupe? That way, you can put your mods on, slap the sucker on the dyno and adjust your curves accordingly.

Just a suggestion.

2002 NBP RSX-S
Hondata, AEM V2, DC SS adapter w/ metal bushings

 

I'm not certain of the specific way the G35 PCM is programmed so I don't know if the following is a viable answer, but here goes:

Many OBDII systems have limited "learned" input at WOT ( open loop ), so anything which improves airflow too much can cause the engine to run leaner - at WOT. There is limited learning capability at WOT, most is in closed loop, so the PCM would not necessarily richen the WOT mixture in response to increased airflow.

Under this scenario, the easiest way to get the A:F back into line would be to go to larger injectors. The PCM would not know that the larger pills are in, it would follow the WOT map for the smaller injectors, resulting in a richer condition - which is what may be needed. I will again qualify this answer since the PCM may be programmed differently than those others I'm more familiar with....please correct me if I'm making the wrong assumptions.

But the dyno should have given you the actual A:F ratios, so you'd know if you were too lean. We need more info!

 

That is true that there is typically not much learning applied to WOT, however there doesn't need to be - the mass airflow sensor (MAF) is relied on by the ECU at WOT to determine fueling, and any change you make to increase the mass of air flowing through the engine will be detected immediately, and the ECU adds fuel accordingly. That has been my experience with a variety of late model cars even with OBD-II. That all works until you max something out (either the MAF flow range or the injectors), but for something simple like an exhaust or intake that isn't going to happen.

Adding injectors without any ECU changes may indeed richen it at WOT but will probably cause drivability issues at part-throttle and idle (where it does learn), as the O2 sensors are going to be reading way rich for the default fueling and the ECU will attempt to take out fuel until it can get the O2 readings it likes. If it has to take out too much and hits a limit, it will probably set a fault code.

2004 Black 6MT

 

Sometimes installing large injectors works fine, sometimes the PCM cannot compensate under open loop conditions. The last two OBDII vehicles I fitted with larger injectors were able to trim the A:F to decent levels without setting a code. But - the injectors were only slightly ( 10-15% ) larger, and because they were matched instead of bulk items they may have actually resulted in little total enrichment.

It is very typical for cars with bulk injectors to have one or two cylinders running richer than the others, so fitting matched injectors of the same nominal flow can actually increase engine power by leaning the A:F ratio that the O2 sensor sees. Thus larger injectors may not give that much richer of an A:F ratio under that circumstance.

[ You're right, speed-density systems are the ones I have the most experience tuning. ]

 

Thanks for all of you input guys! I have one other possible scenario that I would like to put on the table for discussion. Is it possible that the stock ECU program will allow just so much additional air volume and no more. Maybe I have reached that threshold with my exhaust. What kind of numbers are other G35s getting with air intakes and/or plemums and/or exhausts? Are there any G35s that have dynoed over 243HP and 237ftlbs of torque WITHOUT any ECU modifications? Maybe once you reach a certain level, the ECU just cuts back any additional mods unless you go for the Technosquare ECU upgrade.

 

That's just about what I was going to suggest. The ECU is definitely involved here. My sedan ECU would actually keep the throttle plate partailly closed (a deliberate detuning of the engine). Doug's characterizatio is right on IMO.

My $0.02:
Exhaust mods (especially headers) allow a lot more air out per unit time, thus the cylinder gets evacuated much quicker, the piston next in line comes down a little faster due to less resistance from the previous cylinder, effectively causing increased the piston travel speed (rpm) witht he same load on the engine. This sucks more air in a single piston cycle with the same amount of gas sprayed in, thus leaning the mixture. Normally (with optimal mixture) the timing of the ignition spark provides for a flame advance rate for a most optimum position of the piston passed TDC (Top Dead Center) at the time of the maximum flame-induced pressure (forgot the real term) in the cylinder. Now that you are leaner than normal, the mixture's flame advance rate (burning rate) has increased, and the timing is adjusted by the ECU to be cloer to the TDC (retarded) then before (when the piston is ont he way up) to prevent pre-detonation. This, however, prevents your piston from being in the optimum position at peak pressure point, thus reducing the torque with which the crank is turned = less power.

I am sure I mucked up some of the terms. So forgive me. This is why an ECU upgrade for guys like us (die hard modders, that is) is IMO necessary.

Gurgen

 

I could buy this, especially since the throttle is electronic and can be partially closed by the ECU if it so desires (how cruel!) BUT aren't there Z guys pushing almost 300whp on the stock ECU (i.e. they have cams, headers, more serious mods)? I would just hope the limit is a little higher, if such a limit exists.

2004 Black 6MT

 

If that were true, how come the Crawford plenum mod on my G35 sedan (after the ECU update that brought up the mixture from 10.5 stock to the 'normal' 12.5) my airflow mixture was at 14.1. The greater air distribution to the front cylinders and thus greater airflow through the tube needed (and thus the MAF) should have registered and the mixture should have stayed at 12.5, but it did not. That's a fact, saw it with my own eyes.

Gurgen