Octane requirement?

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Erik the Awful

Supporting Member
Supporting Member
Joined
Aug 9, 2019
Messages
7,624
Reaction score
15,542
Location
Choctaw, OK
When storing for long periods,
Dont. Gas nowadays doesn't have the shelf life that gas used to. Even non-ethanol goes bad in about 6 months. Buy it and burn it. Drain your lawn equipment between yard seasons. Put drain valves on anything that has to sit and is too big to tip over.
 

L31MaxExpress

I'm Awesome
Joined
Apr 21, 2018
Messages
5,892
Reaction score
7,542
Location
DFW, TX
Old thread but yea, no need for anything over 87 in these trucks unless you do internal engine mods.

When storing for long periods, you might be able to find a farm store/station that sells low octane non-ethanol for cheaper than 91/93 non-eth.
Not exactly true! My van stock had a ton of knock retard on 87 in hot weather with the stock PCM tuning. Eventually I just tuned it for 91 and was done with it.
 

Schurkey

Supporting Member
Supporting Member
Joined
Jan 14, 2018
Messages
10,970
Reaction score
13,759
Location
The Seasonally Frozen Wastelands
The "exception"...or the guy who connects a scan tool and actually researches what the engine is doing.

Wild guess: He knows when the knock sensor is unhappy, AND the ability to do something about it. Other folks pay no attention.
 

someotherguy

I'm Awesome
Joined
Sep 28, 2013
Messages
9,804
Reaction score
14,177
Location
Houston TX
The "exception"...or the guy who connects a scan tool and actually researches what the engine is doing.

Wild guess: He knows when the knock sensor is unhappy, AND the ability to do something about it. Other folks pay no attention.
Extraordinary claims require extraordinary evidence. :)

Richard
 

Erik the Awful

Supporting Member
Supporting Member
Joined
Aug 9, 2019
Messages
7,624
Reaction score
15,542
Location
Choctaw, OK
Racing in Colorado last month, our BMW's ECU was pulling timing. Stock engine, running a custom tune on 87 octane. Adjusting the tune didn't help. It was unseasonably warm and the air was thin. It's not extraordinary. You just don't notice it because you're not watching your data stream all the time. Modern engines with knock sensors do a great job of working around detonation and keeping you from noticing.
 

L31MaxExpress

I'm Awesome
Joined
Apr 21, 2018
Messages
5,892
Reaction score
7,542
Location
DFW, TX
It's amazing how often you turn out to be the "exception" ... ;) :rolleyes:

Richard

Extraordinary claims require extraordinary evidence. :)

Richard

This was a minor amount of knock retard compared to what it would do at times on 87 octane and the stock tune with the factory 195°F thermostat and factory 5 bladed fly swatter fan blade. It still jumped up around 3° knock retard when the transmission shifted into 2nd gear. I think this was with headers, small cam but stock compression ratio. Cam really did not change its tendency to detonate as the 9.4:1 compression ratio, detonation prone GM dished piston design and thick OEM head gaskets that kill quench were all still in place. Also this had to be when I was playing with trying to get it to run well on 87 given that it is maxed out at 25-26° of timing. Vortec heads typically make peak power between 29° and 34° BTDC. I could get it to stop detonating but it was a dog with the timing pulled. I put 91+ octane in the tank, cranked the timing up and it ran much better. On 91 octane with a coolant temp of 176-182°F I ran 29° at 4,800 and 32° at 5,600 in the timing map. Shifting at 5,100 had me about 31° and 5,500 about 32°. On E85 that same setup liked 34° of timing and all of that at 2,400 rpm. Torque was way up, 330 @ 2,700 vs 310 @ 3,100 and peak hp went from 257 at 4,900 to 272 @ 4,700. At around 1,500 rpm E85 and the high timing gave as much as a 35 ft/lbs gain, it absolutely loved the higher octane and full timing advance at low rpm. My E85 timing table had as much as 8-10° added in places.
The waviness in the maf airflow reading also points to this being before I upgraded the valve springs. I was getting valve float so severely before I changed the springs that it lost 30 hp at the wheels between 5,150 rpm and 5,350 rpm. Post springs the power leveled out and it would rev cleanly to 5,500 and stay within 10hp of the peak at 4,900.
You must be registered for see images attach
 
Last edited:

0xDEADBEEF

Supporting Member
Supporting Member
Joined
May 3, 2021
Messages
2,482
Reaction score
6,109
Location
127.0.0.1
Seeing some knock retard on a log doesn't mean you must run high octane. The ECM is doing its job.

If I can actually hear it, that's when I run higher octane and start investigating.
 

L31MaxExpress

I'm Awesome
Joined
Apr 21, 2018
Messages
5,892
Reaction score
7,542
Location
DFW, TX
Seeing some knock retard on a log doesn't mean you must run high octane. The ECM is doing its job.

If I can actually hear it, that's when I run higher octane and start investigating.
While what you are saying is true, that does not mean that the engine is making as much power as it can. If you eliminate the knock retard with higher octane the engine will make more power. Even the old black box had octane based learning that allowed it to advance the timing more for high octane fuel. The Gen3 LS based controllers like the 0411 and the P59 I run my 383 on have dual timing maps. The 0411 could pull enough timing on its own to prevent knock retard however it ran like a dog with less timing.
 
Top