Knocking with high comp head

Paul from Maine

Paul from Maine

Senior Member
Put on a high compression head. Lots more power, but she knocks unless I retard the timing way back. Then she gradually loses power and starts bucking, so I have to advance the timing. But then she knocks again. Do I need to run premium gas? Buy new coil? Put the old head back on? Suggestions?
 
Compression knock-

theres so many different things that can cause this. Whos head did you buy?
If it were my head, The first thing i would figure out is which cylinder its coming from. Then from their it could be anything from, the spark plug sticking to far down into the combustion chamber, to the block being decked to many times, making the piston have to much up-stroke, (i hope this is the right word) and the piston will petrude over the top of the block, which would cause the knocking noise. My guess, because its the most common, is that the block has been decked too many times. If you have a local machine shop, or a friend with a mill, get them machine the one combustion chamber, that is causing the knock, to releave some of the compression. Depending on how many times the block has been decked, the amount to machine off varies. Thats the hard part to figure out.
 
Model A heads have neither a tight "squish" nor any vortex in the charge, both of which reduce knock and octane requirements. The plug placement isn't ideal, either. My 8:1 head likes 92+ octane. On 89 regular it knocks at W.O.T. no matter how much you retard the spark. Henry's heads were 4.22:1 'cause gas was 60 octane on a good day. I remember when leaded premium was 100+ octane, leaded regular was 94, better than today's premium.

My cranking pressure with this Super Brumfield head is 160 psi.
With big valves & Stipe 340 cam it needs 92+ octane.
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Not enough information here. What kind of head and is the combustion chamber flycut? Since it "knocks" with spark advance, the first thought is preignition or detonation. Check timing. Check the piston "pop up" above the block by using Larry Brumfield's solder method--put some solder through each spark plug hole, over the piston, crank the engine, and measure the thickness of the compressed solder, if it is compressed at all. You can also measure the pop up with the head off.
 
Hi Paul, How high is the compression? Also, are you running a lightened flywheel? I have found in my own car that with the high comp head and a lightened flywheel that the engine is very sensitive to the proper timing! The engine runs like a top, but I need to constantly tweak the timing lever to keep it both at power peak and keep it from knocking. I cannot just set it and forget it, when road conditions vary.
Keep in mind that starting with 1932, the engines were equipped with centrufigal advance, and then later on centrufigal with vacuum trim. With higher compression and lighter flywheels, the engines became more sensitive to the exact position before TDC that the engine needed to run most efficiently. This is not the pain in the neck it might sound like. I've gotten so used to my engine and the timing lever position for hills, etc., that the whole thing is second nature for me. By the way, I'm assuming that the basic timing set for your A is correct at TDC. Happy Motoring.
 
Good points here. IMHO you really need to check timing. There are aircraft engines out there with 7.0:1 compression ratios that run just fine on 87 (R+M)/2 auto gas, with cylinder head temps over 400F. Pump octane should not be a problem for your engine. Your symptoms sound more like pre-ignition than autoignition (spark knock, or detonation). Pre-ignition is ignition that occurs BEFORE the spark plug fires and is usualy caused by glowing particles or a hot spot, but it always precedes the spark plug firing. The cylinder will not make power (sound familiar?) because the fire starts too soon, expanding the gases WAY before TDC, forcing the piston to "fight" the expansion on its way up the cylinder, and adding to the losses in the cycle. By the time the spark plug fires, much of the charge has been burned - again: loss of power. Since you've just replaced the head, there shouldn't be any carbon that could cause a hot spot and I assume your new parts don't have any sharp edges or casting parting lines that would glow when hot either (check the beautiful pic of Mike's head). Autoignition occurs AFTER the spark plug fires and causes a sharp spike in pressure and temp, usually resulting in the "pinging" we used to hear before knock sensors. Tests have shown that peak power is made during light, or incipient autoignition. Severely advanced timing can cause the same symptoms as pre-ignition. Higher octane gasoline can cure autoignition, but won't cure pre-ignition or timing that's too far advanced. $0.02.
 
i did that & found a lose center main brg . sorry but its the truth ........... steve
 
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