[steve25]

The run on conditionis really is called dieseling, because just like a diesel motor the fuel and air mixture is lighting off without spark just like a diesel due to 4 things.
1) octane level.
2) cylinder temperature/ Intake air temperature.
3) cylinder pressure.
4) a combination of all of the above.
The other part of this all that is bad is that the motor is likley pinging to some level or the other at certain rpms and part throttle openings also !

[Formulajones]

Originally Posted by promptcritical

A little off subject perhaps. Why does a motor "run on" with "too much timing" when shutting it off? There is no spark when the motor is shut off, so why does the ignition timing matter?

Just like Steve said. There is unburned fuel in the combustion chamber, and if things are hot enough, it lights off after shut down.

A lot of people seem to want to run their engines with really high thermostats, couple that with the under hood heat, high ambient temps outside, and a low octane fuel that lights off easier, you get dieseling or run on.

[Chief of the 60's]

You left out
Running rich
Carbon build up
Idle too high

[Formulajones]

Originally Posted by Chief of the 60's

And that is why guys running E85 are constantly monitoring every gallon they put into their race car.

What most people seem to forget is that ethanol is alcohol and washes cylinders clean as they run resulting in short engine lifespans. Why do you think guys running alcohol need to re-ring their engines every few seasons.

You pick up more power but IMO its not worth the added wear.

Agree. I personally don't care for the stuff because it's just not consistent enough from tank to tank. I've simply learned to deal and tune with it at the 10% level that fluctuates a bit as I mentioned, only because that's forced upon us. I have no interest at all using 85% for anything.

[Formulajones]

Originally Posted by Chief of the 60's

You left out
Running rich
Carbon build up
Idle too high

There are lots of reasons, but I find if what I mentioned is kept under control then the 3 you mentioned here has less influence.

I have our Z running rich and idling high, and after 3 years of daily driving I'm sure it has some carbon build up. But it never diesels on shut off.

It's 11:1 compression with iron heads, and is run exclusively on 91 octane, no cheap 87 for this thing, I run 36 degrees of total timing in it, I have it idling around 1,000-1100 rpm, and I keep engine temps around 175 degrees pretty much all the time. The hottest it's run is 185 when it's 110 degrees outside. Shuts off perfectly.

[Chief of the 60's]

Originally Posted by Formulajones

There are lots of reasons, but I find if what I mentioned is kept under control then the 3 you mentioned here has less influence.

I have our Z running rich and idling high, and after 3 years of daily driving I'm sure it has some carbon build up. But it never diesels on shut off.

It's 11:1 compression with iron heads, and is run exclusively on 91 octane, no cheap 87 for this thing, I run 36 degrees of total timing in it, I have it idling around 1,000-1100 rpm, and I keep engine temps around 175 degrees pretty much all the time. The hottest it's run is 185 when it's 110 degrees outside. Shuts off perfectly.

That engine temp is really good for a Pontiac. Idle seems rather high. Hows that work for you? I am assuming its an automatic. Doesn't it hit hard going into gear?

[Formulajones]

Originally Posted by Chief of the 60's

That engine temp is really good for a Pontiac. Idle seems rather high. Hows that work for you? I am assuming its an automatic. Doesn't it hit hard going into gear?

It's a 4 speed. Stock idle is 900 but I bumped it up a tad, cam is a bit rambunctious.

[promptcritical]

Thanks for the replies. I do know what dieseling is and how it works. What does ignition timing have to do with it? Is it a second order effect? Like making the combustion chamber temperature "higher" leading to dieseling? The spark timing by itself won't affect anything since the ignition is shut off when it is happening...hence the name - dieseling.

["QUICK-SILVER"]

Originally Posted by promptcritical

What does ignition timing have to do with it? Is it a second order effect? Like making the combustion chamber temperature "higher" leading to dieseling?

Timing to slow will turn exhaust valves red hot.

Clay

[Cliff R]

BP gas has had 10 percent ethanol in it for many years. Many States don't or didn't mandate that they post that information at the pumps. When all this first started there were only about two dozen States that labeled the pumps as containing up to 10 percent ethanol.

Whether it did or didn't used to have ethanol in the fuel really doesn't matter in the big scheme of things. I've been using 93 octane in my engine from BP for quite a few years with zero issues. I've even put 89 octane in it and once in a while 87 octane.

It runs fine on both, but with 87 octane in the hot/humid summer months and engine fully warmed up and heat soaked I've had it "run-on" a couple of times at shut-down. Doesn't do that with high octane and the tune is perfect. I'm a little outside the "norm" for compression, it calculates at 11.3 to 1 if you do the math on it, 455 CID, 74cc heads and flat top two valve relief pistons, etc.

The bad news with fuels containing ethanol is long term storage. It will separate and also soak up some water in a vented system. I deal with this all the time in the shop as we're well known in this area for getting small power equipment up and running when the owners pull them out and they will not start or run well.

Something related to this thread that's very interesting is that we've had to richen up quite a few small power equipment engines to run on this "new" fuel. Case in point. Recently a customer we've done some carb, transmission and engine work for drags a Honda 3000 Watt generator up here. It's seen little use but will not start.

He put Stabil in the fuel and put it away a few years ago. I found nothing wrong with it other than the fuel wouldn't burn. Yes, we drained some out and dumped it in the gravel driveway and it was very difficult to light. It acted more like kerosene than gasoline so we drained the tank, put in fresh fuel, cleaned out the carb and it fired right up.

It as "hunting" terrible and when we hooked two 1500 Watt heaters up it would only carry 92 volts. I very gently "tipped in" the choke flap and it came right up to 110 volts. So I removed the carb and increased the jet orifice by .001". This helped but didn't cure it. Went another 001" and better, then opened it up one more time and it would carry full load at the right voltage. It also quit "hunting" at no load.

Before I made the tuning changes I mentioned to the owner that it "hunted" a lot at no-load and he said it's been doing that but was fine under load. Well it wasn't "fine under load" clear down at 92 volts, glad he didn't try to run any electronics with it during power outages!

I've had to do the EXACT same thing scores of times with these small engines to get them up to par for this "new" fuel. It's easy with some that have adjustable carburetors like chainsaws and string trimmers (most require removing the limiter caps), but many are "fixed jet" and you are going to have to get your precision bits out or go thru the drama of hunting up larger jets.

It's not much different with tuning our Pontiac or any other automotive application. They will more times than not enjoy more fuel and often a little more timing on this new fuel than they needed with older fuel blends. Some actually run very poorly until you fatten them up some, others are OK with older tunes on them but even those more times than not run better with you throw a little more fuel at them.......FWIW........Cliff

[TAQuest]

Originally Posted by promptcritical

A little off subject perhaps. Why does a motor "run on" with "too much timing" when shutting it off? There is no spark when the motor is shut off, so why does the ignition timing matter?

Octane booster will stop this.

When I set my timing by ear, part of what I listen for is dieseling on shut down.

When I hear dieseling on shut down I either retard the timing, add octane booster, or both.

[TAQuest]

Originally Posted by "QUICK-SILVER"

Timing to slow will turn exhaust valves red hot.

Clay

This is how you know you went too far retarded. The headers will glow.

[AG]

Originally Posted by TAQuest

Octane booster will stop this.

When I set my timing by ear, part of what I listen for is dieseling on shut down.

When I hear dieseling on shut down I either retard the timing, add octane booster, or both.

I would be skeptical about the octane boost claims of additives. For example to increase 16 gal of 93 octane to 95 octane, a 12 oz can would have to have an octane rating of 116 which is plausible. To increase 16 gal of 93 octane to 100 octane, that 12 oz can would need to be 170 octane which is impossible. You need to know how much the additive will increase your gas to know how much you need for your requirements.

[TAQuest]

Originally Posted by AG

I would be skeptical about the octane boost claims of additives. For example to increase 16 gal of 93 octane to 95 octane, a 12 oz can would have to have an octane rating of 116 which is plausible. To increase 16 gal of 93 octane to 100 octane, that 12 oz can would need to be 170 octane which is impossible. You need to know how much the additive will increase your gas to know how much you need for your requirements.

It's trial and error.

First you find a reason you need it, use your ears.

Add a bottle and listen again. If it helps, you nailed it. If not then it wasn't an octane issue to begin with or you need even more booster and/or more tuning.

It's a combination of listening, tuning and octane adjustment.

It works.

If you have run on there's a good chance you have detonation also. It's a clue.

For example;

I have a high compression 400 with a small cam. It runs perfect but when using pump gas it pings and runs on. I retard the timing until I start losing performance. Bad throttle response is the first thing you notice. Then I go with the most retard I can and keep performance. If I still have ping and run on I add octane booster until the ping goes away. The run on goes away also. Then after a few runs I check the plugs for signs of detonation. If they look good then I have found my baseline. I always keep some octane booster in the car because you never know when a gas station is going to mess you up with something different your engine doesn't like.

[PDC]

My reluctance to use any of the parts-store octane boost products is that I am unaware of a single one that does not coat your spark plugs (and combustion chambers?) with those nasty red powdery deposits.

[TAQuest]

All octane booster isn't created equal, just like fuel.

There's a lot of things to consider so you have to research the boosters. I did that and have found that 108 at Advance works great for me. Not just on paper, in the tank.

[TAQuest]

Originally Posted by PDC

My reluctance to use any of the parts-store octane boost products is that I am unaware of a single one that does not coat your spark plugs (and combustion chambers?) with those nasty red powdery deposits.

Nope. Not enough to matter. Maybe if you drive it as a daily driver. I can't attest to that. My plugs do not get fouled enough to cause any problems.

Would you rather have some red deposits or pieces of metal because you didn't control detonation?

[Chief of the 60's]

Put a couple gallons of C10 or 12 in per tank. Screw that "trial & error" bottled crap.

Or go back to what I already mentioned and take a degree or two of timing out.

Or spend the rest of the summer like this while your car sits...

[TAQuest]

Originally Posted by Chief of the 60's

Put a couple gallons of C10 or 12 in per tank. Screw that "trial & error" bottled crap.

I like the racing fuel also when there's a pump available. Takes all the guess work out of it. But that's only if you use it by itself. Adding small amounts to a tank is a lot of trial and error. Worse than octane booster, in my opinion.

Nice dance you got going on in your video. You could do better showing yourself dancing around your car. I'll pay for that vid. lol

[Chief of the 60's]

My cars run fine. I don't have to dance. LOL

Besides, I'm a lover not a dancer.