Premium ve Regular

[Walter F.]

I just saw a video in which the owner of a C5 Corvette says he uses 87 octane even though Chevrolet claims using lower octane will effect performance. I have and 84 Corvette which requires 87 octane. However the engine has been modified. It has and Xram after market intake along with and 85 fuel pump, in larged throttle bodies and fuel pressure regulator and a performance chip with a 8746 ECM.. The person who modified this engine said to use 93 octane. I know a larger amount of fuel and air is going into each cylinder. The ports on the new intake are 1/4 of and inch bigger then the stock crossfire intake. Does using a higher octane make any real difference in performance on such modern cars, even those which have been modified like mine ?

[Michael F.]

Re: Premium ve Regular

ecm pulls timing if detonation detected and c5 including zo6 run fine with 87 but might be little slower in performance but most would never notice

[Michael W.]

Re: Premium ve Regular

I just saw a video in which the owner of a C5 Corvette says he uses 87 octane even though Chevrolet claims using lower octane will effect performance. I have and 84 Corvette which requires 87 octane. However the engine has been modified. It has and Xram after market intake along with and 85 fuel pump, in larged throttle bodies and fuel pressure regulator and a performance chip with a 8746 ECM.. The person who modified this engine said to use 93 octane. I know a larger amount of fuel and air is going into each cylinder. The ports on the new intake are 1/4 of and inch bigger then the stock crossfire intake. Does using a higher octane make any real difference in performance on such modern cars, even those which have been modified like mine ?

Unless the mods you've made are inducing detonation and causing the timing to be pulled as described by Michael above, using higher octane fuel would be a waste.

[Duke W.]

Re: Premium ve Regular

If you look at C5/6 owner's manuals most will say "premium fuel recommended, but not required", except Z06. The PCM has mulitple spark advance maps and the detonation sensors can either temporarily retard timing at high load or select a less aggressive spark advance map. I believe the C7 LT-1 engine is similar, but have not seen a C7 owners manual. The supercharged Z06 probably requires premium as do most current turbocharged engines. At worse you might be able to detect slightly less low end torque, and depending on driving conditions you might measure slightly lower fuel economy if you track it carefully. The above applies to virtually all modern cars with naturally aspirated engines.

[Michael J.]

Re: Premium ve Regular

My '12 ZR1 requires premium fuel.

[Brian D.]

Re: Premium ve Regular

I use 89 octane instead of 91, in my 2006 coupe. It's a base engine, not Z06. In 5+ years, I have noticed no reduction in performance, or torque.

[Duke W.]

Re: Premium ve Regular

I just saw a video in which the owner of a C5 Corvette says he uses 87 octane even though Chevrolet claims using lower octane will effect performance. I have and 84 Corvette which requires 87 octane. However the engine has been modified. It has and Xram after market intake along with and 85 fuel pump, in larged throttle bodies and fuel pressure regulator and a performance chip with a 8746 ECM.. The person who modified this engine said to use 93 octane. I know a larger amount of fuel and air is going into each cylinder. The ports on the new intake are 1/4 of and inch bigger then the stock crossfire intake. Does using a higher octane make any real difference in performance on such modern cars, even those which have been modified like mine ?

Modifications to improve air flow may have a small effect on octane appetite. As long as the CR has not been raised and the spark advance map has not been made significantly more aggressive, follow the owner's manual recommendation. I don't recall if '84 engines have detonation sensors (I don't think they do.), so if the owner's manual says premium, that's what you should use.

If it only recommends 87 PON regular, try a few gallons when the tank is near empty, and run this test for two or three small additions of 87. If you don't detect any detonation, 87 is okay. If you do detect detonation, try some midgrade (89), and test again until you determine by testing what the engine's octane appetite actually is.

Engines that lack detonation sensors need enough octane to keep the engine out of significant detonation. Higher octane than this offers no performance or fuel economy benefits for the additional cost.

My '88 MBZ 190E 2.6 has no detonation sensor and only an advertised CR of 9.2:1, and the owner's manual recommends 91 octane fuel. I only drive it during the winter months in SoCal, and even with a more aggressive than OE spark advance map it doesn't detonate as long as coolant temperature is below about 85C, and ambient air temperature is no more than about 70F. If above these levels, I get some transient detonation with my normal 2000 RPM shifts or if I pull from 1500 or less in a higher gear. The solution for the above is to upshift and downshift at higher revs - what I call "driving around the detonation", but a little transient detonation (about a second) will do no harm.

With the OE spark advance map the engine would not pull in fourth or fifth below about 2000, and fifth was not useable below about 45 MPH. With the more aggressive map it will pull from as low as 1200, so I can upshift sooner - into fifth at about 35, and it will pull from 30. Around town fuel economy improved from about 18-20 to 22-24. Highway fuel economy is unchanged at 26-30. Engine thermal efficiency is greatest at high load and relatively low revs. In fact, under most operating conditions the greatest thermal efficiency is obtained on the ragged edge of detonation.

As is typical of OE setups, the spark advance map and fuel recommendation is very conservative because the average driver doesn't understand how to recognize detonation and "drive around it".

Duke

[Joe L.]

Re: Premium ve Regular

Modifications to improve air flow may have a small effect on octane appetite. As long as the CR has not been raised and the spark advance map has not been made significantly more aggressive, follow the owner's manual recommendation. I don't recall if '84 engines have detonation sensors (I don't think they do.), so if the owner's manual says premium, that's what you should use.

If it only recommends 87 PON regular, try a few gallons when the tank is near empty, and run this test for two or three small additions of 87. If you don't detect any detonation, 87 is okay. If you do detect detonation, try some midgrade (89), and test again until you determine by testing what the engine's octane appetite actually is.

Engines that lack detonation sensors need enough octane to keep the engine out of significant detonation. Higher octane than this offers no performance or fuel economy benefits for the additional cost.

My '88 MBZ 190E 2.6 has no detonation sensor and only an advertised CR of 9.2:1, and the owner's manual recommends 91 octane fuel. I only drive it during the winter months in SoCal, and even with a more aggressive than OE spark advance map it doesn't detonate as long as coolant temperature is below about 85C, and ambient air temperature is no more than about 70F. If above these levels, I get some transient detonation with my normal 2000 RPM shifts or if I pull from 1500 or less in a higher gear. The solution for the above is to upshift and downshift at higher revs - what I call "driving around the detonation", but a little transient detonation (about a second) will do no harm.

With the OE spark advance map the engine would not pull in fourth or fifth below about 2000, and fifth was not useable below about 45 MPH. With the more aggressive map it will pull from as low as 1200, so I can upshift sooner - into fifth at about 35, and it will pull from 30. Around town fuel economy improved from about 18-20 to 22-24. Highway fuel economy is unchanged at 26-30. Engine thermal efficiency is greatest at high load and relatively low revs. In fact, under most operating conditions the greatest thermal efficiency is obtained on the ragged edge of detonation.

As is typical of OE setups, the spark advance map and fuel recommendation is very conservative because the average driver doesn't understand how to recognize detonation and "drive around it".

Duke

Duke-----


A knock sensor was first used on Corvettes for the 1982 model year. That sensor was GM #1997463, aka Delco 21-388. As far as Corvettes go, it was a one-year-only part and uniquely configured. For 1984 it changed to GM #1997562 and what became a more "traditional" configuration, at least for later 80's and 90's Corvettes. The 1984 sensor is currently available as GM #10456287, aka Delco 213-325. The 1982 sensor is GM discontinued and difficult to find.