# 93-95 octane engine using 87 gasoline, how worried should I be? **URL:** \ **Category:** Factual Questions\ **Created:** [August 8, 2021, 3:52pm UTC](https://boards.straightdope.com/t/93-95-octane-engine-using-87-gasoline-how-worried-should-i-be/948241 "2021-08-08T15:52:02Z")\ **Posts on this page:** 1\ **Showing post:** 34
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 10, 2021, 10:07am UTC](https://boards.straightdope.com/t/93-95-octane-engine-using-87-gasoline-how-worried-should-i-be/948241/34 "2021-08-10T10:07:57Z")
> [@Crafter\_Man](#): > > Related question: > > I drive a car with a stick shift. I will hear knocking when I am at a relatively low RPM, I am in one of the higher gears, and I step on the gas. > > Example: I finally reach town and slow down to 30 MPH. I am still in 5 gear, but everything is fine. I then exit the town and give it some gas. The car doesn’t want to accelerate much because of the high gear and low RPMs, obviously. But I also hear knocking. I can make the knocking go away by going to a lower gear and getting the RPMs back up. > > What’s up with that? By downshifting and using a lighter throttle setting to achieve the same acceleration that you had in the tall gear at low RPM, you are relieving two of the conditions that are conducive to knock: 1. The lighter throttle setting reduces the peak pressures and temperatures in the combustion chamber. The pocket(s) of unburned mixture that were previously prone to detonation on a hair-trigger are now more stable, and can be combusted in the normal fashion without detonating. 2. By operating at a higher RPM, the combustion takes less time. This is because of in-cylinder turbulence, which scales with RPM and results in stretching/distortion of the flame front that speeds up the combustion process; that RPM-scaled turbulence is the reason an engine can both idle at 600 RPM and make power at 7500 RPM. The total time required for combustion matters for detonation because of a fuel property called ignition delay. Rather than copying and pasting, I will refer you to this post of mine last summer that explains the issue in detail: > [@Using too high octane gasoline can damage your engine?](https://boards.straightdope.com/t/using-too-high-octane-gasoline-can-damage-your-engine/918546/25): > > It’s slightly more complicated than that. Upthread, I said this: The interesting thing is that pretty much any gasoline engine will, at some point, expose not-yet-burned mixture to a temperature well in excess of gasoline’s autoignition temperature. This is true even for engines with modest compression ratio. If you’re running an engine at wide-open throttle (WOT), peak cylinder pressure during combustion can be over 1000 psi. This is before combustion is completed, which means that ther… --- _[View the full topic](https://boards.straightdope.com/t/93-95-octane-engine-using-87-gasoline-how-worried-should-i-be/948241)._