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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link="#467886" vlink="#96607D" style='word-wrap:break-word'><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'>Bill,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'>It is the compression ratio of the engine, not the cylinder compression, that is related to octane rating.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'>Generally, the higher compression ratio, the more need for high octane fuel.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'>My Healey has a 10:1 compression ratio, so I add an octane booster to each tank of gas.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'>My TR3 engine has whatever the factory compression ratio was, so the octane rating is not that important. But, I still put “Premium” gasoline in it because that is all that is available near me that does not contain corn. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'>All that said, in theory, most engine will perfume better with higher octane fuel, and they should get better fuel milage as well. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'>Non ethanol gas will result in around 10 percent better fuel milage.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'>But if you are running gas with ethanol, I doubt the additional cost of “Premium” fuel it is worth it for “normal” driving. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'>Check the owner’s manual for the TR4 and see what it recommends for fuel octane rating.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'> <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'> Ron<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Arial",sans-serif'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'> Triumphs <triumphs-bounces@autox.team.net> <b>On Behalf Of </b>Bill Beecher<br><b>Sent:</b> Friday, April 17, 2026 5:07 PM<br><b>To:</b> Randall <triumphs@autox.team.net><br><b>Subject:</b> [TR] PSI vs Octane<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Good evening all,<o:p></o:p></p><div><p class=MsoNormal>I have always run regular 87 octane in my TR3(‘66 TR4 motor), no knocks or pings. Don’t know what got me to thinking but there has to be a relation between compression psi and the octane rating I should use. My compression across all four is 175-180, would I be getting improved performance from a higher octane gas? Or just wasting $$ ?<o:p></o:p></p><div><p class=MsoNormal>All the best,<o:p></o:p></p></div><div><p class=MsoNormal>Bill B<o:p></o:p></p></div><div><p class=MsoNormal>TS30766L<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>“The harder I work, the luckier I get<span style='font-size:19.0pt'>” H. Ford</span><o:p></o:p></p></div></div></div></div></div></body></html>