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Exhaust port matching


Lenny
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It's easy to see that there is more power to be found in matching the exhaust ports. If you look at the pic, It shows the head, head gasket and the exhaust manifold. Look at the size of the holes in the head compared to the maniflod. The difference is huge. The gasket shows the overlap. If you look, you can see on the right most head port that the respective manifold port almost overlaps it so far that the exhaust port hole almost extends off the available gasket face on the head. They can not be totally matched becaust the exhaust prots are so big but I did open up my head ports for an approximate 30% gain in port size. I was able to open up the ports quite a ways in for much better breathing.

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Lenny

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As I see it, and someone please correct me if I'm wrong. but small engine ports opening into larger exhaust ports is OK, but large engine ports opening into small exhaust ports would be disastrous and would need correcting. Someone explain if there would be turbulence that would be a real problem with this set up or not. Would we be better off with smaller exhaust pipes that match the engine ports?

Thanks,

Kinarfi

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As I see it, and someone please correct me if I'm wrong. but small engine ports opening into larger exhaust ports is OK, but large engine ports opening into small exhaust ports would be disastrous and would need correcting. Someone explain if there would be turbulence that would be a real problem with this set up or not. Would we be better off with smaller exhaust pipes that match the engine ports?

Thanks,

Kinarfi

There are a few things that happen. When the exhaust goes around the square corner it creates turbulence which reaches back into the main stream of exhaust adding resistance. The whole idea is to have the flow of exhaust be as laminar as possible which is the most efficient way to move it. Also, an engine without any exhust pipes will not run as good as one with proper exhaust. The exhaust has inertia as it's traveling out. With the exhaust being contained in the exhaust tube, as the piston finishes pushing out the exhaust, this inertia helps to create a pressure drop at the exhaust port which helps to scavenge more exhaust gas out of the cylinder. Without the pipes, this doesn't happen. Yes, it would be better if the exhaust tubes matched the ports in size. There is actually a formula to determine the tube size based on a lot of factors including where you want your peak power or torque to be. On a two cycle engine you will see the exhaust pipe come out then expand rapidly to just taper back down and finish with a small pipe (called stinger). Exhaust comes out, expands to suck the cylinder cleaner of exhaust gas then compacts quickly to create a reverb of pressure back at the engine to shove some gas back into the cylinder. See, on a two cycle, it can scavenge enough to pull out some of the intake air/gas mixture into the exhaust pipe before the piston closes off the cylinder side wall port. This allows for an extra shot of gas/air mixture but because it is in the exhaust port, the back pressure reverb shoves it back into the cylinder just in time. Again, the pipe geometry all change depending on which RPM you want peak torque or power.

Lenny

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There are a few things that happen. When the exhaust goes around the square corner it creates turbulence which reaches back into the main stream of exhaust adding resistance. The whole idea is to have the flow of exhaust be as laminar as possible which is the most efficient way to move it. Also, an engine without any exhust pipes will not run as good as one with proper exhaust. The exhaust has inertia as it's traveling out. With the exhaust being contained in the exhaust tube, as the piston finishes pushing out the exhaust, this inertia helps to create a pressure drop at the exhaust port which helps to scavenge more exhaust gas out of the cylinder. Without the pipes, this doesn't happen. Yes, it would be better if the exhaust tubes matched the ports in size. There is actually a formula to determine the tube size based on a lot of factors including where you want your peak power or torque to be. On a two cycle engine you will see the exhaust pipe come out then expand rapidly to just taper back down and finish with a small pipe (called stinger). Exhaust comes out, expands to suck the cylinder cleaner of exhaust gas then compacts quickly to create a reverb of pressure back at the engine to shove some gas back into the cylinder. See, on a two cycle, it can scavenge enough to pull out some of the intake air/gas mixture into the exhaust pipe before the piston closes off the cylinder side wall port. This allows for an extra shot of gas/air mixture but because it is in the exhaust port, the back pressure reverb shoves it back into the cylinder just in time. Again, the pipe geometry all change depending on which RPM you want peak torque or power.

Lenny

Lenny,

I am just curious as to why you had to tear your engine down.Hope it is nothing serious.

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Lenny,

I am just curious as to why you had to tear your engine down.Hope it is nothing serious.

I pulled the head to add extra head gaskets to lower my compression. I'm running a supercharger making my compression ratio 15.6 to 1 at full boost of about 10-1/2 PSI. I was running racing fuel but it still had a lot of spark knock which I didn't like as it can damage the engine. Adding the gaskets will bring the compression down to 11.58 to 1 which will be more manageable and I probably can get away with premium gas. I figured that as long as I had the head pulled, I would go ahead and pull the engine and open it up for a check to see if I damaged anything. I found a broken ring on #3 piston. Didn't damage the cylinder and it still ran ok but that cylinder was lower on compression checks so I went ahead and put in a set of rings, honing all the cylinders a bit. When I say it ran ok, it did sort of. I didn't do a lot of tuning because I need to go to wide band MAP and exhaust sensors. I don't figure that the stock computer can handle the wide band sensors so I'm going to a Megasquirt ECU. Also with this, I can tune tune it for maximum torque which is what I really want. I can even set the rev limiter higher if I choose but don't think I will. If I did I wouldn't want to go past about 7000 RPMs without stronger pistons and rods.

Lenny

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  • 1 month later...

Lenny,

I have run a turbo at 10psi on my T2 for the last 1000 miles or so (1500 total on engine). I was very careful about cleaning and oiling the airfilter and using a prefilter (both stock), but It looks like I have been eating dirt anyway (blue/white smoke on deceleration). I have not disassembled yet, but the symtoms look very much like I may have to do a ring job.

Long story short, where did you get your rings? With the factory closed, I can't seem to find a source for parts. Also, any tips or tricks you can pass on for the remove & replace?

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Lenny,

I have run a turbo at 10psi on my T2 for the last 1000 miles or so (1500 total on engine). I was very careful about cleaning and oiling the airfilter and using a prefilter (both stock), but It looks like I have been eating dirt anyway (blue/white smoke on deceleration). I have not disassembled yet, but the symtoms look very much like I may have to do a ring job.

Long story short, where did you get your rings? With the factory closed, I can't seem to find a source for parts. Also, any tips or tricks you can pass on for the remove & replace?

Years ago (30) I rebuilt a small clark forklift that was built in 1952. It was and is still a sweetheart, about 42" long excluding forks and 28" wide. Perfect for a small shop where you have to manuver in tight places. Anyway, Clark no longer had any piston rings so I looked in Thomas Register for companys that manufacture piston rings. They now have a Thomas Register web site. I called the manufactures and told them my piston specs, outside diameter, ring grove diameters and ring grove widths. They were abe to match up a set of rings for me that have been in now for 30+ years. They make a huge variety, making rings for engine manufactures around the world. I would be susprised if one of them couldn't come up with what you need. Running a turbo, be sure to check your ring gaps. With a turbo, you will run warmer and a tight gap could ruin your engine. The Trooper manual has the ring gap specs. Also remember to watch for which side of the ring goes up. Reintall all your pistons and rods with caps in the same location and direction that they came out of. When assembling, be sure to check the valve clearances. Mine were getting real tight. Their adjusted by using different shim washers. I beleive that Honda CRB motorcycles use the same shims. In my case, because everything was on the tight side, I made a collet type fixture to hold the shims and ground them to the thickness I needed on the lathe. Good luck and let me know how you come out on the rings. If you find a supplier, it needs to go in the parts list at the top of the forum. As long as you have it apart, match all your ports from throttle body all the way through to the catalytic converter. They are all badly matched. It's a wonder it puts out more then about 35 HP.

Lenny

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