Question for the S/C owners...

Have any of you tried using the regular 87 octane gas on a normal basis and noticed a difference?

I've been giving my engine the 91-93 octane gas since I've had it, and it's always felt as if my engine gave off a slight sluggish feel. But I started using the cheap, regular lower octane (87) fuel for my last two tanks, and have noticed an immediate 25% increase in mileage(even after a relentless lead foot, due to the fact that I experienced a noticable increase in power).

I have my reasons as to why I'm getting better mileage, and more power, but I'm curious first to know if anybody has considered what I'm saying or if I'm the first.

It may be just you... or it could be just me, I used midgrade and had some knock when accelerating. I dont dare try regular. Again it could just be me.

"Ye canna change the laws of physics"

I see a new piston or set of valves (if you are lucky) in your future.

If you want to test for real power, put it on a dyno. Otherwise this seat of the pants stuff is in your head and what you want to feel.

You have a supercharger. It's a forced induction engine. Despite the stupid sticker Nissan puts on the gas cap door, use Premium or you risk damaging your engine. Esp. in the summer when the heat is up

Check your timing.. It could be WAY late...

Dont use the low octane gas. It'll knock, hesitate and cause damage in the long run. It's not worth trying it. Sorry, buying high octane fuel comes with the ownership of a SC Xterra. I know, it sucks!

Don't like it? Sell it or buy a pedal bike!

Put in some racing fuel (103 octane or higher), then tell me if it's still sluggish.

I'll let you be the lab rat for saying that 87 octane gives you better gas mileage. I'd rather put bleach in my oil...

Back in my racing days, a wise man once told me this...."Your car will always run the fastest it ever has right before the engine blows up" I and most of the racing community have our reasons for that as well...

I'll second that saying..
My grand national ran unbelievably well, later than day while pulling in my driveway I threw a connecting rod.. oops..

The supercharger doesn't engange until WOT, so until then, technically the engine acts as a naturally aspirated engine (with the same compression ratio as the NA engine), with a supercharger to finely tune the amount of air entering the engine.

I've watched my boost gauge, not even surpase 10 vac when I've accelerated. My theory is that the only time it would be best to have premium gas is during WOT. Other than that, why not use gas that is simply easier to combust. Our engines have ECU's to deal with the variables of heat, air, and fuel. By using the 87 octane gas, I'm having the engine have to send in less gas per cylinder, which means less air. For the love of god, our engines were created in the 80's, just because you slap on a supercharger that only works during WOT, doesn't mean you need gas constatly prepared for WOT.

My blower comes on strong at 3000rpms. Maybe if you keep it under that you'll get away w/reg. gas, but I think you'll just destroy the engine in the end.
Course, I've always been the type to keep my timing advanced to the ragged edge. But the cost of ethal is climbing, ain't it.

If by WOT, you mean Wide Open Throttle, that is not how it works according to the manual.
From the Description & Operation section of the Supercharger:
"SYSTEM DESCRIPTION
This system controls supercharged air according to the engine operating conditions.

This control operation is accomplished through the ECM and the Supercharger Bypass(SCB) valve control solenoid valve.

When the solenoid valve is ON, the SCB valve shuts. Then intake air is lead to supercharger, and supercharged air is sent to the cylinder.

When the solenoid valve is OFF, the SCB valve opens. Then intake air is sent to the cylinder directly.

When the ECM detects any of the following conditions, current does not flow through the solenoid valve:

Engine stopped
Engine starting
High- load, high-speed engine operation
Excessively low engine coolant temperature
Mass air flow sensor malfunction
Engine coolant temperature sensor malfunction
Throttle position sensor malfunction"

Mav63, Does that Manual say anything about adjusting the Throttle Position Sensor?

I have a BR-3 diagnostic tool that shows the Idle setting of the TPS at 0.0%. With it disconnected it reads 7.1%, this is what I assume it should read, but the sensor trips out really bad if I try to adjust it.

Thanks!

Conundrum, this is without a diagnostic tool other than a voltmeter checking DC Voltage at the ECM connector:

1.Start engine and warm it up to normal operating temperature.
2.Stop engine (ignition switch OFF).
3.Turn ignition switch ON.
4.Check voltage between ECM terminal 23 (Throttle position sensor signal) and ground. Voltage measurement must be made with throttle position sensor installed in vehicle.
Throttle valve condition
Completely closed .15- .85 V (a)
Partial open between (a) and (b)
Completely open 3.5- 4.7 V (b)

Thanks Mav, I'll check it out tonight when I get home.

I used premium fuel the first few months I owned my 2002 SC Xterra. I tried 87 octane and it ran exactly the same, no pinging whatsoever. Now I use 87 all the time. The engine is tuned extremely rich at WOT ( I checked it) and as long as the engine doensn't ping or detonate then I see no reason to use premium fuel.

yay, somebody that knows what they are talking about! (when it comes specifically to this engine)

How do you know it's not pinging?

My owners manual (2005 S) said I was supposed to use 87 octane. Perhaps your S/C has similar needs.

By-the-way, If you happen to have a problem with your S/C, the first thing they are going to ask you is "Are you using Premium?". At about 15,000 miles they replaced my spark plugs, then O2 sensors, then Knock Sensor, then I made them put the original heat range plugs back in when they said they now were going to replace the S/C alltogether.

Since then it has not exhibited the problems that I was experiencing, but I definately won't do anything to lose my warranty. The S/C alone was about $1,500 without the labor.

I just recently had to have the Power Steering Gear box replaced too, due to binding while turning. Oh, ya, for another 1500! I'd say keeping the warranty intact is a good idea!

Normally when an engine is being fed an insufficient amount of octane you can here it ping while under load. I have not heard any pinging and I've been driving the vehicle for 3 years now. When I am towing a heavy load I try to use higher octane fuel just as a safety measure. I use the 87 octane for just driving around town and long highway trips.

I don't understand what octane rating has to do with supercharger problems. The supercharger has nothing to do with the type of fuel you choose. It's just a large air pump. I've used synthetic oil in my engine since I bought it new. I truly believe synthetic oil is a very good idea for forced induction engine since they are more susceptible to wear due to the higher heat and stress on the S/C and engine components.

I think you answered your own question. If you ping when you use 87 like most of us that have a SC do, then the supercharger will compound the issue by adding more air to an already lean mixture. I think we will have to agree to disagree, while it may be feasable I dont think I will be running 87, i will stick to what nissan recommends. While i am no mechanic to come up with a mechanical reason behind why when I went to 87 it pinged, all I know is that when I went to premium, it stopped pinging. It could be that your timing is retarded so much the engine can use 87, but you arnt getting full performance out of the engine. Who knows, but if it works for you go for it. My only concern is that there is a reason why premium is "recommened", Id hate to see some people with SC start using 87 and have issues.

I don't know what you mean by answering my own question because I don't remember asking a question. What do you mean by adding more air to an already lean mixture? Who said the mixture is lean? I know for a fact the mixture is richer than 10:1 (pegged my meter) during boost conditions.

I can't answer why my engine doesn't knock and a few other people have knock when using a lower octaone fuel. Could be different timing, could be poor fuel quality. From what I've been reading there aren't enough people trying lower octane fuel to get a real poll on the topic. Do any of the "pinging" SC Xterra have other mods like intake, exhaust, SC pulley, etc?

IMO, manufacturers make sure to cover their ass whenever they can. Premium fuel is often recommended in applications where it is not always needed. Vehicles from the manufacturer are overdesigned to accomodate for issues such as "a bad tank of gas", etc. The SC Xterra is not being pused to the limit by any means. The supercharger is well within it's efficiency range, the engine is tuned extremely rich, and in all honesty is pushing a modest amount of HP for a SC V6.

People shouldn't be worried about trying a tank of 87 octane to see if they hear any pinging, expectially if they do a lot of highway driving. If it pings then use higher octane (and make sure your timing is correct), but one tank of gas is not going to self destruct your motor.

The SC is increasing compression, and with increased compression you are seeing detonation aka pinging. Diesels use high compression to cause the fuel to detonate as soon as it is injected, you are injecting fuel with the intake stroke, and using the spark plugs to light it. The pinging is the fuel lighting up before the spark fires, as in before it is suposto. The fuel is burning & causing the pressure in the cylinder to climb before it is at top dead center (tdc). This makes the crankshaft push the piston up into high pressure expanding gasses.. If it pings to earily, and at a high enough throttle, you can bend the connecting rods between the piston & crank.. then you get a new engine, as the crank is usually damaged, the piston moves farther down then it is designed to, and the bent rod can / will contact the inside of the bore, putting shavings in the oil, and dropping them on everthing, destroying the oilpump, which will cause you to loose all oil pressure in a few seconds, let alone the giant dent in the side of the bore itself.

Agreed, but this is very highly unlikely on a stock SC Xterra. Expecially since I have tried 87 octane with no audible detonation. What you are describing is what would happen to a modified engine (exhaust, intake, pulley, etc.)without the proper reliability mods (MAF sensor, pump, ECU programming, etc..)

I was referring to this quote when I said I don't understand what fuel octane has to do with a malfuntioning supercharger.

Sorry for dredging up an old thread, was searching for something else and apparently I lost track of this the first time...

Your basing this on testing (please site source or methodology) or just pulling this statement out of your ass because it sounds good?

Your engine is just as likely to be damaged from inaudible pre-detonation as it is by audible pre-detonation. Just because you can't hear it doesn't mean it's not going on.

When functioning, your knock sensor can hear it - but I wouldn't want to rely solely on that since there isn't an easy way to tell when your knock sensor is functioning properly either.

It's there as a fail safe. Those are Nissan's words, not mine.

To me fail safe does not equal everyday normal operation.

Um, it's simple physics. When you compress air, it heats up.

Period. Boyle\'s Law .

A supercharger under boost increases the temperature of the air/gas mixture. If the octane of the gas is low enough and the air temperature present is high enough, the gas spontaneously combusts (pre-detonates, knocks, whatever you want to call it). As was pointed out earlier in this thread, that is the principle that diesel's work off of.

If it pre-detonation occurs at the wrong time (say when the piston is still traveling up to compress the air/fuel mixture before it's ignited by the spark plug) damage can occur.

This has nothing to do with the ECU, MAF, etc. Those devices can not change the combustion properties (octane) of the fuel - only your choice of fuel's octane content can affect that.

So you may get lucky and be lucky for a while, but all it takes is one bad pre-detonation and you just blew a ring, blew a seal, blew a valve, blew a valve sleeve or blew a hole in the top of your piston - that explosion has to go somewhere. When things are working properly, the explosion pushes the piston down and it's converted to work - but if the piston is coming up, something has to give. Usually it's something expensive.

That's also why you have a harder time with pre-detonation in the summer - the ambient air temperature is already higher, so the compressed air/fuel mixture is also pushed higher by your forced induction and you are again pushed to that threshold of temperature of the mix vs. the detonation point of the fuel being used. If they cross, the fuel ignites. That's why cars with higher boost superchargers or turbo's have intercoolers - to cool the compressed air/fuel air, because even with higher octane there is a point of diminishing returns. Eventually the only way to solve the problem is by controlling temperature.

Again, you can not change the laws of physics (at least not easily enough to where it would be practical outside of a lab and for durations longer than fractions of microseconds).

I highly recommend you get a copy of Maximum Boost by Corky Bell . If you are going to be fooling with things you don't understand, you might want to get educated before handing out bad advice to people.