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Cam vs Exh Manifold vs Reversion

Old 11-09-2008, 09:17 PM
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Default Cam vs Exh Manifold vs Reversion

Looking for some feedback regarding exhaust manifolds and water reversion. I'm building an engine (de-stroking a SBC 400 to a 377) for an aluminum jet boat. Worked with Comp Cams to spec out a cam for my application, and they came back with "Grind # CS XR282HR-10". Being relatively new to custom engines for jet boats, I'm wondering at what point you need to worry about water reversion back into the cylinders. Thought I'd throw my cam specs out here, and see what kind of feedback I got from you folks -- do I need to worry much about reversion running this cam, and if so, what kind of exhaust manifold recommendations would you make? Thanks in advance!

Grind # CS XR282HR-10
Lift: Int = .510” Exh = .520”
Duration @ .006: Int = 282 degrees Exh = 288 degrees
Duration @ .050: Int = 230 Exh = 236
Lobe lift: Int = .340” Exh = .347”
Lobe separation = 110 degrees
Valve timing @ .006: Int open at 35 btdc, Int close at 67 abdc
Exh open at 78 bbdc, Exh close at 30 atdc
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Old 11-10-2008, 06:45 AM
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The 110 lobe separation may be the issue as most of the "marine" cams run 114 and some 112, the lower the # the more prone to revision...Rob
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Old 11-10-2008, 06:51 AM
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I would have to assume if you are de-stroking this engine, you plan to run it at high RPM's? If you plan on running a 110 lobe separation you better have an exhaust that dumps the water further down stream. Years ago I ran a Comp Cam with a 110 lob separation and had troubles with idle. I will admit it ran good on the top end. I changed it to a Crane cam with a 112 and lost 300 rpm.

Last edited by 1BIGJIM; 11-10-2008 at 09:38 AM.
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Old 11-10-2008, 07:36 AM
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Were you planning a set of high performance exhaust manifolds with your new build? If so a set of dry exhaust ( water jacketed but not injecting water into the exhaust stream) would enable you to run any cam you want.
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Old 11-10-2008, 10:35 AM
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With Jet boats your set up is going to greatly influence your cam & engine selection. Let us know some more details, what type of jet, what boat, weight etc.. A little planning ahead of time can really pay off. I've worked on and built commercial, pleasure, and race boats for years and can probably help you out. Feel free to email if you have any questions. [email protected]

http://www.youtube.com/watch?v=M5rWO2qL6WQ
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Old 11-10-2008, 01:34 PM
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Thanks for the feedback. Will try to answer all the questions in one post.

Regarding intended operation -- I chose to de-stroke the 400 less for high rpm reasons, and more for durability/longevity under sustained load, ability to run a much better rod:stroke ratio and high compression ratio, a little better fuel economy,... Am building the engine to get peak torque around 4500 rpm, and will select impellers to give me a WOT around 5000 rpm. Boat should be able to hop onto plane at 2700 rpm, and cruise at 3200, giving me almost 2000 rpm to play with.

Regarding a "high performance dry exhaust" -- my goal was to NOT go that route. Would like to stay with a traditional wet exhaust like a GLM, Kodiak, ... and then fab extensions (if needed) to introduce the water to the exhaust further downstream. By staying with relatively moderate duration specs on the cam (230 intake and 236 exhaust) I was hoping to limit the amount of valve overlap, even with the shorter lobe separation, and not have to worry excessively over reversion.

Regarding boat specs -- here they are:
Hull is 21' in length, 6.5' bottom, with a 11 degree deadrise and delta pad. Dry weight should be around 2800# - 3000#.
Pump will be a new 2-stage Scott, set up for whitewater and aerated water conditions. Intended use is a "do everything" recreation boat capable of whitewater (class 3 and 4), shallow water (5-6"), fishing, and skiing. Key in doing that is keeping it light with decent HP/TQ (hence the SBC rather than BBC). My engine has AFR 195 heads, a Callies crank, 10.0:1 compression ratio, and unless I get convinced otherwise, the above listed cam from Comp Cams. Based on calcs from both my builder and Comp Cams, engine should pump out 450-475 hp, and similar numbers for torque.

I feel pretty comfortable with the path we're going down. I just don't have any practical experience to draw from regarding different cam specs and their impact on reversion, so I appreciate any feedback you can give. Thanks again!

(By the way -- great forum!!!)
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Old 11-10-2008, 02:44 PM
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Originally Posted by maverick2
Looking for some feedback regarding exhaust manifolds and water reversion. I'm building an engine (de-stroking a SBC 400 to a 377) for an aluminum jet boat. Worked with Comp Cams to spec out a cam for my application, and they came back with "Grind # CS XR282HR-10". Being relatively new to custom engines for jet boats, I'm wondering at what point you need to worry about water reversion back into the cylinders. Thought I'd throw my cam specs out here, and see what kind of feedback I got from you folks -- do I need to worry much about reversion running this cam, and if so, what kind of exhaust manifold recommendations would you make? Thanks in advance!

Grind # CS XR282HR-10
Lift: Int = .510” Exh = .520”
Duration @ .006: Int = 282 degrees Exh = 288 degrees
Duration @ .050: Int = 230 Exh = 236
Lobe lift: Int = .340” Exh = .347”
Lobe separation = 110 degrees
Valve timing @ .006: Int open at 35 btdc, Int close at 67 abdc
Exh open at 78 bbdc, Exh close at 30 atdc
While lobe separation plays a role in this, the duration of valve over lap is the real culprit in reversion. Most commonly you would run at least a 112 lobe separation. The 110 narrows the torque curve dramatically. But all of this is really null and void until you pick an exhaust system. Water jacketed headers would accommodate the camshaft you have chosen while almost all water cooled manifold systems (except restrictive log styles) will have reversion by .230 @.050

So if your going to try and by a manifold and riser type system make sure the people you buy it from understand your cam spec's and will guarantee "NO REVERSION". The only way we could do such a thing is with a custom long style riser. In your application this would be custom because the down slope is steeper than say stern drive style exhaust systems.

I'm sorry to say while we handle I think most every brand of exhaust from Stainless Marine, Imco, Hardin, SS, Lightning, Dana. The only way to truly be safe at these cam spec's is with a tubular header. Remember this is merely a suggestion.

Last edited by CPPerformance; 11-10-2008 at 02:52 PM. Reason: typo
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Old 11-10-2008, 05:10 PM
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We ran a Bullet Racing Cam and Stainless Marine Exhaust in our 383's with on problems. The cam specs CHS 288/296-10HR, [email protected] Int 223 Exh 231, Lift Int .530 Exh .548 @110 CenterLine. They made 425 & 427hp on the Dyno, I hope this helps

Joby
Attached Thumbnails Cam vs Exh Manifold vs Reversion-img_1997.jpg   Cam vs Exh Manifold vs Reversion-img_9842.jpg   Cam vs Exh Manifold vs Reversion-img_2301.jpg  


Last edited by formula 382 sr-1; 11-10-2008 at 05:12 PM.
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Old 11-10-2008, 05:18 PM
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Thanks. Nice pix -- great looking installation!
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Old 11-11-2008, 08:14 AM
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It seems like GLM or Kodiak manifolds would have an adverse effect on hp numbers. Like running cast iron oem exhaust manifolds on a drag race car. There are better choices.
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