Reply to "Questions regarding George's 275/285 Custom Street Cam Profile"

Engines with small valve/in-line valve cylinder heads rely on the cam to make top end power, because the size of the intake port is so small it skews the engines power band towards low rpm. So the cam and single plane intake manifolds are used to build high rpm power. The cams role in building a performance street motor equipped with that style of cylinder head is very important. The effects of overlap are not amplified by such engines, so you can get away with a significant amount of overlap without ruining drivability. This is the type of engine 99% of enthusiasts are accustomed to.

With the big intake valve and canted valve geometry of the 351C 4V the effects of overlap are amplified. This is a good thing for a race engine, overlap can be used to scavenge exhaust gases and get the intake charge flowing early which improves volumetric efficiency. Drivability (i.e. performance below 3000 rpm) isn't important to a race engine.

While amplifying the effects of overlap is a good thing for a race engine, its a bad thing for a street engine. We have to minimize overlap to optimize low rpm power and maintain drivability. Many journalists over the years have written "its easy to over-cam a 351C 4V", without really explaining why. I've just explained why.

The large port/large valve heads dominate the power band of 351C 4V engines having power bands that extend to 6000 to 7000 rpm. In one way the cam is just along for the ride, its job is mostly to open the valves. The cams role is not as important as it is with a small valve/in-line valve engine. But there are things you can do with the cam to make things worse, that's what you want to avoid: (1) opening the exhaust valve too late causes high rpm torque to fall-off like a brick, (2) too much over-lap softens low rpm torque, and (3) closing the intake valve too late causes low rpm reversion while lowering dynamic compression. Narrow lobe centers create conditions #1 and #2. Combined they have the effect of making the torque curve (power band) narrower and steeper. Free flowing mufflers or an un-muffled exhaust system can help alleviate problem #1.

Since the big intake valve and canted valve geometry of the 351C 4V amplifies the effects of overlap, a camshaft with 114° LSA has the same effect on a 351C 4V as the effect a cam with 110° LSA has upon an engine with small valve/in-line valve heads. In the same way 112° LSA has the same effect on a 351C 4V as the effect a cam with 108° LSA has upon an engine with small valve/in-line valve heads. This is an aspect people can't wrap their head around. The indoctrination people get via magazines, internet and television makes people think they are losing out on something with the wide 114° LSA. It just isn't so with the 351C 4V, not if drivability is important.

This is something I discovered in the 1980s, and the reason I began having my own cams ground back then. I experimented with 110° LSA cams a few years back, and didn't like the results ... not for a street engine where drivability is an issue for the owner.

Combined with the 4V cylinder heads the cam I've specified provides good drivability AND bitchen performance. This I guarantee. I've learned over the years to trust the heads to be the star of the show, they've never let me down.