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Octane requirement

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Re: Octane requirement

Postby mgrant » Wed Mar 27, 2019 7:42 pm

Benny wrote:
chariotdriver wrote:
Benny wrote:Went for a run today. Had about 30 gal of super in tank. Put a little over 60 gal of 87 on top of it. Wanted to see how the 575 raptor would run on low grade fuel. After 1.5 hrs of running I plugged into the ecm and watched while we ran another hr.

Here are pics of what it thought of the 87. Sure it was a mix so I will put more 87 in and share the results.


Any theories on what is going on? Usually mixing 87 with 91 would result in an octane in between the two; not a boost for the whole mix. 87 in the LSA resulted in a little rattle/knock followed by a WOT in the low 3000rpm range.


Not sure. I ran it at wot yesterday for a short time and still pulled 3950 rpms. I saw a little knock retard for a brief moment in the mid range. (Up to 2 deg) I will put more 87 in it and post up results. Maybe tomorrow.

Would save a good deal of cash if it likes 87.


What is your WOT rpm with 92 octane?
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Re: Octane requirement

Postby Benny » Wed Mar 27, 2019 9:21 pm

WOT is around 3950 with 92 just like with 87.

What is the chance that both stations I filled at have 92 coming out the 87 nozzle??

Whatever it is I'm not complaining for sure! Just wish I would have tried 500 hrs ago. My wallet would be much fatter.
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Re: Octane requirement

Postby mgrant » Thu Mar 28, 2019 5:50 am

Keep in mind, you're testing in cool weather now and seeing the occasional knock. But next summer the game will definitely change, and so I'd highly recommend running a higher grade of fuel during those months.
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Re: Octane requirement

Postby Benny » Thu Mar 28, 2019 7:22 am

I didnt think of that. Will watch as the weather gets nicer.
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Re: Octane requirement

Postby mgrant » Thu Mar 28, 2019 8:32 am

Do you have a way to datalog several minutes of run-time?

Not sure if you can do this with Indmar's ecm, but you might also consider setting up an alarm to the helm to let you know if the engine experiences >2-3* of knock retard, in which you can shut it down and add a couple cans of octane boost to the tank...just a thought.
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Re: Octane requirement

Postby Benny » Thu Mar 28, 2019 10:40 am

The scan tool I have was the cheap way to go so I don't think you can log info. The knock retard I have seen so far was very brief. Most of the time nothing. Not sure there is really anything to log that would be useful until I get some steady retard.
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Re: Octane requirement

Postby Penguin » Thu Mar 28, 2019 10:46 am

You guys should move this thread over to the mechanics or projects area. I understand that you’re experimenting with fuel octane with Indmar engines, but what you’re learning could be of real value to the Kodiak and Marine Power guys as well. I’m keenly following your conversation and I have GM 6.2 DIs.
Looking forward to hearing the report on 87 octane
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Re: Octane requirement

Postby Riverjohn » Thu Mar 28, 2019 11:48 am

Penguin wrote:You guys should move this thread over to the mechanics or projects area. I understand that you’re experimenting with fuel octane with Indmar engines, but what you’re learning could be of real value to the Kodiak and Marine Power guys as well. I’m keenly following your conversation and I have GM 6.2 DIs.
Looking forward to hearing the report on 87 octane

Most on here see all the posts by going to "Board index" and then "View New Posts". No need to look at each one for new threads. :Drink:
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Re: Octane requirement

Postby Penguin » Thu Mar 28, 2019 12:15 pm

I do mean chicken from a smart phone, which requires I look at each topic for new posts...
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Re: Octane requirement

Postby Riverjohn » Thu Mar 28, 2019 1:07 pm

Penguin wrote:I do mean chicken from a smart phone, which requires I look at each topic for new posts...

No actually it doesn't. I can do it from my smart phone also.
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Re: Octane requirement

Postby whee » Thu Mar 28, 2019 1:58 pm

Riverjohn wrote:
Penguin wrote:I do mean chicken from a smart phone, which requires I look at each topic for new posts...

No actually it doesn't. I can do it from my smart phone also.


Yep. Click on the magnifying glass then “view active topics.”
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Re: Octane requirement

Postby mgrant » Thu Mar 28, 2019 6:44 pm

Benny wrote:The scan tool I have was the cheap way to go so I don't think you can log info. The knock retard I have seen so far was very brief. Most of the time nothing. Not sure there is really anything to log that would be useful until I get some steady retard.


It'll probably be summer time before you'll see a consistent knock retard, and probably only when you're in boost. However that is when damage will most likely occur from detonation, and it can happen in only a few engine revolutions.

Anyone know if the SC Raptors are running forged or hypereutectic pistons? I know the stock LSA run hypereutectic pistons, and they have been known to scatter when ran on low octane in hot weather.
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Re: Octane requirement

Postby Benny » Thu Mar 28, 2019 8:30 pm

mgrant wrote:
Benny wrote:The scan tool I have was the cheap way to go so I don't think you can log info. The knock retard I have seen so far was very brief. Most of the time nothing. Not sure there is really anything to log that would be useful until I get some steady retard.


It'll probably be summer time before you'll see a consistent knock retard, and probably only when you're in boost. However that is when damage will most likely occur from detonation, and it can happen in only a few engine revolutions.

Anyone know if the SC Raptors are running forged or hypereutectic pistons? I know the stock LSA run hypereutectic pistons, and they have been known to scatter when ran on low octane in hot weather.


Good info, thanks. I will watch close. I assumed the ecm's ability to adjust was to protect the engine.
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Re: Octane requirement

Postby Penguin » Thu Mar 28, 2019 9:03 pm

Mgrant, doesn’t the knock sensor serve to keep the engine out of detonation?
And if your engine performance suffers from retarded timing only part of the time but does fine with lower octane fuel most of the time, wouldn’t one of the methanol/water injection systems readily available be a more cost effective way to maintain peak performance without having to wast money on high octane fuel when you don’t need it?
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Re: Octane requirement

Postby mgrant » Thu Mar 28, 2019 9:43 pm

Yes, water/methanol injection definitely does allows you to run lower octane fuel, boosting 87 octane to well over 100... an old tried and true method ever since the days of World War II aircraft. The beauty of modern day methanol injection systems is you can program to activate only under boost conditions, allowing you to plug along on 87 octane but have 100 octane under boost.

Yes knock sensors do retard timing, however one must remember that knock must occur in order for the sensor to record it, and the ECM to adjust accordingly. If you jab wide open throttle on a supercharged motor on a hot day with low octane fuel, those few milliseconds of detonation can do serious damage to a hypereutectic piston before the ECM can retard timing or put the engine into limp mode...cast/hypereutectic pistons can shatter to pieces, leaving a bare rod to punch a hole in the cylinder liner. Most modern-day engines use oil squirters for the pistons to help keep them cool, but that only buys a few tenths of a octane point on a SC engine. Forged pistons are a lot stronger won't shatter catastrophically, that's why most high performance engine builders use them religiously, though detonation will still damage them. FYI, although the LSA uses hypereutectic pistons, the LT4 uses forged pistons with oil squirters, as does the Hellcat crate engine.
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Re: Octane requirement

Postby Penguin » Fri Mar 29, 2019 12:56 am

Mgrant, I think methanol/water injection doesn’t get the attention it deserves in the jet boating world to address the issue of detonation happening infrequently enough that using more expensive high octane fuel exclusively is an unnecessary expense. As you pointed out it’s technology that started out in WWII aircraft but then found its way into the turbocharged ‘62-‘63 Oldsmobile Jetfires, Saab’s in the late 90’s, and some BMWs currently I think.
The injection kits on the market today may be a little spendy, but the reviews I’ve read seem very positive, and with the fuel cost savings I suspect the pay back on the investment is pretty quick. Like you said, they can be programmed to activate at predetermined boost pressures so that the engine never gets into detonation to start with.
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