Step okay from exhaust port to header tube?

[andyf]

I'm working with a BB Mopar engine with fairly small and rectangular shaped exhaust ports. PipeMax recommends a 2.125 header tube for max power but the transition from the exhaust port to the flange will be fairly large. Obviously guys have been running this type of setup for a long time but I'm just wondering if maybe there isn't a better option? Maybe this is one reason why step headers seem to work fairly well? If the first step is closer to the port size then there isn't such a large transition. Or does the big change in area not cause a problem on the exhaust side? The gas is coming out of there fairly fast and as it cools down it expands so perhaps the sudden change in area is okay? Here is a picture just to show the size of the change.

flange.JPG

[bigjoe1]

I run stuff like that all the time with great success-- NO PROBLEM !


JOE SHERMAN RACING

[77cruiser]

How much power that you need that big of pipe?

[andyf]

The engine makes a little over 900 hp with the existing 2 inch dyno headers. PipeMax recommends a 2.125 so I'm going to build a new set. I bought the 2.0 inch headers back when the engine was making 750 hp.

[Lenni AMC]

When the exhaust gas cools down it shrinks in volume it does NOT expand.
For example thats the reason why we make more power when the intake charge is cold vs hot. Since you can get more oxygen in the same engines displacement when the air is cold than you could fit if the air was hot.
So the conclusion is when gases and all other materials are cold they are smaller in size or volume vs when they are hot

but for my $.02 the step in the flanges does not make a noticeable difference in power unless the header tube sticks inside the exhaust port and therefor acts as a restriction...

[mag2555]

Leave that mismatch just the way it is and you will be rewarded with a motor that comes on the cam sooner and also Exh flow numbers that will pick up anywhere from 3 to 5% above .400" lift! With a header tube that matches the gasket ID!

[engineguyBill]

You will be OK, since the header flange is smaller than the exhaust ports.

BUT I question the necessity of 2.125" primary tubes for your application. It is my contention that these large tubes MAY be effective if your heads (especially the exhaust ports) are ported by a professional head modifier/expert. The exhaust port modifications should be done by someone who has access to a flow bench, as just hogging out the ports will not necessarily increase the flow/efficiency.

[Ron E]

Andy, just to prove nothing is simple...we've always picked up flow with headers that match the openings. We try to make the port to pipe size/shape transition as gradual and smooth as possible.
Bill...I'm seeing the flanges as bigger than the port openings. And, with 900 HP out of a BBM, I'd bet plenty of flow bench effort was made in the exhaust ports. 2-1/8" pipes seems in line with that output.

[#84Dave]

The primary pipe ID of the header is solidly linked to peak TORQUE rpm, similar to intake manifolds. The following equation for determining the relationship between pipe ID size, single-cylinder displacement, and the peak-TORQUE rpm point is very helpful for determining your engine properties/needs.
Peak TORQUE RPM = Pipe ID cross-section area x 88,200
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single-cylinder volume (cubic inches) -84Dave-

[rfoll]

I'm having difficulty with the math of that formula. A 1" area pipe would have a peak torque of 88,200 RPM

[CamKing]

I usually recommend that you start with a pipe that's as close in area to the exhaust port at the flange, without being smaller, and then step up to the larger size, before the first bend.

[900HP]

Just read this.......
http://www.exhausting101.com/?p=37

[#84Dave]

The pipe formula...... one more time: Peak Torque RPM = Pipe ID cross-section area(square inches) x 88,200/(divided by) single-cylinder volume in cubic inches. -Dave-

[900HP]

The pipe formula...... one more time: Peak Torque RPM = Pipe ID cross-section area(square inches) x 88,200/(divided by) single-cylinder volume in cubic inches. -Dave-

That's all well and good but it doesn't take into account existing exhaust port cross-section. There are reasons why header primary tubes are or are not stepped.

[Tuner]

Sandwich another flange with tapered cones that merge from the port to the pipe ID? The Cyclone "Anti-Reversion" headers is a fond recollection. This is an opportunity to try something like that with straight stubs or tapered from port to pipe. On the dyno you could prototype using a thick spacer/flange with the merging taper in it.