Valve knock under full throttle? 68 charger

[euroz06]

Hi everyone,

I have a fully restored car, stock 383, starts every time, stock air filter. I go cruising every weekend, driving around town etc. I don't usually floor it, normally driving under 3-3.5k rpm.

This weekend however, my buddy got himeself an R33 GTR, so we decided to have some fun on some straights. When I floor it and keep it to the floor, I hear this horrible vavle knock, which is never present when driving normally/medium load. What is going on? Engine has 6k miles after rebuild.

 

[493 Mike]

Are you trying to say "spark knock" or actual valve train noise? What grade fuel are you using?
Mike

 

[DanielSlocum]

never heard of "valve knock". you may have a condition known as spark knock or pre-ignition. that happens usually when the fuels octane rating is not high enough.
if your compression ratio is around 9 to 1 or higher,then you need to be burning premium fuel to prevent the fuel from igniting prematurely which causes a knock combined with a drastic power loss. this assumes iron cylinder heads. aluminum heads generally resist pre-ignition much better than iron where you may not need premium until you are at 10 to 1.
too much ignition advance can also cause or contribute to the problem. good luck.

 

[F4R/T]

Spark knock if your timing is off or crappy fuel, you aren’t floating the valves are you? Dial you timing in and go from there with a good tank of fuel.... unless your are way too lean..I would expect it to hiccup then.

 

[Cranky]

Hi everyone,

I have a fully restored car, stock 383, starts every time, stock air filter. I go cruising every weekend, driving around town etc. I don't usually floor it, normally driving under 3-3.5k rpm.

This weekend however, my buddy got himeself an R33 GTR, so we decided to have some fun on some straights. When I floor it and keep it to the floor, I hear this horrible vavle knock, which is never present when driving normally/medium load. What is going on? Engine has 6k miles after rebuild.

View attachment 1580809

Right, need more info. If you aren't hearing 'rattle' or pinging at part throttle and the valve train is quiet, you are probably experiencing engine ping....too much timing can be one culprit. Does it make the same noise when the engine is cold? Take it out and try it. If it's quiet or not as loud, it's probably pinging. Your spark plugs will most likely should signs of it happening. You do NOT want to keep on driving it hard if that's what's going on. If the temp of the engine goes above 180, that can also contribute to it. If the car runs good and you don't want to bump the timing down, try some octane boost. I used to add some av gas to my stuff to help it quit pinging....

 

[euroz06]

Its a loud metallic clanking sound (ratteling) very clearly heard in the cabin... could be spark knock. Otherwise, no smoke, runs smooth as butter otherwise, on warm engine starts right away.

I run regular gas, i tried running higher octane, but then get blue smoke out of the exhaust (on idle and otherwise). Car runs normal temp, around 180 even on a hot summer day (i live in Miami and have a secondary temp gauge under the dash). I don't know what the compression is, I didn't do the restoration, but when i got the car, it was pretty much stock (even stock headers), so i assume stock compression.

So I should change the sparks, and do timing adjustments? I can do the sparks myself (any recommendations for plugs? what should be the gap?), but should i have some pro shop do timing? I've never done timing adjustment. Are there any specific specs for timing?

 

[RemCharger]

Could it just be a good ol fashioned rod knock? Can you hear it when revving it up under the hood in park?
Maybe go by a local mechanic shop and let them have a listen

 

[euroz06]

When i freely rev it (no load), no alien noise. cruising on the highway at 3.5k rpm, no noise. moderate acceleration (half throttle), no issues. Only when I floor it and keep it floored in gear.

 

[euroz06]

im in miami, finding someone who knows how to do things is difficult or extremelly expensive.

 

[Kern Dog]

It does sound like detonation.
This occurs when the octane requirements of the engine are not met. I’ve dealt with this very thing. A high compression engine can idle and cruise fine on regular octane fuel but then knock once full throttle is applied because the engine sees higher cylinder pressure under full throttle.
There are several crutches to stop this and only a few proper “fixes”.
Use higher octane fuel.
Reduce total ignition tuning.
Replace pistons to lower compress ratio.
Sometimes one can get a bad batch of fuel and the car will not run right. Modern cars with computers have the ability to compensate to some degree and the car will still run okay but old cars like ours are not able to “self adjust”.
My own car had this exact condition as yours…. Idling and even up to half throttle it was fine. At full throttle it sounded like a wrench in a steel can.

 

[Michael_]

I run regular gas, i tried running higher octane, but then get blue smoke out of the exhaust (on idle and otherwise).

That's interesting.

Edit:

I noticed you have an aftermarket MSD Distributor and Coil.
I agree you should start by adjusting timing and running high 93 octane fuel.

 

[pnora]

Looking at his motor he has MSD with no vacuum advance. He needs to check his total timing. My bets he has heavy springs and the total is well over 36 degrees. Heavy springs will cause more advance to pull in at higher RPM's. Either get the dizzy tuned or run higher octane. Premium fuel is not going to make it smoke blue.

 

[Jerry Hall]

Premium fuel when your car was new, was at least 98 octane. If you're running 87 octane regular fuel then this is your problem. If you retard the timing enough for it to not ping, you can throw performance out the window.

 

[hunt2elk]

What is your total timing? Try knocking it down a couple degrees and trying that. Also, if you have light springs in the distributor, it may be coming in to fast.

 

[pnora]

What is your total timing? Try knocking it down a couple degrees and trying that. Also, if you have light springs in the distributor, it may be coming in to fast.

Lighter springs it would rattle at lower rpm's. He says 3,500 and under he does not notice it. Heavier springs is keeping the timing lower for most conditions but allowing more advance over 3,500 rpm's. A 5 minute timing check would verify this.

 

[Michael_]

I suggest he trys again with 93 octane before touching the timing/distributor.

 

[hunt2elk]

Lighter springs it would rattle at lower rpm's. He says 3,500 and under he does not notice it. Heavier springs is keeping the timing lower for most conditions but allowing more advance over 3,500 rpm's. A 5 minute timing check would verify this.

Agreed, I missed that.

 

[AR67GTX]

I think it’s detonation (pinging) from his posts but those dual air conditioning belts can sometimes raise a racket if not properly tensioned and all alternator and air compressor bracing in place. I had owned my 67 GTX with AC for nearly 20 years before I realized I was missing a lower alternator brace to the engine. Revs much quieter now. It has a bone stock 68 440 in it and anything over 34 degrees combined timing runs the risk of pinging on 93 octane.

 

[euroz06]

So shouldnt be getting blue smoke on 93? I thought these were supposed to run on regular ‍

 

[euroz06]

If blue smoke was a glitch that one time, im think i should try 93, and maybe change the plugs…? What plugs/gaps yall recommend?