Yerf Dog 3206 & 3209 Go-Cart Exhaust

Yerf Dog Go-Cart Exhaust Upgrade

I've been thinking about easy ways to uncork a little more power from the Howhit GY6 based engine in my Yerf Dog 3206 go-cart, and I decided to do a simple high flow exhaust.  Since the 3206 go-cart is basically the same as the Yerf Dog 3209 (other than color) this information would apply to both of these Yerf Dog go-carts (and other models based on the same engine).  One of the things that is important to me is no excessive noise.  As I've gotten older, I am less tolerant (don't enjoy) very noisy vehicles.  In my younger days, the noisier the better!  I can remember an old clunker Honda CB360 motorcycle that I bought many years ago for something like $75.  I went through it and rebuilt the engine and then hot rodded it.  This was back when I was in high school.  One of the things I did was I took the exhaust into metal shop, cut them open, gutted them out, and then welded them back together.  At 9000RPM, the bike sounded like a drag racing funny car!  It was still slow by modern motorcycle standards, but it sure "sounded" fast!  No more of that sort of thing for me.  Besides the fact that I don't like noisy exhausts myself, I also don't want to irritate others in areas where I drive our Yerf Dog 3206 go-cart.

The Yerf Dog 3209 & 3206 go-cart GY6 engine header is a 2 stage that starts off with an outer diameter (OD) of approximately .85" and then steps up to an OD of around 1.1" before it enters the big canister muffler. Then the outlet of the muffler is necked back down to a smaller diameter with an  inner diameter (ID) of only around .67".  This is a pretty small outlet.  No doubt, the added restriction of this smaller exhaust outlet helps keep the exhaust quieter.  While I did want a relatively quiet exhaust, I didn't want to choke off my Yerf Dog 3206 go-carts engine excessively. 

The ID of the 1st stage of the header is also small and is only around .74".  This is what bolts up against the exhaust port which has an ID of around .955"!  Such an abrupt step down at the exhaust port is not good for max flow!  No doubt, the smaller pipe diameters on the stock system restrict flow but they also help restrict noise.  The exhaust system on the kart is very short, so there is not a lot of room to muffle the noise before the exhaust is released into the atmosphere.  Typically, a longer exhaust with more pipe should deaden the sound a little more.  So, Howhit probably restricted the flow to get an acceptable noise level from such a short exhaust system on the Yerf-Dog 3206 and 3209 karts.  In addition, it might have been done in part to increase back pressure in an attempt to boost low end engine torque.  Overall, the stock Howhit muffler is a nice setup. It looks good with the chrome heat guards. It's relatively quiet. I even debated whether I should even mess with it at all. I decided to try a higher flow exhaust on my go-cart and see how it worked.

I grabbed a cheap, old Supertrapp muffler off eBay for only around $15 + shipping.  The Supertrapp I bought off eBay is one of the universal types with 3" diameter diffuser discs.  It is made for smaller displacement engines and has a 1.125" pipe going into it.  Supertrapp mufflers are basically "glass packs" that have a tuneable diffuser disc setup on the end.  I had a Supertrapp just like this on an old Honda XL250 bike I once owned.  It was the same basic muffler. The Supertrapp on my XL250 was sort of loud because I ran it with a lot of discs much of the time. The nice thing about Supertrapps are that they are tunable for noise and flow by adding or taking away diffuser discs in the end.  Another nice feature of Supertrapp mufflers is their diffuser design results in a  USFS approved spark arrestor.  This feature was important to me in case I would have an opportunity to drive our Yerf Dog 3206 go-cart in areas that required a USFS approved spark arrestor.

With the muffler in hand, I then began searching for the tubing.  Mandrel bent tubing (with constant inner diameter throughout bends) is not easily found in very small sizes.  I knew that I wanted mandrel bends on my Yerf Dog 3206 go-cart to get maximum flow.  Compression bent tubing crimps the tube at the bends and can kill flow.  After quite a bit of searching, I finally found a source for smaller diameter mandrel bent tubing.  I ordered some 18 gauge 1-1/8" OD mandrel bent U's from Bassani Manufacturing.  The bends were cheap at around $5 a piece, but because of a $25 minimum I needed to buy 5.  The U shape of the mandrel bend is cut into the sections needed to route the exhaust the way you want it, and then the sections are butt welded together.  I used an abrasive cut off saw to cut the sections.  This makes nicer cuts.  In the past, I have also used a hacksaw to cut mandrel bend U's when making a car exhaust, but it's hard to get good butt joints for welding because a hacksaw does not always give a nice straight cut.  Tubing could also be cut with hacksaw and then the ends could be squared off on the side of a bench grinder wheel. 

I made an exhaust flange for my Yerf Dog 3206 go-cart from some 1/4" steel plate that I had laying around.  No milling machines here.  I only wish!  I used the stock flange as a template and I created the new flange using a: bi-metal hole saw, drill, hacksaw, die grinder, and bench grinder.  No doubt, it would have been much easier to make on a mill, but I don't have access to this equipment so I substituted hard work and sweat for the milling machine and the flange turned out fine.  I welded everything up using my cheap "spatter" flux core MIG wire feed welder.  With the 18ga. tubing (.049" wall), it was difficult to get a nice weld with good penetration without burning through.  In the end, the welds looked terrible and I was not happy with it, but it worked fine.  TIG welding would probably work much better on tubing like this, or at the very least a good MIG welder run with shielding gas. 

I stripped the paint off the old Supertrapp since it was rusting and looked pretty bad.  I then shot the entire exhaust with some high temp BBQ paint.  Here's how it looked after I was done...

Yerf Dog 3206 Go-Cart Stock Exhaust and Custom Supertrapp Exhaust

A simple piece of steel flat stock was used to make a exhaust hanger to mount it to the stock Howhit muffer mount.  Here's how it looked when I installed it on the kart.

Custom Supertrapp Exhaust Installed on Yerf Dog 3206 Go-Cart

I first drove our Yerf Dog 3206 go-cart with 8 discs on the Supertrapp.  Way too loud!!  I then went down to 6 discs.  This cut the noise some, but still too loud.  The engine definitely seemed to like less restriction and the engine seemed to rev up quicker.  I went down to 5 discs and the sound was better.  Still too loud for my tastes (and the neighbors!) so I went down to 4 discs and this helped, but then I began to feel the extra backpressure choking off the performance.  Probably the best overall balance of noise/power on our Yerf Dog 3206 go-cart was with 5 discs.  In my opinion, it's still too loud.

If I welded up another complete exhaust for our Yerf Dog 3206 from scratch, I would probably use 1" OD tubing.  The 1-1/8" OD tubing is too large in more than one way.  There is one issue of gasket sealing at the exhaust port.  I made it work, but 1" tubing would be a better choice.  Besides, the exhaust gasket has an ID of around .875" so a 1" OD tube with .049" wall thickness will be around 0.9" ID and match pretty good to the stock gasket.  I did research other exhaust gaskets at motorcycle shops and it would be possible to find a gasket with a larger ID that could work, but then the gasket would have too big of an OD.  The  cylinder head would need to be removed and a fly cutter used on a milling machine to enlarge the diameter of the counterbore that acts as the exhaust gasket seat.  This might not make sense unless you pull the exhaust off and look at the exhaust port on the Yerf Dog 3209 or 3206 Howhit GY6 based engine to see what I am describing.  I won't get into all the other gory details, but I now believe that for most people 1" OD tubing would be a much better overall choice for a custom exhaust on this kart.  Bigger is definitely not always better.  Too big of a header can kill low-midrange power and torque.  1" tubing should work better.

Keep in mind that this custom exhaust is loud.  I may just run with the stock Yerf Dog 3206 go-carts exhaust in areas where people may be annoyed with noise.  Then I can bolt on the custom exhaust for wide open area where the extra sound will not disturb people.  I plan to wear ear plugs either way so the louder Supertrapp won't bother me so much.  Now that I've actually done this, I can say that for most people, the stock Yerf Dog 3209 or 3206 go karts exhaust is probably a pretty good choice when you consider overall noise level, performance, cost. 

YERF DOG 3206 GO-CART EXHAUST UPDATE - 02/20/04

I finally got tired of the noise of just the Supertrapp muffler on our Yerf-Dog 3206 go-cart.  I decided to experiment more with modifying the stock Howhit exhaust because it was relatively  quiet and the large muffler cannister on our Yerf-Dog 3206 did not seem to be too restrictive - besides the small outlet and also the small primary tube of the header.

First thing I did was to cut out the small .74" OD primary header pipe and weld in a section of a 1" OD mandrel bend that I bought from Bassani.  I bolted this to our Yerf Dog 3206 kart and took it for a spin.  I was disappointed because with all the hassles of enlarging the primary header tube size, there didn't appear to be much difference in performance over a stock exhaust.  If anything, it seemed like I might have lost some of the low end punch on our Yerf-Dog 3206 go-cart with nothing to gain in the top end. 

At that point, I really began to suspect that the small outlet on the stock Yerf Dog 3209 & 3206 exhausts was a big potential restriction.  I took a 1" bi-metal hole saw and removed the center pilot drill bit and proceeded to cut out my stock Yerf Dog 3206 go-carts muffler outlet.  This was quick and easy to do.  The I took it out for a test drive.  It was noisier, but it didn't sound too bad.  Our Yerf Dog 3206 go-cart now had a more aggressive tone to it.  It was a little too loud for my tastes, but not as bad as the custom exhaust I tried with only the Supertrapp muffler.  I then decided to utilized that Supertrapp muffler and weld it onto the end of the stock Yerf-Dog 3206 go-carts muffler.  The picture below shows what I finally came up with.     

Modified Exhaust on Yerf Dog 3206 Go-Cart

I first drove the kart with 8 discs and was very pleased at the performance and relatively quiet sound output.  I'd say that the exhaust was quieter than stock, but now our Yerf Dog 3206 go-cart now seemed to run much stronger.  The power difference I noticed may be bigger than other people.  We live at higher elevation (over 5000ft) where carbureted engines tend to run too rich.  The free flowing exhaust not only appears to have reduced backpressure and increased performance that way, but now the air/fuel is also probably leaner and closer to what it should be at this altitude.  In any case, I am pleased with the increase in power from such a small engine. 

I also test drove our Yerf Dog 3206 with 10 Supertrapp discs and then 12 discs.  After doing so, I will probably go back to between 8-10 discs to find the best balance of performance and noise.  Since the Yerf Dog 3209 is the same setup as the 3206, both go-carts could run this sort of exhaust.  For the person  who doesn't have a welder, perhaps just doing the simple mod of using a 1" bi-metal hole saw in a drill to cut out the small outlet would improve the overall performance.  Just beware that it will be a little noisier. 

YERF DOG 3206 GO-CART EXHAUST UPDATE - 05/27/04

Because I did some porting on the cylinder head and opened up the exhaust port size some more, I decided to modify my exhaust header by cutting out the section of 1" OD tubing and replacing it with 1-1/8" OD tubing right up at the cylinder head.  The inside diameter of the 1-1/8" tubing was a better match to my enlarged exhaust port and larger exhaust gasket that I used.  I had the GY6 head machined with a larger counterbore on the exhaust port so that I could use a bigger gasket.  The stock gasket works fine with the stock exhaust and the tiny primary tube diameter, but it has a small inside diameter that was a bottleneck after modifying my exhaust port and installing a custom exhaust system.

UPDATED FEEDBACK - After it was all said and done and I reflect back at many of my modifications (including this exhaust upgrade), I would recommend most people just leave "good enough alone"!  For example, the stock exhaust system that comes on the Howhit engine works well and gives the engine more off the line torque and acceleration than my modified exhaust.  So, for most people, the stock exhaust is probably a good way to go.

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