Detonation Question

I

itstom

Member
The car is a '41 coupe with +.060 bore, 3.75 stroke, dual 97's, L-100 cam, Navarro heads. Although the heads are stamped 8.25, I believe they have been milled (I got them used). My rudimentary cc-ing gave me a combustion camber volume of about 60cc's. A chart I found online puts my CR at about 9:1.

Needless to say, it knocks under moderate to hard acceleration. A rebuilt distributor was installed which solved a horrible overheating problem but the knock is still there. So far I have adjusted the advance brake screw on the dizzy in about 3.5 turns and the knock is still evident.

Other than pulling the heads and reworking the chambers, what can I do here? Does air-fuel ratio effect detonation? Would a different heat range plug help? Or is my only "easy" fix to continue to reduce my advance?

Thanks in advance (no pun intended),
Tom
 
Sounds eerily familiar to something a friend of mine is going through right now. Same kind of heads and everything. He stole those heads!
If he figures it out, I'll pass on the information.
 
The combination of the ignition timing, dynamic compression ratio, and the fuel octane have to be in balance. If the ignition timing numbers are reasonable I would first suggest moving up to a higher octane gasoline. The A/F ratio can affect knock but unless the carbs are way off I doubt there is much knock relief available within them. Likewise the plug heat range is an unlikely suspect.

It is difficult to make solid suggestions without knowing what the existing ignition advance is, what the jets are and what fuel is being used.
 
Thanks, Kube. Maybe if your friend had paid full price for a set of heads instead of being so cheap he (and I) would be out bird-doggin' chicks instead of monitoring the fordbarn all the time.
 
Thanks JWL. I did run a tank of premium through it with no discernible difference. However, that was the first thing I tried so it was before the adjustments to the dizzy. I'll try again next fill up. I put the jets in a year ago before the cam and heads and will have to check on the size tomorrow. Memory ain't what it used to be.
At that time the mixture looked good when checked with a Color Tune kit.

I'm not sure if my inductive timing light will work on 6 volts but I'll work on that as well.

Thanks.
 
itstom said:
The car is a '41 coupe with +.060 bore, 3.75 stroke, dual 97's, L-100 cam, Navarro heads. Although the heads are stamped 8.25, I believe they have been milled (I got them used). My rudimentary cc-ing gave me a combustion camber volume of about 60cc's. A chart I found online puts my CR at about 9:1.

Needless to say, it knocks under moderate to hard acceleration. A rebuilt distributor was installed which solved a horrible overheating problem but the knock is still there. So far I have adjusted the advance brake screw on the dizzy in about 3.5 turns and the knock is still evident.

Other than pulling the heads and reworking the chambers, what can I do here? Does air-fuel ratio effect detonation? Would a different heat range plug help? Or is my only "easy" fix to continue to reduce my advance?

Thanks in advance (no pun intended),
Tom
Click to expand...


Tom,

After building a few of these front mount helmet distributors ii have found quite a few that run 22-24 degrees and ramp it up pretty quick.
I can slow one down a bit for you with a spring adjustment and a positive advance stop of a few lower degrees.
We do these pretty reasonable at $ 48 labor plus parts be glad to set ya up something.
I even have a loaner or two laying around for you to just try at no charge......:eek: Just let me know .
 
Tom,
This is a stretch but I had exactly the same problem with a stock '35 21
stud motor. The heads had been milled so in desperation I tried 2 head gaskets
which did not help at all. In further desperation I changed the intake manifold, it
was a genuine Ford aluminum single 2 barrel. Voila..the problem was solved. The
heat riser passage had burned thru into a runner or runners. The '35 always ran
extremely well with no other annoying characteristics except detonation. I removed
the extra head gaskets and all remained well.
I'm hoping JWL gets a look at this response and will render his opinion because I
really only understand that the inlet change worked and when I ran water into
the heat riser some flowed out the carburetor bores of the even then ancient '35
intake.
Charlie ny
 
My engine in my 40 Convert is a 59A bored to 3 3/8" with a 4 1/8 stroke using Navarro heads and three carb intake with three 97s. Ignition is Mallory Unilite . When I first ran this engine like you I had bad detonation. The advance curve in that distributor was too much too fast. I installed heavier springs and adjusted the initial advance and premium gas is a must . It took a while to get it there but runs like gangbusters now without detonation . One other thing here I almost forgot is that my block has been relieved which helps lower the compression . I believe the Navarro heads flow a little better than some of the others ,they arent as choked off from the combustion chamber to the valve pocket.
 

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You can use your timing light by hooking it to a 12 volt battery & use it on your 6 volt system.
Gary. This was meant for itstom pushed wrong button.
 
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Check your plug reach against the length of the threads in the head. I had the same problem with the head threads being to long.
Butch
 
Charlie ny said:
Tom,
This is a stretch but I had exactly the same problem with a stock '35 21
stud motor. The heads had been milled so in desperation I tried 2 head gaskets
which did not help at all. In further desperation I changed the intake manifold, it
was a genuine Ford aluminum single 2 barrel. Voila..the problem was solved. The
heat riser passage had burned thru into a runner or runners. The '35 always ran
extremely well with no other annoying characteristics except detonation. I removed
the extra head gaskets and all remained well.
I'm hoping JWL gets a look at this response and will render his opinion because I
really only understand that the inlet change worked and when I ran water into
the heat riser some flowed out the carburetor bores of the even then ancient '35
intake.
Charlie ny[/QUOTE
I'm not JWL, but undoubtedly that exhaust leak was overheating the incoming charge, which invites detonation. Cooler charge results in less chance for detonation. In these higher compression instances, it is important to have a tight combustion chamber over the piston - like .050 clearance. It is also important that the cylinder head contour match that of the piston so you can get a consistent tight clearance.
Click to expand...
 
I also am running Navarro heads on an 8ba, 3 5/16"X4", pockets over valves raised a little due to valves touching them, Schneider cam with .365 lift, ported but relieved very little, more just removing sharp edges, running 4X2's but fuel injection and working on the detonation problem also. I am running 15º at 2200 rpm under full load and pulling cool air into the air cleaner bypassing the hot air from the radiator and use premium gas. It seems okay thus far into the summer and still great power. I also have a cooler the fuel return line runs through before the gas returns to the tank helping to cool the returned gas.

Vergil
 
A load of great advice. A quick try is also a cooler heat range plug to see if it helps. According to Barney Navarro, the flathead has the original "quick burn" combustion chamber. Cooler heat range is one plug related step in trying to reduce pre-ignition, indexing the plugs is another. Cooler plugs have worked for me in Ford 2.3L turbo motors. Funny thing, read about cooler plugs and indexing in a Ford bulletin at the dealers, when I asked the service manager what he thought, he said it wouldn't make any difference. Tried it anyway, it worked. The next model year tcoupe came with the cooler range I used.
 
russcc said:
A load of great advice. A quick try is also a cooler heat range plug to see if it helps. According to Barney Navarro, the flathead has the original "quick burn" combustion chamber. Cooler heat range is one plug related step in trying to reduce pre-ignition, indexing the plugs is another. Cooler plugs have worked for me in Ford 2.3L turbo motors. Funny thing, read about cooler plugs and indexing in a Ford bulletin at the dealers, when I asked the service manager what he thought, he said it wouldn't make any difference. Tried it anyway, it worked. The next model year tcoupe came with the cooler range I used.
Click to expand...



I haven't tried changing heat range in plugs but I do have my plugs indexed, not for detonation but after three times of torquing heads from building it, pulled a spark plug from #1 cylinder and noticed ground on the spark plug was just kissing the valve, 1,2,3 &4 were the same so decided to index all the plugs while raising 1 thru 4 plugs. It does have 160# cranking compression or did the last time I checked it which doesn't help with the detonation either.

Vergil
 
Using another head gasket will definitely lower the CR. Unfortunately it will also reduce the turbulence required for complete combustion. Keeping a tight chamber can reduce detonation. When building an engine the combustion chamber is a critical component of the build. You just can't buy a set of heads and slap them on and expect them to preform. Tuning is another part of Building an engine. Measuring the A/F and the spark curves is part of the build.
 
Wow, great discussion. Thanks for all the ideas. Here's where I admit to some more ignorance. I never really understood plug heat ranges and the numbering system. The plugs in the car are H10's. So what would a cooler plug be? Also, as far as indexing the plugs, which way should the gap be facing?

I'll be trying to get some more info. this week as far as where the advance is now and check #'s of compression. No A/F meter but I'll recheck with the color tune.

Thanks again for the help. The Fordbarn at its best!
 
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