ENGINE PING

[88 G-WAGON]

O.K I just had my engine rebuilt, block decked, heads massaged, .30 over, 1 larger than RV cam, Air Gap intake, Holley 4bbl, Edelbrock headers, no cat. The problem is that it has a slight ping in the what it sounds like is the #8 area, the timing is set at 6 and the vacuum hooked to manifold not ported (both ways have been tried no difference). I runs strong, the ping is mainly when in gear but can be heard sometimes at idle. The builder says that it is nothing and to drive the heck out of it, the engine has a warranty. The only thing that has not been changed is the muffler (Vector turbo) and it is about 6yrs old. I took it to a trans shop and they said that a tranny would not make this noise (new flexplate,no cracks) and that it could be the muffler, loose baffle transfering ping to the front, could this happen? HELP PLEASE.

[andy d]

mebbe the head was massaged and decked a bit too much. Put a tank of 91 octane gas in it and if the ping disappears, you will know it isnt the muffler or flex plate

[88 G-WAGON]

Have always used 91.

[Herk]

If the engine is actually "pinging" at idle you have a problem. Pinging is a term used to describe pre-ignition.

If it is pinging at idle it would tend to rattle like hell with even a sligt load (having you vacuum advence hooked up incorrectly might actually help this).

If changing the base timing (try backing it down 3-5 degrees) does not change this, you are not pinging.

If your timing is set at 6 degrees BTDC with the vac advance hooked to manifold. Its WRONG. The advance unit adds about 12-15 so either your base timing is 6-10 ATDC or your are idiling at 18-21 or so. Neither situation is desirable.

Because you say that connecting the advance to ported or manifold made no difference, I would suspect it doesn't work. Idle speed should have gone way up when you hooked it to manifold vacuum.

My money is on an exaust leak...

[GWChris]

I'm assumng you meant the timing was set to 6BTDC without vacuum hooked up. I'd bet on an exhaust leak too - it's amazing what they can sound like.

[kennyh]

If it is detonation, it is not "nothing" and will cause problems, beginning with overheating and ending with a hole or two in pistons.

I believe the vacuum advance is supposed to be hooked to ported vacuum, as it will need more advance with more throttle/higher RPM. Hooking it to manifold vacuum will give it full advance at idle, which it does not need, and less advance when wide-open.

Higher compression may require a change to higher octane to avoid knock. My wife's 98 GC with the 5.9 Magnum demands premium or it pings like crazy, and it's a computer-controlled engine that's supposed to use a knock sensor to eliminate knock.

Don't ignore it, if it is actually detonation. An exhaust leak would be not a large problem. Heck, Flint uses it as an audible location device on his Wag, i.e. we can tell who's coming before we can see him.

[GWChris]

One more time - ported vacuum and manifold vacuum are the same, except that ported is blocked at idle. It is only used to increase combustion temps at idle for emissions.

[Herk]

Not exactly. Ported or timed vacuum as it is sometimes called gets it's signal from a port which is above the throttle blade at idle. So in that respect it is "blocked" at idle. Then as the throttle blades open they expose the port to both manifold vacuum and a venturri erffect signal caused by high velocity air rushing past the port the result is generally a bit more depression than manifold vacuum alone. By the time the primaries are past 1/4 of the way open the airflow is away from the port and the vacuum will be similar to manifold vacuum or less. By 1/2 throttle the ported vacuum signal will be almost zero, regardless of manifold vacuum.

Think about why vacuum advance is fitted to engines. At light throttle such as cruise we want to be able to run a very lean mixture (17:1 or so) for fuel economy and cool running (actually lean of peak EGT). Your power valve is closed in this condition. This lean mixture burns more slowly than the normal (power valve open) mixture, and requires more advance. Once we increase load on the engine, we want the mixture to richen for optimum power (13:1 or so) so the power valve opens. Now we want the vacuum advance totally out of the picture or we will be way over advanced and will lose power and possibly "ping" due to pre-ignition. We also don't want too much advance at idle for smoothness, emissions and ease of starting. In fact we want to be somewhat under advanced at idle You'll probably notice that if you advance the timing to 10-15 BTDC the idle speed will increase quite a bit. The point of highest RPM would be the optimum and most efficient advance for that condition, but idle speed woudl be high, andquality would be poor tending to hunt or surge. This is because idle mixture is again quite lean and inconsistent due to low port velocity. We could lower idle speed by closing the throttle blades some, but this reduces the metering signal in the carb and is may not function well. Bad for emissions (high HC due to incomplete combustion). So if we keep the advance low at idle we need a little more airflow through the carb which results in more consistent mixture. Remember carbs meter fuel based on airflow and must have a minimum amount of flow at idle to work well. If the carb is metering well, we get complete combustion and a stable smooth idle with low emissions. The carb will be easy to "tune" at idle in this condition. We also don't want too much static advance at idle because although the idle mixture is lean, once the throttle is open the mixture riches and we want the correct mechanical advance for power at low RPM. Starting the engine with too much mechanical advance is problematic as well, at cranking speed peak cylinder pressure may occur well before TDC and the engine will fight against the starter. Low advance at idle may not be the most efficient condition, but it is the most stable condition.

With regards to raising combustion chamber temps for emissions reasons, the opposite is true. High combustion temps increase NOx emissions considerably. That is why compression ratios took a dive in the early 70's. The EGR system further lowers combustion temps by contaminating the fuel/air mix. We don't want the EGR operating at idle or at full power so it too typically operates off ported vacuum.

So why does the CTO switch thingie send manifold vacuum to the advance when the engine is cold? Because the choke is designed to release early for emissions, but the engine still needs a higher idle during warm up and converter light-off. It's a crutch.

But, you are not running the factory carb, EGR or (I'm assuming) a catalytic converter. Your carb is not calibrated to run those emission control accessories but is calibrated for performance. Therefor I'd set everything up in a classic performance configuration, optimize meachanical advance for power and vacuum advance for fuel economy.

If you are pinging on a single cylinder (and I have my doubts you are) you need to figure out why that particular cylinder is different. Ignition wire "bleed over", sucking oil through the intake port or something like that comes to mind.

Unless you raised compression substantially, the octane requirements should be similar. In fact the longer duration cam you have will reduce low RPM cylinder pressure making the engine less likely to ping off idle.

[GWChris]

Here's what it looks like according to the Jeep FSM - note that ported vacuum never exceeds manifold:

[BarryL]

we want to be able to run a very lean mixture (17:1 or so) for fuel economy and cool running

Lean will run hotter. Rich runs cooler. An air cooled engine (think VW) will run hot if run to lean. They are really air and gas cooled.

[Ristow]

ay idle,the carb functions of the vacuum signal,not airflow. sucks the fuel out of the idle jet orifice. the idle screw adjustst how much fuel comes out. the idle fuel system has air mixed into the fuel before it gets to the orifice as well.

Chris is correct on ported and vacuum levels. there is no measurable venturii effect on the timed (ported) port. it's simply another vacuum port located a little higher than the rest.

i run my engines at 12-15 degrees initial advance.i am rewarded with spot-on throttle response,and glass smooth idle and easy starting.

[88 G-WAGON]

Engine starts and runs great, it pings even when running straight headers and no exhaust,doubt its exhaust leak. The compression is up to 9:1. I have installed new plug wires (Accel 8mm). I could never get my ported vacuum down to 0, the lowest I could get it was 5, the carb is a Holley 4160 elec. choke,vacuum secondaries and I adjusted the vacuum canister down to move at 7,per the vacuum guage. When I moved the timing from 12btdc to 5btdc and changed to manifold vacuum it decreased slightly, still starts great. I am still running the stock ignition. Last week I had a valve hang up so I pulled the engine and took it back to the builder and we removed the heads and all of the pistons looked the same (black), and everything was tight.

[Ristow]

sounds like you may have the primarys open a bit too far at idle. since you have a holley,you can open the secondaries a bit too compensate,which allows the primary side to be closed some. there is a set screw that sets how far the secondarys close at rest.turn it in a half turn,and reset idle speed. will also allow better idle mix tuning.

if you find idle speed too high with main idle speed screw backed all the way out,then you'll want to beck the secondary set screw out a bit again,closing the secondaries.

i don't think you'd hear pinging with open headers.

[710 Burner]

If you can hear it over open headers, my bet is a lifter/valve train problem. I would start by running a compresion check on that side, possibly followed by visual inspection under the valve cover.

[Herk]

Rich=Cool Lean=Hot? Up to a point, but there is a mixture at which EGT peaks. Otherwise temp would be infinately hot with no fuel!

At darn near $5 a gallon for avgas, I tend to cruise 50 degrees lean of peak. I just bump the mixture knob in before increasing manifold pressure to climb. Lycomings and VW's cool the same way, air and oil.