Still in the planing stages of building a new motor I have switchable exhaust and was looking at stainless marine exhaust 520 sm350ks kit it states " Not recommended for large camshaft engines." but no where tells the limits of the cam ?
right now the plan is 4.030 or 4.040 dart block 383/385 with afr 195 heads Inconel exhaust valves 9.74:1 CR with .041 Quench and either a
comp cam xm270hr Duration @ .050 (Int/Exh): 218/224; Gross Valve Lift (Int/Exh): .495/.503 or
Lunati 20080721 Duration @ .050 (Int/Exh): 219/227; Gross Valve Lift (Int/Exh): .515/.530
leaning twards the Lunati cam with slightly more duration and lift but dont want any problems with reversion
I assume the risers are shorter on the switchable SM kit to clear the silent choice valves ?
I think the cams are made to work with stock exhaust so should be the same lenght as the switchable SM exhaust ?
But what does adding the flow of the heads and extra stroke do to reversion problems ?

IMO, the switchable exhaust is for stock motors. With those small cams you posted, you might be allright. Only way to tell would be a reversion check after idling. Buy risers that mix water into exhaust as close to transom as possible.

Former setups....

355ci XM276HR cam, needed risers 16" longer than stock to prevent reversion.

385ci 234/240 cam, needed 22" longer than stock.

Built a really similar engine a few years ago for my friends Classic 18 Donzi. Started with a Mercruiser 377 longblock. .040 over for 385 bumped compression to about 9.7 used a nice set of vortec heads with a very nice valve job and a very small amount of port work mainly just a clean up of rough edges. I used a GMPP LT4 hot cam kit I think it is 218/224 and .525 lift with the included 1.6 rocker arms. Had to machine valve guides for the additional lift and cut spring seats for larger diameter springs. This little engine ran great with stock Mercruiser manifolds and elbows. I think there was definitely some potential gain from stainless marine exhaust but he sold it before he upgraded. Stock 350 boat would do 58-59 I ran it 70 gps with the 385 I love that combo and would build it again in a minute

Thats good news im not opposed to using vortec heads just looking to get the most out of it. I just thought the AFR would flow better and the Aluminum would help combat detonationand with bonus of weight reduction. I've read you can go 1 point higher on compression but wasnt going over 10:1. I always run ethanol free 90 octane but never know i might get somewhere i cant get it. WIth a better flowing head does it make reversion better or worse ? I'd think the velocity would be less with larger port so the reverson pulse would be less or am i thinking wrong ? Open to other cam sugestions that are proven reversion free

Ryans boat was fast as f**k and he had it many years and used alot.

Staying with his recommendations will be a smart move.

btw: he is talking about inner tube length when saying riser length, I believe. Some weld in inner extensions, thus why I’m mentioning this.

Haha! I miss that boat! SB is correct when I talk about riser length. It's how far past where water is mixed with exhaust stream on a stock setup.

Vortec's are a great budget head. But, a 170cc iron head can only do so much on a 383ci setup. Go with the AFR head for all aspects you mentioned. Mine were early 90s 190R and they rocked! The newer ports are even better.

On a performance engine you want to mix h20 into exhaust stream in the riser as far away as possible from the port. Since you are buying new risers its a no brainer.

[QUOTE=Ryan00TJ;4771002]

Vortec's are a great budget head. But, a 170cc iron head can only do so much on a 383ci setup. Go with the AFR head for all aspects you mentioned. Mine were early 90s 190R and they rocked! The newer ports are even better.

Agree 100% our build was for a boat used in salt and fresh without closed cooling so we wanted to stick with iron heads. It was a budget thing add aluminum and Closed cooling and it was a significant cost increase

This is the cam I use CS XM 270HR-12 roller cam. .495 / .503 w/1.50 roller rockers. 218 /224 duration @.050. 112 Lobe separation This is in a 385 SBC 9.5 CR with World Products Cast Iron Heads. Stainless Marine exhaust with NO reversion problems. Idles at 750-800 RPM.
Hope this helps

Thanks for all the replies I'll try the xm270hr cam
I'm not looking to make it the fastest boat ( wouldnt be playing with small blocks ) just looking for a little more power and keep it reliable !

ryan I’m new to the boat world but building the same 355 now, where do I get the exhaust I need?