Cam vs Exh Manifold vs Reversion
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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
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
#3
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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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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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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
http://www.youtube.com/watch?v=M5rWO2qL6WQ
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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!!!)
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!!!)
#7
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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
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
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
#8
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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
Joby
Last edited by formula 382 sr-1; 11-10-2008 at 05:12 PM.
#10
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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.