<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div dir="ltr"></div><div dir="ltr">Gordon,</div><div dir="ltr"><br></div><div dir="ltr">Regarding the differences between UK and US octane ratings, scroll down to the section \u201cMeasurement Methods\u201d in this entry.</div><div dir="ltr"><br></div><div dir="ltr"><div style="display: block;" class=""><div style="-webkit-user-select: all; -webkit-user-drag: element; display: inline-block;" class="apple-rich-link" draggable="true" role="link" data-url="https://en.m.wikipedia.org/wiki/Octane_rating"><a style="border-radius:10px;font-family:-apple-system, Helvetica, Arial, sans-serif;display:block;-webkit-user-select:none;width:300px;user-select:none;-webkit-user-modify:read-only;user-modify:read-only;overflow:hidden;text-decoration:none;" class="lp-rich-link" rel="nofollow" href="https://en.m.wikipedia.org/wiki/Octane_rating" dir="ltr" role="button" draggable="false" width="300"><table style="table-layout:fixed;border-collapse:collapse;width:300px;background-color:#DFDFE1;font-family:-apple-system, Helvetica, Arial, sans-serif;" class="lp-rich-link-emailBaseTable" cellpadding="0" cellspacing="0" border="0" width="300"><tbody><tr><td vertical-align="center"><table bgcolor="#DFDFE1" cellpadding="0" cellspacing="0" width="300" style="font-family:-apple-system, Helvetica, Arial, sans-serif;table-layout:fixed;background-color:rgba(223, 223, 225, 1);" class="lp-rich-link-captionBar"><tbody><tr><td style="padding:8px 0px 8px 0px;" class="lp-rich-link-captionBar-textStackItem"><div style="max-width:100%;margin:0px 16px 0px 16px;overflow:hidden;" class="lp-rich-link-captionBar-textStack"><div style="word-wrap:break-word;font-weight:500;font-size:12px;overflow:hidden;text-overflow:ellipsis;text-align:left;" class="lp-rich-link-captionBar-textStack-topCaption-leading"><a rel="nofollow" href="https://en.m.wikipedia.org/wiki/Octane_rating" style="text-decoration: none" draggable="false"><font color="#000000" style="color: rgba(0, 0, 0, 1);">Octane rating</font></a></div><div style="word-wrap:break-word;font-weight:400;font-size:11px;overflow:hidden;text-overflow:ellipsis;text-align:left;" class="lp-rich-link-captionBar-textStack-bottomCaption-leading"><a rel="nofollow" href="https://en.m.wikipedia.org/wiki/Octane_rating" style="text-decoration: none" draggable="false"><font color="#A2A2A9" style="color: rgba(60, 60, 67, 0.6);">wikipedia.org</font></a></div></div></td><td style="padding:6px 12px 6px 0px;" class="lp-rich-link-captionBar-rightIconItem" width="36"><a rel="nofollow" href="https://en.m.wikipedia.org/wiki/Octane_rating" draggable="false"><img style="pointer-events:none !important;display:inline-block;width:36px;height:36px;border-radius:3px;" width="36" height="36" draggable="false" class="lp-rich-link-captionBar-rightIcon" alt="wikipedia.png" src="cid:E0BA1B17-465C-4826-99A5-4437448F0723"></a></td></tr></tbody></table></td></tr></tbody></table></a></div></div><br></div><div dir="ltr">Tom</div><div dir="ltr"><br><blockquote type="cite">On Sep 12, 2025, at 3:50 PM, Gordon Reddy via Rovernet <rovernet@rovernet.org> wrote:<br><br></blockquote></div><blockquote type="cite"><div dir="ltr">\ufeff<div dir="ltr"><div class="gmail_default" style="font-size:large">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.</div><div class="gmail_default" style="font-size:large">My assumption is that 100octane is the UK's "high test" and 93 or 94octane is the NA "high test".</div><div class="gmail_default" style="font-size:large">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. </div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, 12 Sept 2025 at 11:38, Jim Pile via Rovernet <<a href="mailto:rovernet@rovernet.org" target="_blank">rovernet@rovernet.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><div style="font-family:arial;font-size:16px"><div></div>
<div dir="ltr">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!</div><div><br></div>
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On Friday, September 12, 2025 at 12:13:54 AM EDT, Gordon Reddy via Rovernet <<a href="mailto:rovernet@rovernet.org" target="_blank">rovernet@rovernet.org</a>> wrote:
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<div><div id="m_-868739363161110169m_-5428827404417969173ydp570649eayiv6391056927"><div><div dir="ltr"><div style="font-size:large">Interesting, check this out.</div><div style="font-size:large"><a shape="rect" href="https://aircooledbug.co.uk/usa-versus-uk-fuel-octane/" rel="nofollow" target="_blank">https://aircooledbug.co.uk/usa-versus-uk-fuel-octane/</a></div><div style="font-size:large"><br clear="none"></div><div style="font-size:large">Gord</div><div style="font-size:large">also </div><div style="font-size:large">1972 TVR Vixen 2500</div><div style="font-size:large">...and if I am crazy enough 19?? Citroen 2CV panel van</div></div><br clear="none"><div id="m_-868739363161110169m_-5428827404417969173ydp570649eayiv6391056927yqt65703"><div><div dir="ltr">On Thu, 11 Sept 2025 at 18:05, Gordon Reddy <<a shape="rect" href="mailto:gorgowof@gmail.com" rel="nofollow" target="_blank">gorgowof@gmail.com</a>> wrote:<br clear="none"></div><blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div style="font-size:large">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. </div><div style="font-size:large"><br clear="none"></div><div style="font-size:large">Gord</div><div style="font-size:large">67 P6 TC2000</div><div style="font-size:large">66 P5 3 litre</div><div style="font-size:large"> </div></div><br clear="none"><div><div dir="ltr">On Thu, 11 Sept 2025 at 17:54, Jon Stanley via Rovernet <<a shape="rect" href="mailto:rovernet@rovernet.org" rel="nofollow" target="_blank">rovernet@rovernet.org</a>> wrote:<br clear="none"></div><blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div>Hello all, <div>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. </div><div><br clear="none"></div><div>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?</div><div><br clear="none"></div><div>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. </div><div><br clear="none"></div><div>Regards, </div><div>Jon</div></div>
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