Effects of high octane fuel in 115 Pro XS CT 4 Stroke?

[Emilime75]

I've read the Fuel and Additives thread created by Don, and that SUBSTANTIALLY higher octane fuels in Merc 4 strokes leads to poor performance, higher fuel consumption and the possibility of carbon build up.

With that, I may be experiencing the poor performance after filling up at my marina. Too early to tell on the fuel consumption and carbon build up.

Boat is a 22' pontoon with lifting strakes and under skinning. Engine is a 2017/18 115 Pro XS CT SN#2B501861. Story goes like this -

Bought boat about 2 months ago with about 1/3rd tank full of unknown fuel. Changed oil/filter, lower gear oil, spark plugs and in line fuel filter. Filled it up rest of the way with local gas station 89 octane E10(didn't know any better at the time).

Boat started and ran fine for the first 3-5 outings. With a light load and smooth water, we were able to reach about 5800 RPM and about 26 MPH. Not too terrible for a pontoon with a 115, but thought I could play with props later to try and better that.

Next few outings we loaded the boat up some more. 2 anchors, life jackets, Lilly pad, towables, coolers... Still got to about 5800, but lost 2-3 MPH, kind of to be expected and still no real signs of issues.

Eventually we ran low on fuel and had to refill. Decided to do so at our marina. It took 26 gallons in a 29 gallon tank. The very next outing and no other changes in load and previous water conditions, we maxed out at 5400 RPM and a whopping 15 MPH. Also, on this outing, the very first time we went to start it it fired up like normal, but died a second or 2 later. Restarted and this time was fine. Several other restarts throughout the day were also fine.

So, is this the kind of performance loss one could expect from higher octane fuel? I asked the marina what was in their tanks, and was told it is ethanol free, 91 octane with marine additives that bring it to 93 or above - not to sure on the accuracy of that. Thought maybe I picked up some sediment from the bottom of the tank and clogged the new fuel filter, even though there were still 3 gallons in there, have another filter ordered.

RPM and MPH conformed via VesselView Mobile, no codes.

[EuropeanAM]

Definitely suspect the fuel is playing a big role here (some other factors may be atmospheric conditions and how clean the logs are).

[berudd]

Regarding your question on the 91 octane fuel, no the higher octane is it causing any problems other than it cost you more money tha 89. While an engine tuned to run on 89 will see no benefit from 91, it will see no detriment either. Some guys will swear it does. They are wrong. I’ve watched a test with automotive engines and they ran 87, 91 and 112 and the engine made the same power, and this it key, they did it with the same total timing. “Conventional wisdom” says higher octane gas burns slower and that’s the problem. But if that were true, the above test would have need to advance the timing to account for the slower burn to see peak power. That was not the case. Maybe their would be an issue if you ran something waaaay out of the ordinary like the 150 they ran in fighter planes in WW2 but you’re not so don’t worry.

The only thing hurt by running 91 octane in an engine tuned for 89 is your wallet.

[TritonTRXV8]

Never really bought into it before myself but after seeing it first hand it does have effects. I think all the newer 4 strokes other than upper hp and performance engines are rated for 87 not 89 octane. My friend has a 90/65 jet its a 2.1l so the current engine and he kept having this horrible misfire at random times and took it to the shop couldnt figure it out coil packs changed all that ran all over hooked to the laptop couldnt find anything. Got to where it would run poorly every trip. Finally he said idk what the deal is i spend the money on 91 octane fuel and run all merc products and i said well lets switch to 87 octane from what i read on here so we pumped the tank out and filled with 87. Never had an issue again that was 2 years ago. So ill have to be on the side of it does make a difference.

[Emilime75]

Definitely suspect the fuel is playing a big role here (some other factors may be atmospheric conditions and how clean the logs are).

How would one go about logging, what's needed and is it obtainable by the public? I'm accustomed to logging cars, do it regularly with one of them, but don't know what's involved with a marine outboard. Currently using a VesselView Mobile for engine monitoring.

[fishon!sc]

How would one go about logging, what's needed and is it obtainable by the public? I'm accustomed to logging cars, do it regularly with one of them, but don't know what's involved with a marine outboard. Currently using a VesselView Mobile for engine monitoring.

When he is talking about logs. He means your pontoons are dirty. Sounds like water in your fuel. It is easy to check. Pour some in a glass jar. You will be able to see if there is water in it.

[EuropeanAM]

Regarding your question on the 91 octane fuel, no the higher octane is it causing any problems other than it cost you more money tha 89. While an engine tuned to run on 89 will see no benefit from 91, it will see no detriment either. Some guys will swear it does. They are wrong. I’ve watched a test with automotive engines and they ran 87, 91 and 112 and the engine made the same power, and this it key, they did it with the same total timing. “Conventional wisdom” says higher octane gas burns slower and that’s the problem. But if that were true, the above test would have need to advance the timing to account for the slower burn to see peak power. That was not the case. Maybe their would be an issue if you ran something waaaay out of the ordinary like the 150 they ran in fighter planes in WW2 but you’re not so don’t worry.

The only thing hurt by running 91 octane in an engine tuned for 89 is your wallet.

Bruce- can't compare Speed-Density fuel injection systems to a Mass Airflow system in an automobile.

And... it's also not an apple-to-apple comparison, as these engines are under full-load from the moment they are placed in gear (unlike auto engines).

Emilime75: "Logs" would be the pontoon hulls of your boat. Even a little bit of growth on them can cost several MPH (back when I kept a Tri-Toon in the water all summer, I had to send the kids under the boat with scrub brushes every 2 weeks). Saw as much as 10 MPH loss in top speed JUST from "pond scum".

[Emilime75]

Got it, misunderstood and thought we were able to log engine parameters like IATs, fuel injector duty cycles, timing... We do have some growth on the logs, as we do keep it in a wet slip, it's not terrible yet, but there is some as it's been in the slip for about 2 months now. I'll scrub it off and see what happens.

However, I still don't think this explains the drastic change over a 2 day period. We're talking roughly a 4-500 RPM and 7-8 MPH loss, plus the fact that it started and died that one time after the fill up.

[EuropeanAM]

Agree that it's not the ONLY problem, but it is likely a significant contributor to the loss of performance.

[berudd]

Bruce- can't compare Speed-Density fuel injection systems to a Mass Airflow system in an automobile.

And... it's also not an apple-to-apple comparison, as these engines are under full-load from the moment they are placed in gear (unlike auto engines).

Don,

That's interesting. Do you have any info you can share on how a SD system would be more adversely affected than a MAF system. That would be interesting to see. True that the LS3 comes with MAF but its not too uncommon for racers and tuners to switch to SD do to simplicity and reduced air restriction and high RPM that can come with MAF.

As far as the test I was referring to, it was on a dyno so that was WOT and full load through all of the test. If anyone is curious who ran these tests, its not some random shade tree mechanic. It from a show called Engine Masters on MotorTrend OnDemand. Look it up is you want to see who is on the show but combined these guys probably have 100+ years of experience with this stuff and one of them has decades as an engine tuner and dyno guy. I only mentioned that point out these guys know what they are talking about. I went back and watched the episode (S6 E81) and they ran the engine on 87, 91, 112, 116 and even E85. They did a timing sweep in all test and all made peek power at 29* of total timing, even E85. That is what tends to kill the myth of higher octane fuels burning slower. If they really did, you would need more ignition advance to start combustion sooner to achieve max power. Higher octane gas is more resistant to detonation so one could say that lower octane fuel will detonate sooner, or maybe faster if you will, and that's where the myth comes from. The first 4 fuels all made within 1 or 2 numbers of the same power and torque and peek HP was at 6400 rpm in all 4 tests. That variance is more likely just differences in runs and not indicative of any real change in engine performance. Changes in water and oil temp during the pull can result in more change than was seen here.

So, the engine in this test runs fine with fuel that is 25 octane points higher than the 91 one is it recommended for but the 115 ProXS won't run on fuel that is 4 octane points higher? That seems a bit hard to believe, IMO. Is it a 100% apple to apples test? No, but it also not wildly different. The ProXS is making significantly less power per cu in than the LS3 in this test so maybe it is at a really, really low state of tune with a compression ratio so low it could run an extremely low octane fuel.

I think fuel could be a problem but I am skeptical it is the 4 octane points different. I'd be more suspicious of the quality of the specific fuel dumped in the tank and whether or not it was free of water or other contaminants.

Also, let's not forget we are talking about a situation where the OP has just purchase a used engine. That means the presence of any pre-existing problem with the engine is complete unknown. Maybe he was just lucky it ran fine we he got it because he was just lucky. That happened to me when I bought a used Mercury 60 Fourstroke. It ran great until it didn't. Ran through all kinds of stuff, including fuel, until we discovered the thermostat was clogged. Replaced it and problem solved.

[berudd]

Got it, misunderstood and thought we were able to log engine parameters like IATs, fuel injector duty cycles, timing... We do have some growth on the logs, as we do keep it in a wet slip, it's not terrible yet, but there is some as it's been in the slip for about 2 months now. I'll scrub it off and see what happens.

However, I still don't think this explains the drastic change over a 2 day period. We're talking roughly a 4-500 RPM and 7-8 MPH loss, plus the fact that it started and died that one time after the fill up.

Looking back over your OP. Do you happen to recall if air temps we significantly higher when you experienced the worst performance? That can have a significant impact. Also, as I think you are aware, s 22' pontoon is a lot of boat for a 115. A seeming small change in engine RPM can have a larger impact on speed as more of those big pontoons are in the water. As I mentioned above, you don't know what you don't know about this boat and motor. A lot of boats sit for a long time before they are sold as the owner tries to decide to sell it or not. Sitting unused and actually need to issues. As you mentioned, it might have gotten crap in the tank or you might have some varnish forming from sitting so long. Even something like rodents and insects making a home in the wrong place can cause issues. If the tank is not venting properly, you could be starved for fuel. Realistically, there could be all kinds of things that might be causing issues.

That growth on the logs could be a big deal. We had a deck boat once that sat in the water if it would get so bad 140hp would not get it on plane. Clean it and see what happens.

As to the it dying at start up one time, I would completely disregard that at this point. That's a single data point so it really means nothing. If it becomes a common occurrence, then it becomes something worth paying attention too. I don't think I have ever owned an internal combustion engine that hasn't done that to me at least once.

[Emilime75]

Berudd, thanks for the input. I'm far from an expert, but I'd have to say that I do agree with your thoughts on higher octane fuels - generally speaking, the higher the octane, the less it is susceptible to detonation(pre ignition, ping, knock...), therefore can withstand higher compression ratios and timing advance. However, there are limits to that, and while higher octane fuels have a higher resistance to detonation, they also have less explosive energy and rely on higher compression ratios. With that, if the Pro XS 4 stroke is a low compression motor, than the high octane fuel could, in theory, be causing a lack of power. Does anyone know what the actual compression ratio of a 115 Pro XS 4 stroke is?

Something else that just came to mind - What is the operating temp range for these motors? I seem to recall seeing around 167 on my VesselView Mobile.

Touching on some of berudd's other points - Ambient temps that day were actually lowest than any other time we've been out with this boat. I know what you're saying about a "new to me boat/motor", but a dozen or so previous outings didn't hint to any kind of problems, and this was a drastic change in a short amount of time. Sediment picked up when we ran low on fuel - have new filters that showed up today, we'll see. We'll also be scrubbing the growth off of the toons this weekend. Hard to disregard the starting/dying, even though it was just one time - if that was the only issue, then maybe I could/would. But, with also experiencing a performance loss, I have to believe they may be related.

[berudd]

I hope you’re able to get it sorted soon. Where’d you get that info about higher octane fuel having less explosive energy? If that was the case, I would think the same engine would make less power as the octane was increased but that’s not the case, at least in the test I was referring to.

[Emilime75]

Went out last night, took a few things off of the boat to help lighten the load.

First start it died again, restart was fine. Hardly anyone out and light wind, so lake was quite smooth. Maxed out at 5700 RPM and 18 MPH. Water/engine temp maxed out at 136, I think the 167 I mentioned previously was a figment of my imagination.

Berudd, about the higher octane fuel having less explosive energy, if you google studies and information on octane you can read about it. Same goes for E85, the benefit of it is that it burns much cooler allowing for more timing, the caveat is that it takes more of it which is why most cars' fuel systems need to be upgraded to run it. I don't know all of the details of the test you're referring to, did they monitor things like fuel injector duty cycles, IATs, AFRs...?

[Mechanic Bob]

Thought maybe I picked up some sediment from the bottom of the tank and clogged the new fuel filter, even though there were still 3 gallons in there, have another filter ordered.

To you statement:
-Fuel pickup tube is at the bottom of your gas tank. Also there is a small screen (typically) on your pickup tube.
-Gas floats on top of water.

Therefore, just because you got low on your fuel level, did not lead to you getting trash or water in your system.*
*having an empty tank, can lead to more condensate in you gas tank.*

[gehol]

You are correct Don. It has been argued too much, I should have stayed out of it. Won't happen again.

[berudd]

Went out last night, took a few things off of the boat to help lighten the load.

First start it died again, restart was fine. Hardly anyone out and light wind, so lake was quite smooth. Maxed out at 5700 RPM and 18 MPH. Water/engine temp maxed out at 136, I think the 167 I mentioned previously was a figment of my imagination.

Berudd, about the higher octane fuel having less explosive energy, if you google studies and information on octane you can read about it. Same goes for E85, the benefit of it is that it burns much cooler allowing for more timing, the caveat is that it takes more of it which is why most cars' fuel systems need to be upgraded to run it. I don't know all of the details of the test you're referring to, did they monitor things like fuel injector duty cycles, IATs, AFRs...?

The idea about different timing is was pretty much the primary thing they were testing in the show I mentioned. They found that all for gasolines and well as E85 made peek power at the same total time. They were challenging the idea where a guy wants to go faster to he dumps in higher octane so he can add more timing. That proved to be false. Now, if you had an engine that barely runs on 87, you might have to retard timing to prevent detonation in which case, yes you could running higher octane fuel with more timing. But that's only because the engine was detuned to run on 87. The did make more power with E85 but at the same timing. But, I believe is because you can run E85 richer than just gasoline (or E10 really). The stoichiometric ratio for E85 is 9.8:1 versus 14.7:1 for gasoline. You make more power with E85 because you can burn more of it with the same amount of oxygen in your intake charge. And, while they used an engine with port injection in this test, if you run E85 with a carburetor you will also get a little intake charge cooling (not talking combustion temps) which might also give you a little bump in power.

[EuropeanAM]

Gasoline has 124,000 BTU per gallon, does not matter if it is 87 or 93 octane.

Actually- it can vary from 112,114 BTU/GA to as much as 124,340 BTU/GA.

This has been argued in depth far too many times- and much of the data that's being used does not pertain to the application (or is completely inaccurate).

The following link actually details this in the chart provided:

https://afdc.energy.gov/files/u/publ...ison_chart.pdf

Let's get back on track now:


Emilime75: Have you run your engine on the 87 octane fuel yet?

[Emilime75]

Emilime75: Have you run your engine on the 87 octane fuel yet?

Don, I have not. I pulled the boat out on Sunday and have it at the house, it currently has a little less than half a tank full of whatever it was we filled up with at the marina. Should I siphon it out and refill with 87, or would topping off whatever is in there be OK?

[EuropeanAM]

I'd probably be inclined to run that fuel level down to about 1/4 tank and then start refilling with 87.

Your call- just keep in mind that if you have half a tank of one octane, and top it off with another octane, you're now somewhere in between (average).