Octane Explained

Date: Thu, 29 Oct 1998 14:43:21 -0500
From: "Power British Perfrormance Parts, Inc."
Subject: Re: TR-2,3,4A - Retard timimg for use with 92 octane gas?

John,

You should use the lowest octane fuel you can provided the engine shows no signs of detonation. Octane actually makes the fuel harder to ignite, hence it's use in very high performance engines. These engines make their power from the compression, not the octane in the fuel. An engine's power output is a funtion of the pressure created in the combustion chamber above the piston. The more compression, the higher the pressure at the moment of ignition and the more "push" down on the piston.

As the air/fuel mixture is compressed, the temperature goes up. The more compression, the higher the temp. At some point, the fuel may spontaneously ignite prior to the spark plug firing. This is detonation and it is very bad for an engine because it often occurs just before the plug fires. You then end up with two flame fronts in the cylinder that crash into each other which can crack pistons and rings, etc. Most often in TR engines, I find cracked ring lands (the area between the topmost two rings) to be the most likely victim of detonation damage.

Octane is added to the fuel to reduce the chances of detonation. In doing so it actually slows the combustion process. The lower the octane, the faster the flame front spreads thoughout the combustion chamber - this yeilds higher cylinder pressure just after the piston passes TDC, and theoretically more power for any given compression ratio provided there is no detonation. Bottom line - for best performance, run the lowest octane you can that does not cause detonation.

As far as timing goes, you must do some experimentation to find the best combination for your engine. Most motors will make good, reliable power with a full advance of 28 - 34 degrees total timing. Double your distributor advance, then subtract from your desired total timing to find your initial timing. (e.g. If your distributor weights are stamed with a 12, then that is 12 degrees distributor advance. 12 degrees x 2 = 24 28 - 24 = 4 degrees before top dead center initial timing.) You must make timed runs and compare results to find what is the best for you. The amount of advance the engine will tolerate depends on the quality of fuel you're using, the general condition of the engine, and the atmospheric conditions in which you live. First, make sure your carbs are tuned properly and that your car is fully warmed up by driving for a few miles. Then find a flat length of secluded roadway to make some tests. Start on the low end of the scale, set your timing, and make a timed acceleration run in top gear between 30 - 60 mph (helps to have an assistant in the car with you to record times with a stopwatch). Advance the timing a few degrees (try 2 degrees to start) and retest. If at any point, you hear detonation, stop the run and retard the timing until the detonation goes away. You will find your runs get faster to a point and then fall off again. If you continue advancing, detonation will set in. Once you find your fastest timed run, set the timing to that figure provided it is at least 4 degrees away from the point at which you heard detonation.

I bet if you spend an afternoon experimenting, you'll be surprised at how much better your car will run afterward!

BTW if you find that the car idles too high after setting to your best performance, you'll need advance weights with more timing built in to bring the idle back down again.

Regards,

Brian Schlorff

Power British

http://www.powerbritish.com/~britcars