Hello:
Headers can help in the power department by getting more exhaust out (as exhaust gasses can't burn twice) that otherwise take up valuable room inside the combustion chamber for the fresh incoming charge of fuel and air.
In my case (with aftermarket heads and a ported Intake), headers should have
really helped as the engine's airflow increased considerably on the Upstream side. If you are increasing airflow/fuel
into an engine, it will also require the ability to vacate that increase on the exhaust side to maintain a balance between the two.
More often than not on the 8.3 Viper engines, headers have provided less of a power increase than I've seen on other engines.
Headers work and are more than just free-flowing pipes to help exhaust gasses get out of the cylinder.
They work on "rarefaction" as well.
When the piston is on the way down during the Power Stroke (nothing to do with Ford Diesels here
), the Exhaust valve opens and exhaust gasses begin their journey out of the cylinder and out the exhaust system. The piston (now on it's way UP) pushes most of the rest of the exhaust out.
As the exhaust is leaving the cylinder (piston still rising), the Intake Valve Begins to open and the fresh air/fuel charge coming into the cylinder gets an assist by the draw of the Exhaust gasses
leaving the cylinder.
You may have heard the term Overlap (when both the Intake and Exhaust valves are open) in camshaft terminology and that is what/when it is.
Now the theory comes into play: When the exhaust pulse is travelling down the Header tube it hits the collector and a negative pressure pulse goes back towards and into the exhaust port which can partially block the new air/fuel charge from leaving the cylinder (a good thing). These pressure waves (Rarefaction in action) assists exhaust scavenging and increases torque.
WHEN this occurs is a function of camshaft design, the length of the exhaust header pipes and the diameter of the header pipes. That is why a properly designed long-tube header helps produce torque as there is enough pipe for the good stuff to be able to work their magic.
Shorties do not have enough pipe to work as above. But they could help IF the exhaust manifolds were restrictive to start with, in which case anything would help. On a race engine, Shorties can be beneficial for other reasons.
More often than not, headers seem to do little to increase performance (much) on the 8.3 and it could be because of a low overlap camshaft as many manufacturers are going with.
I hope you didn't glaze over too early as this can be pretty dry stuff but isn't easy to try to explain without creating a baseline then yakking' from there.
As for ported heads: Flow numbers tend to sell cylinder heads and I don't really know of any other way of trying to promote them if I were a head porter.
Do ported heads help power production? Yes, absolutely, but only to a certain point when comparing flow numbers. A head that can flow 350 cfm does not automatically make more power than that same head ported to flow say, 310 cfm, for example. All heads have there limits and sweet-spots.
If you are out shopping for ported heads don't believe the one with the high(er) flow number is the best one to get. With that, there are several ways to "cheat" cfm numbers that don't translate into better performance once they are bolted on.
Our o.e.m. Gen III heads flow 270 c.f.m. (cubic feet of air per minute).
Aftermarket Striker heads flow 330 c.f.m. but keep in mind that increase was NOT from porting the oem head but from a replacement aftermarket head.
That increase in c.f.m. the Strikers brought to the table (coupled with advanced port and combustion chamber design) and a small camshaft, provided an increase of 143 horsepower to the wheels on my engine and the identical power number on another SRT-10 (regular cab) with the same camshaft.
Would either of these engines produce more power if say the Striker heads were ported to flow 400 c.f.m.? No.
At a certain point, increased flow (c.f.m.) becomes detrimental to engine performance in the form of air fuel separation (incomplete combustion) and/or port stall where efficient airflow drops dramatically.
MPH at the end of the 1/4 mile is a great indicator as to whether an engine picked up power or not.
I've seen ported heads that made a dramatic difference and produced big MPH gains. I've also seen some really high-flowing ported heads that made NO difference or actually lowered MPH.
Tearing these engines apart will often tell the tale. You can SEE evidence of combustion on the piston tops (and often the area immediately above it on the combustion chamber.) The idea here is to have as much of the cylinder showing evidence of combustion as possible. An engine with lackluster performance likely shows a lack of combustion).
The worst example (best example?) of a high-cfm porting disaster I saw years ago was on a 440 piston that had a burn spot about the size of a silver dollar. The rest of the piston was silver as was the combustion chamber. This meant this engine (HEAD) was experiencing massive Air/Fuel separation. As fuel is heavier than air, if the path through the heads and valve isn't properly designed, fuel and air (still in a vapour form and ready to be burnt) will part company and combustibility drops substantially.
Liquid fuel doesn't burn and the silver areas on the piston top indicated that.
Often when someone is bolting on some shiny new ported heads, a number of other engine changes occurred as well: An increase in compression, a changed cam profile, exhaust changes, intake changes, a tune, weight reduction, gearing, tires, etc; and it's often difficult/impossible to say how much of a performance increase
was attributable TO THE HEADS.
Rather than going off of flow numbers, look at track performance, combustion burn-patterns inside the engine (not often an option) and to a lesser degree, dyno numbers.
But to close with Headers/Exhaust, I spent almost as much on the exhaust system as I did on cylinder heads. The performance difference wasn't even close to being comparable.
If this generates more questions, fire away and I may even be able to answer them.
I hope somebody got something from this LONG post and even if you disagree with it...that's something!
Ronnie