Its always been my experience that running more compression in a radical motor helps cover up a loss in low end torque that may have been caused by a larger port or a radical camshaft, but you are the engine builder so I would imagine you'd have better insight into this matter than I would.

 

Go look up dynamic compression ratio on here, that will explain alot about why the extra compression helps the cam and low end TQ output. The larger port is not as big of a deal here, but when you have too much, more camshaft is just going to hurt you more.

Bret

 

Think about it as the volume that is pulling on the port, not on how it gets the volume or at what speed the piston is going down in the bore. The more times it needs to pull on that volume (RPM) the more air it needs to flow. The rate at which the air/fuel needs to fill that volume will go up with RPM so therefore the velocity of the port flow will go up .(remeber that you have less time to fill that port when the RPM increases) There is always a point of dimishing returns, and once you go over that peak, you are just fighting the system to make the HP peak any higher.

When the velocity becomes a restriction you can always slow it down with a larger hole. You get the air in 2 ways, one with speed the other with volume (CF) so you can either flow more (Which is usually thru a bigger hole) or you can flow it faster. At one point the speed is going to hurt you. This kind of shows why the smallest port that can easily do the job is good, to get the velocity up where you want it up. It also shows that when you have too small of a port it's never going to flow enough, or where velocity is too high it hurts the performance.

This doesn't give you the had numbers on what works the best on what motor, but it gets you closer to figuring out the problems.

Now if you can have a nice fat mid lift part of your curve vs a flatter peaker flow curve you are ahead of the game. Flow is more important than the size, but when you are tring to get the best combo together they all count alot.

That make more sense?

Bret

 

I think I see what you are saying. So you are saying that smaller intake runners will help "cover up" an oversized cam, whereas larger intake runners will only add to the problems an oversized cam can have?

 

Here's my claim to fame (I've used it a few times before) and it
helps me understand flow dynamics in a motor:

Compare your intake runner size to a drinking straw and a toilet
paper roll.

Imagine your lungs representing the displacement.

Spread some popcorn kernals on a plate (they represent air/fuel
mixture).

Try to suck up a kernal with the toilet paper roll.

Do you think it would be better to use the straw? Why?

What would happen if you used larger more powerful lungs behind
the toilet paper roll?

As the piston accelerates down into the bore, it create a low
pressure area. The air and fuel above the cylinder (including
head chamber volume) rush down according to the differential
in pressure.

If you have larger valves and runners, the differential in pressure
between the the cylinder and intake runner is minimized.

Once the piston is moving and air flow is constant (velocity is
achieved), volumetric efficiency improves because the tuned length and 'diameter' is reaching optimum for that RPM.

I know this doesn't answer the question directly, but it helped me
to understand that bigger isn't always better...and all this talk
about "matching the system" holds alot of water in the world
of Horsepower and Torque.

 

define "oversized cam".

what he's talking about is cranking compression. the static comp and the closing of the intake valve determine the in cylinder compression or "dynamic comp". with a "large" camshaft comes more overlap and a later intake valve closing point. close the intake earlier and you have trapped a larger portion of the intake charge. close it later and you have trapped less. the more air you trap, the more torque you make and the more responsive the engine is in the lower rpms. too much will/may cause some trouble. so its good to know where your at, hence the dynamic comp calculation.

large camshafts are not necessarily the problem. they are a necessary evil depending on the goals of your build. what does seem to be a problem for most street engines is people using large cams with too wide a lobe sep angle. if you have been around race engines for any period of time you will see that there is a right and wrong lobe seperation for every app. best bet is, if you'd drop some @.050 duration and tighten the lsa, you'd make more power but most guys run too much duration and compound the problem with too wide a lsa. this is particularly true of the late model builders who spec cams like they are trying to pass emissions testing. which is fine if that is the goal. but judging by the number of gutless cats out there, i'd say that some are missing the boat.

JB

 

You're exactly correct here. TQ is a direct product of a certain cylinder pressure, bore area, and stroke at a given engine rpm. The end result is the same with both bore/stroke combinations if your cylinder pressure is the same. One of the keys here is a smaller bore engine will hit it's flow limit sooner because the bore size is shrouding the valves, as you mentioned elsewhere. A longer stroke engine will also require shorter connecting rod, which is almost always the wrong way to go.

However, for most of us with LT1 engines we just can't overbore them much so a longer stroke is the most popular way to get more cubic inches.

 

I remember reading about the rod lenght that 5.7 vs 6" yielded the same under the curve torque but in a different powerband...

I dont see how the longer stroke wont give you more torque down low, its hard to internalize or visualize as its what I have "seen" always...

 

A quick calculation on a spreadsheet will tell you all you need to know. Bore area times stroke length times cylinder pressure. If they're the same cubic inches the first two will have the same product.

 

if you will think about "where" maximum cylinder pressure takes place and "where" it is most beneficial you start to paint a better picture of the situation. its a floating parameter that is totally dependent on the purpose of the engine.
i don't buy into the "longest rod possible" theory in all cases. just based on my experiences with competition engines.

JB

 

Jeff,

Yeah agree on the Rod Length setup.

We got into it pretty heavy here:

http://web.camaross.com/forums/showt...hreadid=114767

I still think that is one of the better threads I've had the pleasure of being in on the forum. Learned some good stuff there.

Good quote from that thread

"This might explain why Smokey liked long rods and Grumpy liked short rods. Smokey worked mostly with lower RPM, lower duration cams on durability circle track engines and Grumpy made the high winding 331cube SBC famous. There is no question that the longer durations on cam lobes are seen in drag racing engines. Get a little more life out of the engine and run a little less timing since you have to use stock parts. Makes sense now why Grumpy and Smokey had the battle they had about this."

I tend to side with Smokey on the rod length for street motors though.

Bret