Custom Stainless Steel Exhaust

[davery]

generally the tubes are at least 1.5" and the collector is 2.5". Our header steps from 1.5 to 1.75 per tube into a 2.5" collector. Dyno testing has proven a substantial hp and torque improvement with no other changes to the engine. At peak hp we had a 10% increase, peak torque was a 15% increase. What is really telling is the power and torque increase at 6000rpm. The stock hp and torque curves begin to drop at 6k, while our header/exhaust kept the curves nearly flat so that we had a 15% hp increase and 20% torque increase at 6k. This indicates the stock exhaust system doesn't flow enough at high rpm

So the total area of a cross section of all the 1" tubes would be about 7 sq in. The total area of the four 1.75" tubes would be about 9.6 sq in. The area of a cross section of the 2.5 collector would be about 4.9 sq in. That is a 37 % increase then almost a 50% decrease. Interesting.

[davery]

there need to be some backpressure.

No there don't.

I've never understood the concept of back pressure. It would seem to me that the most efficient engine would be one that discharges the burnt gas from the combustion chamber as quick as possible. Any impediment, such as back pressure, would seem to slow that discharge down.

[Redline]

Remember that the four individual runners are not all flowing at the same time. Pulse, pulse, pulse, pulse.

[So Cal Mark]

"So the total area of a cross section of all the 1" tubes would be about 7 sq in. The total area of the four 1.75" tubes would be about 9.6 sq in. The area of a cross section of the 2.5 collector would be about 4.9 sq in. That is a 37 % increase then almost a 50% decrease. Interesting."

Where did you read 1"? Collectors work really well if there is a venturi effect

[bradartigue]

Like seeing a single pipe performance exhaust, they haven't been that prevalent and they should be.

[v6spider]

....Where did you read 1"? Collectors work really well if there is a venturi effect

Yes DO tell.. There should be zero back pressure.. venturi effects have to do with carburetion .. IMO

Rob

[So Cal Mark]

venturis work with any air flow, not just carbs. Look at a merge collector from Burns and you'll see a venturi at the end of the collector.
Remember the air cleaners on old Chevys? They had a snorkel that seemed WAY too small to feed the carb but that snorkel was a nice long venturi

[v6spider]

venturis work with any air flow, not just carbs. Look at a merger collector from Burns and you'll see a venture at the end of the collector.
Remember the air cleaners on old Chevys? They had a snorkel that seemed WAY too small to feed the carb but that snorkel was a nice long venturi

I'm aware of this.. But the intake air is being pulled into the engine ... And exhaust air is pushed out. So the effects of restrictions react differently I think. No different than mounting a fan in front of the radiator as versed to behind. It more efficient to pull the air through it than to push it through.. In other words the force drawing the air into an engine is stronger than the force pushing it out.. Of course this is just my theory..

[davery]

"So the total area of a cross section of all the 1" tubes would be about 7 sq in. The total area of the four 1.75" tubes would be about 9.6 sq in. The area of a cross section of the 2.5 collector would be about 4.9 sq in. That is a 37 % increase then almost a 50% decrease. Interesting."

Where did you read 1"? Collectors work really well if there is a venturi effect

That should be 1.5". Which would give a cross section area of .75 x .75 x 3.14, which is about 1.76. Times four tubes is about 7 sq in. Too much calculating in my head.

[timinator]

It more efficient to pull the air through it than to push it through.. In other words the force drawing the air into an engine is stronger than the force pushing it out.. Of course this is just my theory..

Find another theory.

[davery]

venturis work with any air flow, not just carbs. Look at a merge collector from Burns and you'll see a venturi at the end of the collector.
Remember the air cleaners on old Chevys? They had a snorkel that seemed WAY too small to feed the carb but that snorkel was a nice long venturi

So what you are saying is the decrease and then increase in the cross section area creates a venturi effect which increases the flow (speed) of the exhaust gas, thus making the exhaust system more efficient. Venturi effects are created when there is a gradual decrease, but what about when the gas in a 2.25" pipe hits a 1.75" opening to the converter? I'm assuming the OEM converter is 1.75". Would there be a benefit to switching to a converter with a 2.25" opening? Thanks for all the info, this is a great thread (although, sorry to the OP about hijacking it).

[So Cal Mark]

yes there is a huge advantage to matching the catalyst size to the exhaust tubing size. Even better to go a bit larger with the cat.
Just a comment about a previous post concerning air flow and pushing and pulling. The correct term is high pressure and low pressure. High pressure will flow into low pressure areas, if you can speed up the flow you'll be better off

[v6spider]

It more efficient to pull the air through it than to push it through.. In other words the force drawing the air into an engine is stronger than the force pushing it out.. Of course this is just my theory..

Find another theory.

This pretty much covers what I'm talking about in a different way.. My point is the technique you use to improve flow in the exhaust system is different than what you would use to improve flow on the induction end.
http://www.enginebasics.com/Engine%20Ba ... 0Flow.html

[bobplyler]

And 2 strokes use VERY different size pipes. It's called an expansion chamber.
https://en.wikipedia.org/wiki/Expansion_chamber

[timinator]

This pretty much covers what I'm talking about in a different way.. My point is the technique you use to improve flow in the exhaust system is different than what you would use to improve flow on the induction end.
http://www.enginebasics.com/Engine%20Ba ... 0Flow.html

Try this instead:
"At this time a few numbers will put the value of exhaust pressure wave tuning into perspective. Air flows from point A to point B by virtue of the pressure difference between those two points. The piston traveling down the bore on the intake stroke causes the pressure difference we normally associate with induction. The better the head flows the less suction it takes to fill (or nearly fill) the cylinder. For a highly developed two-valve race engine the pressure difference between the intake port and the cylinder caused by the piston motion down the bore, should not exceed about 10-12 inches of water (about 0.5 psi). Anything much higher than this indicates inadequate flowing heads. For more cost-conscious motors, such as most of us would be building, about 20-25 inches of water (about 1 psi) is about the limit if decent power (relative to the budget available) is to be achieved. From this we can say that, at most, the piston traveling down the bore exerts a suction of 1 psi on the intake port Fig. 3.
The exhaust system on a well-tuned race engine can exert a partial vacuum as high as 6-7 psi at the exhaust valve at and around TDC. Because this occurs during the overlap period, as much as 4-5 psi of this partial vacuum is communicated via the open intake valve to the intake port. Given these numbers you can see the exhaust system draws on the intake port as much as 500 percent harder than the piston going down the bore. The only conclusion we can draw from this is that the exhaust is the principal means of induction, not the piston moving down the bore. The result of these exhaust-induced pressure differences are that the intake port velocity can be as much as 100 ft./sec. (almost 70 mph) even though the piston is parked at TDC! In practice then, you can see the exhaust phenomena makes a race engine a five-cycle unit with two consecutive induction events".