exhaust flow for old log manifolds vs center risers

[jay dezman]

77 vintage 351 merc outdrive powered boat.<br /><br />thinking of repowering. how much horsepower equivalent can be put thru both style exhausts?

 

[Scaaty]

Re: exhaust flow for old log manifolds vs center risers

I don't think it would matter one bit

 

[Bondo]

Re: exhaust flow for old log manifolds vs center risers

Ayuh,..... I'm with Scaaty on this 1..........<br /><br />They're Both rather Restrictive,....... No HP Differences.......<br /><br />And,.....<br />I Doubt you'll Find center risers to Fit a Ford/Merc..............

 

[mkast]

Re: exhaust flow for old log manifolds vs center risers

Small block Ford center risers on EBay ala Dennis Moore.

 

[Haut Medoc]

Re: exhaust flow for old log manifolds vs center risers

I don't think there would be that much of an advantage....JK

 

[Boomyal]

Re: exhaust flow for old log manifolds vs center risers

mkast mentioned SBF center risors by Dennis Moore, so I went to take a look. I'm still running the original oem round log style and they are getting very expensive should I need to change someday.<br /><br />This is what I saw;<br /><br />

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<br /><br />My question to the vendor should not be in doubt here. I asked if this was not perhaps a "formula for an exhaust leak"? Their reply was simply "could be"????<br /><br />The price of $369 per complete pair (item #4626553905) was certainly attractive but I would not use them.<br /><br />What say ya'll??

 

[jay dezman]

Re: exhaust flow for old log manifolds vs center risers

guess i was clear on my request....sorry for that.<br /><br />i wasn't looking so much for a comparison of one type to another but the fact that the largest 351 based engine i recall is (was)around 250hp in the day.<br /><br />if i have them rebuilt with the idea of more power (like possibly a stroker crank to 392 ci) at what point does the exhaust become tooooo restrictive to make any additional hp building fruitless? can you get 350 hp to breathe thru them or even 400 hp (and yes i'm aware the old drives are on egg shells at 300 hp)

 

[qaztwo]

Re: exhaust flow for old log manifolds vs center risers

I saw those too, think about all the weight your saving by using half the bolts.

 

[Buttanic]

Re: exhaust flow for old log manifolds vs center risers

Don't really see the problen with the one bolt. On the small block Chevy the two center ports only have 2 bolts, there is none in between the two center ports, never has been even on automotive engines. I believe even the OEM Mercruiser manifolds have only one bolt on each end. If it were a thin plate flange like on tubular headers there could be possible leakage but a big rigid casting ain't likely to bend or warp.

 

[ron7000]

Re: exhaust flow for old log manifolds vs center risers

I have a 351w with osco center risers. I've never had an exhaust leak due to it having only one bolt per exhaust port. My manifolds are going on 10+ yrs old and are great. I've only been through 1 set of risers. Every other year I've been pulling the manifolds to shake out the scale and rust which is probably why they've lasted so long.<br />I have a 1976 model 233, if yours is the same and want the kind I have I can look up the part #'s for you. Next to the new motor, these were the next best thing I did repowering the boat.<br /><br />As for hp rating, I repowered with the 285hp 351w HO. Looking at these, or any marine manifold really, they are non restrictive and the riser outlets are typically 2-3" (each side!) which is plenty big enough so I don't think you are ever going to max it out. Plenty of SBC's making 400-500 rwhp going through 1.75" headers. The biggest constriction or flow reduction is where the exhaust mixes with the raw water at the riser outlet, and then when that mix flows down the Y-tube and through the outdrive in mercruisers.<br />Solution to that would be to run thru-hull exhaust and have the water mix in to the exhaust as far back as possible.<br />If I had to make a guess, I'd say the center risers would flow better because there are less 90 deg turns in the flow compared to the log style manifolds. But I doubt it's anything significant. The center riser style have the greatest advantage by being less wide and freeing up engine bay room. With the old merc logs, you couldn't reach the spark plugs, now with the oscos it's almost fun to change them.. almost.

 

[ron7000]

Re: exhaust flow for old log manifolds vs center risers

Originally posted by ron7000:<br /> I have a 351w with osco center risers. I've never had an exhaust leak due to it having only one bolt per exhaust port. My manifolds are going on 10+ yrs old and are great. I've only been through 1 set of risers. Every other year I've been pulling the manifolds to shake out the scale and rust which is probably why they've lasted so long.<br />I have a 1976 model 233, if yours is the same and want the kind I have I can look up the part #'s for you. Next to the new motor, these were the next best thing I did repowering the boat.<br /><br />As for hp rating, I repowered with the 285hp 351w HO. Looking at these, or any marine manifold really, they are non restrictive and the riser outlets are typically 2-3" (each side!) which is plenty big enough so I don't think you are ever going to max it out. Plenty of SBC's making 400-500 rwhp going through 1.75" headers. The biggest constriction or flow reduction is where the exhaust mixes with the raw water at the riser outlet, and then when that mix flows down the Y-tube and through the outdrive in mercruisers.<br />Solution to that would be to run thru-hull exhaust and have the water mix in to the exhaust as far back as possible.<br />If I had to make a guess, I'd say the center risers would flow better because there are less 90 deg turns in the flow compared to the log style manifolds. But I doubt it's anything significant. The center riser style have the greatest advantage by being less wide and freeing up engine bay room. With the old merc logs, I couldn't reach the spark plugs, now with the oscos it's almost fun to change them.. almost.