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Rod Knock vs. Detonation

4896 Views 5 Replies 4 Participants Last post by  rustedduster
how can I tell the difference between the two? my 351m in my 79 F100 is making a knocking/rattling noise at about 50+ mph. i dont have a tach. so ill have to estimate it at 2000+ rpm or so. oil preassure is ok by the factory guage (always in normal range), a little on the low side when at normal operating temp and high when cold. when cold, the engine makes less noise and im able to cruise a little bit faster with out getting the noise. it seems lately though im able to go almost 60 mph without hearing it. let me know if yall have any ideas or similar experiences. thanks.
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I guess you could try backing off the timing just a bit and see if it changes.
i think i found the problem. i believe it was detonation occuring when the advance on the dist. kicked in. ill give it a better test after work tonight. thanks for the advise Clint.

<font size=-1>[ This Message was edited by: rustedduster on 2/25/06 12:48am ]</font>
In my experience a rod will knock every time the cylinder fires, so it should knock at idle.
Rod knock isn't always apparent at idle due to the oil films ability to cushion the crank/rod. Rod knock will get worse/louder with rpm and isn't dependent on load at all so you can hear it while sitting still.

Detonation or spark knock is a bit more difficult to detect and occasionally can't be heard at all. At best it will have the effect of making the engine sound similar to a diesel or it can best be described like a handful of marbles rattling in a coffee can. Detonation in street vehicles will generally present itself in low rpm/low vacuum/high load/high temperature situations.

A good example would be my 78 F250 4x4 400 4spd pulling a 10,000# trailer up a steep hill in 95* weather in 4th gear at 2000rpm with my foot to the floor, damn thing sounded like a Detroit Diesel in a dump truck.
thanks for all the input. i have definately come to the conclusion that it is detonating. after pulling the hose off the vacum advance on the dist. and taking it up on our Turnpike i was able to get it up to 80 mph without a sound now. i hope when i get a chance i can bring down the timing and enable the advance again all will be good.

now another question. when i rebuild the motor, what should i do or avoid doing to decrease this aparently common detonation problem? i was planning on swapping in a 400 crank and pistons (9 to 1 flat tops if possible), a towing/rv cam, edelbrock intake, a 4 bbl holley, and a few other trick parts like headers, double roller timing chain with adjustable keyways, MSD 6A box, and other such parts, i guess my question really is, will any thing i stated increase or decrease detonation or is it more in the tuneing? from my understanding the higher compression ratio may aggrivate detonation.

thanks again all i really appreciate all the help.
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