Boosting octane level
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Laughing Gas
Joined: May 2002
Posts: 873
Likes: 37
From: Ohio
Boosting octane level
I searched the forum and didn’t really find the answer to my question, so looking for active members to fill me in. I have a classic car with a 429 engine and for the 25+ years I’ve owned it, I have just purchased premium pump gas and added a lead substitute. Recently, I dug into it a bit and discovered that even premium fuel does not come close to what was recommended in the manual, as far as octane level. Here in Ohio, I don’t drive it more than about 500-800 miles each year and it doesn’t normally ping, but I wondered if the right gas would help the performance that much. I started adding an octane booster, but I don’t think there is one available that would raise it to level of around the recommended 100. Is there anything out there that would help raise it and also one that includes a lead substitute? I don’t run it hard, but it would be interesting to see what this engine is capable of doing with proper fuel.
Lead Driver
Joined: Nov 2000
Posts: 6,639
Likes: 349
From: Albuquerque
I think most octane booster claim about a 2 point increase in octane. What they don't say is it's only .2 . i.e. 92 to 92.2. Ive heard of a new product lately called race gas that might be worth looking into.
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Laughing Gas
Joined: May 2002
Posts: 873
Likes: 37
From: Ohio
I get that about the 2 pts. When you read it closely, .2 is really what they mean. Might as well save my money, not worth anything!
Lead Driver
Joined: Sep 2003
Posts: 7,018
Likes: 210
From: SoCal
Use octane booster only if the engine is knocking under load running the recommended timing. The only time you get better performance with higher octane fuel is if you can advance the timing to the higher knock threshold, up to a certain point.
The idea is to achieve peak combustion pressure just as the piston starts coming down from TDC, so the combustion can produce the most amount of work. But for a lot of engines, that amount of pressure can cause pre-ignition; the unburned portion of the A/F in the chamber spontaneously ignites from the pressure generated by the ignited flame front. So retarding the spark causes the peak pressure to occur after the piston has already started descending, reducing total pressure. But this wastes the combustion energy into heat, which makes the engine run hotter, which itself can cause pre-ignition in severe cases.
The other extreme (if your fuel can tolerate it) is too much timing advance, which causes the peak combustion to occur BEFORE TDC, but this doesn't happen very often.
The idea is to achieve peak combustion pressure just as the piston starts coming down from TDC, so the combustion can produce the most amount of work. But for a lot of engines, that amount of pressure can cause pre-ignition; the unburned portion of the A/F in the chamber spontaneously ignites from the pressure generated by the ignited flame front. So retarding the spark causes the peak pressure to occur after the piston has already started descending, reducing total pressure. But this wastes the combustion energy into heat, which makes the engine run hotter, which itself can cause pre-ignition in severe cases.
The other extreme (if your fuel can tolerate it) is too much timing advance, which causes the peak combustion to occur BEFORE TDC, but this doesn't happen very often.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Sometimes you can purchase avgas, 100LL, at a local airport. Don't tell them you're putting it in your car. You can also purchase specialty fuels for racing by the barrel, delivered to your door. Pricey stuff!
Thread Starter
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Laughing Gas
Joined: May 2002
Posts: 873
Likes: 37
From: Ohio
Use octane booster only if the engine is knocking under load running the recommended timing. The only time you get better performance with higher octane fuel is if you can advance the timing to the higher knock threshold, up to a certain point.
The idea is to achieve peak combustion pressure just as the piston starts coming down from TDC, so the combustion can produce the most amount of work. But for a lot of engines, that amount of pressure can cause pre-ignition; the unburned portion of the A/F in the chamber spontaneously ignites from the pressure generated by the ignited flame front. So retarding the spark causes the peak pressure to occur after the piston has already started descending, reducing total pressure. But this wastes the combustion energy into heat, which makes the engine run hotter, which itself can cause pre-ignition in severe cases.
The other extreme (if your fuel can tolerate it) is too much timing advance, which causes the peak combustion to occur BEFORE TDC, but this doesn't happen very often.
The idea is to achieve peak combustion pressure just as the piston starts coming down from TDC, so the combustion can produce the most amount of work. But for a lot of engines, that amount of pressure can cause pre-ignition; the unburned portion of the A/F in the chamber spontaneously ignites from the pressure generated by the ignited flame front. So retarding the spark causes the peak pressure to occur after the piston has already started descending, reducing total pressure. But this wastes the combustion energy into heat, which makes the engine run hotter, which itself can cause pre-ignition in severe cases.
The other extreme (if your fuel can tolerate it) is too much timing advance, which causes the peak combustion to occur BEFORE TDC, but this doesn't happen very often.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Guess I don't understand the reasoning. If the engine calls for 100 octane as you say and you want to try it, that's going to be about the only way. Avgas or race gas, nothing else is going to come even close. If there's a race track near you there may be a retail pump that sells it.
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Lead Driver
Joined: Mar 2004
Posts: 8,924
Likes: 153
You really need to know how much compression the motor has ? 100 octane ? wow that's a lot for a motor I presume hasn't had much if anything done to it.
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Laughing Gas
Joined: May 2002
Posts: 873
Likes: 37
From: Ohio
Guess I don't understand the reasoning. If the engine calls for 100 octane as you say and you want to try it, that's going to be about the only way. Avgas or race gas, nothing else is going to come even close. If there's a race track near you there may be a retail pump that sells it.
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Laughing Gas
Joined: May 2002
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Likes: 37
From: Ohio
Last edited by unibody62; Jun 12, 2018 at 03:14 PM. Reason: Attach picture
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Originally Posted by unibody62
My reasoning is based on the reply by xlt4wd90, stating that as long it isn’t pinging, 100 octane isn’t going to give me more HP. I have no expertise in performance, so I’m engaging the help of the members. The other factor is the price of AV or race gas. I can imagine it’s expensive, but haven’t priced it yet.
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Laughing Gas
Joined: May 2002
Posts: 873
Likes: 37
From: Ohio
Lol. Tedster, you are making me feel bad for treating my Ford that way. I’ve owned the car since 1992, and it had just 24,000 miles on it then. All the tuning is according to the spec. The carb has been rebuilt, so possibly they re-jetted it but I don’t recall. I feel bad now 😢. I need to take better care of my ride.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
I have to wonder, since even the operator's manual calls for 100 octane, that the ignition timing hasn't been retarded at some point, in order to run 87 pump gas. Ignition timing has to be advanced in order to take advantage of high octane fuel all things being equal.
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Laughing Gas
Joined: May 2002
Posts: 873
Likes: 37
From: Ohio
Yes, just to clarify, it does not run well at all on 87 pump gas. I run the premium fuel plus (lately) an octane booster. Isn’t high test usually 93 octane? And I always do the timing myself, so it’s according to spec. No wonder you were confused, thinking 87.
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Laughing Gas
Joined: May 2002
Posts: 873
Likes: 37
From: Ohio
I had some time sitting at the tire shop, so I did some browsing for products. This Royal Purple sounds promising, boosts 30 pts and also has a lead substitute. This may work.
https://www.summitracing.com/parts/rpo-11757
https://www.summitracing.com/parts/rpo-11757