Lowering the LSA doesn't always mean you'll get a nasty idle.

Overlap is what you hear and just because a cam is a 104, 106, or 108 (or anything in between) LSA, doesn't mean it will have a nasty idle.

I can grind a very tight advertised duration, small .050" duration camshaft on a 104 LSA and it will not thump. Or.....I can grind a very large advertised duration on a 114 LSA and it will sound completely nasty.

Multiple variables play into how much overlap (sound) you have. The LSA is just one of them and isn't the major proponent.

In addition to all of that, it's generally thought of that a 110 LSA is a "good street cam", a 112-114 is "an EFI cam", a 108 is a "drag race cam" and a 106 is a "circle track cam". Those thoughts and ways of thinking couldn't be further from actuality/reality. Widening the LSA doesn't always mean it will have a smooth idle and narrowing the LSA doesn't always mean it will have a rowdy idle. My pulling truck camshafts are on 119-121 lobe separation angles and sound like the engine is leaving the face of the earth.

Thumpr cams sound like they do not just because the LSA is on a 107, but because the advertised durations are very high as well. These cams have overlap numbers in the 90's. They are made for pure sound, not efficiency or horsepower. As a matter of fact, for anything but the most horrible performing factory heads, they are putting more horsepower out the exhaust than making it usable.

www.customfordcams.com

Hello again TassieCobra. Going sideways only slightly....can you tell us more about your Cobra build?
You mentioned a "higher end Cobra build with an aluminium body" .... just wondering, is it an AP variant, with a Kirkham body on an AP aluminium alloy monocoque? Curiosity piqued ;)

Cheers!
Glen

Now, I'm going to throw another angle at you. From what I understand, there are still quite a few 351 Clevelands down under.

Cleveland 351 engines were built in Australia at Ford's Geelong plant (near Melbourne) from 1972 until 1982. They went into locally built Falcons and Fairlanes; don't know what the numbers were, but it would have been a popular option.
The 351C engines were used in the almost legendary Falcon GTHO Phase II and Phase III cars that had significant racing success here ( but - just occasionally - were beaten by those pesky little 6 cylinder Holden Torana XU1 upstarts).

Cheers,
Glen

In my personal experience, you are correct, Brent. If the engine is under-valved, has poor porting and/or other intake/exhaust design demerits, tightening lobe centers might help offset, but not correct, the basic design deficiencies at a given rpm. However, it will likely not produce an increase in the rock-n-roll quality of the idle. The additional dimension of camshaft duration needs to be present also.

As you pointed out, the changes are purely for auditory satisfaction and not necessarily a performance improvement — a lot of power is lost out the exhaust. This type of cam mimics the sound of a sixties race engine. More significantly (and again, as you pointed out), it can diminish the low-speed performance in the rpm band the car will be most frequently driven in.

I left the duration parameter out of the explanation because I suspected the OP would use something along the lines of a Thumper profile, duration-wise. However, something to consider for those enamored with the 60's race engine style idle is that, again as Brent pointed out, your engine's volumetric efficiency at lower engine speeds is compromised. That means the engine will not want to pull the car's weight in the 1000 to 2000 rpm range, in higher gears, without bucking and farting, making the driving experience, once again, unpleasant.

If you use tall gears and 295-ish tires like many of us do, driving at highway speeds of less than 2000 rpm will also be an unhappy experience. Downshifting to 4th gear will correct the engine bucking problem, but a sustained 2100 to 2500 engine rpm at highway speeds gets old quickly and is hard on the ears.

Let's not also forget that the in-town driving experience will continuously require a conscious selection of the correct gear to maintain in-traffic engine speeds above the engine's bucking and farting rpm. Once again, hard on the ears and minimally tiresome. In the end, you will have created a car that is not fun to drive, which is why many of us built these things in the first place. Sadly, it will also not be a very good performing car either.

Without generalizing too much, on my street engines I pick the intake duration for where I want peak power, use a closer LSA (108* or so) to get more mid range torque and add a few degrees exhaust duration to extend the power band. This gives me good torque and power over a wide RPM range, perfect for a performance street engine.

The intake .050" duration (along with the displacement and cylinder head flow) does control where the engine will peak the horsepower.

However, the LSA is just a number that plays into how much overlap the camshaft has. It doesn't change where the peaks occur or how broad the curves are, the overlap does that.

Exhaust duration should be based on how well the cylinder heads flow on the exhaust side, or if the application is a high/low rpm application, or if the application is forced induction, etc.