RMC List - Octane boost question

Wed Apr 29 00:14:49 EDT 2026

Dylan:
You are correct that dieseling is a functiion of the timing and the octane
rating.  What I am trying to do is to keep the octane of the (alcohol-free)
gas I use as high as economically reasonable without having to sacrifice
performance by retarding the timing to allow for a lower octane.  The last
time I filled one of my cars at the Green Mountain Mobil station with the
91 octane non-ethanol gas (late last year) it was $8/gal!
I too have a Volvo, a '67 122S with B18B motor and 10.1:1 compression.  It
ran on for a second or two today after I killed the ignition, with 87
octane gas in the tank.  I'll be adding some of my VP Racing octane booster.
Andy has a point about hot spots, often carbon deposits, I think, from an
overly rich mixture and lack of use or only short drives.

Garrie:
119 mph!!  Nice to get a can of that 100 octane avgas and blend in a bit.
Can I borrow your credit card?

Best,
Michael

On Tue, Apr 28, 2026 at 4:15 PM Dylan Berichon via RMC-List <
rmc-list at corvair.org> wrote:

> Dieseling is usually the result of a timing issue in relation to the
> octane fuel you are running.
>
> While not on my Corvair, I had this issue with an old pushrod Volvo that
> had a real flame flower engine under the hot.  Hotter cam, bigger pistons,
> etc...
>
> If I adjusted the timing to avoid dieseling, it would ping from time to
> time under heavy load.
>
> Ultimately I adjusted the timing to avoid pinging, yield the best power,
> and just learned to slip the clutch when turning the car off.  (I'd put the
> car in 2nd gear, slowing let off the clutch while also stepping on the
> brake while turning off the ignition.)
>
> Perhaps not the most eloquent solution, but it worked.  I suspect it would
> also work on the Corvair.
>
> All this goes out the window if you have a Powerglide though.
>
> Kindest regards,
> --
> Dylan Berichon
> IHeartMedia Denver <https://www.iheartmedia.com/>
> Oregon State Beavers Broadcasting <https://osubeavers.com/>
> Learfield <https://www.learfield.com/>
> (503) 427-8305
>
>
> On Tue, Apr 28, 2026 at 5:05 AM Garrie Fox via RMC-List <
> rmc-list at corvair.org> wrote:
>
>> Interesting!
>>
>> I owned a '65 180 convertible Turbo which I sold at the Flagstaff
>> National.
>>
>> I never had a detonation problem after I added a Dale Mfg Turbo
>> retard/advance  unit.
>>
>> I ran colorado base fuel Octane.
>>
>> There is a long uphill run on HWY 94 east of the Springs. I always used
>> it to check Turbo Boost.
>>
>> If it was running well, the gauge indicated 14 +or more.
>>
>> I bought Marlyn Lyles. PPCC member, 66 Aztec Bronze 180 Turbo for the
>> storage fees owed in the 90s
>> He stated that the Corvair Blew Away the Big Mustangs going west our of
>> Denver to the Continental Divide.
>>
>> I will admit to using aircraft 100 octane aircraft fuel. But only once.
>> It was available with  credit card at a El Paso County airport.
>>
>> It was for a Binion FI car that I still have.
>>
>> I also determined on that runway at night that a Binion FI Corvair red
>> lined at 119 MPH. The '66 Sports Sedan had a 3:27 diff ratio.
>>
>>
>>
>>
>> Regards,
>>
>> Garrie Fox
>> 719.338.5051
>> (texting not available)
>>
>>
>> On Monday, April 27, 2026 at 11:00:33 AM MDT, Eric Schakel via RMC-List <
>> rmc-list at corvair.org> wrote:
>>
>>
>> Michael,
>>
>>
>>
>> I’m curious about the rpm and load where your Spyder detonates. I had a
>> couple of 180 turbo Corsas in my youth, and recall that even with the 98
>> octane leaded ‘Hi-Test’ of the era, there was always a little buzz of
>> detonation on throttle tip-in. I initially addressed it by opening the
>> passage to increase the accelerator pump shot.
>>
>>
>>
>> Turbos are an awesome secret weapon. Linae’s Mazda CX-5 has a turbo 2.6
>> four, and the thing runs a quarter-mile as quick as my 1971 455 Pontiac
>> Trans Am!
>>
>>
>>
>> Eric
>>
>>
>>
>>
>>
>>
>>
>> *From:* Michael Brittan <mibrittan at gmail.com>
>> *Sent:* Monday, April 27, 2026 9:17 AM
>> *To:* gearhead066 at gmail.com
>> *Cc:* Rick Beets via RMC-List <rmc-list at corvair.org>
>> *Subject:* Re: RMC List - Octane boost question
>>
>>
>>
>> Hi Eric,
>>
>> Now you're talking!  A 140 with 10.4:1 compression and a turbo.  In the
>> interests of Corvair science though, you'll have to run the experiment on
>> two cars - one with fuel with ethanol and one without.
>>
>> FYI, a can of VP Racing Octane Booster (purchased at Walmart) added to a
>> full tank of ethanol-free 87 gas took care of the detonation on my Spyder.
>> This can was the cheaper of the VP options so I'm not sure if it contained
>> alcohols.  The can blurb did not provide the ingredients.
>>
>> Best,
>>
>> Michael
>>
>> Michael
>>
>>
>>
>> On Sun, Apr 26, 2026, 10:22 PM <gearhead066 at gmail.com> wrote:
>>
>> Michael, for what it’s worth, I’ve ignored all warnings and online
>> threads talking about detonation and engine wear and simply used standard
>> Tier 1 91 octane pump premium with ethanol at Denver’s altitude for years.
>> I dislike the hassle (and occasional clothing disaster) of diddling with
>> octane boosters and fuel additives. Even in engines with cylinder heads
>> milled to raise compression to a true 10.4:1 range, detonation and wear
>> have not been issues. I can’t comment on a turbo engine at this point, but
>> I’m thinking of tossing a turbo onto a set of 10.4:1 compression 140
>> big-valve heads to see how long it takes to melt the piston tops. If I do
>> that and ethanol helps with piston top life, I’ll let you know!
>>
>>
>>
>> Eric S.
>>
>>
>>
>>
>>
>> *From:* RMC-List <rmc-list-bounces at corvair.org> *On Behalf Of *Michael
>> Brittan via RMC-List
>> *Sent:* Friday, April 24, 2026 1:47 PM
>> *To:* rmc-list at corvair.org
>> *Subject:* RMC List - Octane boost question
>>
>>
>>
>> Hello All,
>>
>>
>>
>> I like to use only non-ethanol gas in my cars (Spyder and 2x164/110
>> Monzas).  87 non-ethanol is available in my area of SE Denver at a
>> reasonable price.  Preferred 91 octane non-ethanol is available at the
>> Mobil station in Lakewood, but at a huge price premium and a 30-mile
>> round-trip (+- 2 gals of gas consumption right there) from my home.
>>
>>
>>
>> I can eliminate the pre-ignition with expensive octane booster containing
>> MMT.  Cheaper brands contain alcohols, so defeat the purpose of using
>> no-ethanol gas and don't raise the octane as much per ounce of dosage.
>>
>>
>>
>> I'd welcome members' views on using such octane boosters and whether they
>> may have preferred brands.  Could any harmful side-effects be experienced
>> with our trusty Corvair engines using these boosters?  What do members feel
>> about using standard pump 91 octane containing ethanol?
>>
>>
>>
>> Best,
>>
>> Michael
>>
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