View Full Version : What Octane will you be running this summer?

03-17-2008, 10:46 PM
I know this has been debated some on here. I have 05 Tahoe with 5.3L in it and it only requires 87 Oct. Why does Indmar recommend 89 in their engines? I Ran 89 in it all last Fall... but, with Gas prices climbing... just curious what everone else does!

03-17-2008, 10:59 PM
I am thinking 89, just cause they recommend it. Don't want the engine knocking or ticking.

03-17-2008, 10:59 PM
I know the manual recommended 89 but I have been running 87 for past 2 years and with no problem. I dont think it will hurt anything. I have many people who have indmar engines (not only Moomba) and they all said they used 87.

03-18-2008, 10:50 AM
i generally run 89 in everything. don't really know why. i don know where i could get 93 around here.

03-18-2008, 11:38 AM
I plan on running 89, but I will try lower octane once I get used to how the engine should run. This is what I do in all my vehicles. As long as there are no audible adverse effects, I will run the lowest octane possible.

03-18-2008, 03:42 PM
I believe these motors have a knock sensor, so I would assume running lower octane would not harm the motor, but if it does sense detonation it will pull timing out of the entire motor reducing your horsepower. I guessing its a pretty simply knock sensor. Not like the ionization pattern detecting knock sensors on each cylinder of the new BMW M3's V8 (sick car! even sicker motor!!). But I'm not sure how much it will affect horsepower.

03-18-2008, 03:54 PM
I will probably use both 87 & 89. Ran 89 most of the time last year with the occasional 87. I couldn't tell any difference between the 2.

03-18-2008, 05:57 PM
I was actually told by my dealer to run 87 and maybe once a month run the 93. I ran 87 all year and before putting her up for the winter I ran a tank of 93 with Stabil all is well....

03-18-2008, 06:16 PM
Mix it up, Fill the 30Gallon gas caddy with 89 and all the 6 gallons with 87 so what ever is not used by the jet ski goes in the moomba.

I can't really tell a difference even running 2000 pounds ballast

03-18-2008, 07:22 PM
I've always run 87 octane and 10 cents added to $1.30/gallon was a big deal. Now at $3.50 a 10 cent jump is no big deal.

03-18-2008, 10:12 PM
round here, it is no longer a 10c increment on 89/93.. it is more like 20!
Wifes car prefers 91-93! Fortunate to get 89!

I suspect I will mix in some 87 with 89.. I have read where running 93 if not required can be a waste.. although, if you mix 93 in with 87, i guess it is a wash..

Market up today, oil down this week! KEEP GOING! SEND OIL to $50/bbl

03-19-2008, 09:40 PM
Diesel was 4.19 today and is expected to go to 4.40 by the weekend around here, cost me 133.00 to fill my truck today. My wifes car runs on 93 and when we try to run 87 we loose performance and mileage. I ran the Mid grade in my boat last summer and I suppose I'll do the same this summer. I'm taking it in the shorts any way you look at it!

03-19-2008, 09:45 PM
My wifes car runs on 93

What is she driveing that requires 93.....way to curious.

03-20-2008, 01:12 AM
Every car I've owned required 92+.

currently ML-500.

Though I get 22mph in a V-8 SUV so the 93 is not as bad now as it was when I was driving sports cars.

(just don't ask me what replacing the spark plugs costs)

89 is cheap ! I keep the octane at 89 by adding 1/4 tank of 92+ anytime I go longer than 6 weeks without a refill. 87 is NOT cheaper... you are losing milage and adding wear and dirt to the engine. I'm sure Larry from Indmar (EngineNut) will explain better than I ever can.


03-20-2008, 10:27 AM
I will run Mid grade. I dont even want to think about how expensive gas in general is going to be this summer. One thing is forsure, Indmar recommends Midgrade thats what I'll use. The season is short enough I dont want to do anything that might cause downtime so I can save a few bux!



03-20-2008, 03:45 PM
I get 22mph in a V-8 SUV

Joe - I think you need a new vehicle. :lol: - Deerfield

03-20-2008, 08:45 PM
Jesse, my wife has a Jaguar, and we have tried running 87 and 89 octane. Today I was getting 30mpg at 65 mph. On the lower octane we get around 23 - 24 and the car is very sluggish.

03-20-2008, 08:53 PM
Jesse, my wife has a Jaguar, and we have tried running 87 and 89 octane. Today I was getting 30mpg at 65 mph. On the lower octane we get around 23 - 24 and the car is very sluggish.

I hear you on that one. My wife has to run 91 or higher as well. She drives one of the RX-8 with the rotary engine. I love how it drives, but it sucks fuel like a private jet! I guess that is just the price you pay!!!

03-21-2008, 09:09 AM
Acura TL.. Ran 87 in it this week and did not really see an mileage change/performance change.. Right now, she still can't drive, so I drive her to work and back (90 miles for me to take her to work and then to mine-2x a day) so I am putting it to the test.

I think this summer, i will most probably run 89 with some 87 mixed in.. may even splurge with a little upgrade in there.
Any mathematicians wanna do some figuring?
Note, for all my adult life, mid grade and hi grade fuel was always 10cents upcharge. Now, with prices so high, the mid is now 15+ cents higher and premium 10-15 higher. SO, here is the math question:
Difference in 87 and 89 is minimal(2), then 89-93 is 4.. Would one be better off buying a majority amount of 87 and then topping off with 93?
Anyone wanna take a stab at formulating that?
My point: 93 is a better value than 89.... More gain in octane per cost increase...
If 93 was only $0.10 more than 89, you would seemingly be better served by mixing 93 and 87.. SLED and ZEGM will have to engineer this one..

03-21-2008, 06:16 PM
Seems reasonable to me. I'm also wondering about octane boost. We use Klotz octance boost for our Champ engine and get a 6 octane increase/gallon. I don't actually have to purchase this stuff so I don't know how that works out price wise.

Lets Z engineer that one :lol:

ian ashton
03-21-2008, 08:26 PM
All this talk of the wife's cars; my girlfriend (Jessica) has a Dodge Caliber, 34+ MPG, lol. Granted, its a tiny little crapbox, but she only has to drive to work and back, and for $200/month it couldn't be beat! If we want to look good we take my car 8)

03-21-2008, 09:59 PM
More octane is NOT more power, the octane needs to match the compression ratio. More Engine Compression is more power and thus high octane is required. Little explanation, gas does not instantly explode, it burns and gases expand pushing the piston down on the power stroke(ok relatively speaking it is quick). You want the maximum amount of burn to happen the nano second the piston passes top dead center to get the biggest push down on the piston. In order to do this you need the spark to happen before TDC (advanced timing) because again you are starting the burn but it has just started and has not reached all of the fuel in the combustion chamber. If the engine is high compression and you use low octane then the gas is ignited by the pressure in the cylinder "before" the spark plug ignites it, now the maximum burn is before the piston TDC (top dead center) and you hear all of the play in the piston, rod, crank as it gets the poo kicked out of it on the way up. If you have a low compression engine and you use high octane and you loose power as it burns slower because the pressure is lower in the cylinder (doesn't have the pressure to help the burn). You can offset this some by advancing the timing of the engine to start the burn sooner with the spark plug. High octane gas ignites at a higher pressure (less volatile) than low octane does, this combined with the correct ignition timing and high compression of the engine will allow it to burn at the correct time. This allows a High Compression engine make MORE power. The best Octane to use is the first grade that you do not hear detonation, nothing more. On knock sensor vehicles the best fuel to use is the factory recommended fuel. You can go with a lower grade but expect to loose some power.

03-21-2008, 10:32 PM
Right on Z (the other Z). That is what I've been reading but I am an idiot when it comes to engines. What I also read is if you use a high octane in an engine designed for a lower octane and you do notice an increase in power then the engine (or electronics) needs to be checked. Question I have is on my 99 350HO (carburated) the octane suggested is 89 so does that engine have a higher combustion than the standard 350?


03-21-2008, 10:33 PM
ian - Please....no four letter words (well, actually it was seven, but you know what I mean). Not a big deal, just something I'm asking and would appreciate on the BB. Thanks. - Deerfield

03-21-2008, 10:37 PM
I also noticed that our new 2008 LS is required to run 89 octane. Ok this engine in the on road application (Chevy 350/5.7L) was rated to use 87 octane. That combined with the fact that these engines run cooler suggest to me that they are coming with higher compression than what they used for on the road usage. Cooler temperatures means less pressure too. I will try to find the compression ratio for these engines but if I was to guess I would say they might be around 9.5 to 1.

ian ashton
03-21-2008, 10:55 PM
ian - Please....no four letter words (well, actually it was seven, but you know what I mean). Not a big deal, just something I'm asking and would appreciate on the BB. Thanks. - Deerfield

Are you kidding me? :roll: :lol: Edited per your request.

03-22-2008, 09:32 AM
Z is of course correct. Another factor with high octane fuel is the actual burn time. The higher the octane the slower the burn. Again this is applicable to high compression engines and the smaller combustion chamber area. A flash burn on low octane fuel would render the engine inefficiant and in some cases (like our race engines) could actually cause damage and catastrophic failure. High Octane slow burn, therfore power is generated farther doen the stroke pushing the piston farther along instead of just say shoving the piston down the cylinder.

Engines equiped with DET sensors allow you to use Fuel other than recommended without damage but do not nessicarily mean that you are getting the most out of your engine both in terms of economy and power.

03-22-2008, 09:09 PM
ian - Thanks! :lol: - Deerfield

03-24-2008, 08:09 PM
Good replies. The delta between 87 and 89 is pretty insignificant when it comes to a 9.5-1 compression. One small difference is most refineries put a better additive package with the 89 which does keep your engine a little cleaner. However with the hours we put on our boats even you high use guys I'm not sure it matters. What matters more is where you buy your gas. 89 octane gas that's been sitting for months (low turnover station) or in old tanks (old gas station) probably has more condensation and dirt then newer 87. I have a 12:1 68 mustang and run race gas but in a pinch had to run 92 octane when the track ran out on a busy weekend. Difference in my quarter mile times was .004 only 20 minutes apart on runs.


03-24-2008, 09:31 PM
I have been doing some calculations on using some 87 oct and some 93 oct... Based on prices this weekend at Shell... $3.08/87 $3.20/89 $3.34/93; You only save $2.00 by mixing 87 and 93 to reach 89 oct.... not worth the trouble! This was based on a full tank.. even less on half a tank.

SO, I am guessing I will run 89 most of the time... may slip a little 87 in every now and then! WE NEED RELIEF on FUEL PRICES BAD!


03-24-2008, 10:21 PM

I believe you are the first one to hit the nail on the head. WE NEED LOWER GAS PRICES..... PERIOD! :shock:

03-24-2008, 11:19 PM
On my big boat I used to pay 50 bucks to go fishing. Then gas went up, I then enlisted a fishing partner and we each pay 50 bucks. Well it is up again so now we are taking applications for a 3rd fishing partner! Please send resume here. Ah ...any women who apply will get extra points as we have to try to balance the gender issue, you know those stupid federal laws. And this applies to any guy who brings his hot wife too, however she must also be able to open beer bottles up. 8)

03-24-2008, 11:28 PM
Hahaha…I think I like you ZEGM!!!

03-24-2008, 11:37 PM
Just trying to make sure everyone has a good time! :lol:

Here is the real gas hog at my house, 2 x 150 Evinrudes, but at least they are Fuel Injected!!!
And just to let the ladies know Mrs. Z MADE the boat dealer throw in a new porta pottie when we bought it!!!!

http://i237.photobucket.com/albums/ff136/zegm/DSC00029.jpg (http://s237.photobucket.com/albums/ff136/zegm/?action=view&current=DSC00029.jpg)

03-24-2008, 11:38 PM

03-25-2008, 12:02 PM
Seem to recall reading in Indar literature they recommend 89 because boats tend to sit more than cars and the gas can loose octane. Altitude has a huge impact on hp. Haven't had any issues with 87 in my 325 EFI for 3 seasons.

03-25-2008, 05:52 PM
That makes sense! However our boat doesn't sit long after the first time in the water.
Yes we live 8 feet above sea level and during the winter the cars run really good here!!!

03-25-2008, 06:49 PM
87 octane and the occasional fuel treatment can. Mid summer I'll run sum super though it.