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Thread: expansion chamber seals and tuning

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    Default expansion chamber seals and tuning

    Hi all, I'm thinking I will go ahead and give expansion chambers a try on my B-Mod. I want to try individual "dual" chambers, then try a single pipe for experiments. There is no way I could afford them, so I will make them myself. I'm a good welder. My knowledge though is still pretty limited about chambers. What I would like to know about is how sliding pipes are sealed...is it a special o-ring? The highest temp one I could find is only good for 600-degrees.

    Also, if my motor makes max power at 7000rpm with factory exhaust, is that what I should start with for calculating tuned length? Basically, what range of rpm would I need to tune for on a sliding chamber? THANKS VERY MUCH!!

    The calculated wave speed for a Y-80 is 1703-ft per second @ 15hp per 175cc cylinder (30hp 350cc total).

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    Team Member ProHydroRacer's Avatar
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    In PRO the sliding pipes are not sealed. Use an inner and outer straight pipes, you want them to easily slide, about 1/32" clearance all around. We slide them around 4". All the way in for max RPM and all the out to retune for lower RPM. PRO run in the neighborhood of 9-10,000 RPM to 12 - 14,000 RPM. Check out if you can find it "Two-Stroke Exhaust Systems" by Roy Bacon - Lodgemark Press LTD, England

    I think, calculate for 7,000 Rpm and what ever RPM it runs thru the corners.

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    David Weaver David Weaver's Avatar
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    What is the power plant being used? If Yamato, I suggest that speak with Bill Van.

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    Thanks guys! I'm glad that I wont need to worry with a seal! What I have is a Yamato-80. I have a runabout that I set up for B-mod.

    What it has is 166-degrees exhaust timing. The "C" Yamatos are only a couple degrees less with less compression. But the tuned length between the two at a given RPM is only around 1/2" distance according to my calculations. They all use about a 1.5" to 1.6" diameter feed pipe to as for port area. I am planning to run it as soon as I can save up to afford all the protective gear I will need later on, brand new.

    I do plan to give it a try with the "stock exhaust" just for a challenge. I'm a Yamato believer, and I honestly think one can be run that way in B-Mod with the right touches and make a good showing. But I imagine that pipes might be good to have ultimately. The "C" mods are running around (7800?) or more though they are a 6600 Rpm motors as stock, but my Y-80 is only safe up to 8000RPM according to the owners manual. I don't know what the Mod guys run the Y80 at in MOD class though.

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    Try the Expansion chambers for Dmod Hydro at Hydroracer. Nbass has something to say about Yamato pipes.

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    Quote Originally Posted by rumleyfips View Post
    Try the Expansion chambers for Dmod Hydro at Hydroracer. Nbass has something to say about Yamato pipes.
    Thanks! It took a while but I found it.

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    Team Member JohnsonM50's Avatar
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    Quote Originally Posted by champ20B View Post
    Thanks guys! I'm glad that I wont need to worry with a seal! What I have is a Yamato-80. I have a runabout that I set up for B-mod.

    What it has is 166-degrees exhaust timing. The "C" Yamatos are only a couple degrees less with less compression. But the tuned length between the two at a given RPM is only around 1/2" distance according to my calculations. They all use about a 1.5" to 1.6" diameter feed pipe to as for port area. I am planning to run it as soon as I can save up to afford all the protective gear I will need later on, brand new.

    I do plan to give it a try with the "stock exhaust" just for a challenge. I'm a Yamato believer, and I honestly think one can be run that way in B-Mod with the right touches and make a good showing. But I imagine that pipes might be good to have ultimately. The "C" mods are running around (7800?) or more though they are a 6600 Rpm motors as stock, but my Y-80 is only safe up to 8000RPM according to the owners manual. I don't know what the Mod guys run the Y80 at in MOD class though.
    I've done some C-stock racing in 500mod. I didn't raise the motor or anything, [still a C] You can make a good show as a stock but will find the top mods likely out of reach. My 302 C has read up to 7400 on the tiny tach. I would imagine I could do a little better trying to raise the stock motor & maybe try a 4 blade prop in 500 but running the same thing in 2 classes was more appealing.

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    Quote Originally Posted by JohnsonM50 View Post
    I've done some C-stock racing in 500mod. I didn't raise the motor or anything, [still a C] You can make a good show as a stock but will find the top mods likely out of reach. My 302 C has read up to 7400 on the tiny tach. I would imagine I could do a little better trying to raise the stock motor & maybe try a 4 blade prop in 500 but running the same thing in 2 classes was more appealing.
    I bet it would keep up with the front runners, maybe even win a heat or two with a mod gearfoot. If you had one on the side, you could easily switch it out (just two fasteners and a gasket on my Y-80 for example). The three blade prop would give the best of both worlds (turning and speed) over a four blade in my opinion though. For example, I once saw a test on T.V where a high performance center console w/twin outboards was being run with different props. A four blade was tried and had good cruising efficiency and good pull, but it really lacked at high rpm/high speed performance. The guy on the show said it was great for rough water though. A three blade regular looking prop was the best of all!! ... Racing sometimes has similar results with props. The pro motors do run up to (5) blades though most who win have the usual (3). I'm also often told to this day, by local area "v-drive; hot boat" guys that (2) blades are still the fastest for straight top end sometimes. I actually have a (2) blade I designed, and that thing bites hard and is really fast....it puts a good load on the engine just right, even at 1/2" deep.

  9. #9
    Tim Weber
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    A good mod will clobber a c-stock.

    My 500MR is about 5 mph better than my c-stock and my stocker is no slouch. I'm finding about 500 more rpm with lot's of punch with the mod.

    As far as props the three will run just as fast as the four blade but what I've found is the four holds your speed better in the turns so the eta with the 4 blade for me is better.

    Tim

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    At Moses Lake, the 500MH were 11 seconds and 6 mph faster than CSH. DSH was one second and .3 mph faster than the 500 mods.

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