Mohr performance header
baz:
Anyone on here running a mohr performance stepped header?
Would love to hear some feedback on them.
Sarge:
I don't have a Mohr Performance exhaust... but I do have a stepped header (from A1). My engine size is 2017cc. I'd been running an 1 5/8 A1 and after doing some reading, asked Tiger to add some 1 1/2" diameter x 3" sections at the head. This simple modification woke things up! Read on...
How Do Stepped Headers Work?
A tuned length primary exhaust header pipe creates a rarefaction (negative low-pressure wave) that's reflected back up the pipe. On a standard header the negative wave occurs at the primary tube's open or collector-end due to sudden gas expansion into the larger collector and consequent velocity decrease. With the proper tuned-length for the combo, the wave arrives back at the exhaust valve during the valve overlap period when both the intake and exhaust valves are open, in theory completely scavenging the cylinder of residual gasses.
Incrementally increasing primary tube diameter before the collector—aka a "stepped header" design—creates additional rarefactions. The steps' location on the pipe and the amount of increase between each step can be used to tune the strength of the various reflected waves. The greater the step-up amount, the stronger the negative pulse. On the other hand, too many or too much step-up weakens the "primary" rarefaction of the main wave at the collector. A typical stepped header has one or two steps ahead of the collector, and each step is progressively -inch larger.
Stepped primary tubes can be used to create multiple peaks in the power and torque curves and to dial-in at what rpm they occur. One step may be adjusted to enhance the torque peak, and the second for peak power. On the other hand, multiple rarefactions may trade-off peak power in favor of more area under the curve. Stepped headers tend to be very finicky, and to actually see gains over a conventional primary tube requires closely coordinating the steps with the camshaft and induction system. In fact, a tuned induction system with controlled plenum volume and runner lengths acts very much like a header, but on the induction side, giving the tuner yet another tool for careful crafting of the power and torque curves.
baz:
A guy who knows his stuff tells me the primaries are too long on the mohr header. He also suggested something like you had A1 do for you with your system by restricting the id at exhaust ports.
I like the look of the mohr system and it clears a berg traction bar which I want to fit to my car. I'd like to hear how it sounds too, along with some feedback from someone with one before choosing to buy one.
andrewlandon67:
Quote from: Sarge on March 11, 2022, 14:10:13 pm
I don't have a Mohr Performance exhaust... but I do have a stepped header (from A1). My engine size is 2017cc. I'd been running an 1 5/8 A1 and after doing some reading, asked Tiger to add some 1 1/2" diameter x 3" sections at the head. This simple modification woke things up! Read on...
How Do Stepped Headers Work?
A tuned length primary exhaust header pipe creates a rarefaction (negative low-pressure wave) that's reflected back up the pipe. On a standard header the negative wave occurs at the primary tube's open or collector-end due to sudden gas expansion into the larger collector and consequent velocity decrease. With the proper tuned-length for the combo, the wave arrives back at the exhaust valve during the valve overlap period when both the intake and exhaust valves are open, in theory completely scavenging the cylinder of residual gasses.
Incrementally increasing primary tube diameter before the collector—aka a "stepped header" design—creates additional rarefactions. The steps' location on the pipe and the amount of increase between each step can be used to tune the strength of the various reflected waves. The greater the step-up amount, the stronger the negative pulse. On the other hand, too many or too much step-up weakens the "primary" rarefaction of the main wave at the collector. A typical stepped header has one or two steps ahead of the collector, and each step is progressively -inch larger.
Stepped primary tubes can be used to create multiple peaks in the power and torque curves and to dial-in at what rpm they occur. One step may be adjusted to enhance the torque peak, and the second for peak power. On the other hand, multiple rarefactions may trade-off peak power in favor of more area under the curve. Stepped headers tend to be very finicky, and to actually see gains over a conventional primary tube requires closely coordinating the steps with the camshaft and induction system. In fact, a tuned induction system with controlled plenum volume and runner lengths acts very much like a header, but on the induction side, giving the tuner yet another tool for careful crafting of the power and torque curves.
Sarge, when you say "woke up", did you gain more at the top end, or was it more under the curve as your post mentions? Did you have to make any jetting changes when you stepped it up? And where did your 3" section come from?
I'm glad to see some of this stuff being talked about around here, I've been curious about it for some time but not enough to really look into it.
Sarge:
I had my changes made in 2013 when I got rid of the sidewinder system I'd been running. I made no jetting changes and the wake-up call seems more in the low-end. My motor is pretty much old school... a 125 Engle cam and some Fumio cylinder heads with 40mm int X 35mm exh valves and 9.0:1compression. The 3" sections were fabed up by Tiger at A1.
I recently ran across a picture of Bill Schwimmer's engine while it was out of the car a while back and noticed he runs a stepped header as well but probably larger diameter (bigger engine). Sometimes, ya just gotta try stuff.... ;D
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