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I am seeking some info on running expansion pipes on 20H in 250mod No I dont have a tower exhaust for merc , so is this a viable alternative to provide better performance? Thanks for any comments, I did see a expansion chamber on a Dmod in Richmond Ill. and he said it was strong and could run up front, is this accurate?
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Call Frank Erion at 269 720 1337 -
The 20H is a "deflector" type of engine as opposed to a "loop" charged engine. Deflector engines don't respond as well to a expansion chamber as loop charged engines. For that reason a set of megaphone pipes is almost as good as an expansion chamber setup and it's a lot easier to do and has a broader range than a fixed expansion chamber. For that reason the Merc mods running deflector pistons and aren't loop charged still use megaphones as opposed to an expansion chamber. Could it be better?? The short answer is yes, but not as much as you might think, and it's harder to get it to work without making a variable length pipe. With a "slipper" pipe, yes but that's even more difficult to build and get to work. If you have a dyno and can figure out what length of pipe you need for each speed then you have a better chance to get it to work. As with anything else, to get the advantage out of it you're going to have to do a lot of work.
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Agreed. I'll add, though, that megaphones help only by helping draw air thru the engine, what we call 'scavenging'. The conversion exhaust housing on a 20H is essentially just a pair of megaphones tuned for 6500-6800 RPM.
It's possible to 'over-scavenge' an engine. When you do this, you draw some of the incoming fuel-air mixture completely thru the engine and out the exhaust ports so that it never gets burned. What a correctly tuned expansion chamber does is to provide a reverse pulse to push this spilled-over fuel-air mixture back into the cylinder just before the piston closes the exhaust ports on the upstroke -- hence better combustion and more power.
So yes, an expansion-chamber exhaust system will be 'pipier' than a megaphone system. Making the system adjustable by sliding the chamber in and out over the exhaust header (a 'slipper' pipe) broadens the overall powerband.
So does adding water injection to the exhaust stream, which can be as simple as drilling a 0.040-inch (#60 drill bit) hole into the header up quite close to the exhaust port and diverting some of your cooling water to this hole. This works by cooling the exhaust stream and slowing it down, which makes the chamber seem longer and hence tuned for a lower RPM. I use a foot-operated valve for diverting the water to this hole.
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Thank you for the comments very good. Due to deflector pistons in the 20H does this pose one from using a expansion tuned exhaust as fuel will pass through the combustion chamber prior to closing of exhaust plort as it is pulled out by draw from chamber exhaust? I have heard this argument and we would prefer to use a megaphone but 250 rules prohibit it and only allow for quick silver tower exhaust or expansion chamber, we dont have a club foot so would like to use a expansion chamber, Any thoughts guys?Comment
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Goller ran chambers on a 30 inch deflecter in the seventies and how many nationals did it win? I guess Goller didn't get the memo! Just sayingComment
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Good luck however you go, but my opinion is it will be a lot easier and less expensive to get the merc popper housing than to develop the tuned expansion chamber
But if the real fun for you is the technical challenge of designing and building the pipe then go for it
I started down this path with my OMC 25mod and gave up. I was just talking about this with Ed Runne and I may revisit it with his help, but mainly for the tech challenge, it is not an easy path to performance on the deflector piston engines.
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Any pics of that Goller chamber set up?Originally posted by Steve Dunn Racing View Post"Keep Move'n" life is catching up!
No man's life, liberty or property are safe while the legislature is in session.Comment
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Try Googling :
Boat Racing Facts Deflector engine exhaust - schematic for Mercury 25SS
Hydroracer Wraparound chambers - picture of above exhaust
Hydroracer Expansion chambers for DMH
Hydroracer Megaphone theory 101
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It seems that we will need to make a slip chamber to allow for adjustment , does any one have pics of the Goller pipes / Or does anyone have pipes for a 20H that worked well?Comment
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Google Johns old merc site Bob Goller. Scroll down for a picture of Goller Cmod pipes.
The Schmidt built pipe looks like a slider too.
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Goller 30ci pipes.jpgOriginally posted by ZUL8TR View Post
Here ya go, Pete: http://johnsoldmercurysite.com/phpBB...it=cmod#p29998 (This site requires a paid registration, so I posted the picture here as well...)
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thx for the pics I really appreciate them does any one have D mod expansion pipes for sale? Or 20H pipes for sale?Comment
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-of course if you have too much time on your hands,
or do not believe any of what you hear on this site,
[and I do not
] you could try this approach;
Maybe plug one set for short course, the other for long, or,
let it all hang out for the kilos
There is a 'capped meg pipe' that works on a Hot Rod but I can't find a pic .
Brian Hendrick, #66 F
"the harder we try, the worser it gets"
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Speed doesn't come easy, it takes a lot of work and sometime it's easier to do what we've always done. The problem is you get what you always got! Go for it with the chambers because when u get right u will see a performance avantage. If u need some help give me a call and I'll tell u what think I know. 843 455 1393Comment
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