98RedBird

iTrader: (77)

I'm just wondering why a "big" cam for a 347 is considered a small cam on a 400+ ci motor?

G Atsma

Because larger displacements move the air faster, keeping flow velocity up and averting reversion better than smaller displacements.

Kfxguy

iTrader: (1)

because a bigger motor "sucks" on the air/fuel charge harder at idle, allowing less reversion and charge dilution. Thats the short of it.

Kfxguy

iTrader: (1)

lol i was typing as you were typing and pushed submit too late....at work haha

00pooterSS

iTrader: (40)

Generally speaking cam sizes go with cubes. The more cubes an engine has the bigger the air pump it is. Larger cams allow more air in and out.

Bigger air pump, bigger air demand, bigger cam necessary to match the air pumps demand.

Tuskyz28

Stroke eats duration.

Kfxguy

iTrader: (1)

JoeNova

Larger engines will move more CFM of air, allowing them to maintain higher idle and part throttle manifold vacuum with a larger cam, causing them to idle and cruise better.

A smaller engine will fill the cylinders faster. With a lot of duration, the valves will be held open longer than it takes to fill the cylinders, so the air stagnates and stops moving, reducing vacuum and idle quality.

bammax

That was actually a really concise answer. You have to know that an engine is an air pump and what a cam actually does to understand it though.

.boB

iTrader: (1)

They do, and they don't. I know, that makes perfect sense.

What everyone above said is true, as long as you're talking about a small block. As you start adding bore and stroke, of course you increase air:fuel requirements. That's obvious. So you have to open the valves longer and longer to feed the cubes at the same rate. A bigger bore and stroke with a stock cam won't be fed well, and will make poor power.

Also, because it's a small block, you also get a small intake and small heads, with small ports. Obviously, you can make them bigger than stock. But overall length and width limits that. At higher power levels, a "bigger" cam will help compensate for the small heads and intake.

Take a look at a big block engine, with big displacement, big heads and a big intake. The heads are big enough to swallow a beer can! But despite a 30-40% increase in displacement, similar cams will produce about the same characteristics and power levels in the big and small block engines.

NSFW

This was by far the most helpful explanation. Thanks!

Ls7colorado

iTrader: (3)

You know I never really thought about it, but it really doesnt make any sense at all why a large motor would perform any better with more duration than a smaller engine, unless of course the heads were limiting flow on the larger engine

bammax

It makes perfect sense. The cylinder is bigger and needs more air to fill. The only way to get more air is to increase the duration that the intake valve is open or to increase intake air velocity.

bigsapper

And yes, the heads are different. Larger valves & ports.

SoFla01SSLookinstok

As there are lots of things that can make an engine run different with the same cam. Thinking that valve relation to the piston will be the same with the same stroke even when having a bigger or smaller piston/bore. As when you put a longer stroke crank in the engine. The valve relation to the piston changes. As this is what I thought ate up duration the most. How much would just changing the bore eat up duration. It would be cool to see the exact same combo on say a 5.7 put on a 6.2. Would the peaks change rpm or would they stay the same & just gain everywhere?

Ls7colorado

iTrader: (3)

G Atsma

Not quite. If both have the same volumetric efficiency, the 400 WILL pull more air than the 350 by volume since it is a bigger engine, and thereby run smoother on the same cam because more air within the same timeframe is moving faster, using the cam's capacity to a fuller extent.

NSFW

I suspect the OP's question may also have been about the effects of a given amount of overlap on the idle and driveability of a small engine vs a large engine (346 vs 427, for example).

99 Black Bird T/A

iTrader: (4)

Why do larger cube motors seem to tame big cams?

I think of it like this, it's sort of a hokey analogy. It's like filling two different sized buckets up with water.

Pretend we have the following...
One gallon (like a gallon of milk) is 4 quarts which is 231 cubic inches. Which is basically the size of a big V6 engine That is about a 3.8 liter V6.

You have two buckets to fill up, a one gallon or 231 cubic inches and a two gallon bucket or 462 cubic inches ( 454 V8 punched .030 over is ~460 cubes.)

You want to fill both buckets with water with your garden hose. The garden hose turned on flowing water is like the intake valve turned on by cam duration flowing the air and gas mixture. The time the water hose is turned on is basically like cam duration turning "on" the intake valve.

Filling the one gallon bucket takes a duration of 60 seconds of time with the garden hose "on" .

​​​​​Filling the two gallon bucket takes a duration of 120 seconds of time with the garden hose "on" .

The bigger bucket can absorb more garden hose "on" (up to 120 seconds) duration than the smaller bucket can (filled after only 60 seconds.)

Analogy wise - Cam duration in big and small engines works in a similar way to filling up the buckets.

BTW some of it is Perception. What's viewed as a "big cam" often comes from being more familiar with the smaller / medium cams typically used on smaller cube LS motors to achieve the desired result but not being as familiar with the cam size needed in a big motor to get the desired results.

Ls7colorado

iTrader: (3)