Scrambler Exhaust

I'm doing a low key KZ400 rat-scrambler build. Not anything drastic like hacking the frame, destroying original parts, and whatnot, but you know: dual sport tires, belly pan for the engine, already installed a flat handlebar (dirt bike) - which I'm probably going to probably add a riser for (sits a little low for my liking).

My question is about exhaust. I've noticed a two of three new "retro" bikes being made into the scrambler style are using low exhaust on the bottom of the bike (Ducati Scrambler, R9T Scrambler). The only one using a high exhaust is the Triumph Street Scrambler. Is it okay to use low pipes for offroading assuming I am not gonna be bushwhacking my bike (sticking to known maintained, nontechnical trails / forest fire service roads with very mild overgrowth if any)?

I love the look of high pipes on these twins when done right, but my concern is the exhaust getting in the way of the kickstarter as well as my soon to be ammo can saddlebags (much cheaper than commercial hard or even soft) and I sure as hell want the high pipes if I'm doing them to be on the right side of the bike...being on the left just looks weird to me (and they get highlighted when the bike is resting on the kickstand). My other concern with high pipes is I want a somewhat quiet bike. Shorties are a little loud for me and most "moderate" mufflers (like kerker) are way to long to fit with my ammo cans.

I'm not going for overall aesthetics (like most cafe racers are nowadays), but rather functionality, whilst trying to make it look the best I can.

My desired end result would look like this but with 50 caliber cans instead of the .30/5.56:

As I see it, your choices would be.....

1. Use a single ammo can like in the picture, but on the other side.

2. Run the pipe slightly lower so the muffler(s) would be just under the can.

3. Snake the pipes inside the frame behind the engine and put the muffler(s) under the seat.

4. Build a low system.

Personally, I prefer a high system with no cans. If you need to carry stuff, you can use a tail bag on the seat, or a tank bag, or a backpack, or some combination of them.

P.S. looking at the picture you posted, I think that if the pipes were parallel to the ground, and the mufflers positioned before the can, the two small tailpipes should go just under the can. I see several mockups in your future.

Or you could get crazy and sculpt the back side of the can to mold around the pipes.

Because of the 360 crankshaft, you could maybe make the pipes into a 2-into-1 system and have it shoot out under the ammo can.

YamaKawa wrote:

the problem with many custom high pipes you see on the web is that they aren't good systems. For example, these pipes shoot straight at the shock which will quickly cause failure, particularly so if used on rough terrain where the shock is forced to move rapidly generating even more heat. They will leave a carbon stain on your shock, frame and ammo can. Its impossible to tell here, but that system may not have a "X" or cross over for scavenging, so free horsepower is being given up. Its way too short for a street bike as well. It looks like a factory triumph scrambler exhaust that someone cut the mufflers off, because, you know, that makes horsepower :laugh:

Since your stated desire is functionality first and looks second, and you want to ride this thing in the dirt, you are going to want to preserve torque and low RPM power. Particularly so on the already anemic kz400. On a relatively stock engine my advice would be as follows, take it or leave it:

Measure your exhaust port, you want to the ID of your exhaust tubing to be equal, or a tiny bit larger than this. It is going to be pretty small on the 400, probably like 1" or 1.25". Having tubes this small might look weird. The factory pipes are double walled to prevent chrome blueing and because everybody wants big diameter pipes for the looks, but the inner pipe of the stock exhaust will be the size you want to use. The larger the diameter tube the slower the exhaust gas velocity, all other things remaining equal. Go bigger diameter and this will lead to reversion problems at low RPM. This is the number 1 problem with custom systems I see, too big of tube diameter.

You are going to want a X pipe, crossover, or a 2 into 1 system to help with scavenging. Its free horsepower.

Primary length needs to be considered to maximize power. Shorter primaries= high rpm power, longer primaries= low RPM power. Measurement starts at the valve seat. When in doubt copy the factory primary length. There are calculators out there for determining this.

Total length of the system needs to be considered as well. again short=high rpm long=low rpm. When in doubt copy the factory system as it is a probably good compromise for a stock engine.

a free flowing muffler is important for power gains, use a perforated core that has the same diameter output/input pipe. stay away from louver type cores. Smaller diameter cores can be used to quiet the bike down but they will be more restrictive.

If you're only going to be blasting fire service roads an under-slung exhaust will probably be fine, particularly if you have a skid plate. I have rallied street bikes on NFS roads for years and only have ever bashed exhausts when I went on 4 wheeling trails. Another plus side of an under-slung exhaust is it gives you room for the necessary system length, going high pipes means the system has to go back further to reach the desired length. (Look at the length of factory high pipes)

If you decide to go high pipes (they do look awesome) I second Loudhvx and think doing a 2>1 would be best as it will be easier to squeeze 1 muffler past the amo boxes rather than 2. A 2>1 will cost less, and solves the issue of the crossover for you as well. Collector size is equally important here.

I don't remember how close the kick starter was to the engine on my 400 but when I was playing with the idea of high pipes on my 750 twin I considered adding a stop to prevent the kick-starter from swinging in too far. This is why 70's Honda's had their scrambler pipes on the left side of the bike, but I totally understand the desire to keep it on the right side, or the "show me" side as my buddy calls it.

Anyway there is a ton of of other things to consider as well but those are the big ones.

Here is the stainless 2>1 exhaust system I built for my Kz750 with a custom muffler. 1.5" primaries, to a 1.875" collector, with a 1.75" muffler core. notice the length. Its pretty quiet at idle but once over 3k its opens up. Its about as loud as a kerker on a kz1000 with the street baffle. A good compromise on sound IMHO.