[Rovernet] P5B V8 ignition timing

[Gordon Reddy]

Tom, thank you very much!
For a guy that uses Wikipedia all the time (and donates to keep it going)
it didn't even cross my mind to "Wiki" the subject.
I skimmed the article and intend to read through it in depth tonight. So
many answers to so many questions about fuel!
Cheers,
Gord

On Sat, 13 Sept 2025 at 04:59, Tom Rymes via Rovernet <rovernet at rovernet.org>
wrote:

> Gordon,
>
> Regarding the differences between UK and US octane ratings, scroll down to
> the section “Measurement Methods” in this entry.
>
> Octane rating <https://en.m.wikipedia.org/wiki/Octane_rating>
> wikipedia.org <https://en.m.wikipedia.org/wiki/Octane_rating>
> [image: wikipedia.png] <https://en.m.wikipedia.org/wiki/Octane_rating>
> <https://en.m.wikipedia.org/wiki/Octane_rating>
>
> Tom
>
> On Sep 12, 2025, at 3:50 PM, Gordon Reddy via Rovernet <
> rovernet at rovernet.org> wrote:
>
> 
> Just found another chart which says 100octane UK equals 97.5octane NA
> while another site says it equals 93octane NA so it's a slippery subject.
> My assumption is that 100octane is the UK's "high test" and 93 or 94octane
> is the NA "high test".
> I usually run Shell Vpower 91octane in my Mazdaspeed3 (2.3L, 200psi
> compression, turbo/intercooled) and when I race it I sometimes change the
> tune to use Shell Vpower 93octane.
>
> On Fri, 12 Sept 2025 at 11:38, Jim Pile via Rovernet <
> rovernet at rovernet.org> wrote:
>
>> Really good info, Gordon. I always wondered if 93 octane was sufficient
>> for the TC, with its 100 octane sticker. Now I know that the US rating
>> would be closer to 95. I run on 93, which is readily available on the East
>> coast, which seems to be just fine. However, I admit to pumping in just a
>> little racing fuel per tank (103 octane), which really perks the TC up a
>> notch!
>>
>> On Friday, September 12, 2025 at 12:13:54 AM EDT, Gordon Reddy via
>> Rovernet <rovernet at rovernet.org> wrote:
>>
>>
>> Interesting, check this out.
>> https://aircooledbug.co.uk/usa-versus-uk-fuel-octane/
>>
>> Gord
>> also
>> 1972 TVR Vixen 2500
>> ...and if I am crazy enough 19?? Citroen 2CV panel van
>>
>> On Thu, 11 Sept 2025 at 18:05, Gordon Reddy <gorgowof at gmail.com> wrote:
>>
>> Interesting, I just returned from the UK yesterday and while there
>> noticed octane ratings there are higher. I believe its a different rating
>> system for octane which I was going to check on this evening. I remember my
>> P6 TC2000 had a tag stating use 100 octane but it ran fine on 92. My P5 3
>> litre runs fine on 91 but its certainly not 10.5:1 more like 8.5:1 It would
>> probably run fine on 87. More knowledgable minds will hopefully contribute.
>>
>> Gord
>> 67 P6 TC2000
>> 66 P5 3 litre
>>
>>
>> On Thu, 11 Sept 2025 at 17:54, Jon Stanley via Rovernet <
>> rovernet at rovernet.org> wrote:
>>
>> Hello all,
>> I'm trying to tune a stock early 67 P5B V8 with high compression 10.5:1
>> and is currently set to factory setting of 6 degrees BTDC. The non ethanol
>> fuel I have access to around here in Texas is 90 octane, and adding in a
>> boost probably pushes up to around 92, but the engine was designed and
>> tuned for 100 octane.
>>
>> I'm curious if anyone here has a high compression V8 and what timing they
>> set the engine to and what fuel? Some sources say retard the idle timing
>> possibly all the way to TDC?
>>
>> Right now the engine runs poorly when cold and on choke, but eventually
>> once warmed up seems to run well. Both carbs have been rebuilt.
>>
>> Regards,
>> Jon
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