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Thread: 2.5 exhaust chest mods....
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06-17-2017, 12:46 PM #46
Thanks! The wall thing was done by T-Rex, that's the way he does it. Not sure if he was doing it to gain the extra width or just the bit he uses is too wide to stay between them lol. I'm not quite finished with that one. Still needs some clean up and polishing.
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06-17-2017, 12:48 PM #47
The little wall, what's left of it it still stock thinkness just much shorter. Guessing it's around .040 tall,
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07-14-2017, 05:28 PM #48
Got some more pics of chest I just finished with the drawings on it. Pics look different than actual due to reflections and such so I gave two different angles.
The sharp edge at top of port is the one that causes a venturi type effect as exhaust passes over it to pull exhaust from cylinder, I like this edge to clean and crisp.
The lower angle is one I feel needs to be changed depending on rpm, 30 degree works well with most river runners. This one is on a 45 degree, which doesn't show good in pics because it all blends in. By increasing angle it (1) shortens distance between cylinders to allow pulse from cylinder above it to get to edge quicker. (2) also makes for a less of a chance for exhaust from other cylinders to enter that cylinder as the exhaust passes over it.
This is the setup I have found that has made the most power over the widest rpm range for me so far. I havnt yet ran the 45 degree angle, I suspect I will loose some low end. It's an experiment setup I'm trying to see if my evaluations are correct. Someone may can add to it, this is just what I have figured out from looking at T Rex work and my own discoveries fooling with a heavy boat. I discovered the low end gains by accident, I was actually expecting to loose bottom end and gain top end by opening up exhaust, but in reality actually picked up bottom end and top end. I suspect is was due to the fact I had by particular set up actually choked down. Suspect wouldn't see same results on a stock setup, I say all this to say the exhaust needs to be cut in relation to the amount of air/fuel the mota can take in at a given rpm. This is info most don't like to share, so I have set out to figure it out on my own. I know some of it is against the grain, but this is what has worked really well for my setup and what I'm doing with my motas. I'm still learning, so prob still has room for improvement,
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07-14-2017, 05:40 PM #49
Don't you be copying me boy.
If I don't ask any questions, I'll never learn anything.
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07-14-2017, 06:12 PM #50Screaming And Flying!
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Distance C , ex port at cyl sleeve to ex port at cyl sleeve, next in firing order
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07-14-2017, 06:25 PM #51
Correct, that distance is shortened with more angle downward being it's more of a direct path. My theory anyway.
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07-14-2017, 07:45 PM #52Screaming And Flying!
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You really can't do too much about the 1 out of three distances that's the long vs the two which are equally short in the firing order. Meaning, two are side by side in pulse distance to tuned pair, the third jumps over the neighbour to reach the next to close. Longer distances to travel later pulse, if you believe in that sort of thing. If not it's like a Warner Bros cartoon where the character doesn't fall because he didn't study gravity. Nobody has made a crank with firing order 1-2-3-2-1-2-3-2.....lol
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07-15-2017, 07:48 PM #536000 RPM
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This is how Junior Eaves did it. Bottom end mid range and top end were all there. His numbers were used for years in the racing/production world. But there was a hand in the motors that a mill just cant do. These pics are of a block of mine that I coated and they don't show up as well as I wish but maybe you can get the idea.
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07-15-2017, 07:49 PM #546000 RPM
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07-16-2017, 08:26 AM #55
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07-16-2017, 08:31 AM #56Screaming And Flying!
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Does the thread mention port degrees?
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07-16-2017, 08:48 AM #57
Not other than the chest volume needs to match the ability of getting air into mota. I feel the exhaust is choke down factor on a 2.5 when you start increasing amount of air/fuel into mota and running intake durations above 120-22 degrees. I feel this is why I experience an increase on bottom end also by increasing exhaust volume and lowering back pressure. This is confirmed by statements about Mr Eaves chest also making good bottom end. Eaves looks to be doing same basic idea and using around a 60 degree lower angle and keeping a sharp lower ramp exit to increase velocity. BTW I think Eaves was the MAN!! And thanks for those pics!
Last edited by FORBESAUTO; 07-16-2017 at 08:56 AM.
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07-16-2017, 09:12 AM #58
I was kind of thinking the same thing ...
Closer they can get those fishin blocks to have the same angles and volume as a 280, the closer they will be to the height of development , before Merc called it quits ..
Puttin bolts on the cover kinda makes it where everyone can become a rock star ..
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07-16-2017, 09:48 AM #59
Us po folks can't afford no 280
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07-16-2017, 12:03 PM #60
I've been told a 280 won't be worth a darn on my heavy boat, due to not having enough bottom end and how hard you have to turn em to get its full potential. One size don't fit all. When mother merc designs engines it's made to perform well on a variety of applications, except for the hi po's which where for a more specific purpose. For us guys that are stuck somewhere in the middle we are better off starting with the more torque of a fishing mota and customizing it to the specific need of what your trying to occomplish. Plus the learning from modifying is the enjoyment of it. Too boring buying a runner and not touching it. Lot more fun to take a turd and make it run.
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