I'm having a serious issue.
 

Hi everyone! I'll get right down to the point. I bought my 350z from Atlanta, GA. I was there for about a year before I recently moved up to Iowa. Now down in Atlanta, I pumped premium 93 octane. Once I hit the Midwest, I noticed that the gas stations only offered 91 octane. At first, I didn't think much of it, but as the months have passed, I've noticed my car is making weird sounds as I accelerate. My RPM will suddenly jump a little bit and then drop back to normal starting at about 2k-3k RPM. I'm ASSUMING it's due to the 91 octane and my fuel/air ratio. Not to mention, I'm 890 miles north of Atlanta. So I think the attitude might be effecting it too...
Give me your thoughts and comments.
Oh, and I've already tried octane booster. So that's a no go.

Well if you tried octane boost and it didnt cure the problem why would you assume its the 91 pump gas? Doubt that is the issue - but you havent provided enough info to diagnose further.

Iowa min/max elevation ~490/1600 ft.
Not exactly Denver.
Lotsa stations here with 93 advertised (SheLL),
if they're accurate.

Air/fuel is automatically compensated for (altitude), as is
anti knock timing adjustment

Code reader might spot a sensor
issue

RPMs with fan on and fan off.

Well there are no Shell gas stations in DUBUQUE, Iowa. Sorry for not specifying. And the gas grade is the only thing I can come up with. That and the elevation would be the only difference. On my way to Iowa, I pumped gas at Tennessee and it was 93. Just fine. Then again in Illinois, and it was 91. That was when I immediately noticed the problem in sound and performance. I'm not so much worried about the performance as I am about my pistons. I did a bit of research, and learned that if could "pinking" my piston crowns. Just a diagnostic alone at the shop will cost $100. I'm just hoping to get an answer here and save a little bit.

You should read up on why different parts of the USA offer varying grades of fuel...

I doubt the issue is caused by 91 octane fuel...

probably save a whole lot by taking it in and getting a diagnosis.
some shops as low as $25.
AutoZone, O'Reilly, Advance parts store
may be free hookup and they can do it for you.
Hell some even help you install
stuff in the parking lot.

The more months/miles/damage accumulates the more expensive it could be.
months and miles have already gone by and it could be lotsa stuff causing this.

It could be unrelated to fuel completely.

Is it a manual transmission? How many miles on that clutch?

Does your "My RPM will suddenly jump a little bit and then drop back to normal starting at about 2k-3k RPM" event occur more easily at 5th/6th gears than at 1st/2nd? If so, then your clutch disk is just worn.

^^^^^^^ Time for a new clutch

cold slippery streets and summer tires would explain same.
what about the pinking pistons?

Its not the fuel…I'm in colorado, I've taken my z up to 10k feet ASL (leadville, over the summer) running 12psi pushing just as hard as I've always have no issues…My normal elevation is 5400' and I use 91 octane and no issues…

I think this is the very first sign of your clutch slipping…could be a sour tank of gas, but I'm thinking its the clutch plate is worn.

Hmm... I didn't really think of my clutch plate. To be honest, I didn't think that'd have any effect if I wasn't actually clutching. This is happening while I'm already in gear accelerating. And yes, It's happening more in 5th and 6th. Problem starts in 3rd though. 1st and 2nd are still fine.

Beast, your car’s engine is normally connected solidly to the rear wheels. If the engine RPM increases, there must also be a correspondingly increase in the road speed. The only weak link in this chain is the clutch, if as others have pointed out, the clutch is slipping it could account for what you are experiencing.

Your owner’s manual only calls for 91 octane fuel, so you’re fine with that. The effect of altitude on your car’s need for high octane gas is the opposite of what you are assuming. At high altitude the air is thinner by volume, so each cylinder will reach a lower mean effective pressure than at sea level. This means the engine will make less power and will need less anti knock value in its fuel (lower octane). For example regular gas is 87 octane all over the USA except in Colorado and the other mountain states, where it is 85 octane.

Thanks Jennifer and everyone else who commented! I will definitely have my clutch looked at. I just wish a clutch kit wasn't so damn expensive.... Not to mention installation...
-__-"

dont freak out…if I were you, considering this is the very very early stages of your clutch losing grip(if this is indeed what it is) it will get progressively worse, you slip a hundred RPMs now, next week you'll slip a few hundred RPMs, then a few weeks a thousand RPMs…this will be the progression…and logically it will slip more when you hammer the throttle versus feathering the throttle…

so if the RPMs are raising but your speed is staying the same then it's for sure your clutch…give it a few weeks and see if the problem gets worse…someone correct me if I'm wrong, but the damage is done, you can't undone the clutch wear and driving with (short term approximately 300miles) clutch that slips wont ruin anything but the clutch…

I toasted my 2nd clutch back in 2008 and drove from Albuquerque NM to Durango Colorado and back (200miles)…I essentially had to drive the car at no more than 50% throttle…

look into the Jim Wolf Technology clutch & flywheel combo…good stuff…

please keep us posted on this...

I talked to several different mechanics today, and they all agree that it's my clutch. Unfortunately, I'm probably already passed the early stages. I noticed this issue back in October, but I honestly thought it was because of how cold it is here in Iowa, vs where I was before in Atlanta. My quotes ranged from 700-2000 depending on the brand. I'm looking to get a new clutch and fly wheel. If I could stay under 1.5k, that'd be great. It's so damn expensive...

In these situations I always suggest wearing a very low cut top. Oh. Wait. Never mind.

OP my clutch recently went out as well and I ordered my parts through THMotorsports, exedy replacement clutch and a basic lightweight flywheel and did the swap myself on a saturday using one of the DIY guides on here and all in all cost me about 700 bucks. BTW im not exactly a master mechanic and I didnt really have any trouble with it. Ive got about 2000 miles on the clutch and flywheel so far and all is well.

Thanks Austin! I'll take a look at that!

I know how it is OP. I have to drive 20 miles from home just to get 93. I live in Iowa too, but northwest side.