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From Hughes Engines
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World of Mopar wedge engines
Issue #7 Let's talk Octane
I’m amazed at the number of people who think nothing of spending thousands of dollars on machine work and high performance parts and then try to “save money” (by handcapping) the engine with 87 octane gas. (Probably read it in a chat room).
87 Octane is made for low compression, low output engines where cheap operating costs are important, tire shredding power and quick ¼ mile ets are NOT important.
Do they think they know something no one else does? 87 octane is as good as 93 or 110 octane, just cheaper! (Maybe they read it in a chat room, too!) If they wanted to save money, forget the stroker kit, roller cam, and ported heads. Get an electric car they accelerate like a dragster - and the chinese manufacturer of batteries - will love them!
What is octane? What does it mean? The octane rating tells you how much pressure (in the combustion chamber) the fuel will tolerate before it explodes (That is called detonation) instead of burning very rapidly which is what you want. “So what? Say the chatters,” “don’t diesels live on detonation?” Yes they do! Chaters in their ignorance have stumbled into some truth. Have you ever compared the weight of a diesel piston or connecting rod to a gasoline engine part? Why? Detonation is like a sledgehammer driving the piston down and the engine’s parts must be VERY STRONG AND HEAVY to tolerate this hammering. Not only that but diesel engines are not built or designed to turn the rpm of a gasoline engine. You can forget 5000+ rpm.
Gas engine parts will be destroyed when detonation occurs. The goal is to get the air/fuel mixture in the combustion chamber to burn as fast as possible without exploding-detonating.
How do you do that? The best answer is to raise the octane. This means the gasoline in the air/fuel mixture can tolerate more pressure in the combustion chamber (higher compression ratio) before it explodes - detonates. There are also some mechanical changes that can be made to the engine that will help decrease the potential for detonation. Polish any rough cast areas in the combustion chamber and run the piston as close as possible to the cylinder head.
A small, tight, compact, smooth combustion chamber will tolerate more cylinder pressure (Higher compression) before detonating than a large, rough chamber.
So, am I saying you could build a higher compression engine and still use 87 octane? Yes, it is possible to use 87 octane in a “relatively” higher compression, higher than normal for an 87 octane engine. This would take extra work and experience of the engine builder and cost a “few dollars more.” And for the difference in time and cost, you could probably afford 93 octane or higher.
Octane rating of gasoline tells you how much pressure (in the combustion chamber(PSI)) causes gasoline explosions instead of burning which is detonation.
There are some things you can do when building an engine to prevent or at least reduce the potential of detonation. If the spark plugs heat range is too hot that will encourage detonation. The combustion chamber must be very smooth.
A tight piston-to-head clearance reduces the amount of the air/fuel mixture that can enter this area to burn, thus in the contrary reducing the heat in that area which discourages detonation. The tight piston-to-head clearance also speeds up the burn. If you are limited to 87 octane gasoline following these steps will allow you to make more power without detonation - but that is a lot of work and it is easier to find the higher octane gas.
Then there is the poor missguided individual who thinks he can build a high compression engine that needs 110 octane gas when he’s racing but only needs 89 octane gas when he is cruising. Yes, they are out there! Where do these people come from? They may be in chat rooms “helping” others build their engines too!
Now here is something that I’m sure never happens so I don’t know why I’m mentioning it, but, if you have a service(gas) station and the supply truck with tank full of 87 shows up and has left over 87 octane(which you must pay for even if you can’t take it all) what will you do?! Just put it in another tank - The 93 octane tank, it won’t hurt anything - will it? But that never happens, does it? At least not at the station where you buy gas, right?? Try another station if your engine starts pinging since your last fill-up.
The faster the air/fuel mixture burns the less spark advance is required, which is a good thing, power wise.
Higher octane fuel can tolerate more combustion chamber pressure before it detonates. (i.e. higher compression ratio).
