Detonation vs Preignition? 62 84hp

[joelsplace]

I have been running 87 octane in my 62 A/C PG 84hp in the cold weather with factory ignition timing. No issues.
It hit 80 degrees yesterday and it started sounding like a rod knock from idle to about 1300 rpm. Above that no issues even at full throttle. I backed off the timing and put almost 8 gallons of 93 octane in it. The timing helped and the fuel cured it.
I've never had this issue before. Octane/timing issues have always shown up in high throttle situations.
How do I pull some timing out only at low RPM? Different weights or is this a overactive vacuum advance?

[bbodie52]

Higher ambient temperatures and increased engine heat can also promote pinging and other signs of detonation — especially under acceleration, when climbing long uphill grades, etc. I believe that the manual transmission 80 hp engines had a 8.0:1 compression ratio, but GM increased the compression ratio to 9.0:1 in the Powerglide cars. That is the same compression ratio found in the 110hp and 140hp engines. I have found that I can get away with the low octane fuels often in the cold winter months, but have to upgrade in the spring and summer to a higher octane fuel to avoid engine pinging. Your driving habits and hilly terrain, and the load you are carrying in your Corvair can all impact the type of fuel you will need.

There might be some carbon build-up and related deposits in the combustion chambers that are part of the cause of your experience with engine pinging in hot weather. There may also be some variables in the actual octane rating of the fuel brand you are purchasing. In any case, it is important to avoid engine detonation in the aging Corvair.

Make sure there are no air leaks in the upper shroud. The upper heater system hose is a common problem area that can cause the engine to run hot at that cylinder.

Colder spark plugs might make a difference... but probably not that much. Proper tuning settings are important for good fuel mileage and performance, and to avoid detonation. With an air-cooled engine, adequate fuel octane is also an important factor. I know fuel is expensive, but you may just have to tolerate the need for a higher octane fuel in your vintage Corvair.

[joelsplace]

Thanks Brad but I know all that stuff.
I probably gave too much info and asked too many questions at once.
1) Has anyone else had these symptoms? What fixed it?
2) Why does it do this with almost no load and then is fine under heavy load?
3) What is the best way to take out timing below 1500 without retarding overall timing?
4) How do I know if the vacuum advance is causing the issue by being overly sensitive? I guess I can unplug it but I really don't want to let it knock at all to test it.

You did get me thinking about spark plugs. I read that the theory behind extended tip plugs was that they ran cooler at high RPM and hotter at low RPM. I don't have extended tip plugs as far as I know (I've only had one out and it wasn't) but maybe I need to run cooler plugs. I always thought octane problems showed up only under high load high heat situations but this car will actually knock at idle.

[RexJohnson]

Sometimes high loads are more at low speeds than high speed. With modern car when they go into lock up converter in high gear at 1500 rpms it is easy to get an engine miss if the ignition is get old. Drop the same car at the same speed into a lower gear and the miss-fire goes away.

[Dennis66]

This just suddenly started? Have you pulled the cap and checked the movement of the advance system? It makes me wonder if maybe a spring could have broken on one of the weights. Dennis

[American Mel]

Thanks Brad but I know all that stuff.
I probably gave too much info and asked too many questions at once.
1) Has anyone else had these symptoms? What fixed it?
2) Why does it do this with almost no load and then is fine under heavy load?
3) What is the best way to take out timing below 1500 without retarding overall timing?
4) How do I know if the vacuum advance is causing the issue by being overly sensitive? I guess I can unplug it but I really don't want to let it knock at all to test it.

You did get me thinking about spark plugs. I read that the theory behind extended tip plugs was that they ran cooler at high RPM and hotter at low RPM. I don't have extended tip plugs as far as I know (I've only had one out and it wasn't) but maybe I need to run cooler plugs. I always thought octane problems showed up only under high load high heat situations but this car will actually knock at idle.

Two words, . . . . . . . Black Box!
3D programmable timing.
kmart356 has been preaching it for years.
https://www.cbperformance.com/product-p/2013.htm
See his latest thread here, viewtopic.php?f=225&t=19132

[joelsplace]

Ok Mel, I should have said easy way. I do agree that the Black Box is great.

Dennis, I didn't think to check the timing again so I guess a broken spring could be an issue. The symptoms and way it came about with higher ambient temps didn't have me thinking in that direction. Thanks for that.

Rex, That is true but this does it no matter how light I am on the throttle unlike octane issues I've had in the past that were totally load dependent and only showed up with a lot of throttle. Thanks!

[66vairguy]

I have been running 87 octane in my 62 A/C PG 84hp in the cold weather with factory ignition timing. No issues.
It hit 80 degrees yesterday and it started sounding like a rod knock from idle to about 1300 rpm. Above that no issues even at full throttle. I backed off the timing and put almost 8 gallons of 93 octane in it. The timing helped and the fuel cured it.
I've never had this issue before. Octane/timing issues have always shown up in high throttle situations.
How do I pull some timing out only at low RPM? Different weights or is this a overactive vacuum advance?

If the "knock" only happens at light throttle, then yes the vacuum advance is probably the culprit. Over the decades I read about a few Corvair folks limiting the vacuum advance to cure your problem. Was it a weak vacuum can spring or just a just a normal issue??? I don't know.

If you go full throttle from a start and there is no ping off idle to 1,300 RPM, that would confirm a vacuum advance issue. Keep in mind sometimes the vacuum advance mechanism gets "sluggish" off idle so you get too much advance for a second or so.

Detonation is an explosive burn instead of smooth burn and makes a sharp metallic noise. Pre-ignition is when the timing is so advanced the combustion happens too far before TDC creating a very high compression pressure. It is usually silent (water cooled enigines) and will eventually put a hole in the piston, not to mention beating up the rod bearing. Back in the pre computer/knock sensor days folks would advance the timing 2-4 degrees for more power. It worked until they "holed" a piston.

[joelsplace]

This sounds like a serious rod knock. Nothing like normal detonation where it sounds like bbs in a can. Can't really think of a good way to describe the sound that I'm used to.

[Nashfan]

This sounds like a serious rod knock. Nothing like normal detonation where it sounds like bbs in a can. Can't really think of a good way to describe the sound that I'm used to.

Does the noise go away if you let off the throttle? I had one like that on mine. Drove me bats ass because it only did it when cold, defied timing changes and no matter what, would go away when letting off the throttle. In other words, noisy ping like at 1000 rpm and let the throttle off at 1000 rpm. Noise went away completely just like flipping a switch!. I was beginning to suspect sticky wristpins because of this ping like noise going away when it got warm.
When I took the engine apart, yes there was a bunch of sticky wristpins! One was so tight it felt like it was ready to seize the engine. This probably didnt help either.

[joelsplace]

It has only happened a couple of times. The first time it kept knocking even at idle. I thought for sure a rod bearing let go.
It did it after running the A/C for 10 minutes or so. I turned off the A/C and got it above 1300rpm or so and it quit. The next time it did it was on a warmer day without the A/C. I put in almost 8 gallons of 93 octane and it quit doing it within about a mile and hasn't done it since and I've driven it in much warmer temperatures.

[66vairguy]

Nashfan makes a point about "piston slap" due to tight wrist pins. I ran into that once.

Joel, if the issue has gone away with new gasoline, it might have just been a bad batch of gasoline.

A buddy called the other day to say he had not started his Corvair in a year and it was running awful. I told him that even with a gasoline preservative gasoline should not be burned in an engine after about 8 months when the gasoline is in a VENTED tank. As much as I hate to pump a tank out, I've done it rather than risk running an engine on old fuel.

[joelsplace]

Mine was doing it hot not cold so the wrist pins are out.
The gas was fresh and worked fine for the first 8 gallons.

[Vairone]

Detonation is an explosive burn instead of smooth burn and makes a sharp metallic noise. Pre-ignition is when the timing is so advanced the combustion happens too far before TDC creating a very high compression pressure. It is usually silent (water cooled enigines) and will eventually put a hole in the piston, not to mention beating up the rod bearing.

In very simple terms:
Detonation occurs after the spark event, pre-ignition happens before the spark event.

Because detonation occurs after the spark event, it can be prevented by reducing the timing. Thus, timing which is too far advanced may cause detonation. Detonation occurs slightly before, or slightly after TDC.

Since pre-ignition happens before the spark event, it can not be stopped by reducing the timing. Pre-ignition can occur near BDC and is not caused by excessively advanced ignition timing.

Both detonation and pre-ignition will sound similar to the naked ear.

Combustion pressures are much higher during pre-ignition, than detonation. In high output engines pre-ignition will damage an engine in a few seconds - or less. The same engine could survive running into detonation for much longer without damage.

[azdave]

I don't think anyone has mentioned carb jetting as part of the equation but a richer mixture will run cooler but won't help your gas mileage. I run 1-2 steps larger than stock on my 110 and 140 engines here in Phoenix. I sometimes will hear 2-3 knocks when leaving a stop light on a hot day. It is when the timing is transitioning between no-load idle and loaded engine settings. It's fine once I'm more heavily into the throttle.

[joelsplace]

I'm not having any issues where a main jet would help. This is at idle and just off idle.
New info: Mechanical timing does not advance too quickly. It actually advances much slower than it should. No mechanical advance where the knock occurs. It does have vacuum advance on just opening the throttle as you would expect. I'm thinking it needs a restriction in the vacuum line to slow down the advance along with a check valve that lets the vacuum dump quickly so it can back off at normal speed. Anyone tried that?
I tried a tank of 89 octane and the problem returns but not as bad.
Price premium is .80 on 93 and .40 on 89.