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Discussion Starter #1 (Edited)
I know this has been discussed before. What is the recommended exhaust duration on a bbc with cast marine manifolds, isn't it around [email protected] The cam that came out was 254 and .600 on a 110 wich I think is a little much on the duration.
 

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northern member
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flat tappet ???? transom ex ???? even a f/t with 240 and 110 would be too much for prop ex. imo . merc cams were like 220 with the ex. c/l @ 120 .
 

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Or Seth, either one
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The guys who can send you in the right direction are gonna need to know:

The type of cam your talking about ie. hyd/solid, roller/flat tappet
Engine size
Compression Ratio
Intake: single plane, dual plane, bolwer, etc
Intended max RPM
and Exhaust: through hub or through hull
 

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The manifold have nothing to do with cam timing events. The pipes and exit point do.
I've run logs with 300+ degrees adv on a FE Ford and never had any issues.
yeah, but since yer old you probly puddled the intake...hil:)
 

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Discussion Starter #7
I know this has been discussed before. What is the recommended exhaust duration on a bbc with cast marine manifolds, isn't it around [email protected] The cam that came out was 254 and .600 on a 110 wich I think is a little much on the duration.
Sorry guys. It is Hyd flat. Through transom into what formula calls a "silent thunder exhaust". Basically a swim deck that acts like a big muffler. 9.8 to 10 compression. No captains choice.
 

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northern member
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Sorry guys. It is Hyd flat. Through transom into what formula calls a "silent thunder exhaust". Basically a swim deck that acts like a big muffler. 9.8 to 10 compression. No captains choice.
I'd say somewhere around 230 on a 112LC advanced 4or5* 550 lift .
 

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how did it idle with the cam that was in it ? I would expect it was a little too high for the clutch dogs on the gears to be happy . a stock mercruiser cam in the magnum's was a crane 132561 , I like the crane 134241 or a comp xm270h as they have less seat dur. for basically the same .050 dur. and lift as the 132561 and have a 112LC . anything more is going to need better manifolds or headers and the idle is hard on the clutch dogs .
 

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how did it idle with the cam that was in it ? I would expect it was a little too high for the clutch dogs on the gears to be happy . a stock mercruiser cam in the magnum's was a crane 132561 , I like the crane 134241 or a comp xm270h as they have less seat dur. for basically the same .050 dur. and lift as the 132561 and have a 112LC . anything more is going to need better manifolds or headers and the idle is hard on the clutch dogs .
Only the Alpha has "clutch dogs". The 454 "MAG" used with Alpha drives in '86 - '88 and were not really a MAG. Just a label Merc gave it. 3/8 rod bolts, oval port heads, cast crank, cast pistons, etc.

I didn't see where he said it was an Alpha? If it is, they are designed for a 650 rpm shift max.

The issue with the manifolds is not so much the poor flow as much as the extremely short elbow used with silent thunder exhaust. The water mixes with the exhaust very close to the bend, so any cam with much overlap WILL revert water. Anything tighter than 114 and over 230/236 @ .050 with that elbow will put water in the cylinder. Merc had huge problems with that when the first design BBC with HR cams hit the market. The stock elbows are horrible
 

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northern member
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thanks BD Marine for the additional specific info for merc risers . I didn't see where he said it wasn't a Alpha either . I am not a Merc ceritfied tech but have been around mercruisers and rebuilt a few myself , but not TRS or Bravos . Don't Bravo's have dog clutches in the upper gearcase ?
 

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thanks BD Marine for the additional specific info for merc risers . I didn't see where he said it wasn't a Alpha either . I am not a Merc ceritfied tech but have been around mercruisers and rebuilt a few myself , but not TRS or Bravos . Don't Bravo's have dog clutches in the upper gearcase ?
You're welcome. No, they have a cone clutch.
 

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Discussion Starter #14
You're welcome. No, they have a cone clutch.
The cam that was in it was [email protected]/.600 lift on a 110. It also had Rhoads lifters to calm it down a little?? It is a brovo 1. I had to use the throttle for the idle. You could easily get it low enough to get it into gear but couldn't get it to idle in gear. I got pretty good at not beating on the drive in and out of gear. I did have a fair amount of water in one cylinder when I tore it down. It is a 370 hp magnum/1989 fourmula 232ls. I never really put much thought into reversion. The guy who specked the cam was supposed to be a boat guru. I will check into those part numbers. Thanks to all
 

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Discussion Starter #15
Isky

You're welcome. No, they have a cone clutch.
the cam I "thought" I wanted to run is am Isky 280/292-.564/.590-232/244 @.050 on a114. Sounds like this is too big on the exhaust duration? They also have a 270/280-.543/565-221/[email protected] on a 114. What do ya think ?? My riser elbows dump about 4" from where the hose clamps on.
 

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The cam that was in it was [email protected]/.600 lift on a 110. It also had Rhoads lifters to calm it down a little?? It is a brovo 1. I had to use the throttle for the idle. You could easily get it low enough to get it into gear but couldn't get it to idle in gear. I got pretty good at not beating on the drive in and out of gear. I did have a fair amount of water in one cylinder when I tore it down. It is a 370 hp magnum/1989 fourmula 232ls. I never really put much thought into reversion. The guy who specked the cam was supposed to be a boat guru. I will check into those part numbers. Thanks to all
The Bravo can "safely" shift up to 800. But that is a true 800, most tachs are not accurate at idle.
 

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the cam I "thought" I wanted to run is am Isky 280/292-.564/.590-232/244 @.050 on a114. Sounds like this is too big on the exhaust duration? They also have a 270/280-.543/565-221/[email protected] on a 114. What do ya think ?? My riser elbows dump about 4" from where the hose clamps on.
The distance from the bend in the elbow to where the water mixes with exhaust is whats important. I personally would rather see a cam with 6 degrees longer exhaust duration than 11 or 12. I would lean towards the smaller one unless the elbow was long.
 

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Discussion Starter #18
Cam

The distance from the bend in the elbow to where the water mixes with exhaust is whats important. I personally would rather see a cam with 6 degrees longer exhaust duration than 11 or 12. I would lean towards the smaller one unless the elbow was long.
thanks for the info. I was planning on running a set of comp 1.8 intake and 1.7 exhaust rollers I have owned. So I bet the smaller cam will work with that rocker combo and I know that will increase the intake duration a bit. I also have another set of aluminum 1.7s if that won't work. I am trying to keep the crowers that came off the engine
I will measure the riser and post that tonight. I thought it was about 4''

Thanks for the opinions!
 

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Discussion Starter #19
Puc

thanks for the info. I was planning on running a set of comp 1.8 intake and 1.7 exhaust rollers I have owned. So I bet the smaller cam will work with that rocker combo and I know that will increase the intake duration a bit. I also have another set of aluminum 1.7s if that won't work. I am trying to keep the crowers that came off the engine
I will measure the riser and post that tonight. I thought it was about 4''

Thanks for the opinions!
heres a pic if the riser
 

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The distance from the bend in the elbow to where the water mixes with exhaust is whats important. I personally would rather see a cam with 6 degrees longer exhaust duration than 11 or 12. I would lean towards the smaller one unless the elbow was long.
heres a pic if the riser
Like BD posted earlier, its not just the height of the elbow, its the distance from the bend, or the manifold to where the water and exhaust merge together. The farther down line the two come together, the more overlap can be tolerated.



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