What octane to use on the HHR
Senior Member
Joined: 06-21-2006
Posts: 3,100
From: Chicago
I use 87 for my everyday driving and will use 97 on trips. This trip to the dells I used 87 and averaged 31.4 mpg, then compare it to one of my last trips with 91 that averaged 32.9.
The old saying goes that the mileage you get directly relates to what your pocket book can endure.
The old saying goes that the mileage you get directly relates to what your pocket book can endure.
97% were do you get that
Senior Member
Joined: 07-13-2006
Posts: 1,567
From: Geneva, FL.
{" The mileage difference? In a practical sense, this bit about high octane giving more fuel economy than low octane is B.S. Sure, there might be a very slight difference but to measure it would take some very precise driving and very accurate measurement of fuel flow, both of which are beyond the common method of reading the mileage traveled divided by fuel used base on gallons listed on the gas pump at fill-ups.
Also, the savings in mileage are probably negated by the increased cost of premium gas."}
YES! Someone that knows about a reed vapor test for testing fuel!
unless you have a high comp. 10.5 + you won't see or feel anything.
Also, the savings in mileage are probably negated by the increased cost of premium gas."}
YES! Someone that knows about a reed vapor test for testing fuel!
unless you have a high comp. 10.5 + you won't see or feel anything.
Platinum Member
Joined: 05-09-2006
Posts: 6,805
From: "Upland" Mesa, Arizona
There seems to be some misunderstanding about the 2.4 Ecotec in HHRs and octane.
GM recommends 91 because that is the octane which gives best performance. For about 20 years GM engines have used feedback control of spark advance. A "knock sensor" (KS) detects detonation and when the ECM "sees" a KS input, it retards the spark a degree or so. If the detonation goes away, the spark advance is held there momentarily then advances back to the base timing. If that level of "knock retard" (KR) doesn't stop the detonation, the EMC dials back the spark a little bit more, then "listens" again. This process goes on continuously whenever the engine is running.
Knock retard allows the HHR's engine to run on 87-octane with a spark advance curve optimized for 91 without detonation.
What's the tradeoff? Retarded spark under high load when running 87 octane.
The mileage difference? In a practical sense, this bit about high octane giving more fuel economy than low octane is B.S. Sure, there might be a very slight difference but to measure it would take some very precise driving and very accurate measurement of fuel flow, both of which are beyond the common method of reading the mileage traveled divided by fuel used base on gallons listed on the gas pump at fill-ups.
Also, the savings in mileage are probably negated by the increased cost of premium gas.
I only put 91-octane in mine when I know I'm going to run it real hard in warm or hot weather. Otherwise, I run 87 in cool weather and 89 in hot weather.
GM recommends 91 because that is the octane which gives best performance. For about 20 years GM engines have used feedback control of spark advance. A "knock sensor" (KS) detects detonation and when the ECM "sees" a KS input, it retards the spark a degree or so. If the detonation goes away, the spark advance is held there momentarily then advances back to the base timing. If that level of "knock retard" (KR) doesn't stop the detonation, the EMC dials back the spark a little bit more, then "listens" again. This process goes on continuously whenever the engine is running.
Knock retard allows the HHR's engine to run on 87-octane with a spark advance curve optimized for 91 without detonation.
What's the tradeoff? Retarded spark under high load when running 87 octane.
The mileage difference? In a practical sense, this bit about high octane giving more fuel economy than low octane is B.S. Sure, there might be a very slight difference but to measure it would take some very precise driving and very accurate measurement of fuel flow, both of which are beyond the common method of reading the mileage traveled divided by fuel used base on gallons listed on the gas pump at fill-ups.
Also, the savings in mileage are probably negated by the increased cost of premium gas.
I only put 91-octane in mine when I know I'm going to run it real hard in warm or hot weather. Otherwise, I run 87 in cool weather and 89 in hot weather.
BINGO, BINGO, BINGO
Somebody that finally makes sense and writes intelligently.
Oh, catching up on old threads and saw your posts SO, even though it's a bit late.....
Member
Joined: 08-30-2007
Posts: 61
From: Vallejo, Ca
This dates me but I remember when I had an Alfa Romeo back in 1970 and used leaded Chevron Custom Supreme at 110 OCTANE!! When they did away with it and the octane of supreme started it's long spiral down the AR started to run like a dog and I got rid of it. But 110 octane, it was like rocket fuel.
Senior Member
Joined: 08-29-2007
Posts: 256
From: CenCoast California
If you're not, I'm sorry, but your mileage test is not accurate.
Senior Member
Joined: 08-29-2007
Posts: 256
From: CenCoast California
This dates me but I remember when I had an Alfa Romeo back in 1970 and used leaded Chevron Custom Supreme at 110 OCTANE!! When they did away with it and the octane of supreme started it's long spiral down the AR started to run like a dog and I got rid of it. But 110 octane, it was like rocket fuel.
I'll betcha that stuff and 2 or 3 grams per gallon of TEL. Damn...it's wonder we all didn't die of lead poisoning.
Last edited by Hib Halverson; Sep 14, 2007 at 09:39 PM. Reason: added content and typos