2.5 exhaust chest mods....

[texasvector]

The roofs have an angle too..not stra8ght across like your cut forbes....not knocking just inspecting

[FORBESAUTO]

You want sharp edge at top of port opening in chest below cylinder above it, mine actually has some angle but it's downward also. The amount of angle depends on width of exhaust port., I do not raise the top of the port opening in the chest any, just angle it straight to port width in sleeve/ block. The wider the port the more the angle.

[FORBESAUTO]

The roofs are actually radiused into the chest also, picture doesn't show it well but not the same 30 degrees. The roof angle is also changed with slightly wider at bottom to promote downward flow. With a piston in cylinder near closing position you can see this angle and understand what I'm talking about. Most blocks come wider on top which seems would cause last part of exhaust to blow upwards.

[FORBESAUTO]

I wish I had a pic before polished so you could see more about it, but from what I can tell looks to be close to what you have. The shiny doesn't show the radius well, blends in. I did remove a lot of metal on roof exit into chest though, can see by distance of opening in roof to seam in block

[texasvector]

A : angle this area somewhere in 30-45* range depending on rpm you are wanting to achieve...coming from cylinder the volume of the port gets larger?

B: this should be sharp......does angle of edge matter....1st block looks straight across...2nd looks angled

C: is the depth of this area same as bottom of channel a stock block would be? is the chest milled flat or angled towards ports (i.e. 15*)


please correct me ....add to it.....take away.....draw at free will!!!

[patchesII]

Find a picture of a stock drag. Copy that as close as you can

[FMP]

The roofs are actually radiused into the chest also, picture doesn't show it well but not the same 30 degrees. The roof angle is also changed with slightly wider at bottom to promote downward flow. With a piston in cylinder near closing position you can see this angle and understand what I'm talking about. Most blocks come wider on top which seems would cause last part of exhaust to blow upwards.

Consider this, degrees of formation for pulse exit changes with crank speed. Also that port is open for a number of degrees before anything comes out. It's been written as much as 15* +/- before pulse wave exit, then time the travel to tuner end and scavenging of spent charge. So piston travels x degrees down pulse blasts out into chest pulsing a high pressure wave. You talk of piston down from roof and port exposure with shape of worked area. How many degrees of crank rotation are you looking at after port opening? Meaning what is the improvement helping, scavenging or direction of pulse. It will have effect on both but your piston down comment got my attention.

Also Texasvector , 1/8" humps any pics?

[FORBESAUTO]

A : angle this area somewhere in 30-45* range depending on rpm you are wanting to achieve...coming from cylinder the volume of the port gets larger?

B: this should be sharp......does angle of edge matter....1st block looks straight across...2nd looks angled

C: is the depth of this area same as bottom of channel a stock block would be? is the chest milled flat or angled towards ports (i.e. 15*)


please correct me ....add to it.....take away.....draw at free will!!!

A- correct, I have seen some drag blocks with this cut at 60 degrees. This is the angle I have found is increased as rpm increases. I'm building my first mota with 45 so don't know results yet how affects low end torque. I feel it will loose some.

B- yes, sharp edge entering chest has same arc shape as port. There is an angle angled backwards some to the opening of port in sleeve, the wider the ports are cut the more the angle, I don't raise the opening at chest entrance to decrease angle, the more angle back actually helps flow downward.

c- so far have only cut just slightly deeper than original trench. Scared to remove too much, afraid of cracking from heat. I cut it straight between cylinders with a slight arch from side to side. Have also ran some with little wall in chest against divider center milled down also giving it a little more width. T-Rex does it this way. I've found the more you can flow in this area the better. I have basically taken same powerhead with a lot of mods and gradually opened up chest flow in baby steps and found that each time I've increased flow I have picked up all the way around.

[FORBESAUTO]

Consider this, degrees of formation for pulse exit changes with crank speed. Also that port is open for a number of degrees before anything comes out. It's been written as much as 15* +/- before pulse wave exit, then time the travel to tuner end and scavenging of spent charge. So piston travels x degrees down pulse blasts out into chest pulsing i a high pressure wave. You talk of piston down from roof and port exposure with shape of worked area. How many degrees of crank rotation are you looking at after port opening? Meaning what is the improvement helping, scavenging or direction of pulse. It will have effect on both but your piston down comment got my attention.

Also Texasvector , 1/8" humps any pics?

I mentioned with piston installed because without it it's hard to notice the shape I'm talking about without measuring, with piston in and coming up to top of port you can see it by using piston top as reference. I'm mainly concerned about the end of exhaust with piston coming up, original shape is larger at top of opening and smaller at bottom, which I feel caused the last amount of flow to go upwards. I straighten it out and favor bottom just a tad to force all flow to go downward.

[FORBESAUTO]

I also taper down the upper ridge near the coolant passages on outer walls to increase flow if you didn't notice it.

[90 5.0]

I also wanted to find a 1977 175 block...heard they looked like drag block now.....or pic

interesting I've never heard that before, I might look for one to compare to an early 200, would be very strange for an early 175 to have a much bigger exhaust than an early 200 but hey it's Merc so who knows.

[patchesII]

I've got a drag block I can copy....it has "humps" welded back in below exhaust port only on outside.....about 1/8" tall....follow?


I also wanted to find a 1977 175 block...heard they looked like drag block now.....or pic

I wouldn't worry too much about what was done in 77 on a 175

[90 5.0]

I wouldn't worry too much about what was done in 77 on a 175

V6 outboards were brand new back then basically. I guess it is possible that Merc made the exhaust big to up horsepower, realized it was killing bottom and went smaller again until the drag came out.

But you think they would have figured that out BEFORE it went to production but who knows

we do know that a lot of the early consumer motor stuff "became" the high pert standard later(like single ram trim for example!)

where's Fugimo? Maybe he has a pic of a 77' 150 exhaust??

[FORBESAUTO]

Heres a pic of one with wall milled down at divider and not polished yet so may can tell more about it. The wall is left sticking up around 0.040

[W2F a V-King]

That's some amazing work there.

So that .040 wall doesn't melt away? That must have been discovered after many trial and error attempts, and a few beers in between!!

I know that kind of work takes patience, and practice.