[Rovernet] P5B V8 ignition timing

[Gordon Reddy]

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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