Apparently heads that have 200cc or larger runners are better suited for more track-oriented racing engines, 160-180cc heads are better for the street. Generally when I notice people say this in reference to performance parts (like cams and intakes and so forth), when they say "better for the street", they really mean "better low-end torque" and better throttle response. I've seen this many different places and I just assumed it was true.

If larger runners in a cylinder head are better suited for all out flow and peak power, then why is it that the TPI systems that came on these cars, with their long, long runners are somehow tuned for torque? If less volume meant more torque and more useable power, then why didn't TPI systems have much shorter, smaller runners?

Obviously there's a difference between the cylinder head runners and the intake runners, but I dont think the incoming air and fuel cares very much that they're different pieces of metal.

Obviously I'm missing something. Maybe something to do with fuel/air velocity?

 

Mixing ideas.

Port volume on intake ports is just to compare one head to the other in relative terms. The distance from the intake face to the valve doesnt change so its easy to compare one to the other. What you're wanting to really know is the cross sectional area, and compare those to the TPI runners. The smaller heads have a cross sectional area much closer (but still bigger) than the TPI. The length of the TPI runners themselves gets into harmonics and other.. IMO... nonsense, creating the power curve of a typical TPI engine.

 

Cross section is the issue, aka "flow area". Take the relatively small air volume you are putting into the cylinder on the street, dump that into a huge port volume, and the speed of the air/fuel moving into the cylinder will drop - when that happens, the filling of the cylinder is reduced, reducing efficiency in low engine speed conditions. On the other hand, with a "race" engine turning at high RPMs, you want to be able to move as much air/fuel volume as possible into the cylinder, so a small port will restrict movement of the air/fuel into the cylinder.

TPI runner length is a pulse tuning issue, not volume. A shorter, fatter runner may have the same volume as the long, skinny stock runner, but will support more power and higher RPM operation than the stock runner. Again, cross-section or "flow area" comes into play, but pulse tuning is the bigger player.

If you want more discussion about "pulse tuning", we can do that.

 

The runners on a Tuned port manifold (factory or otherwise) work on the same principle as a big church Pipe Organ. The long organ pipes resonate at a low frequency ( rpm) . as the pipe length shortens the tuned resonance frequency ( rpm) increases also.

long runners=low to mid rpm torque boost but limited rpm and peak horsepower.

short runners= torque boost at mid and higher rpm, lots of rpm and horsepower.
low rpm torque is compromised as a result.

 

Actually, what ALL 3 OF THEM said...

Another issue is that it's VERY BAD to have the head port cross-section larger than the intake port. The IDEAL situation, though very hard to achieve in practice, is to have the cross-sectional area uniformly decreasing as it approaches the valve; that way, the charge doesn't speed up to pass through some smaller area up in the first part of the intake tract somewhere, then slow down and "pool" (sometimes literally, letting the fuel fall out) in a bigger area farther downstream. Having heads with runners that are too big will do that: you end up adding 8 little plenums right behind the valve, where the air comes in, and it slows down and becomes relatively "stagnant", instead of rushing in all at the same speed it was travelling at at its fastest in the port, as fast as it can past the valve and into the cyl.

Like most everything else, port size is part of the overall SYSTEM, or combination. All of the parts need to be sized and otherwise selected to work TOGETHER, without any parts mismatched in either the "too big" or "too small" direction.

 

wow you got all the big boys in on this one, kudo's for a very good question.

think of it as water flowing thru tubing.

one tube is perfectly straight and does not change in diameter

one tube is straight and then balloons out right at the end before the water comes out.

now imagine pressurized water (at the same psi) is travelling thru both tubes, which one would fill a bucket faster?(this relates to volumetric efficiency), and also, how far from directly underneath the end of the tube could you place that bucket to catch the water?(this relates to port velocity).

The straight tube at the same psi, will out flow the ballooned tube, in both psi and gallons per minute. the water pooled in that balloon on the end is only going to seep out over the edge, it will be turbulent instead of a stream, and it will have zero forward velocity.

now AT THE SAME PSI use a city sewer pipe vs the copper tubing, if psi is maintained, the sewer pipe will blow that bucket right over while the tubing fills its own. one or the either works, not a mixture of the two.

did that make any sense? was trying to explain it metaphorically and that doesn't always work.