a question of octane

[HMeeder]

Should I be using premium?

The original owners manual insists on a minimum of 98 octane, but that ain't happening in sunny SoCal unless you buy racing or aircraft fuel. From what I understand, octane ratings are not figured the same way now as they were when our cars were new.

So, anyone know what the real deal is? I know that modern fuel management systems make premium fuel a waste of money except for only the most high performance cars, but that wouldn't apply to us. I have tried tanks of hi-test and regular back to back, but can't discern a difference.




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Herb
'72 1.7 Tangerine 'Teen
'74 2.0 Red Rustmobile

[agent914]

To keep it simple. If you engine doesn't detonate or have pinging w/87 octane, use it. If you have a motor w/ a higher compression ratio, turbo or s/c then it is recommecded a higher octane be used.

What the higher octane gas does is slow down or delay the burn rate of the gasoline. The higher the octane the later it explodes.

[John Rogers]

A good place to test for the spark knock is to climb up the hill on I805 over I8 in high gear which puts about the heaviest load on the engine, much more than revving at the Q. If it knocks you'll hear it and on our stock 914 it knocks so I run the "NEW" 91 octane. See you tomorrow....

[Will98D]

Dave Darling's informative website covers this. Go to http://www.dgi.net/914/tech/faq.html
and then read number 20. The manual probably is quoting the German ROZ, which would be more like mid-grade in the US. He has a chart that spells out what fuels with what engine, too.

[Kevin@ojai.net]

Yea, Whats the deal with the 91? I want my 92 octain back!

-Kevin

[HMeeder]

I was mostly just curious. I have never gotten any pinging under any load.

The manual says "98 ron" whereas the pump says "91 roz/ron" I want to know who this Roz person is and what is she doing with Ron?

An idle mind is a dangerous thing.



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Herb
'72 1.7 Tangerine 'Teen
'74 2.0 Red Rustmobile

[Dave at Pelican Parts]

There are three different octane ratings for any given fuel: RON, MON, and AKI. RON is the Research Octane Number. MON is the Motor Octane Number. AKI (Anti-Knock Index) is the average of those two, and it's what US pumps are labeled in.

ROZ is the German term for RON. (MOZ is the German term for MON.)

For any given batch of fuel, the RON rating will be about 8-10 points higher than the MON rating. This means the AKI will be 4-5 points lower than RON for the same fuel.

If the owner's manual calls for 98 RON, you'd prefer to run 93 AKI (US pump octane). Since we can't get that, try the 91 and see if you get knocking or pinging when going uphill on a warm day. If not, stick with the 91--or run it almost dry and try 89 and see what happens. If you do, you may have to retard the timing or start adding octane booster.

Note that the US-spec 2.0 requires 91 RON, which is 86-87 AKI. Regular-grade fuel is just fine. (Well, mid-grade in those areas that carry 85 octane fuel as their low-end stuff.)

--DD

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Pelican Parts 914 Tech Support

A few pics of my car: http://www.pelicanparts.com/gallery/Dave_Darling

[campbellcj]

FWIW I run 87 (regular) in my stock-engined 73 2.0 most of the time; never had any pinging. It's a low compression engine...

I do usually put at least 89 (mid-grade) in it when I'm doing hard canyon/mtn driving or track events, especially when it's hot out. I may be wrong on this, but my understanding is that pinging/knocking is more likely when you're running at say 230-240 degrees than 180-190...I figure it's cheap insurance anyways.


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Chris C.
73 914 2.0
70 911E Targa
campbell.chris@gte.net

[sammyg2]

I ran premium gas (91 oct.) for the first six months I had my current car, and then decided I didn't need it and switched to the midgrade. I still run premium through about every third tank, because I like the additives. My engine doesn't seem to mind the mid grade at all, and I have never heard any pinging no matter how bad I lug the motor, like when I leave a stop light in third gear. My motor has 9 to 1 compression ratio, BTW. ;-)


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Gerald Gore II (Sam)
73 914 350 small block