2.5 exhaust chest mods....

[FMP]

That's another good read

[Iowafshr]

So being a complete noob where should I start?
I may be able to get a machinist to do some milling for me, but he has no experience with this type of work. Any direction I can give him? Is it possible to do by hand? This is my 225 Promax. Any pictures of the block set up for machining?

[FORBESAUTO]

A lot depends on what all your doing to the motor as to how you want to do your chest. The operational rpm of your build, and amount of increase in air/fuel into engine helps decide on chest design. The more air/fuel you begin to cram in intake side you begin to increase back pressure on exhaust side and the more you have to open in the right places to balance it back out. Give us some info on what all is being done. If no other significant changes, also want minor to little changes in chest for best all around performance, all things need to be done proportional, if that makes sense.

[dwilfong]

So being a complete noob where should I start?
I may be able to get a machinist to do some milling for me, but he has no experience with this type of work. Any direction I can give him? Is it possible to do by hand? This is my 225 Promax. Any pictures of the block set up for machining?

Have him run a 1" ball end mill down the center . don't go any deeper than the grove that is in there now you have circled in red. Then just smooth out the sharp corners in the pocket. that block has the second cut in the ex so it is easy to clean up and make good flow.
My 240 Sport jet had the same chest. This eng pops right off from idle with 300+ port timing and the ex port opened up to 280/300 size. 45deg chamfer in the sleeve and full radius of the top shelf in the block.

[Iowafshr]

Any suggestions on the front? Currently running stock laser injection with Brucato ACU. I don't plan to run over 8k and would really like my gains in the low to mid range.

[FORBESAUTO]

There is a lot of variables of what can be done and how much. If porting is left alone, I would shave .050 off front half surface where intake attaches, this decreases cc volume and increases velocity and also gives a better signal to reeds by moving it closer to crank. I would stuff front half on the lands between cylinders by filling in the shelf in half (1s pic). Then fill in oil pump void to make cyl 2 cc volume equal to the rest (2nd pic), cut rod slots in block to straighten out path behind sleeve (3rd pic) that pic has the whole opening milled over, I would only cut the slot relief in your application because the more metal you remove you are increasing cc volume and reducing cc pressure and will need to do more stuffing to get cc pressure back where it needs to be, smooth out all casting slag in front half and block as can be seen in pics 1 and 3, Then I would add a 1 inch intake spacer between reeds and intake to straighten out intake path on the laser injection intake tract. Now a lot can be done in cylinder porting, but gets very detailed and a whole nother subject and also changes how I would do front and rear sections. On the exhaust side your goal is to have no backpressure but keep it as small as possible to just achieve no back pressure so that tuning pulses ( not to be confused with gas flow) are stronger, and better velocity.

[hoop235]

Forbesauto,

What did you use to fill in that area at the oil pump? Also, do you still use a sealing ring on that filled in area?

BTW: Great thread and thanks for all of the info from everbody!!

[FORBESAUTO]

I use splash zone epoxy to do the filling mainly because it's ease of shaping and holding shape. It takes some time to get used to working it. Main thing is you don't want to use much water with it, mainly at the end for final smoothing and shaping. I've found if mixture has much water in it, when it dries out it will leave little voids that look like air pockets and will be weak. Done correctly and the stuff is almost Bullet proof. That particular land doesn't use a crank ring normally. I use my sanding tube that I use to remove ring wear grooves from block and half to make sure the epoxy is shaped true and even.

[FORBESAUTO]

When you you fill the oil pump void, you will not be able to use the block off to hold bushing in. I also fill the oil pump hole in the half, knock out bushing and fill bushing hole in cylinder floor

[hoop235]

Do you "rough up" the surface any before applying the epoxy? Like 80 or 120 grit paper?
Interesting about filling up the void on the intake, I havn't seen that one listed before. Thanks!!

I currently building my first 260 and have worked the exhaust chest over by hand so I know my angles are not all the same like yours are but I hope I'm in the ballpark. I had someone local that's very familiar with these motors give me some tips and then he looked at it and said it looked good to him. Half the fun is experimenting with these things!

[FORBESAUTO]

Yes I rough it up with a burr, then when you first start applying the epoxy with a thin coat and rub in with fingers until it gets a hold and won't roll up.

[hoop235]

Thanks again for the info and let me add that's a great looking job you did in that area!

[Iowafshr]

Thanks for all the info. This info used to seem like it was a kept secret by engine builders. Maybe I just wasn't looking hard enough. Someone should put together a wiki on all the mods and processes for these motors like the LS/sloppy mechanic's community has.

[QuarterHorse]

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[Keaten]

Wat kind of hp would a merc 2.0 make with 7 petal front horn and chest work like pics before? And wat ever is normally done in all other ports etc..thanks