The Cal-look Lounge

Anyone on here running a mohr performance stepped header?

Would love to hear some feedback on them.

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.

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.

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.

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

I know how that goes for sure! I'm hoping to try a set of the Berg grooved venturies in my IDFs this summer, possibly in conjunction with a larger header, which might be fun. My main question is what brought you to the 3" length though? What I've been able to find about stepped headers online seems to show that how far down the "step" is is usually tied to some other metric on the engine.

Ha... that's top secret!! ;)  Maybe it's tied to some part of my anatomy...  All kidding aside, I never gave it much thought other then that I wanted to create some back-pressure.  Tiger did the rest.

I suppose that makes sense! This post has me thinking about ways to cheaply/easily create a stepped header similar to what you run, as I like the low end of the 1 1/2 on my 1915 but I can't help thinking there might be some more up top to unlock with more diameter.

The guy who tells me the primaries are too long in the mohr header suggested a better way is to build your own system. He gave me a sketch of how to build something that performs as well almost as a stinger but super quiet.

Interesting topic for sure, I bet a lot of us are leaving some performance behind due to less than ideal exhaust systems

That absolutely wouldn't surprise me, but given the general attitude of the VW scene (one part has to fit everything AND be super cheap) I'm guessing it's just easier to find a version of something that just works and then market it. Bespoke is expensive, and not everyone has the space/tools/time to devote to becoming an exhaust builder for just their own car. It's a shame too that the VW performance scene was written off by the V-8 companies ages ago, you can dial in every aspect of your Chevy-engined racecar on a number of websites and it'll spit out the exact primary length, diameter, number of steps, and even bend configuration to get the performance you want, and they'll fabricate a pair of headers to those specs and ship them to your door. It really seems that we traded the convenience of ordering a header that we think will work alright size-wise, but we know will fit the same, for the ability to actually have what we need when we got into VWs.

Unless, of course, you actually live in southern California and can just pop over to A-1, but that's not a commitment I'm really willing to make just for an exhaust. It's good that we're getting some of these conversations rolling though, it'd be nice to see some of this come to fruition eventually.

You're right in that bespoke is the only way if wanting to extract the most from your own particular combination. Ive been hunting all over for info on the mohr system but can't find anything, I guess that tells me that they aren't very popular in the scene. Not to say they don't work but as pointed out to me they may work very well with a particular combination of parts particularly cam and heads.

I guess building a system is the way to go. Gotta find someone local who can build exhausts first though as its beyond my capabilities.

I guess I'm "lucky" because I'm OLD.  My first real set of headers came from Four Tuned and were made of 1 3/8" tubing and used heater boxes.  They worked great and were on the car until I sold it in 1973 (I did get rid of the heater boxes at some point!).  The engine at that time was an 1835.  By then, guys had moved on to 1 1/2" merged collector pipes from Dean Lowery & Gene Berg... bigger is better but not necessarily always.  My current exhaust is 1 5/8", so the "step" idea came to mind.

 :D :D :D Inversely proportional???

Hiya Sargie!

 ;D WhatEVER!

Whereabouts are you Baz? Turbo Thomas is good, based in Redditch if I remember correctly.

Hi Neil I'm back in Ireland since 2018.

How to calculate a primary pipe
A good guess is 500 ° - 550 ° C. A good starting point for the primary pipe diameter from the top is 5 - 10% larger area than the exhaust valve. Where the primary pipe enters the secondary pipe (collector), it should be the same area as the exhaust surface of the exhaust valve at full lift or larger. Sheath surface = Valve diameter x pi x valve lift. To calculate the length of the secondary pipe to an engine with even ignition sequence and get the secondary pipe negative pressure pulse to return at the same time as the next primary pipe pulse will be used the formula: secondary pipe length = secondary pipe tuning factor (900000) x root out (average exhaust temperature + 273) to.

Primary Pipe length = primary pipe tuning factor (205800) x root out (average exhaust temperature + 273) / rpm to which the primary pipe is to be tuned.

Sometimes it's easier to believe in something and buy something you believe in
And test in dyno

/// Micke

I'd got a feeling you were. Rich does have (or certainly used to) the back end of a Beetle set up in his workshop complete with mock up motor so that he can build a system to order. Worth a call?

Now THAT's the kind of science I'm looking for! My next question is - what are those equations spitting out? Is it supposed to be length in MM, or volume, or something else entirely?

   
Length in mm on primary
There is a very good simulation program called pipemax
Too bad you do not read Swedish because we have a forum like Like crazy guys like me who exchange ideas about high-level engines (www.savareturbo.se
/// Micke

I’m using the JPM stepped header. On the dyno the power increase versus A1 1 3/4 low down header was over 18 bhp at the wheels. This might not help with the quest for Mohr details but should provide some confidence in the stepped header benefits.

That's quite a gain from just an exhaust system! Good work.

Can we see this sketch?

I'll share when I've had it built, hopefully this summer.