Expansion Chamber on a 4 stroke
An properly built expansion chamber on a 2 stroke actually works like a supercharger. The design of the pipe actually draws the fuel mixture through the combustion chamber into the pipe then the exhaust pulse forces the mixture back into the chamber before the piston closes off exhaust the port in the cylinder. It actually packs more fuel into the cylinder to make more power.
From what I know about a 2 stroke (which isn't much), expansion chambers are good for a particular powerband. They work really well for that rpm range, but they're pretty specific. If you're looking for a good exhaust for a clone, I recommend this 19-930HJRC J-Pipe Header or this 18-930HJ 3-Staged J-Pipe Header. I've got the second pipe, and I added an RLV silencer on mine. I ride my kart through the neighborhood occasionally, and I don't want to make the neighbors mad. It's not terribly loud, but I wouldn't say it's terribly quiet either. It sounds great. I felt a noticeable difference between this and the older screw-on muffler I was using before. If you can weld, I'm sure you could whip up a pipe and add the silencer yourself.
Realizing we're no longer talking about expansion chambers, I understood that single piston four strokes did not benefit by anything other than less restriction, the only law being don't quick-cool the valves. That scavenging only applied with there were two or more pistons at work. Yet I keep seeing tuned pipe references being applied to single cyl engines. Marketing?
Harley Sportsters in the late 90's to early 2000's came with an exhaust was joined together with an additional pipe. I was told once that the pipe connecting the two main exhuast pipes somehow allowed the two cylinders to work together to create more power. I don't know the science behind it, but I've heard similar claims with other 4 stroke (multiple cylinder) set ups, Dave. I have also ridden Harley's with stock exhaust systems before upgrading them with less restrictive exhaust. The back pressure in the smaller stock bike felt great, and I kinda wondered why everyone was jumping up to less restrictive pipes for any other reason other than a good "Harley" sound. I felt like they were much less torquey with the less restrictive exhaust. Most people who know about building horse power will tell you that drag pipes on a Harley are only effective if you're drag racing. Drag racing is one time when you want to get the exhaust gases to exit the pipes quickly. I think the key to all of this is matching your jets to your pipes. You have to have the right amount of air and gas coming in and exiting at a certain ratio. That's where tuning your motor comes into play. We can give you our opinions from out experiences, but every motor is different, and you'll want to tune to your climate, elevation, carb, etc. That's the fun part of building these little motors. They're easy to work on, fairly cheap to modify, and great for learning how to do some of this basic stuff.
Also, H Dave is spot on with the "don't quick cool the valves". I'm sure there is a science behind the custom exhaust, but I don't know that an exact science applies to what you're wanting with your go cart. I would worry more about how pi$$ed off the neighbors will be due to loud pipes than I would about the performance gains. Racing applications may need more attention, but you may not see much of a difference with your set up. If you can weld, you can make something for cheap. I can give you measurements of the pipe on my modified clone and my alky briggs. That might help.
Since we're talking about HP and pipes, I also wondered about the pipe wrap that you see on a lot of racing applications (I have some on my current Harley, on my kart, on my cheap $hi77y Chinese quad, and on both of my minis. The theory is that hot gasses will exit the pipes quicker than the the cooler gases (I think) and the wrap keeps the gases hot. I only use the wrap because I wanted to prevent burning my leg (I removed a couple layers of skin on my shin while getting off of the DB, and my son decided to grab the hot pipe on my kart). I have never noticed any gains in performance, but it keeps me and my sons safe. I'm not sure how much it would help in your application, but it might be nice to prevent burning yourself or someone else.
Also, H Dave is spot on with the "don't quick cool the valves". I'm sure there is a science behind the custom exhaust, but I don't know that an exact science applies to what you're wanting with your go cart. I would worry more about how pi$$ed off the neighbors will be due to loud pipes than I would about the performance gains. Racing applications may need more attention, but you may not see much of a difference with your set up. If you can weld, you can make something for cheap. I can give you measurements of the pipe on my modified clone and my alky briggs. That might help.
Since we're talking about HP and pipes, I also wondered about the pipe wrap that you see on a lot of racing applications (I have some on my current Harley, on my kart, on my cheap $hi77y Chinese quad, and on both of my minis. The theory is that hot gasses will exit the pipes quicker than the the cooler gases (I think) and the wrap keeps the gases hot. I only use the wrap because I wanted to prevent burning my leg (I removed a couple layers of skin on my shin while getting off of the DB, and my son decided to grab the hot pipe on my kart). I have never noticed any gains in performance, but it keeps me and my sons safe. I'm not sure how much it would help in your application, but it might be nice to prevent burning yourself or someone else.
Davidlamb, great post. I ran an early evolution motor, 74 inches, Andrews cams, SS super E with third fuel circuit and a thousand dollars worth of porting. Yeah, I ran the cheapest drag pipes I could find.
The X-over pipe was there because the long, muffled exhaust was too restrictive, so there was room and routing for scavenging. I would imagine the torque curve on stock motors would yield more torque at lower RPM with that X-over than with a drag pipe.
When I consider every high performance application of 4 stroke engines, I keep seeing less restrictive exhaust. Aircraft radial engines, supercharged, yet pipes so short they barely clear the cowl. (God those things sounded bad ass!) Drag racing cars- with a million per car per year running NHRA- straight pipes, one bend.
As far as header wrap, I know with the turbo cars there are claims of performance increases. I don't know why, but it always made sense to keep cherry red bits of metal covered up to avoid fires. We learned that by setting the master cylinder on fire. (engine swap yielded a turbo on the brake side) On an air cooled kart, it seems that getting rid of heat faster is better. ?
I agree with you on making your own pipes. I know some folks make money building and selling exhaust systems, but claims of "tuned" exhaust setups on single bore motors...well, it's fan boy stuff. I've seen so called "tuned exhausts" exiting the head, going up to clear whatever Chinese bit of casting is in the way, then back down, and off to the side to clear a frame. To this, a muffler is added....also "tuned." We accept Visa and Master Card.
The X-over pipe was there because the long, muffled exhaust was too restrictive, so there was room and routing for scavenging. I would imagine the torque curve on stock motors would yield more torque at lower RPM with that X-over than with a drag pipe.
When I consider every high performance application of 4 stroke engines, I keep seeing less restrictive exhaust. Aircraft radial engines, supercharged, yet pipes so short they barely clear the cowl. (God those things sounded bad ass!) Drag racing cars- with a million per car per year running NHRA- straight pipes, one bend.
As far as header wrap, I know with the turbo cars there are claims of performance increases. I don't know why, but it always made sense to keep cherry red bits of metal covered up to avoid fires. We learned that by setting the master cylinder on fire. (engine swap yielded a turbo on the brake side) On an air cooled kart, it seems that getting rid of heat faster is better. ?
I agree with you on making your own pipes. I know some folks make money building and selling exhaust systems, but claims of "tuned" exhaust setups on single bore motors...well, it's fan boy stuff. I've seen so called "tuned exhausts" exiting the head, going up to clear whatever Chinese bit of casting is in the way, then back down, and off to the side to clear a frame. To this, a muffler is added....also "tuned." We accept Visa and Master Card.
I ride it in the neighborhood so I went with this one. I also needed to be as close to the frame as possible so I can transport it without needing to remove the pipe each time. I figured I could just have my buddy make me a custom one out of the DIY kit. But the header/muffler was only $25 versus $45.
