knock during hard acceleration

[Gufshoz]

Hey people,
I have 1,500 miles on a rebuild: cylinders sleeved, new babbitts, new Scat crank, tour ground cam, and a 5.5 Snyder head.
I did a 500 mi break-in drive, 100 mi/day, starting at 25 mph and increasing, daily to 45 mph. I normally drive around in my area and try to stay under 45 mph. Yesterday, I was driving around a corner, revved and downshifted, then floored it to merge into traffic. I had the spark fully advanced (usually I keep it around 2/3) and she started knocking.
I experimented by advancing and retarding the spark to about 1/2 during hard acceleration but not going faster than 45 mph. Hard acceleration at full advance = knock and hard acceleration at 2/3 advance = no knock. Is this just normal pre-detonation due to timing or should I be looking more deeply.
Surely, it's too early to be checking bearing clearances on the crank.
Thanks
P.S.: I've been told that I'm my own worst enemy for imagining problems that aren't.

[J Franklin]

It sounds as if the ignition is too far advanced.

[gdmn852]

Hello, generally with the higher compression head , the spark isn’t as advanced as far, Even with standard compression most people don’t use the full advance.Hopefully the noise is from over advanced spark .

[nkaminar]

Ignition too advanced. Search for the "knee" in the ignition curve. This is the point where there is no increase in rpm with increase in advance one notch but a decrease in rpm with the ignition retarded one notch. Normally you should drive with the ignition retarded one notch from the knee. If cruising down a level highway at a constant speed you can advance the ignition one notch and if driving around town or pulling a grade then retard the timing one notch.
You can find the knee with a fast idle, about 1,500 rpm, or when driving at about 45 mph.

The high compression head needs less advance because the air/fuel burns faster.

If you run the engine with the knocking because it is too advanced, you will ruin the rod bearings.

[Banditorama]

"I've been told that I'm my own worst enemy for imagining problems that aren't."

I can relate

But yes, that knock was almost certainly caused by the ignition being too far advanced. When you're really getting on it you want to retard the ignition some not advance it.

[Gufshoz]

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Thanks guys. That was my first guess because I had never driven with the full advance. Full retard and she starts before she turn’s over completely then I normally keep the spark at about 2/3 advanced and that seems to be her sweet spot. I do retard a little if she lugs at all.

[tinkirk]

Why not use a timing light and start playing with it so when your lever is all the down it will only advance as high as 28
Not that hard to do


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[nkaminar]

If you do not have a timing light you can set the advance lever so that the timing cannot be too advanced by following this simple procedure. Find the knee in the timing (sweet spot). Count the number of notches from the sweet spot to full advanced. Now set the lever down from the fully retarded position that number of notches minus one and time the engine in the usual way. You can start the car with the lever advanced that number of notches or a little less. You can impress your friends as to how slow the car will idle with the lever full up and the hand throttle cracked open a touch.

An alternative is to install a stop on the sector one notch further than the knee. One way to make the stop is to cut a slot in a long nut to fit the sector then tighten it to the sector with a bolt through the nut. Cut the slot perpendicular to the threads so to make a clamp.

[Mulletwagon]

Sounds like you are describing "pinging" rather than "Knocking". Common causes for pinging is usually timing over advanced for the compression ratio/octane rating/fuel mixture (too lean)/engine loading. Experimenting with all the variables will provide more data and insight. I think most of us change the timing to full advance after starting and tend to leave it there. Not much hill climbing here on the Panhandle, but suspect going up a sustained steep hill may require a reduction in timing to avoid pinging. Please share what you discover.

[CWPASADENA]

Originally Posted by J Franklin

It sounds as if the ignition is too far advanced.

I agree,

What you hear is detonation. DO NOT LET YOUR ENGINE DETONATE.
This can destroy an engine in a very short period of time.

A 5.5 Snyder head should not run more than about 26-28 deg. of advance at 2000 RPM.

If you have the ignition set for TDC when the spark lever is all the way up, then all the way down would be 40 deg. of advance. way too much for a high compression head. try running the spark about 2/3 down. that would be about 26 deg. of advance which is about right for a 5.5 head.

Run a little less spark advance around town when the engine is not running as fast.

My opinion,

Chris W.

[Bob Bidonde]

Raising the compression ratio necessitates running with less spark advance. These slides should help you get back to a more normal spark advance lever travel.

[Big hammer]

I’ll add do a search on the internet on how to read your spark plugs, reading spark timing by the color on the ground strap!

[Gufshoz]

These responses have really been educational for me. I had assumed that I would just be using the advance like I did with the stock head. I almost always ran about 2/3 to 3/4 advance. Now I’ll try running in the first1/3 of the sector.
Big thanks to everyone.

[nkaminar]

The reduction in needed spark advance with higher compression heads on a Model A is due to increase in the compression ratio in addition to increase in turbulence in the combustion chamber. This increases the rate that the air/fuel burns. This turbulence is a design feature of the flat head engine head. If you go to an overhead valve configuration, depending on the design, you may not get the turbulence and so you may need more advance than with a stock head.

[eagle]

Ping is your teacher. Advance the spark lever until you hear the ping, then retard a click or two until it goes away. Note the position of the lever, then dont go beyond it. Its no more complicated than that.

[Rob Doe]

Gulf, I'd like to add the following points:

IMHO, when running a high compression head on the Model A, it's important to know exactly where your car is initially timed. What I'm referring to is: It is easy to unintentionally time the car at different numbers of degrees ATDC.

I use a test light to determine how many notches down the quadrant the light comes on after tightening the dist. cam lock screw during the timing process. Generally, it comes on at the second notch down, but I've had it be 1 down also, and 0 or 3 at least once each. The light comes on when the points open. When estimating your advance you would subtract the number of notches for the actual points opening from the quadrant settings.

Our coupe has a h/c head of unknown ratio. The compression tests 110, 110, 110, 115 on 1-4. I have been told it is likely the 6:1. After much effort hunting down vibrations, rattles, tinkles and yes, knocks, our car is now quiet. I originally thought the engine had a rod knock at about 35mph on level roads. But, after replacing the motor mount rubbers, working on the spring tension at the front mount and tightening a lot of fender, shock, etc. bolts, the spark advance location at 40-45mph matches Bidondes' graphic exactly.

Hope this helps, it's been a 2.5 year search for the "Model A Experience"