This is probably a stupid question.... I've read about different size risers on exhaust systems for different applications. What exactly does this all mean?

Just trying to figure it all out.... thanks.

Risers are the top half of the exhaust that bolts to the exhaust manifold.
For higher HP engines, you need larger diameter risers and better flowing manifolds or headers.

Griff is spot on, but to add some meat to the answer:

The riser is the part which changes the direction of the exhaust as it leaves the exhaust manifold. It bends the flow either 90 degrees or about 120 degrees so that the exhaust may then be carried out through either the transom or the drive or both (split system).

As far as I'm aware, most boats these days come with the stock system either split or through the drive (out through the prop hub under water).
This is done to keep the exhaust noise down and not disturb any little cherubs who may be easily offended. (What about the Fish??? Who asks them if they're offended?!?!)

Anyway, stock risers are good as wheel chocks and emergency anchors, or you could chrome them or gold plate them and wear them as jewellery if you like! Other than that, the stock manifolds and risers are choke points. After market risers such as the EMIs, GILs and any other lot I missed are an improvement by reducing weight (usually tubular Stainless rather than Cast Iron) and increasing flow by increasing interior diameter which decreases backpressure and using tubular bends rather than cast elbows. Most types of riser also change the point at which the water that has been used to cool the motor and exhaust goes into the exhaust stream so that it can exit the boat. Allowing the deposit to take place later in the exhaust stream helps prevent 'reversion' which is when water manages to make it's way back into the head via the exhaust ports.

This combination of reduced weight and increased flow leads to increased horsepower (usually) and more horsepower makes baby Jesus happy.

Hope this answers you query!

Next week we tackle the question: "Mummy, Where do Baby Boats come from?"

Remember: Loud Boats Save Lives!

Longer risers allow the use of bigger cams without the problem of reversion.

It does answer my query... Ghostrider that was awesome.

So reversion is the when water and exhaust meet?


When you say longer... is it the height above the exhaust ports or the diameter of the exhaust. That is exactly what I am trying to figure out... bigger cams and riser connection.

Long risers are used to prevent reversion. Reversion is when the engine accutally sucks water back into the engine. So the further away the water mixes with exhaust the better.

Ok.. thanks I understand.

So the last question I still have is.. risers are height and not diameter, right?

Thanks everyone for the help.

Good risers should have a 4" outer diameter with at least a 3.5" inner diameter. The water travels through the exhaust and keeps it cool then exits at the end. The riser fits on top of the manifold. Take a look at some manufactures websites. Eddie Marine, Imco, Stainless Marine, maybe that will help.

Risers are a combination of height and diameter. The best sort would be the high rising long tail length ones as they would have the highest flowing bend and the latest mixing exhaust and water, but they don't always fit under your hatch (as I found out the hard way), the next best ones would be the low rising bend with the long tail pipe which still has a smooth bend with less rise and the long tail pipe helps stop the water getting back up the exhaust passage and into the heads as it mixes further away.

Heres some pics: (the green ones are cast aftermarket EMI Risers, don't laugh, I think they look great! )

Good example long, medium and short