3 inch vs 4 inch exhaust elbows

[greg82255]

1986 Merc 260/Alpha One. I need to replace the manifolds and risers, and it currently has 3 inch risers on it. When running a Mercruiser center rise exhaust is there any noticeable difference in power or fuel economy when using 4 inch risers vs 3 inch risers? I've heard that there's a performance increase with the 4 inch, but I've also heard it makes no difference. If one isn't better than the other, and the size is basically the same as are the bolt holes, why did Merc make both/why do they still?

 

[Bondo]

Re: 3 inch vs 4 inch exhaust elbows

When running a Mercruiser center rise exhaust is there any noticeable difference in power or fuel economy when using 4 inch risers vs 3 inch risers?

Nope...

 

[greg82255]

Re: 3 inch vs 4 inch exhaust elbows

so just out of curiosity.. why did they make both?

 

[Bondo]

Re: 3 inch vs 4 inch exhaust elbows

so just out of curiosity.. why did they make both?

Ayuh,... I guess you'll have to ask Mercruiser.....

 

[sasto]

Re: 3 inch vs 4 inch exhaust elbows

The performance gain that is claimed comes not from the size of the riser but the internal structure of the riser itself. The 4" has less angle of flow whereas the 3" incorporates a rise and fall of a greater angle. In other words the internal flow of the 4" is smoother with less bend than the 3". Not much difference in the internal diameter of each, just a straighter flow of exhaust with a claim of less backpressure.

Noticable difference of power gain? Probably, probably not.

You should be able to find a cutaway view of each and pick the one you feel is best.

 

[sasto]

Re: 3 inch vs 4 inch exhaust elbows

I was passing on knowledge given me and being curious about this myself I found this site. I run a boat with duel inboards. One engine has the 3" the other the 4". The one with the 4" runs better than the one with 3", but these motors being several years old and I can hardly put the claim on the risers. The 4" is reduced to 3" exhaust mufflers and outlet.

http://www.mooreperformance.info/MercruiserRisers.html

Good luck!