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Old 10-01-2007, 01:17 PM
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Marine 383 build

I have pretty much decided on everything but the cam for my MARINE 383 build. No Parts ordered yet.
The block is at the machinists getting worked on.
Parts.
Enginequest Heads 200cc and 72cc combustion chamber.
SB Chevy 383 Mahle -16cc Dish Top Pistons or equivalent.
Scat 383 4340 Standard Weight Crank - 3.750" Stroke - 6" Rods.
Scat 4340 Forged Pro Stock SBC 6.000" I-Beam Rods - 7/16" Cap Screw.
1.5 roller rockers
Roller retrofit lifters.
Q jet marine carb
Clevite 77 bearings
etc etc.
Static compression of 9.1:1
quench .040"

I am looking at two marine hydraulic roller cams from comp cams.
1) CS XM 270HR-12
Duration At 0.05 218 224
Gross Valve Lift 0.495 0.503
LSA 112*
2) CS XM 264HR-12
Duration At 0.05 212 218
Gross Valve Lift 0.488 0.495
LSA 112*

Low speed idle is important. I want an engine that idles and runs quiet but still gets up and goes. I think I can get that with the 264 but will I give up too much not going with the 270, which I understand idles well in a 383 as compared to a 350?

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Old 10-01-2007, 01:30 PM
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Either of those cams will require dry exhaust. There is too much overlap for wet exhaust and reversion will suck water back in the valve. The 264 will give you a smooth idle, the 270 won't, but both are too big for wet exhaust.

I also suggest against the heads. Boat engines need to make all of their power as low as possible. As long as you have a 4-bolt main, all forged components and you're basically making a race engine, then 6000 is OK on a 383 for short periods. Otherwise, no more than 5000 on a cast or 2-bolt bottom end. You need heads that will match your peak RPM, and those heads will be good for more like 6500 rpm. If you want a smooth idle, you're never going to see even close to that.

What is the boat? weight? sterndrive, jet, vee drive? Outdrive ratio? I used to know all of that, but I forgot Lots to consider.

By the way... welcome to our little forums Good to see you on the "dark" side. You'll find some good marine guys here, but mostly automotive stuff. I can't wait until you get that boat running. I want a ride
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Old 10-01-2007, 02:02 PM
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It's funny, you see many of the same names on all of the performance forums .
Anyway. It is going on a small 18 foot deep V with a volvo 280 leg 1.61 ratio. I want to go through the leg for exhaust(quiet) and have been told that those cams will not give me reversion issues. I have not called comp personally though. You obviously have a differing opinion.
I was waffling on the heads, however there I get differing opinions as well.
I have not bought anything yet as i want to be sure I have components that match and will give me what I need.
The block is a 4 bolt being bored .030 over.
Here is a pic of the boat before total restoration (yellowish one), but it gives you an idea of what i am working with. Should really get up and go with a 383. Currently has a very weakly built 350.
camshaft specs -
intake duration is 202 @ 50
exhaust duration is 212 @ 50
intake valvelift is .393
exhaust valvelift is .408
110 degree lobe seperation
valve specs - 1.94 (intake) + 1.50 (exhaust)
compression ratio - 8:1
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Old 10-01-2007, 03:20 PM
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Ah, yeah... I remember now.

The 262 is listed on comp's site as "largest cam for I/O" but they say that because of low end torque requirements for larger cruisers. I called them for my lighter (well, relatively) 19' baja and they recommended the 270. I said won't that have water reversion issues? There was a long pause. Not only did I have to explain water reversion to Comp's tech guy, I had to explain that reversion doesn't just go back in the intake, it sucks in the exhaust as well.

I figured if I had to explain a simple thing like reversion to a comp tech I couldn't really trust them.

I would say a cam of no more than -9* overlap (@ .050) will be fine. The 262 fits that bill barely, but it won't exactly give you a low idle. Pretty much anything with exhaust through the leg will be quiet at idle.

I was confused when we were talking about the 264 cam which is on a 110 LSA. The 262 marine is on a 112. Too... many... numbers... in head... must .... explode
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Old 10-01-2007, 04:00 PM
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"Too... many... numbers... in head... must .... explode "
Know what ya mean. I have been in discussions with people at some of the offshore and marine boards, same places you were when you were researching, as well as working with my machinist who has built a number of marine 383s. He is also and engine builder, but is getting close to retirement From what I learned from them and in discussions off line I found that the heads with the 200 cc runners were correct for what i want to do. The newer technology is making this work well. The two comp cams listed are marine hydraulic roller and the XM 270 HR is listed as the largest for an outboard. That is assuming a 350. By going to the 383 you can run a little larger cam and get the same results.
Anyway I am throwing in the towel with respect to cam selection and am going to work with a reputable cam grinder to get what I need that will work with my exhaust. Let you know how that works.
In the meantime any more opinions etc.
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Old 10-01-2007, 07:46 PM
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That's probably wise. Most of what I know I just learned last spring as you helped out, so maybe we need to increase the knowledge pool

Here you are trying to find out diversified info at a different forum and you get the same damn guy
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Old 10-01-2007, 08:21 PM
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Quote:
Originally Posted by curtis73
That's probably wise. Most of what I know I just learned last spring as you helped out, so maybe we need to increase the knowledge pool

Here you are trying to find out diversified info at a different forum and you get the same damn guy
Yea and this guy doesn't know very much.
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Old 10-02-2007, 06:47 AM
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I maybe off some here, but would the ZZ4 roller camshaft work? It has 208/221 duration @ .050 .474 .510 lift on 112 centers, there`s always several of these cams on ebay at giveaway prices.
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Old 10-02-2007, 08:00 AM
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Wouldn't vortec heads give you the needed low rpm torque a marine engine needs.
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Old 10-02-2007, 09:10 AM
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The problem with vortec heads is that their small 64cc combustion chambers mean that you have to get a dish of about 24cc to get the 9to1 or so cr that you can run on pump gas found at most marinas. To get that dish in a D dish configuration using 3.75 stroke, 6" rods would also require a compression distance of 1.125. This is with a .040 quench. Very difficult to find and that is getting to quite a large dish for a non forced induction engine.
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Old 06-15-2008, 04:02 AM
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Hi - I been lookin gat 383 engines, cams and exhausts. My discussions with both enginebuilders and exhuast companies tell me that you need t obe over 230 degrees duration to get water reversion. I hope they are right as the engine cam combo we are getting is jut under that. Its suppoesed to deliver 405 hp @450ft lbs torque s owe are lokking forward to that. I iwll make sure the exhust flaps are fully functional thoug has recomended by GLM marine manifolds. Good luck!
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Old 06-15-2008, 04:03 AM
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Hi - I been looking at 383 engines, cams and exhausts. My discussions with both engine builders and exhuast companies tell me that you need to be over 230 degrees duration to get water reversion. I hope they are right as the engine cam combo we are getting is jut under that. Its suppoesed to deliver 405 hp @450ft lbs torque so we are looking forward to that. I will make sure the exhust flaps are fully functional though has recomended by GLM marine manifolds. Good luck!
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Old 06-15-2008, 09:36 AM
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Quote:
I figured if I had to explain a simple thing like reversion to a comp tech I couldn't really trust them
ZACKLEE!!! Prolly some kid looking at a monitor full of FAQ's giving you answers he thinks you want to hear LOL


Who is your cam grinder?
Quote:
Its supposed to deliver 405 hp @450ft lbs torque
Those numbers sound good for a boat engine. For any boat, TQ is king, making it in the low end and mid range is what works. I know a guy, John, who held some water speed records and he now messes with and makes silk purses out of sows ears restoring and finishing old cars, all serious street rides. Time he's done they are beautiful and wind up with a new mill. He must still know the boat guys because every now and again one of his beauties comes around with some serious engine, I suspect he's still using marine cams in some of his street rides.
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Old 06-15-2008, 10:51 AM
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What kind of boat are we talking about? Helps if I read I see now. With 9.1 static comp ratio I'd lean toward the larger cam you have listed on the first post to keep the dynamic compression in check. A marine engine operates under a load all the time, to much compression and you will have to run some kind of high octane fuel to keep the detonation away, and depending on your exhaust you might not hear it rattle until it was too late. I ran a 232/236 @.050 cam in my jet boat with a 350 for years and didn't have any reversion problems, and that's with water injected headers. I just put a 383 in it with specs and parts similar to what you are doing, used a hydraulic roller cam with 230/236 duration and 510/520 lift and haven't seen had any problems. If you have an outdrive will it be able to handle the additional power?
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Old 06-15-2008, 09:19 PM
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This is the cam i would use in your marine 383.

http://www.cranecams.com/?show=brows...tType=camshaft
You won't have problems with water reversion or low speed docking or getting up on plane ( low/mid torque) etc with this cam.
You cr is a bit modest. Mercruiser 350's and 383's are 9.3 to 9.4:1 (91 octane marina fuel with knock sensor)

A 170cc vortec head like the RHS 906 vortec or GM small port Bow tie vortec would have been better but you'll be fine using the 200cc Engine Quest heads.

Your stock Marine Qjet will need upgrading and rejetting.
Replace the inlet needle and seat with the Edelbrock .149" high flow needle and seat ***. use a high flow fuel filter and consider a high flow Marine grade fuel pump upgrade.
You can rejet the Qjet useing Edelbrock Qjet rods, jets and hangers.
I highly recomend a dyno test to dial in and jet the Qjet before running this motor on the water. (you can use GM high flow 2.5" Corvette center dump exhaust manifolds to simulate the marine exhaust manifolds for the dyno test. or old marine exhaust manifolds run dry on the dyno) Long tube typical dyno headers will give a different result.
With your total airflow and power increase mods (heads, cam etc), the stock marine Qjet will very likely be lean at WOT, without recalibration. Once recalibrated and upgraded (high flow needle and seat) it will be fine.

Use a Holley #4779 750DP with stock 70-80 jetting as a baseline for your initial dyno testing. Then dial in the Qjet as required.

You should be using a 1.5:1 ratio leg. You may have problems propping the boat properly with a 1.6:1 ratio leg.
The dyno test will tell you where peak hp rpm actually is so you can know the WOT RPM point of maximum power in order to select/dial in the prop pitch required. You will need to play with the prop selection a bit.

You're, right most of the cam company tech line reps have no clue about marine engines or wet exhaust and water reversion.
Use a Dual plane hi rise intake manifold.
Mercruiser and GMPP have cast iron hi rise intake manifolds for Qjet carb flange.
and Edelbrock has a Performer RPM manifold with a Qjet carb flange
Any/either of these will work very well.
If you are using the stock Volvo penta ignition system, the built in rev limiter may kick in before you reach peak power rpm on your non stock engine.
Some stock non race version marine ignitions built in rev limiters (like Mercruiser's Thunderbolt ignition) are quite low (4800rpm)
Your engine should want 5200-5800rpm for maximum horsepower/ top speed performance on the water. The dyno test will tell you the engine rpm to target. Radar/ GPS speed testing on the water will tell you which prop is best.

Last edited by F-BIRD'88; 06-15-2008 at 09:50 PM.
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