2.5 exhaust chest mods....

[FORBESAUTO]

The more angle also shortens the distance between the ports which causes pulses to reach next cylinder quicker for higher rpm. This is my reasoning why the angles are important for the rpm tuning and the opening up of chest to support the breathing capabilities of the mota

[FMP]

I was waiting for you to say it, now I know where you are referring to without looking at the pics. Yup, are you increasing the volume of area just outside the port?

[FORBESAUTO]

Yes, I mill out between ports, cut angles on mill, then blend by hand. Milling between ports only beneficial when increasing engines ability to breath

[FMP]

Sure , but port area size transitions to larger cross section area at what rate or distance from port ? If you increase port size area is it better to maintain that cross section shape through the turn to maintain velocity, short side radius turn .

[FORBESAUTO]

In a sense that's what your doing, the cross section area is much smaller than the port area and you are opening it up by milling it

[texasvector]

Could someone post some of these pics with labeling? I.e. angled areas...."tops"....."roofs".....etc

When I have extra time please.....

[FORBESAUTO]

Man, I ain't smart enough to do that! Roof is top of port opening on cylinder, top that I say needs no angle is vertically on top, the angles go on bottom exit vertically pointing toward exits

[FMP]

Not losing bottom, more cc psi , tighter head, a bit more ign down low , etc

[FORBESAUTO]

I fixed it

[FMP]

Could someone post some of these pics with labeling? I.e. angled areas...."tops"....."roofs".....etc

When I have extra time please.....

You mean, draw

[FMP]

I fixed it

Didn't know it was broken

[FORBESAUTO]

I initially labeled roof incorrectly

[texasvector]

You mean, draw

Crayons are acceptable

[texasvector]

[FORBESAUTO]

That chest/ ports looks pretty healthy and the angle in pic looks to be close to 30 degrees. I know it's hard to tell in pics. I feel 30 degree works good to around 8K and need to increase angle above 8k. I haven't found where backpressure helps anywhere on my motors, just the tuning of angles and tuner lengths/ shapes make a difference. What opened my eyes to this is when I was running Lake Maurapas one day with some really rough waves and was plowing along at 2800-3000 rpm and when I would hit waves I would get around a 200 rpm surge/ spike really quick to where I initially thought I had a loose connection. After I got home and inspected motor, found nothing. Then it occurred to me it was my exhaust holes in bottom of mid coming in and out of the water and the rpm increased 200 rpm every time they came uncovered. Also when taking off with a tall prop and exhaust comes out of water, what happens? The motor turns on and begins running through the rpm like crazy, but is sluggish until the exhaust is relieved.