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Gasoline octane

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jhlash
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Posted on Wednesday, July 14, 2004 - 09:38 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

Recently purchased a 1955 Chris Craft 17' Sportsman with a KBL (131 HP) engine. The original manual states to use 92 octane (research method) gasoline. I know todays gasolines use a difference system for determining octane rating. What gasoline should I be using today for this engine?
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eddie
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Username: eddie

Post Number: 8
Registered: 04-2003
Posted on Thursday, July 15, 2004 - 05:04 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

jh,
If you don't hear pinging when you gun the accellerator in gear or cruising at top speed you're OK. Even a slight ping on hard accelleration is fine. In fact you can pretty much go with the lowest octane that falls into this guideline and be better off. You just don't want a lot of preignition detenation ( pinging ).
Eddie
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mart
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Posted on Sunday, August 08, 2004 - 03:18 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

I agree with "Eddie" about using any grade of gasoline that doesn't cause pre-ignition and pinging, but your statement about modern gasolines using a different method for rating octane is wrong. The "Research" method was and is still used and octane is octane. Research octane is the theoretical resistance to knock based on the percentage of iso-octane in the fuel. Motor octane is the actual knock resistance of the fuel, as tested in a special variable compression test engine and is always a few points lower than the theoretical "research" rating. The octane rating listed on most fuel pumps (pump octane) is the average of the research and motor octanes. This is the way it is and always has been done, at least back as far as the 1930's.

By the way, as a rule of thumb, most modern regular grade gasoline has a pump octane rating (R+M divided by 2) of about 87 to 89 and most premium fuel has a pump octane (R+M divided by 2) rating of 92 to 94. Most regular grade fuel would come in with a straight research octane of pretty close to 92 your manual says you need. Don't forget too, you can play a bit with your ignition timing to get the best use out of the available fuel. You can retard the timing a bit to get by on lower octane fuel (with a slight loss in power and fuel economy) and advance it a bit to get the best power and fuel economy if you have better, higher octane fuel.

One possible concern though that is not necessarily octane related - Your engine was probably designed to run on leaded fuel. Lead was added to increase octane, but as a side benefit, it also lubricated the valve seats. Modern unleaded fuel can be had with almost any octane required, but the lack of lead to lubricate the valve seats can be a problem in older engines without hardened valve seats. For the time being you can add a bit of commercial "lead substitute " to protect the valve seats. Make sure that the additive you buy is a "lead substitute" designed to protect valve seats and not just an "octane booster" that only increases octane. When the time comes for a rebuild, have a machine shop add hardened valve seats intended for a steady diet of modern unleaded fuel
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Gary Patterson
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Posted on Wednesday, August 11, 2004 - 11:37 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

I agree with most of what Mart stated with the exception of the research method test being "theoretical". Both octane systems (Research and Motor) are determined on a CFR variable compression ratio engine, and reflect an equal knock intensity of an iso-octane/n-heptane mixture. There is a special fuel delivery system that holds the test fuel being evaluated, iso-octane, and n-heptane, and blends the reference fuels to enable switching between the fuels.

The difference between Research and Motor octane numbers is the test conditions. Research numbers represent non supercharged operation, and as such have supplied mixtures of ~52C, where Motor Octane has a much higher mixture temperature of 1491C.

The ASTM Test Procedures for the two methods are:
Research Method: ASTM D2699
Motor Method: ASTM D2700

Mart's point about the hardened valve seats is on the money, as is the method to determing the pump octane number.
Recent work (by researchers such as Kalghati at Shell Oil) has shown that research numbers are much more refelctive of how modern non-boosted engines will knock.

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