Detonation: Timing Plugs Compression Heads Etc

[Moparfiend]

I am getting some knock/detonation at WOT. Went one colder plug down didn't help. I am trying various timing and plug combinations to see if I can get it to run well without detonating.


Set up:
Car: 1970 Sat with a A904 and 271 rear
Engine: 1973 340 with an unknown aggressive cam and unknown compression with stock steel heads.
Set up: Have it a running a little rich right now and will get an actual A/F reading later in the week.
Variables and Combinations:
Fresh clean gas
Current set up is base timing 19 with a mechanical advance of limit of 14 via a new bushing. Total timing 33. All in at 2500 or so.

Went from 66 to 65 Autolite's. Still detonates.

In the next day or so when I get time to play with the car again I will try lowering the timing to 17/31 to see if it goes away. Not sure I want to retard it much more than that.

Looking for some reasonable suggestions. Slapping on the new Trickflow's is not reasonable unless you want to purchase them for me Lowering compression via thicker gaskets etc is what I am looking for input wise.

 

[Jerry Hall]

Fresh gas, what octane? Unknown compression and cam profile makes it real hard to figure. Plugs usually make little to no difference in my opinion. Retarding timing and higher octane fuel will make the biggest gains in the elimination of detonation.

 

[Moparfiend]

Fresh gas, what octane? Unknown compression and cam profile makes it real hard to figure. Plugs usually make little to no difference in my opinion. Retarding timing and higher octane fuel will make the biggest gains in the elimination of detonation.

yeah unfortunately unknown cam and compression makes this difficult. I have found the same thing that colder plugs don't really help so its good that you also found this.

 

[steve340]

I would reduce static timing and slow the advance curve down. 2500 all in is to early.
Try 3500 you want to avoid detonation at all costs.
Detonation at higher throttle opening can be caused by a lean condition as well as timing.
If you can hear it over the sound of the exhaust make no mistake it is rattling its *** out.
Bad news.

 

[Demonic]

That's a lot of initial timing, back the 19 off to 17 or so.

 

[lewtot184]

dirty burn is a contributor to detonation. oil and water in the mix is guaranteed detonation. somebody may have put 10+:1 pistons in it with non-quench heads. if so there's no cure with pump gas.

 

[dla4567]

Do a compression check. It’s cheap, quick and easy. Then report back.

 

[Don Frelier]

If it's really rich. Then you can try seafoam or you need to pull the heads and see what you have. A long duration cam helps with detonation so it's likely you have too high a compression for a street engine

 

[Moparfiend]

Do a compression check. It’s cheap, quick and easy. Then report back.

I will do a compression test on all cylinders as soon as I can and report back. How does static compression relate to detonation?

 

[Moparfiend]

That's a lot of initial timing, back the 19 off to 17 or so.

Yes I will try it. At 15 I had no sign of detonation but the engine wants more than that. I might be able to get away with 17. Will report back my findings.

 

[dla4567]

Higher compression is more prone to detonation.
My engines compression is around 180-185 (static), (120-125 running IIRC). With pump gas (93), timing needs to be set at 15 initial, 30-32 all in at 3500. Any more than that and ping ping ping. With 110 octane it runs best at 22 initial, and 38 all in at 2200 no pinging or knocking. Sooooo much more power, quicker response, a completely different engine.
I believe with iron heads around 165 is max. (Rule of thumb).

 

[Moparfiend]

Fresh gas, what octane? Unknown compression and cam profile makes it real hard to figure. Plugs usually make little to no difference in my opinion. Retarding timing and higher octane fuel will make the biggest gains in the elimination of detonation.

93 Octane with added Stabil not sure how Stabil affect octane

 

[Hemirunner]

A 65 autolite isn’t exactly a cold plug and that rear gear is putting one hell of a load on the engine.

 

[PurpleBeeper]

Stabil won't affect octane at all. These guys are correct....turn back the initial timing a little bit & maybe stiffen up the springs in the distributor to slow down the "all in".

 

[Moparfiend]

Ok so I slowed the advance curve as much as possible. This was using the largest springs in the MSD kit. That curve again is the flattest curve I could tune to. The MSD curve in the book shows all in by 5500 RPM. But I am running a 14 degree advance bushing. I did not change my initial timing and left it at 20 degrees. So I am limiting all in timing to 14 I see all in by about 3600 RPM. This makes sense I am shortening the max all in advance by limiting the added mechanical advance to only 14. I am not detecting any more detonation.

With an engine thats power band is up in the 4K-5K range I am lugging it around town. Hell its like at idle when I am going 35MPH so I desperately need a better rear gear as that 271 just isn’t right for this application. I knew this going in. So I still need to figure out what rear end I have to see what my choices are. But I need at least a 355 if not taller....probably taller since this is not a highway cruiser.

 

[Kern Dog]

Yes I will try it. At 15 I had no sign of detonation but the engine wants more than that. I might be able to get away with 17. Will report back my findings.

The initial timing number will have no effect on detonation but the total timing will.
For one thing, the weight of the car and the LONG 2.71 gearing do not help at all. A 3.21 or 3.55 gear (From a 8 1/4" axle in a Durango or Dakota) would improve acceleration and improve mechanical leverage...which does reduce the risk of detonation.
As stated, a delayed or s-l-o-w-e-r advance curve will help. The "All in by 2500" schmere belongs with the other age old terms like 3/4 race cam. Distributors with mechanical advance use 2 springs. One trick is to use one light and one heavy spring. This makes for a two step advance rate where part of the advance comes on quickly while the rest of it is not "all in" until something like 3000, 3500 rpms, give or take.
I dealt with detonation with my 440/493 for years and stumbled on a few winning "fixes". I posted here and maybe 3 other places with responses totalling well over 5000 in number collectively. This was back between 2011 and 2013. I went with thicker head gaskets, a fix that was equally bashed as it was supported. It worked for me after several other fixes did not. 110 leaded gas was the only fuel that allowed me to hit full throttle without any knocking. I tried delayed spark, colder plugs, leaner jetting, richer jetting, open exhaust, different camshafts, different amounts of clearance with rocker arms to valves, lower temp thermostats.....
I went from approx 10.98 to 1 to 10.07 to 1 using a .075 head gasket from Cometic. There have only been a couple of incidents of knocking since then, if I recall right, they were when I drove the car after an extended period of sitting...The gas had surely lost some of it's punch.

 

[Kern Dog]

With an engine thats power band is up in the 4K-5K range I am lugging it around town. Hell its like at idle when I am going 35MPH so I desperately need a better rear gear as that 271 just isn’t right for this application. I knew this going in. So I still need to figure out what rear end I have to see what my choices are. But I need at least a 355 if not taller....probably taller since this is not a highway cruiser.

One thing to know when speaking of gearing: A "Tall ratio" refers to one with numerically low numbers. A 2.71, 2.76, 2.94 is a TALL gear. Many also refer to them as "Highway gears". Taller may be rooted in the theory that a tall person can travel greater distance with fewer steps due to the longer stride. A car with taller gears can travel greater distance with fewer engine revolutions due to its lower numbered gear ratio.
A 3.91. 4.10, 4.56 is a LOW gear, a short gear.

 

[Kern Dog]

Also, I respect that you are out there working on the car rather than just spending time on the joke threads, the political forums, etc. This is a CAR site and I like to see and read from guys that are out there getting busy working on and enjoying these fine cars!

 

[Moparfiend]

Also, I respect that you are out there working on the car rather than just spending time on the joke threads, the political forums, etc. This is a CAR site and I like to see and read from guys that are out there getting busy working on and enjoying these fine cars!

Thanks KD I agree. I am doing the most I can given my shity health issues. But I enjoy this **** so much I just push trough it as best I can.

 

[Moparfiend]

One of the benefits for a car that needs body and paint work lol