2.5 exhaust chest mods....

[Merc 2.5]

Great info! And you're right, it's definitely got my wheels turning. Sounds like crankcase stuffing can help if one goes overboard with a die grinder haha. Thanks for explaining this in detail to me. Anyone can cut what they're told to cut. Understanding why is the key. Thanks again!

If one goes overboard,, you can't stuff it enuff to bring it back. You would need to put it on a light weight hull with stuffing mite work. That would b my guess anyway

[FORBESAUTO]

You dead on 2.5. Use stuffing, angles and shapes to manipulate the ports to pull more rpm than just opening them up. Once it’s gone hard to put it back. The 2.5 can make really good power and pull some really good rpm without getting real crazy with port timing.

[Smalltownbassin]

Right, understood. What im really trying to do is understand the relationship between exhaust and transfer and how that results in power. I understand each aspect individually pretty well but like you said, there are so many variables involved its hard to put it in black and white. Here is a good example of my confusion...
Im sitting here thumbing through an L.A.Sleeve catalogue and comparing exhaust port sizes between HP and year models. What im notincing is very little difference between them but with big HP changes. The only sense i can make of it is that the transfers must be different but they arent listed in the sleeve specs. Of course i understand that there are many other variables considered here like intake volume, velocity, compression and mixture density that add to power output but there must be more of a difference between HP port maps to create such large jumps in HP from one port map to another. And if the difference isnt as much in the exhaust port, where is it? Im not so much looking for a specific answer but more just an understanding if that makes sense.

[FORBESAUTO]

Most has to do with the shape of the ports, width, and the angles in the roofs of the ports. For instance a flatter top arch on an export port will pull more rpm than a more rounded one with same port heights, simple example, but to clarify what I’m saying. The angle and flow direction of charge into the combustion chamber from boost and transfers also makes sizable changes in characteristics of an engine. A lot of people get crazy with duration and blowdown to try make high rpm power and run into all kind of problems, go backwards in power, have meltdowns and such. From my experience, and again I’m not trying to turn 10k on sheet of plywood, I mainly focus on good all around power from 2k to around 7500 with good low end torque, find more gains in shapes, angles, and bottom ports. You don’t need to overboard with blowdown or transfer duration. Plus lots of power is made on the lower entry ports of the sleeve for transfers and boosters, this is often overlooked but plays a big role. We almost always are changing them along with the cuts for them on the pistons. Also on hipo motors a lot of the power is made in the chest configuration, which again changes how it may be ported due to effectiveness in scavanging and such, may not need as much blowdown to get job done

[LakeFever]

If you really want to see how things will flow take a silicone cast of the port, then make a quick mold over it, then put a water hose through it and watch. When you see smooth fast streams exiting? Your winning. When you see turbulent water with spray/splatter? Somethings not right. Of course if you build or have access to a flow bench you can do this with air and measure gains/losses. David Vizard has some excellent info out there on porting and flow. Fluid dynamics is the field this lives in