Better gas = more power, wtf?

I decided, more on a whim then anything else, to put premium gas in the tank tonight. I was rotating the gas in my gas can and had 1/4 tank left. So, it was a mix of premium with a good amount of regular. Probably came out more like mid grade gas with the mixing. That would be 89 octane around here.

For a minute or two it drove weird. Sorta, thug thug thug, I don't want to move. After it settled down, my but dyno said I had more power. Not just a tiny amount either, more like 10-15% more power. Floored it feels the same, or very close. At 35-75% throttle I can feel the difference. Under 35%, little to no change.

This makes no sense to me. This isn't a particularly high compression engine. With it's age and mileage it should have lost some of it's compression too. Changing the fuel octane should not have made much, if any, difference at what I thought was relatively low engine compression.

This has me kinda concerned. If my computer thinks the engine is knocking and is adjusting the timing to compensate that would account for the difference. The question then becomes. Why does it think that, or why is the engine knocking?

Ok, here's the variables I think may make a difference.

Engine: 2.7l 3rz-fe
Temperature: 48f
Elevation: 5480 ft, this is for the city of Littleton I'm about 200 ft higher.

Last spark plug replacement: Just under 3k miles ago. Direct OEM replacements, nothing fancy.

Full K&N air intake, the filter is fairly dirty ATM.

Last oil change: About 100 miles ago. Went from a 10w-30 to mobile 1 0w-30 advance fuel economy.

Seafoam applied through vacuum assist line about 2 weeks ago, about 2 hours after the oil change. This smoothed out the idle weirdness I had, gave me a tiny power increase. It was small enough, the power increase could have been in my head though.

Idle weirdness= Previously the idle would slowly drop to just under 500 rpm. Then shoot up to 1000. Then drift back down to under 500, lather rinse repeat. This is after the engine is warmed up. Now the idle is a nice steady 750.

Last o2 or maf sensor replacement, no idea.

So... Thoughts? Ideas? Something messed up with my engine I need to figure out?

 

Placebo effect.

 

It sounds like your pulling something out of nothing.

 

Uh, no. It's a big enough difference I'm sure it isn't in my head. That's why I'm concerned.

 

Might be real. my 3.4 gets detonation under certain circumstances when running 87. That hurts power. It goes away when 92 is used.

And yes, the knock sensor does pull timing. So if it was pulling a lot of timing and now it isn't you would definitely be able to tell the difference in power. Although knock sensors usually only have a set amount of how much they can affect timing, so if it can only sway timing like 2 degrees, that's probably not gonna be noticable. But if it's like 4-8 that's a big difference.

 

adding premium gas does nothing.
the only differences in regular-super-premium is the octane rating.
im suprised you arnt having spark knock.
the higher octane would cause an earlier detonation (ping)

 

Errmmm....Higher octane makes for earlier detonation?? How does that work??

 

Uh, octane is resistance to detonation? Prevents fuel from combusting too early in high compression vehicles. Mix up your words a bit?

OP, going over your stats again if i had to pin it down to anything I would say that within a two week period you seafoamed, and went to a lighter weight oil. I know whenever i oil change the honda it always feels a little more refreshed. Maybe all the little things added up to something bigger?

What brand of gas do you fill up with? I'm not religious about it but i do believe Chevron and Shell yield better quality gas than Arco or Valeros. Each to his own.

 

Hmm, I honestly don't remember for sure. I think it's a chevron station. Same place I usually get gas.

I guess it might have been a combination of other factors. I'll run the next tank regular and see if I get a power drop. If it feels like I did I'll then run a full tank of premium for a second comparison.

 

I wouldn't say I got more HP from switching gas's, but I did switch from using 91 in my 2001 automatic 3.4l V6 to 87 (2 full tankfuls) and my truck seemed to be idling and starting a lot rougher so I switched back to 91 and it smoothed everything out...

and on a post note: I just changed out my spark plugs (long over due) and fired it up and it seemed even cleaner and almost like it had a lil more pep...

...next is fuel filter

 

Ok, I think I got it figured out. Basically, I think my engine just has a lot of carbon deposits. When I ran the seafoam through it took me a minute to remember I needed to let it sit. So, I probably didn't clean a ton of what was in there out. Either that, or there are just a lot of deposits to remove.

http://zhome.com/ZCMnL/PICS/detonation/detonation.html

I think the type of problem I'm getting here is a non-knocking surface ignition. I had noticed that my engine ran rough at higher throttle percentages, and didn't seem to make much more power floored then it did at 80% throttle. With more air entering the cylinder I was increasing the likelihood of a knock, or abnormal combustion. Using the higher octane fuel I was able to apply more throttle without the power loss of the abnormal combustion.

I'm going to see if I can get a friend to help me seafoam her again tomorrow. That way I can get the engine shutoff a lot faster.

 

i switched to mid grade a long time ago, and at the very least, it seems to last longer.

 

Seafoamed the engine through the vacuum assist line again today. I got the engine shut off much more quickly this time. The first time it only smoked for about 5 minutes. This time it smoked for 43 minutes. Yes, I really am anal enough about my truck to have timed it. I was sitting in my driveway and holding the rpms at 2k.

I did this before work. Driving to work I didn't really notice a difference. On my test drive after work, my usual drive, didn't notice any power difference either. I did notice the engine didn't mind running at lower rpms though. Usually, if I'm moving, my engine got kinda rough under 1800 rpm. Ran it down to 1400 and no roughness.

I won't really be able to tell if this did anything in terms of fixing the abnormal combustion until my next tank of gas. I'll be putting regular back in, thereby removing the "band aid" of the higher octane gas.

 

Modern engines have things called knock sensors. When they detect knocking, the ECU retards your engine timing. That reduces power. Under those conditions, yes, you would have more power.

and higher octane means more knock resistance.

 

I should probably mention that, when I seafoamed, I found an exhaust leak. It's between the engine and the O2 sensors. I'm fairly confident that is what is giving me the rough running at higher throttle positions. Bad data from the front O2 sensor is giving me a somewhat off fuel/air mix.

It looks like it's just the seal between the manifold and the down pipe. Shouldn't be a big deal to fix.

After driving for a while, still on higher octane fuel but after seafoaming, I find I shift up more. Basically, all my shift points are about 5 mph lower then they used to be. Don't know if that means anything, other then I'm getting better at driving a stick maybe, but it is interesting.

 

Better grade of gas is a slower burn rate,,extended push on the piston on the downstroke,,versus cheap gas, that just pops at the top.

 

could also be more low rpm power. The engine in my old car had pretty low compression. Down low it had significantly less power than normal, so i almost always shifted at high rpms because it still had good power when you wrung it out.

 

Fixed the exhaust leak. Actually did that monday. Normal driving, no noticeable difference. Pushing the engine past 2500 rpm, I always do this on my test drive, much better. If the truck keeps performing at this level with the lower octane gas then I'm done with performance mods.

 

Switched back to regular octane gas, if there is any difference it's within the margin of error for my but dyno. Looks like cleaning out the carbon solved the problem.

 

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Without accurate instrumentation anyone can have an opinion...totally inaccurate.

You and I have read that it is a waste to use more octane than the manufacturer calls for. It's probably generally true. Knock sensors are a special case, self adjusting and all that.

I check gas mileage regularly by dividing gallons into mileage covered. It is a pretty consistent method. It doesn't much lie unless you occassionally fill into a "bubble" pocket in a gas tank.

I have had vehicles where high test makes zero difference, I have had an occassional surprising exception.

My motorcyle maker says to use 92 octane. But 95 or higher gives more miles per gallon on my Kawasaki, and zero ping/knock under hard acceleration. Proven fact to me.

Start using ethanol (which has a lower energy density) and guess what ? mileage drops 2 mpg.

So there are unique cases. If gas mileage does not INCREASE over baseline, I wouldn't see much benefit in spending more money unless you are trying to eliminate "ping".

Also don't forget: As manufactured, compression ratio is not quite what the specs say. Because carbon accumulates gradually in the cylinder head and on the top of the piston, it becomes accurate the first year, and without cleaning, goes a little higher as you continue to drive year after year.

In theory higher compression should increase efficiency, but don't bet on being able to measure it.