exaust options

[Abbott Racing]

Those Quincy pipes were nice, and worked well. I drove the rig before he bought it and it was quick and had lots of snot coming out of the corner. It was almost as quick as my rig with the megaphones.

Still looking for my drawings to share with the folks, so be patient......

[Merc55H]

Mike Schmidt pipe design

I am posting the design Mike Schmidt discussed in post # 16 in this forum. Click on the link listed below to see it. If you want a hard copy, right click then either copy it to a word processor or save the image. All dimensions are correct.


http://www.boatsport.org/BSC/Mikepipe.jpg





Ed Hatch
Boatsport.org (shameless plug)

[Merc55H]

Mike Schmidt pipe design

I am posting the design Mike Schmidt discussed in post # 16 in this forum. Click on the link listed below to see it. If you want a hard copy, right click then either copy it to a word processor or save the image. All dimensions are correct.


http://www.boatsport.org/BSC/Mikepipe.jpg





Ed Hatch
Boatsport.org (shameless plug)

[xcelerator]

now that someone mentioned carbs, anyone ever try puttin a mikuni carb VM or flatside on a mod merc? Lots of tuneability.. I was also wondering, could I use one pipe designed for the 55h mod on a 22ci merc 200 25ss powerhead? Lots of port and other work will be done, paded block, etc. Or does anyone have twin pipe plans for them , or know what a similar pipe could be used, such as twin pipes from a yamaha Rz350? Eager to learn about what other people know/have tried, thanks

[Sonny]

Mikuni carbs

straight to the carbs, It seems like the first place to run to to make more power. The very first thought I ever had was putting a set of Mikuni carbs on a merc, But the rules say no way. Maybe just a cute project?? yea, cute until you have to get the distributor moving, then it sucks. I was all upside down with my theories on carbs until a friend of mine reminded me of the obvious, the finish line is half way down the straight away. Here it is in perspective, You build a wild merc deflector I mean a real smoker, you hit 90mph with it, but it only does 40mph arround the corner and takes 3/4 of the straight away to get back to 90. you have done what you set out to do by hitting 90, but you shot you'r self in the foot getting there because a boat that does 60 all the way arround the coarse will smoke you'r ass every time. As far as the expantion chamber goes, 1700 feet per second is the speed of the returning sonic wave, the header pipe length deturmins how long it will take to get back to the combustion chamber . Measure the length you'r exaust ports are open in degrees of rotation. The rear cone shape desides how long that returning wave will last, and the stinger desides how much pressure the pipe bleeds off. I'll be the first to tell you that I am not the brightest bulb in the box when it comes to this stuff. The greatest mind's involved with this stuff are the one's that have already added to this discusion....oh yea, and Bob Goller, but he is aparently unwilling to help.

[xcelerator]

how about a mikuni on a 2cyl 22ci merc? The reed cage is the restrictor, no? Anyone have any experimental experience with modifying the stock reed cages or a custom case? OR that and making the transfer ports bigger in the block. Can someone decribe to me how the "3rd" port intakes work that replace the intake port cover on the block? Thanks,

[xcelerator]

hey sam thanks for clearing me up completely! Do the 3rd ports work better than stock, any bad side effects?

[jeff55v]

Anyone have any experimental experience with modifying the stock reed cages or a custom case? OR that and making the transfer ports bigger in the block.

Shawn, You need to get Harry Brinkman's book, "How to modify your Mercury outboard".
You'll find tons of info in it! I highly recommend it. If you're interested, Let me know and I'll dig up his address and phone #.

[Sonny]

OH BOY, THE BOOK...you have to read the book, it's the Merc bible. There were a very small number of 20H blocks made that were known as stuffed blocks where the crank case was thicker and the air fuel volume of the crank case was cut down. they were declaired illegal to race in stock because they had a distinct advantage over a normal stock powerhead. Point being, when you cut down on the volume of the impending air fuel charge waiting in the block you demand more flow out of you'r reeds. So dont just chase your reeds arround the shop with a grinder or you'll just ruin them. Choose you'r application "long coarse",or "short coarse". Do the math involved with you'r application. And for god's sake man... get the book, you'll pop when you see what's in there.

[xcelerator]

I'll def. get the book! Sounds very interesting...

[seagull170]

Sonny I'm not sure that is the case.
When you stuff a crankcase the rising piston in the cylinder causes a more rapid increase in negative pressure before the reeds open, causing a stronger faster airflow through the carb, this is the point where the intake tract can be tuned up to the point, that when the reeds close there is more than 1 atmosphere in the crankcase.
Now when the piston starts the crankcase compression stroke the piston squeezes the same amount as before but into a smaller space giving a higher pressure that causes more fuel/air to pass through the transfer ports in the same period of time.
The whole point of this operation is to get more air into the combustion chamber, More air = higher compression ratio = more fuel burnt = more pressure on the piston crown = more power to the prop.
Now it gets complicated if you've got magaphones you could be sucking some of the fuel/air out of the exhaust port before it closes, as it comes into the cylinder with more energy therefore faster.
So stuff your crankcase in small steps, the maximum crankcase compression all the books seem to agree is circa 1.5 :1.
But I've never worked on a Merc but it has worked for me on both loopers & cross scavanged power heads without megaphones.
Perhaps I should put a dyno on my birthday wish list.

Charles

[Sonny]

AAAHHHAAA, I hadn't thought of that part of it. But you'r right. I am trying this in an attempt to develop a stronger exaust wave so it can be tuned with a stinger pipe. The thought was to keep the fuel moving rather than allowing it to stand somewhat idle in the block. What I kind of invision happening is a larger demand on the carb's and reeds and a strong push by the negitive pressure developed by the downstroke of the piston. Because like you said, same amount of pressure but a smaller erea to jam it into kind of like puting you'r thumb over the end of a garden hose. But... on the same token, that would mean that you actually hurt the performance by beveling the crank, that would increase the volume of the case. A nessary evil on the counter weights that appose the reed stops. So like these guy's have told me all along mild is better than wild when it comes to the crank. I was going through Brinkmans book the other day and I was fixing to grind the snot out of a crank this weekend seeing as how we are pretty much snowed in. Maybe I should just take a nap instead.

[oupoorsoul]

Hello, I ran across this old thread, I would be intersted in the plans tha Chic drew up as I have a e mod engine to build emodhydro69@verizon.net Thanks Todd