Head and cam decisions

[Anonymous]

Although I have the 300cid head in mind, I'm also curious to hear responses for heads of other motors. If this isn't the right place for this post please mod's correct me. As the time comes closer to build motors (a 300, a 460, and a chebby 2.2L 4 banger) I'm starting to gather information for the heads and have alot of areas that are blurry. I'm wondering what will give more power, a larger combustion chamber because it can take in more fuel, or a smaller combustion chamber that'll have higher compression. Also what about runner volume, is a larger runner better or does it just change rpm range? If I put larger valves in a cylinder head will it get worse gas mileage if I stay out of the pedal, I would think it would get better mileage or the same until you get on it. Sorry that I have so many questions I've been searching here and on other forums/sites but can't come up with answers I can understand.
As far as cams, I'm wondering first off what dictates the rpm range? Is it duration and lift? I'm wondering if it's possible to have a high lift high duration cam for a lower rpm range rather than 4-9k. Again this isn't necessarily for the 300, I know the head can't flow that much, I'm just trying to gain information. Thanks in advance.

 

[Thad]

RPM limit. If an engine can breathe well enough to reach max RPM, the ultimate limit to RPM is the stroke of the engine. A piston can not move faster than the flame speed of the fuel be used.
eg. an engine with a 4.00" can turn only 1/2 the RPM of an engine with a 2.00". Provided they are both capable of reach max RPM and burning the same fuel. Forget 9K rpm the 2.2Ltr 4 banger might make it, but not for long.
The size of the combustion chamber, if it does not limit the size of the valves or shroud them, has no direct relationship to to amount of air/ fuel mixture take in. Compression ratio is not dictated by combustion chamber volume alone, there is also the volume remaining between the piston top and the head at TDC.
Optimum is a small combustion chamber with a good squinch area surrounding it . Squinch area is where there is a flat face of the head  opposite a flat face on the piston.  As the the piston rises and approaches the head the A/F mixture between the two is forced in to the combustion chamber causing turblence promoting a better A/R mixture and fuel burn at ignition.Cam- duration and lift.
Cylinder filling is basically a function of the velocity of the A/F mixture entering the cylinder. Lift and duration regulates the time and valve opening for the A/F to enter and that with piston speed regulates the velocity.
Small lift/ short duration well at low RPM good for torque but A/F flow is limited. High lift/ long duration can generate good velocity at higher RPM but not at low RPM because of lower piston speed. Cams for street performance are a compromise between needed torque and usable HP.
Runner length. There is an inertial wave in the incoming A/F miture that that causes pressure to build against the valve after it closes. The length of the runner dictates at what RPm this occurs. Long runner - low RPM, short runner-high RPM. Most manifolds are a compromise with long and short runners.
Welcome to our world----6s rock.

 

[XPC66]

I'm wondering what will give more power, a larger combustion chamber because it can take in more fuel, or a smaller combustion chamber that'll have higher compression.

The combustion chamber, or more importantly the clearance volume doesn't really have any effect of how much fuel mixture is injested. The swept volume is the primary determinant. The main objective is to maximise the compression within the hard limits of EGT, ignition and detonation and of course mechanical integrity.

Also what about runner volume, is a larger runner better or does it just change rpm range?

Generally the longer runners and larger plenums favour low to midrange.

If I put larger valves in a cylinder head will it get worse gas mileage if I stay out of the pedal, I would think it would get better mileage or the same until you get on it.

You'll use more fuel

As far as cams, I'm wondering first off what dictates the rpm range? Is it duration and lift? ..

Redline is generally preditcable by the spring float/resonance

 

[rbohm]

As far as cams, I'm wondering first off what dictates the rpm range? Is it duration and lift? I'm wondering if it's possible to have a high lift high duration cam for a lower rpm range rather than 4-9k. Again this isn't necessarily for the 300, I know the head can't flow that much, I'm just trying to gain information. Thanks in advance.

8) duration, lobe center line angle, overlap, all determine the rpm range of the cam, along with ramp rates.

 

[Anonymous]

Thank you for the replies, I'm learning alot on this forum. I've been plugging information into a desktop dyno that I have and I'm pretty happy with the crane 272 cam, and I've been using 1.94 intake and 1.6 exhaust valves for the head just to see what kind of numbers it puts down, and I'm very happy with the torque curve, 345@2000 up to 360@4000 then a slow drop off, but I'm not very sure about the hp numbers, starting at 130@2000 up to 290@5000 then a drop off. I've played around quite a bit and noticed that a 1.94 intake paired with a 1.78 exhaust increases power significantly, but I'm wondering if this is just on the desktop dyno, because I've heard that the common believe is that the pressure in the cylinder is high enough to evacuate the exhaust gasses rapidly. I'm wondering if anyone else has built a motor with this cam and significant headwork, and what kind of numbers they are putting down. My whole goal here is to break into the 12's all motor.

 

[Thad]

The 1.94 is a little big and will be shrouded by the wall of the combustion chamber. 1.90 is a better choice. 1.78 exhuast great. In the real world you will need to pccket the valve pocket and clean the runners up to realize the most benefit of the bigger valves.