1968 Charger 440 Pinging when its about 90 degrees out. Need help

[garthed]

Hello everyone. Just so everyone knows where I'm coming from, I know enough about carbs and engines to know I don't know enough. High school auto mechanics 20 years ago.

Here is my problem. I have a 68 Charger RT that I purchased about 9 years ago. I have only put maybe 10000 miles on it since I bought it. The engine had just been rebuilt when I bought it and was told that it had an 11:1 CR. Its got a big cam but don't know the specs on it. It has adjustable roller rockers and running hooker competition headers. The carb is a Holley and I think its an 800 dominator.

Here is my problems two of them. First I have been chasing a stumble right after idol. Second I have pinging at after about mid throttle when its hot out 90 degrees plus. It goes away at WOT and does not do it right after idol. It ping around 2700 under medium to heavy load. I have been running octane booster for along time as I live in CA and all I can get is 91 octane.

So for the stumble, If I give it a lot of throttle fast I would get a massive hesitation or back fire. I was told I should swap out the accelerator pump cam. I did this with the most aggressive cam and it seamed to help. but it was still back firing and hesitating. Then I advanced the timing and wow did the car wake up and the stumble is about 95% gone and can live with it. But it now pings more.

So for the pinging. I unfortunately can not see what degree I'm at as the numbers are gone but there are two mark that the builder painted on. When I adjusted it I guess I added about 5 degree's and is on the lower mark. I'm not sure how vacuum advance works but when I pull the vacuum line the engine runs no different at idol. I have not tried driving it with it plugged in.

Could this pinging be from low octane and high compression? Should I try changing something in the distributor to slow down the vacuum advance? Any help in trouble shooting would be great. It also smells rich at idol.

Thanks
Garth

[68CoronetRT]

My first thought to all of this is try disconnecting the vacuum advance all together(Plug the port from the carb, you can leave the one on the dist open if you want, no leaks!) and go for a test drive. I bet your issues will go away. (Timing wise anyway)

I'm sure Firefighter will chime in and have a few questions....

[Dmichels]

Your problem is caused buy the 11:1 compresion ratio and trying to run it on 91 octane.  IMHO anything much over 10 to 1 and street gas will cause problems. Yes I know there are combinations that work, but usally you need to back off timing so it will not ping. If you have aluminum heads it helps with the higher CR.   Someone tried to solve the problem of detonating by retarding the timing too much, but that kills performance and causes it to run poorly. You need to get the timing back up were it needs to be. I can not coment on how well octane booster works. Can you get 100 plus octane gas from a race shop or air field? THat will really wake up the car. Do not drive the car with it knocking you can hurt the motor. If you can not live with the cost of race/av gas you can try composit head gaskets that will lower the comprestion ratio but probally not engh to run on street gas.   Your other option is to rebuild the motor with new pistons.
Dave

[garthed]

Thanks for the replies. I will try disconnecting the vacuum advance. Higher octane gas is just not available where I'm at. Unfortunately by the time I filled up and drove back home I would have to turn around and fill up because I would be empty. It only pings when its hot out 90+ and after sitting at a stop light or two. If I don't drive it aggressively in these conditions I can prevent the pinging.  Is it ok to drive it this way or when its cooler and not pinging.

[68CoronetRT]

Yea, the other thing is the 11:1 ratio. If you have iron heads that not good. But I bet plugging that line will fix your problem based on when it happens. I was told to never run the advance like that because of this exact issue. If your running aluminum heads then you might get away with 11:1 on 91. Just depends on the motor and what it likes.

[garthed]

Ok I took the vacuum advance off and the pinging is completely gone. Its not as hot today but I notice no pinging at all. But now the motor does not seam as crisp. Seams that the snap it had is gone now. Should I get an adjustable vacuum advance and give that a try?

[justcruisin]

The vacuum advance should not have any effect on performance as it is not in play when you give it a boot full. Maybe it is sticking and causing a problem. Check your timing at idle and then bring the revs up until it stops advancing and note the reading, leave the vacuum unit disconnected while testing. You may need to set it up a little different.

[Cooter]

If the engine responds after unhooking vaccum adv. Then there's the problem.
Most likely, someone has typically hooked it up to straight manifold vaccum instead of ported carb.
Also, many of the stock style distributors have WAY too much vaccum/mechanical timing built into them.
Either you have 17degrees at the crank and make the rest up through mechanical timing, or you have 25degrees mech. Timing and can only run about 10 degrees at the crank for a total of 35-36degrees.
Running all that timing will make it responsive as hell. But at the cost of your piston skirts.

[fy469rtse]

I willing to bet you need to work on your distributor , curve of advance not matching your cam shaft , how much initial timing at what rpm , disconnecting vacum advance should have altered the way car was idling, where is or was that plugged in to , there's a proper port for that, how much advance once it's all in ? Deg's

[garthed]

The advance was plugged into the ported vacuum of the Carb. When I unplug the vacuum advance at idol it did not anything at idol. With it unplugged its starts the same and idols the same but seams to be a little more sluggish at mid throttle and RPM (2300-2700) but does not ping any more. If its cool out under 90 degrees and can run the vacuum advance and it runs great everywhere in the Rpms. I do not know how much timing I'm running initial or mechanical because there are no markings or numbers on the engine or harmonic balancer just two paint marks.

[Kern Dog]

I have been dealing with a similar problem for years.
I've tried different spark plugs, octane boosters, carb jetting, different electronic ECUs, timing settings....
Finally it has come down to a camshaft swap. While some suggested it early on, I was slow to understand the importance. Here is a summary of why it matters. Please forgive me if this seems oversimplified:

An engine with high compression usually needs high octane fuel to resist detonation. The worst are the high compression engines with relatively tame camshafts that produce a smoothe idle. The reason for that is that a smoothe idle is the result of a camshaft with less lift, less duration, less overlap and an earlier intake closing. All of these factors combine to trap MORE cylinder pressure. The result is an engine that runs pretty nice from idle to 5000 rpms, doesn't shake your teeth out of your head at a stoplight and provides plenty of vacuum for the power brakes. Trouble is, the static compression and the camshafts design raise the "dynamic compression ratio" to the point where you need 100 octane fuel to keep it from detonating.
Take that same engine with the high compression and install a wild, race type camshaft in it. Now the idle is choppy and sounds really radical. The cam has more lift, more duration, more overlap and a later intake closing event. The power brakes dont work since the engine only makes 2 inches of vacuum at idle. Somehow it runs without detonation right up to the 8000 rpm redline!

Happiness is found in the middle of this.

[Kern Dog]

My Charger 493 is 10.7 compression with aluminum heads. For years I ran the mopar Performance 292/509 cam. At the same time, I've had detonation problems when the weather got warm. I've always been limited to 30-31 degrees total ignition timing to minimize the detonation. Many similar engines can run it to 35-38 degrees, but mine knocks even worse at that number.
After much internet discussion, I finally admitted to myself that the suggestions to change the camshaft made sense. Then it was a matter of which cam to choose! So many cams, so many "experts" online to suggest their favorites! I got some help from a guy on another site and ordered a Lunati solid flat tappet cam that looks to make a huge difference.
The theory is: A more "racy" cam helps tame the high compression by effectively bleeding off some of the compression by way of the design. In the simplest terms I can imagine, the "bigger" cam sorta leaks at lower rpms and reduces the actual compression that the engine will see. Less cylinder pressure equates to lower octane needs. The tradeoff is that the engine needs to spin faster to make its power.
In the case of my engine, the Lunati cam that I went with has specs that are different in every way. Different in the right direction too.

[garthed]

Again thanks for everyone's replies. I was able to really look for the timing markers today and found them. With the Vacuum advanced disconnected my initial timing is at 10 and bounces to 12. Is this normal?

I drove the car today and got it hot sitting in some traffic and with the vacuum advanced disconnected and got some pinging. This happens at mid throttle under acceleration. If I floor it I don't get pining. If I drive like a grandpa I don't get pining and keep it under 2000 rpm.

I'm not really sure how to set mechanical timing. If anyone had a easy explanation or a link to a website that would help. I know that I will need to get a timing tape. Do I start that tape at the mark on the harmonic balancer?

I'm going to hook back up the vacuum advance to the ported vacuum on the carb and take it for a drive. I tried the vacuum advance on the manifold just for giggles and it seamed to have better throttle response right after idol. I'm confused by that. It pings about the same regardless if its at the ported or manifold vacuum. 

[68CoronetRT]

Yes, it's normal for the timing to bounce a little, I guess this is due to a slightly worn distributor bushing.

I think the guy that posted above has the only definite answer to getting rid of it (pinging) completely.But as far as your timing question goes Ill try to explain it in simple terms.

Your initial timing at idle will be 10-18*, Mechanical timing comes in based on the weights/springs inside the distributor, AKA your timing curve. So lets say in example you have what alot of Mopar guys would probably call the "perfect curve", you start at your 10-18* initial timing and by the time the motor gets to lets say 2200-2500 RPM your TOTAL MECHANICAL timing is 34-38* and goes no more. This is changed and controlled by your springs/weights. So if you start at 12* and end up with 36*, if you change it to 10* then your top end goes to 34* make sense? Then you add your vacuum advance curve(Plugging in your line to the distributor), this is where the timing jumps to 50*+ and leans the motor way out to help burn the fuel more efficiently and therefore saving gas, also where you might start seeing pinging problems.

Ok so how do you check timing? Well you simply find a dial back timing light. You set your timing advance to lets say 14* on the dial and hit the timing marks, as you move the distributor when the marks line up to ZERO you will be @ 14* initial. So to find total mechanical you simply dial in 34-38 and begin raising the RPM until the marks line up. You might have to play with the dial to find out what your total mech is, but once you find it then you know the difference from initial to total mech, and you can adjust at idle up or down. So in example you set it to 14 and find that you stop going up in timing at 38* now you know your curve, so 12* becomes 36* etc...(I kinda repeated myself here).

Then you can get crazy and start figuring out how fast the curve ramps up, or how to slow it down etc... all depends on weights/springs. This is also why we said to disconnect that line, your adding too much timing and it's causing the pinging. So backing off the initial will help on the top end, but sounds like you need more initial and less mechanical.

Hope this makes sense!

[garthed]

Thanks for the info. I notice that its ping regardless if the vacuum advance is hooked up or not when its hot. I guess I'm going to need to get a dial back timing light. Does anyone have a good recommendation for one?