75 Mark IV 460 Spark knock

[75MarkIV]

Hey all!

I'm new to this forum, and wanted to say thank you to everyone for all the information I now have access too! I bought, on a whim a 1975 Silver luxury group Mark IV. It's been a love hate between us, but shortly after I bought it, we discovered it needed a new engine. My dad and I rebuilt the motor with a donor 460 from another 75 Lincoln, and decided to hop it up a little to give it more power. I bought D0VE heads, and had them cleaned up, and new seats and valves. I bought a cam shaft as close to the 70 specs, with springs to match and a timing gear and chain to match as well. I did not change out the intake manifold, and kept the 75 exhaust manifold as I was in a hurry to get it running, and have noticed through the last few years a spark knock at around 55mph, give or take. We have played with the timing as much as possible and its improved, but it still keeps having the spark knock around that speed. I cant remember off the top of my head what the current timing degree is at, but we tried to keep it as close as possible to the 70 specs since the cam, timing, and heads are from the pre emissions specs. Any thoughts on what it could be? I'm running 93 Octane at all times, and use lead additive, or octane booster, and it still has it. The cat is gone as well, cause I figured maybe it was building up back pressure. I love the car, but dont really want to drive it too much with that. Thanks for any tips or advice!

[TonyC]

First off, welcome !

Now, hmmm ... honestly, I'm not sure what to tell you that you haven't already been doing. But if the engine knocks even with octane booster added to 93-octane gas, that is a problem which needs addressing. I can tell you that you don't need the lead additive; you have new components, and Lincoln engines even back then were better at withstanding the "inconvenience" of unleaded fuel than most other engines (the stuff is not real lead, anyway, and its octane-boosting attributes are likely not as good as formulas specifically made to raise octane).

I'm not an expert on 460s, save for mostly historical details. I do know that they have a large customizing support network, so it should be easy to boost an engine's power output. Let's go through the older-spec components you added again: 1970 heads, camshaft, timing set, and valve set. I can only guess, subject to correction from vets who know these engines better...but, if the pistons used in '75 (which I presume you're still running) had a different crown from the ones used in '70, I think that could contribute–I'm guessing the crowns combining with the head surfaces may be bumping the compression even higher than the 10.5:1 the 460s maxed out on in the pre-government years. If that happens, then pump gas won't cut it, even with boosters thrown in. As I already said, that's only a guess from someone who hasn't really done a lot of work on 460s; I've done some, but not enough to compare with other vets.

Retarding the timing settings should help, and you did say it helped a tad...but there are other settings besides the initial-idle timing that have to be adjusted–namely, the centrifugal advance without vacuum, and then the advance with vacuum, both checked at higher revs. If you haven't tinkered with those settings, try that and see whether it helps.

Of course, if others chime in with different advice, try those suggestions as well. Good luck.

---Tony

[Lee]

It wasn't clear to me whether the knock occurs as you accelerate through 55, or whether it knocks at a steady 55 cruise.
If it is the former, it is probably related to a fault in the centrifugal advance, e.g., a broken spring, or some previous owner installed too light springs from a kit. If it is the latter, the vacuum advance might have been replaced with aftermarket, and it has been adjusted to allow too many degrees advance.

Are you still running EGR, and have you also swapped out the carb? I could imagine a scenario where a too-lean mixture at certain speed could contribute to the tendency toward pre-ignition.

As Tony was saying, you'll want to put a timing light (preferably a dial-back) on it to see how much advance you are getting at various speeds, and how much of is attributable to centrifugal and vacuum. With some adjustments, I'm sure you can resolve this.

[75MarkIV]

Hey guys! Thanks for the advice! I’ve been wondering about the vacuum advance myself, but haven’t checked it out yet. It happens as a steady cruising 55ish highway speed. You can really only hear it when the windows are up, so it’s slight, but it’s still there. We did aftermarket pistons that are close to the 75 pistons so we didn’t raise the compression too much. Most of the compression increase came from the heads, and cam work. It has an egr manifold, but it plated off. I’ve thought about doing an intake from a 70 460 to see if that would help?

[Lee]

It’s probably aftermarket, so I think I’d start by finding the right Allen key, and backing off the vacuum advance until you no longer hear the rattle. That doesn’t mean the centrifugal advance is exonerated, and if backing off the vacuum doesn’t work, I’d be looking there.

[frasern]

With those pistons and heads, you are around 9.5:1, the early timing gear has set the cam to 0, whereas stock was, I think -4, so no factory timing spec. will work, you just have to find one that works. I would set it with a light, then try 1 degree in either direction until it runs best, then use that as a base to fine tune the advances. Some 460s of that era also had a vacuum retard, if so, check all the hose routing, I believe the retard should only cut in for starting, but not 100% sure.
The factory intake manifold design really didn't change much over the years (CJ and boss notwithstanding), but the flange did, yours probably has the spread bore with a 4350. An early square bore will accept many more choices, but likely do nothing for the pinging.

[TonyC]

If Fraser's right, and he could very well be, 9.5 compression isn't high enough to require racing fuel; so then it's all a matter of finding the sweet spots in ignition timing, per his and Lee's advice.
Look for a 9/32" Allen wrench; that's the size I have to use on my vacuum-advance diaphragm, which is aftermarket.

---Tony

[75MarkIV]

Perfect! I've really been thinking its the timing, but all this advice is much appreciated and needed. Thank you much guys! When the weather clears up a bit and I can get it out and drive it, I will be making adjustments. We have used a timing light in the past, I'll have to ask my dad what the last timing settings were. Thanks for all the help guys!

[Scoopfab]

From like 1970 on, Ford used a temperature switch inline with the vacuum advance supply. The purpose of this switch was engine idle, related to coolant temp. The normal vacuum line routing to the distributor/switch was ported vacuum, than a third vacuum line to a manifold vacuum source.

So how this works, the switch has two ports normally open, and a third temperature dependent. The distributor will see "ported vacuum" under normal conditions, than at a given high temp, switches the dist. to full "manifold vacuum" thus raising the base idle up 500/600 RPM's to speed coolant flow, and fan speed.

Now my point of this winded post.....
1) if this switch is present?
2) If so, are the vacuum lines in the "correct position" on the switch? If not the distributor will be getting full manifold vacuum all the time, it will take say 15 degrees of base timing, to somewhere in the 50's @ lite throttle cruising speeds, which could lead to your problem?

A quick test would be to accelerate somewhat aggressively, and see if knock go's away? What you are doing by opening the throttle blades, is stalling the vacuum, taking timing away? In any case it sounds like you have to much "total timing" for whatever reason!
Hope this helps in your adventure......