Why is the exhaust favored for duration

[N-gin]

Why are there so many cams that favor duration on he exhaust side.
I don't think I've seen any cams that have a longer intake duration than exhaust.

[rigidthumper]

Kuryakyn  cams buck that trend.

[rbabos]

Why are there so many cams that favor duration on he exhaust side.
I don't think I've seen any cams that have a longer intake duration than exhaust.

Scavenging and to drive tuners nuts with reversion.
Ron

[Barrett]

Smaller valve..

[Matt C]

When the TC first showed up on the scene, the EX port was really bad (until the -06 heads came out).

There are lots of reasons to extend EX duration; Low compression and a poor EX port are the Two big ones,
"Potty mouth"ty pipes is another. I'm sure there are others, but these are the ones that jump right out imo.

[TorQuePimp]

Ever see an exhaust port that outflows the intake?

[speedzter]

And why do some have lower lift on the exhaust ?

[1FSTRK]

Ever see an exhaust port that outflows the intake?

I have on turbo charge engines.

[Deye76]

Kuryakyn  cams buck that trend.

[1FSTRK]

Kuryakyn  cams buck that trend.

But as many found out those cams gave average gains until used with Kury heads they had different exhaust valves and ports.
Most cam companies develop the cams to match the head.
Many head porters develop the head to match the cam.
Very few develop a head and cam package to match the needs of a particular bore and stroke.
This is exactly what is hold the M-8 back

[N-gin]

 My understanding is that the exhaust valve is smaller cause gasses are less heavy. And the gasses are under pressure anyways.

Is there a correlation between cam and exhaust system as far as a stepped head pipe works better with longer duration exhaust, and non-stepped and single pattern?
Or is it just luck of the draw and combo.

Kuryakyn  cams buck that trend.

But as many found out those cams gave average gains until used with Kury heads they had different exhaust valves and ports.
Most cam companies develop the cams to match the head.
Many head porters develop the head to match the cam.
Very few develop a head and cam package to match the needs of a particular bore and stroke.
This is exactly what is hold the M-8 back

That makes sense

[Timinator]

Only atmospheric pressure (14.7 lbs/sq.inch at sea level) fills the intake charge (other than ram tuning in a tuned induction system and exhaust scavenging with headers), and at that, in the allotted duration and rpms involved, the actual pressure differential is a lot less. On the exhaust side several hundred lbs. pressure remain in the cylinder at the beginning of the exhaust event (and that starts way before the piston is at the bottom of its stroke). That being said, when the exhaust valve begins to open, the cylinder pressure drops very rapidly percentage wise, as the piston reaches BDC and continues at a much lower pressure as the piston rises back up to TDC, then as the piston goes back down on overlap until the exhaust valve closes. You have more exhaust volume, but you have hundreds of times more pressure working to extract it. With a well designed exhaust system (headers of some type with low restriction muffler), during the overlap period, the exhaust leaving the cylinder lowers the pressure in the cylinder below atmospheric, even before the intake valve opens, and the actual induction event occurs. Ed Iskendrian of Isky cams in the 1960s called it the "5th cycle", and he called his cams 5 cycle cams. He didn't invent this phenomenon, but got peeps talking and thinking about it.
Some further thought: some have disagreed with my post regarding "80% of the power is made in the top 20% of the stroke." To help in understanding this, and get a better perspective of the goings on during the exhaust event, Google "early Detroit Diesels" and "2 stroke Diesels." Watching the graphic of how they function with 4 exhaust valves and no intake valve will give you some insight into the exhaust stroke and power stroke of your engine and may give you some ideas into improving it.
An easy way to see residual exhaust in the cylinder, is on a blower engine with a poor exhaust or improperly designed cam for the engine. If say the boost seen on the gauge is 10 lbs. and the cam is replaced with one of a longer duration, or the exhaust port or valve is made larger (more efficient), the boost seen on the gauge will drop, sometimes up to 2-4 lbs. The engine's power will definitely increase, but lower boost is seen as a result of the cylinder being scavenged better (less residual exhaust that was taking up space in the chamber that can now be filled with fresh charge) that is no longer backed-up in the intake manifold and has entered the combustion chamber. Boost is actually bad, it is simply potential power that has not entered the cylinder and therefore cannot make power, until the next intake stroke.
Back to the actual question, longer exhaust duration hurts power at lower engine speeds (in most applications) and helps it at higher engine speeds. Most HDs don't run at higher engine speeds, but, most HD riders care more about looks and sound than performance and this is why most HD cams are ground to dual pattern specs to crutch the poor exhaust systems and enhance the Harley "sound."
Many of my customers won't use collectored headers because they "look and sound wrong."
Woods cams are mostly all single pattern, they enhance the midrange, and do it well, but he will also tell you that you need good heads and exhaust to take advantage in the upper RPMs of the cams he sells.
Random thoughts regarding internal combustion engines that are not meant to be argumentative, but informative. Hope I have not strayed too far from the original question....   TIMINATOR

[Don D]

The exhaust is a system and examing the cam only covers one component of that system.
The overlap function must be functional, exhaust suction aids in cylinder fill. Most designs of random parts don't accomplish this and thus reversion and dropping power at higher rpms on motors that have the breathing potential to go higher.
Dynomation is wave action simulation program can help choose components and fit engine demand to parts. Pipe max is good as a guide for properly sized headers.
You will need the heads flow sheets to use dynomation effectively.
Think system design, not good components.

[Matt C]

Ever see an exhaust port that outflows the intake?

Yeah. once

(Designing an EX port on a flowbench will get you into trouble)

[Don D]

And after blow down at bdc there can be pumping losses if the port or exhaust system don't cooperate. If there is pressure at tdc the overlap cycle cant work properly . Guys would be surprised some exhaust systems actually build pressure at high rpm. Those old testers they use for cats would open some eyes. With back pressure the longer the overlap the soggyier they get. Intakes become black from the egr.

[Timinator]

Dynomation is the shiznit for component selection and curve prediction! Been using it for years!
I'll post a link to the 2 stroke diesel site when I get time. Its amazing how many peeps I have shown that graphic on my computer and they thought that it was some April Fools Day prank!    TIMINATOR

[Don D]

On a side note have messed with all -71 detroits, know them well. Still srands the hair up on my arms to hear a screaming jimmy.

[Matt C]

Dynomation is the shiznit for component selection and curve prediction! Been using it for years!
I'll post a link to the 2 stroke diesel site when I get time. Its amazing how many peeps I have shown that graphic on my computer and they thought that it was some April Fools Day prank!    TIMINATOR

You can load campro files (and flowbench data) into it too and get even more amazing results.
I like the wave action displays too.

[Deye76]

"But as many found out those cams gave average gains until used with Kury heads"

I've used them 3 times in the last 12 years, 1 with HTCC heads, one with Dorfman's heads, and the current one with stock heads. No worse than other comparable builds with like compression, pipes etc.
Latest attached.

[Admiral Akbar]

But as many found out those cams gave average gains until used with Kury heads they had different exhaust valves and ports.
Most cam companies develop the cams to match the head.
Many head porters develop the head to match the cam.
Very few develop a head and cam package to match the needs of a particular bore and stroke.
This is exactly what is hold the M-8 back

Care to explain why? Or are you just going to wander off?

[1FSTRK]

But as many found out those cams gave average gains until used with Kury heads they had different exhaust valves and ports.
Most cam companies develop the cams to match the head.
Many head porters develop the head to match the cam.
Very few develop a head and cam package to match the needs of a particular bore and stroke.
This is exactly what is hold the M-8 back

Care to explain why? Or are you just going to wander off?

Why in regards to what?

[Timinator]

I always use MY flow bench data and I've had Comp Cams lobe library on my computer since 2004, Dynomation is just a much better deal than all of the rest that I have tried. But still garbage in equals garbage out. That's why I only trust my own data. Except of course for the giggle factor on some builds on some of these sites! I do believe that there are some unscrupulous shops that "slant" the data in their favor when one of their builds doesn't actually deliver as promised. My HD flow data is developed with the actual intake manifold and carb/Throttle body to be used installed. I know of several shops that use flow guides on their heads when testing, but the air doesn't enter the intake port the same way and amount as it does with the actual intake parts to be used. When done this way Dynomation always shows a lower, but more accurate HP/Torque number. Lotsa shops, lotsa ways to skin a cat.    TIMINATOR

[Admiral Akbar]

But as many found out those cams gave average gains until used with Kury heads they had different exhaust valves and ports.
Most cam companies develop the cams to match the head.
Many head porters develop the head to match the cam.
Very few develop a head and cam package to match the needs of a particular bore and stroke.
This is exactly what is hold the M-8 back

Care to explain why? Or are you just going to wander off?

Why in regards to what?

What does the head and cam package need in reference to bore and stroke other than displacement?  I'll agree that the current cams are a bit off to match the heads.. It will take a while for the better ones to come out.  Also the exhaust systems may need re-thinking.

[1workinman]

And after blow down at bdc there can be pumping losses if the port or exhaust system don't cooperate. If there is pressure at tdc the overlap cycle cant work properly . Guys would be surprised some exhaust systems actually build pressure at high rpm. Those old testers they use for cats would open some eyes. With back pressure the longer the overlap the soggyier they get. Intakes become black from the egr.

When I was a kid I had a 302 Chev that had a set of heads that came off a g gas car , angled milled . 100 , 12.5 pistons , 600 isky roller ect ect in 68 Camaro I had the rev limiter set at 8k and hit it regular . When I first got it together I had a set of small tube headers and hush thrush I think mufflers . The damn engine would or seemed to blow fuel out of the carb when I got into it hard. I cut those mufflers off added a cross over pipe between the collectors and 3 glass packs . It was the turbo mufflers that was causing the problems . From then one the engine would scream , It was a rush to run threw the gears at 8k , That's all the bottom end was good for

[1FSTRK]

But as many found out those cams gave average gains until used with Kury heads they had different exhaust valves and ports.
Most cam companies develop the cams to match the head.
Many head porters develop the head to match the cam.
Very few develop a head and cam package to match the needs of a particular bore and stroke.
This is exactly what is hold the M-8 back

Care to explain why? Or are you just going to wander off?

Why in regards to what?

What does the head and cam package need in reference to bore and stroke other than displacement?  I'll agree that the current cams are a bit off to match the heads.. It will take a while for the better ones to come out.  Also the exhaust systems may need re-thinking.

Did you see the Youtube on the redshift cam? They at least take cfm of the head into account.
The problem is you can make the cam get the most from the head but the head is the problem so you still come up short. The factory head used a poor approach to get stock torque through the EPA that is one reason their Screamin eagle head is a different casting.