I have a 2005RK..... Upgraded to a '96, heads ported, heavy springs, adjustable lifters S&S 585 cam hydraulic pump...etc etc....anyway my question....i often use 93/91 octane gas...but recently i used 89/87 octane...when I was running the higher octane above 70mph the bike seemed to want to "surge" seemed like a hesitation/surge......but when i ran the 89/87 the surge/hesitation went away......I had a dyno done but can't remember what gas I had in it....does it make a difference ? Can the octane of the gas really make a difference.....it's not the first time i changed gas and noticed the difference.....i think i will try sticking to the 89/87....unless it will be disastrous to my engine.
Do the '88 run better with lower octane gas?

Thanks,

 

Mine seamed like it ran better with 87 but also had way to much deto. Need to atleast run 89 I've been told. Mine was tuned with 93 so I'm sticking with that after top end rebuild.

 

If you plan to use a low octane fuel, you should consider adding an octane booster at each fill. Cheaper than detonation taking out a piston top.

 

If need to use an octane booster, i might as well use 91/93.....but then the hesitation/surge will return so what's the point of using a lower octane with a boost vs the 93/91????

 

You'll be fine running the lower octane rating. There are lots of people that do with no real issues. The ECU has the ability to adjust.

 

My whole Family run 87 in their Harley's with no ill effects. It'll be fine.

 

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These days your avg hd has too high of comp and or cyl pressure to safely run lower octane 87/89 fuel all time without detonating esp when under load.

Also.higher octane fuel is a bit harder to ignite/slower ro react & ignite the lower grade fuel so sometimes you need to adv timing a few deg to help with the issue your having.

But if you keep running low octane 87/89 fuel thats ok ONLY IF THE MOTOR DOESNT DETONATE/PING esp under load on a hot day which most hds would have issues with on 91 or 93 let along crappy 87/89 octane fuel.

Detonation/pinging due to running too low of an octain fuel can in some cases crack & or break a piston or piston skirt and can also damage a rod or main bearing too either of which could result in destroying the motor in the process.

So be careful running low octane fuel in a lrg disp air cooled hd v-twin because most of them dont normally like it at all,my stock stock 96" w-only 9.2 compression used to ping on hd's rec 91 fuel & sometimes 93 oct fuel on hot summer days too.

But that all stopped when i got the oil running 50+deg cooler (which also makes motor run significantly cooler at the same time too!) with some mods i did that stopped the detonation dead in its tracks even on 91 oct fuel which also significantly increased hot oil pressure at idle and cruise along with reducing valve noise a lot with a perf other positive things too.

Installing wards cooling rans,ultracool oil cooler with dual elec cooling fans & switching from syn3 20-50 to spectro 20-50 (in std dino/semi blend or full syn,doesnt matter) is the 3 part rcp i did that collectively working together reduced oil temp 50+ deg on my bikes motor on hot 90-95deg HHH summer heat.

FYI,this is the time of yr that they switch to different winter grade / blend formula fuel which can also cause tuning/driveability issues that i have had to deal with 1st hand when dialing in timing & carbs on classic 60's to early 70's street perf muscle cars i work on so keep that in mind in case not all stations and or all grades of fuel have switched to winter blend fuel yet .

Scott

 

The octane of gas does make a difference, but that is another thread.
OP, in your case it sounds more like you were hitting cells with 91 that were not tuned as opposed to tuned cells with lower octane.
most likely you dyno tuned with lower octane and the premium is either hitting lean cells or timing is pulled due to pre-ignition.
while you made the problem over 70 go away I am surprised you did not notice a performance difference at lower rpm.

 

Excellent point, as I have the S&S 585 cam in the motor i felt no sluggishness nor drop in torque, when i take off my rpm usually hit above 4500-5000 before i gear up to the next gear.

I think i will have it re-tuned......as i do like the fact that the surging/hesitation has disappeared....