[Rovernet] P5B V8 ignition timing

[geffandjulie at comcast.net]

Aviation 100 Octane is actually 100 LL …low lead. Environmental zealots have been trying to ground general aviation, but have only succeeded in Santa Clara county, in…you guessed it… California.  I don’t think use of 100LL would affect the engine, but I am not to be trusted on this…

PS: In WWII, air cooled rotary engines needed 140 Octane! I don’t know about the V12 water cooled Merlin, etc. 

 

Geoffrey W. McCarthy MD MBA DipAvMed Col USAFMC (ret)

677 NW Melinda Ave

Portland OR 97210

503-799-3809 (m)

 

From: Rovernet <rovernet-bounces at rovernet.org> On Behalf Of Gordon Reddy via Rovernet
Sent: Friday, September 12, 2025 12:49 PM
To: Rovernet <rovernet at rovernet.org>
Cc: Gordon Reddy <gorgowof at gmail.com>
Subject: Re: [Rovernet] P5B V8 ignition timing

 

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 <mailto: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 <mailto: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 <mailto: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 <mailto: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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