2013 Maxima. Installed Magnaflows and less power/better mileage than stock?
 

They sound descent but the results were the opposite of what I expected. I've lost about 30% power and my mileage has at least doubled, if not tripled. It's a huge difference.

My question is I have a buddy who is on Ram forums all the time and he swears I need to tune it to adjust the PCM to the new exhaust. He said it's breathing better and the computer is adjusting to that so the engine won't need to work as hard. If I do a tune it will be much better performance than stock.

This guy knows a ton about this stuff but I wanted to see if anyone has experience with this with Nissan?

I'm considering going back and putting the stock mufflers back on but I can't tell if flashing in some custom tuning would make a big difference?

I put a k&n filter, did the exhaust, and was thinking of a tune and that was it.

For purposes of discussion I asked him again and this was his (very technical) response:

"So this is going to be a long one. The pcm is calibrated from the factory to work optimally with a stock exhaust system that is quiet but restrictive. So the fuel trims, spark maps, injector pulsewidths, trans shift points(being a CVT it wouldn't be shift points but where it hold the engine rpms while accelerating), and a slew of other parameters are "tuned for a factory setup.

In your case, the restrictive but quiet exhaust creates back pressure which the pcm has accounted for and adjusts everything accordingly. You removed that restriction and now the pcm is trying to adjust for it but the stock tuning parameters only allow for so much adjustment for a certain parameter. That's just one aspect of tuning.

The other more important ones removes the slight delay in the throttle response built into the stock tune to make accelerating "smoother". Then you get into tuning for a higher octane so the engine makes more power"

Sorry, just not believable numbers.

Totally understandable. I'm not new to online forums but it's my 1st post here so I can see how it could look like a troll topic.

Can we just say that my power went down and mileage went up? I'm just hoping for answers obviously.

I did read a post on here that someone said it takes 2 weeks for the computer to adjust to the new settings. I suppose that might be like if you undo the battery to try and get around a check engine light for emissions? It needs so many miles to get proper reads.

Oh. I have pics actually. https://cimg4.ibsrv.net/gimg/maxima....71efe53917.jpg
https://cimg3.ibsrv.net/gimg/maxima....fd683de3f5.jpg
Drove 44 miles and gas barely moved.

I posted this in another thread a while back, wonder if you're situation is just bad velocity of the exhaust air:

Backpressure is bad for performance, period. It's not a lack of Backpressure that was causing your problem. it was a lack of the suction phenomenon called scavenging. This guy explains it quite well:

"Basically, when people claim that a certain amount of backpressure is beneficial, they're mixing up backpressure and scavenging. The latter is a phenomenon where the movement of gases through the exhaust system creates a partial vacuum that can actually suck the exhaust out of the cylinder. A properly-designed exhaust system maximizes this effect across a wide RPM range, efficiently evacuating spent exhaust gases using a precisely tuned and shaped exhaust system."


https://www.roadandtrack.com/new-car...or-horsepower/

This is the part that concerns me about his advice. He sounds like he knows what he's talking about, but the bottom line here, if the new exhaust system isn't scavenging air correctly, it is likely "creating backpressure", which is leaving exhaust your cylinder after the valve closes. This would lead me to believe tuning the car should be last, first determine if the exhaust is actually doing it's job or creating backpressure.

I probably should have mentioned this sooner. The tachometer seems to move a little easier. It gets up to 3,000 or more rpms easier. I feel like the Magnaflows are indeed less restrictive than stock.

However I accelerate less at 2,500+ rpms now then I did with stock. It's like the engine is creating less power at the same rpms. Exhaust being stuck in the cylinders makes sense. Although so does the computer trying to adjust.

I don't think modding intake would help? That would get air in quicker but doesn't help to get it out, which seems to be my problem.

I'm not sure if a tune would help or not but you're saying it probably won't.

I think my options are to go back to the stock exhaust as was designed by Nissan, or try to widen the exhaust somewhere else?

This is interesting. I know I'm posting a lot but hopefully someone else might be learning something. I know I am.

https://en.m.wikipedia.org/wiki/Scavenging_(automotive)

"Modifying the exhaust system, (i.e. modifying the exhaust gas velocity by changing exhaust tube diameters) can detract from the "ideal" scavenging effects, and reduce fuel efficiency and power if not properly planned out and executed.

To increase scavenging potential, the entire path from intake to exhaust to tailpipe must be tuned and in sync with itself. This will ensure that the air flow is never interrupted. The acceleration and deceleration of this exhaust gas is what will hinder the scavenging potential.

For example, fast flowing heads and a tunnel ram intake combined with a poorly planned camshaft and exhaust system will cause the air to "slow down" and "speed up" throughout its journey, thus reducing its scavenging potential. So, to increase scavenging potential, the air must maintain a positive linear acceleration curve."

So it could be scavenging or tuning, w/ I think more emphasis on scavenging. I'm going to stop posting for awhile unless someone posts something. Don't want to overstay my welcome. At least my posts are all in 1 topic.

Yeah, that's along the lines of what I was talking about...

^Sorry I didn't respond sooner. I was confirming what you said.

I reached out to 3 tuners. My thought is that a tune may adjust the timing of when the exhaust valves open/close to align with how the scavenging is working now. I figure if I get a good answer I can share it here.

1 tuner responded and said "go back to the stock mufflers. I wouldn't tune that car. Those types of cars are better stock".

The guy that tuned my friend's Ram said he doesn't tune Nissans or know anything about them.

The 3rd is in DE, came up when I googled Nissan tuners, and hasn't responded.

Did you reach out to Magnaflow? I would at least file a warrantee claim see if you can get your money back, but they may be able to tell you what you need. I'd almost venture to guess they'll recommend a different set of pipes getting from the front to the mufflers. I bet they have a pipe tuned for these cars...

You are a genius my friend. I'll try that.

Haa, thanks, genius I'm not. but being on the outside looking in gives me a perspective free of the frustration you're experiencing...

I talked to Magnaflow yesterday.

The guy I talked to seriously knows his stuff.

He mentioned it could be a bad O2 sensor but probably not since the car is not too old. He didn't think anything past the cat would affect scavenging. I asked about tuning and it's hard to say since every manufacturer is different. However he said tuning often affects Variable valve timing (which I think impacts when the valves open?), when the plugs spark, it can change fuel/air ratios to pump more in since it's getting out faster.

He said the easiest and 1st thing is to clear the codes and he also mentioned they see it a lot where it's psychological. You hear the louder exhaust and your mind tells you to expect more power. He made a joke about driving around with ear muffs on and see what I think.

What made you buy that and how much?

These cars have a really good engine. 290 hp/ 261 torque. Edit: Per the wiki they range from 228-300hp and 246-274 ft lbs torque. Get this: power levels up to 420hp or 1,000+ via turbo charging and proper tuning/breathing. Double overhead cams. I feel that there's a ton of potential there.

I picked up a Pedal Commander but haven't put it in yet (since I can't find anyone to do a tune). $500 for the Magnaflows and $330 for the PC.and I have a fun car to drive around for the next 8-10 years. Seems worth it imo.

I do think the cvt is holding back the engine though. I might do a transmission cooler and will definitely do synthetic. It prolongs the life of the car.

Y pipe! And there is a thread on here about a larger oem CVT cooler from Nissan for hot climates.