[TR] PSI vs Octane

[rjray.architect at gmail.com]

Bill,

 

It is the compression ratio of the engine, not the cylinder compression, that is related to octane rating.

 

Generally, the higher compression ratio, the more need for high octane fuel.

My Healey has a 10:1 compression ratio, so I add an octane booster to each tank of gas.

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. 

 

All that said, in theory, most engine will perfume better with higher octane fuel, and they should get better fuel milage as well. 

Non ethanol gas will result in around 10 percent better fuel milage.

But if you are running gas with ethanol, I doubt the additional cost of “Premium” fuel it is worth it for “normal” driving. 

 

Check the owner’s manual for the TR4 and see what it recommends for fuel octane rating.

  Ron

 

From: Triumphs <triumphs-bounces at autox.team.net> On Behalf Of Bill Beecher
Sent: Friday, April 17, 2026 5:07 PM
To: Randall <triumphs at autox.team.net>
Subject: [TR] PSI vs Octane

 

Good evening all,

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 $$ ?

All the best,

Bill B

TS30766L

 

“The harder I work, the luckier I get”  H. Ford

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