Timing and temperature question

[sparkydave]

This is probably one of those questions that has probably been asked before, but what is the relationship between timing and how hot the engine runs? I've heard that having the wrong timing can cause the engine to run hot, and I assume under that condition that extra heat translates into lost horsepower, but is this when the timing is too advanced (fires too early) or too retarded (fires too late)? I've been wondering what effect changing the timing on mine would have, since the 75-79 models seem to have a wide range of timing specifications. I've got mine at 12 degrees before, but wondered if there was a more optimal setting that might help it run a little more efficiently.

 

[dklawson]

I believe that the more advanced your timing, the hotter the engine runs. Other issues with highly advanced timing are that the engine becomes harder to start and it will ping earlier under load. If you retard the timing the engine is much easier to start but has less power.

I think most engines benefit from running with as much advance as they can stand without pinging. On my Mini I followed the advice of several who suggested bringing the engine up to about 4k RPM and setting the dynamic timing at that speed to 30 BTDC. I have no idea where this puts the static or idle advance. What you've done at that point is set the engine for what most consider to be its maximum acceptable advance. If it pings under load when you do that, you gradually, experimentally back off the timing in 2 or 3 degree steps and repeat your test drives until you're running without pinging under load.

An Australian friend of mine has his timing so far advanced that he has an interrupter switch on the ignition circuit. He doesn't close the switch (allowing the ignition system to fire) until he's got the engine really turning on the starter.

 

[77_MG_Midget]

You can somewhat compensate for the engine running hotter when you over advance engine timing by changing to a cooler burning spark plug. I'd caution anyone about advancing the timing so far as to get spark knock (pinging) under load.

Spark knock is caused by the pre-detination of the fuel, where it burns from one side of the cylinder to the other, instead of from the center of the cylinder outward. This can somewhat be remedied by increasing the octane of the fuel. Octane helps prevent pre ignition of the fuel and helps to stabilize the fuel to burn in the proper pattern.

When fuel ignites on one side of the cylinder first, it "throws" the piston skirt into the cylinder wall, resulting in the "knocking" you'll here. This is VERY VERY VERY bad on an engine and can result in a cracked cylinder wall, broken piston, broken rod, etc.... Never let your engine "Ping" if you want to have it around for a long time. Just my 2 cents worth... /ubbthreads/images/graemlins/smile.gif

 

[aeronca65t]

The book-answer for a '77, non-California spec 1500 Midget is 10 degrees BTDC at 800 RPM.

Like Doug, I set my car so that I know what max advance is....BL lists the standard max timing for a 1500 Midget for 25 degrees at 3300 RPM (but you could easily run a bit more).
Your distributor allows about 16 degrees of total advance, so setting idle advance at about 11 degreees should be in the ballpark for the 25 degree max.
I set mine at 30 degrees (at 5500 RPM). I can still use 93 octane street gas with high-comp pistons and a milled head. When I run leaded race fuel, I run 32 degrees. I've never actually checked my timing at idle. The car starts easy, so I assume it is OK.

The trick of spinning the engine with the starter before hitting the ignition is used in many older aircraft engines (because they run fixed timing, which tends to be a bit too advanced for easy starting).

I have read that excessivly retarded ignition timing can make a car run slightly hot, but I don't recall the reason. It could also cause low power and poor fuel economy.

Too much advance is more risky (but it is tempting, since the car may have a bit more power). As Doug has said, it can cause detonation (that can destroy pistons). My car is unmuffled and I wear a helmet, so it would be pretty hard to hear pinging (especially with my old ears!). Within reason, less advance is always safer.

 

[77_MG_Midget]

Just another thought about over-advanced ignition timing.

Keep in mind that pre detonation occurs when the air/fuel mixture combusts too early or too quickly during the compression stroke. The flame front collides with the upward-stroking piston, increasingly cylinder pressures to levels which can destroy a motor.

Also, due to the increased temperature, unless you change to a colder burning spark plug and richen up the air/fuel mixture, you greatly increase the chances of burning a hole through the top of the pistons, especially if the air/fuel mixture is too lean.

 

[Dave Russell]

Hi Dave,
Too much advance will cause ping, explosive detonation of the mixture, instead of a quick controlled burn. This greatly increases cylinder pressure at a time when the piston, rod, crank are not in the proper mechanical positions to accept the high pressures. Maximum cylinder pressure should occur at somewhere around 20 degrees after TDC. Before this point, the pressure is just trying to push the crank down instead of turning it.

Retarded timing will cause the mixture to ignite late & burn slowly. The mixture will still be burning as it exits the exhaust valve. This can put a great deal of extra heat into the exhaust ports & manifolds. Some of this misplaced heat is transfered to the cooling system. Needless to say, this lost heat reduces power. I have seen engines with retarded timing that had the manifolds glowing bright red.

The optimum advance is likely somewhere in the 30 to 36 degree range at 4500 rpm.
D

 

[Matthew E. Herd]

My exhaust manifold (not cast iron) glows in the dark, and my total advance is approximately 34 degrees on 93/94 octane gas. That may simply be a case of low heat capacity, header wrap, and hot exhaust (nothing to worry about). At any rate, I've got 2000 miles under my belt with this setup, so I've been pretty happy with it.

 

[sparkydave]

Thanks all for all the useful information. I knew that advancing too far caused detonation and that can cause damage. Note to Aeronca, I got mixed up on the spec, and it is at 10 degrees. Now I remember I had to set it halfway between 8 and 12 to make 10 degrees. Maybe something to experiment with a little, but this weekend's project is putting the front end back together (finally!).