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  #31 (permalink)  
Old 10-30-2008, 04:14 PM
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bry,
the stock 2V carb and stock dual plane makes the most low end TQ because each intake stroke is only pulling from one venturi and half the intake area at WOT from idle rpms....

add back the divider wall only cures half the problem....

you are still trying to pull a vacuum from twice the venturi area with 2 of the 4 venturi's with a cam designed for the smaller single barrel charge per intake stroke....

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Last edited by red65mustang; 10-30-2008 at 04:24 PM.
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  #32 (permalink)  
Old 10-30-2008, 06:37 PM
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bry,
a correction....
the Edelbrock secondaries air flow is controlled by the counter weighted "velocity (dependent) valves" mounted above the secondary venturis....
so that when you do first give it instant WOT there is "some" Hg at the primaries "only" to make a decent atomized mix for it to start to wind up....
fair chance that it will start off ok with the divider wall added but then bog due to the cam and ign timing and the air valves opening speed....

it is possible to modify the carb weights and dist timing curve to have a better matched combo but you will basically have a bit less low end TQ than the 2V intake and "maybe" better top end speed by a bit with the divider added back in....

edit add:
fair to good chance that if you had bumped up the base timing just 2* and richened the 2V jets just a bit ,,,it would have preformed same as the 4V does now at mid rpms plus....

Last edited by red65mustang; 10-30-2008 at 06:55 PM.
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  #33 (permalink)  
Old 10-31-2008, 08:01 AM
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I think the Timing is now advanced some, I forget off hand this minute, but when I talked to edelbrock, they told me to do this. It did make soem difference, and that little 305, does make some great power for a small low torque engine.

I think it could have some potential. I wonder if I could get a stock cam, that Mercruiser would use for the 4bbl carbs. Would this make a difference, to complete the setup? Or if I were to get a cam, such as the Performer, to give it some more life? Just throwing out ideas, since you guys seem to be the Masters of Engines, that I have found! I wish that I had found this site along time ago, like just over a Year, before I had done these upgrades.

Also, is it possible to use JB Weld, Just asking, to put the divider back into the Intake? Or build some type of Sheet metal, pieces on either side, but then make sure it is sealed and not leaking.

Bryan
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Old 10-31-2008, 08:44 AM
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Not easily done

Restoring the plenum is best done by welding, as much as I repect JB weld. You are getting a lot of info from knowledgeable people. That being said, I hope it doesn't confuse you or frustrate you. I don't want to add to it. I will say this: higher velocity generally creates more bottom end. On a 305, this calls for a manifold with moderate runner volume in a 180-degree design and a carb like a Holley list 1850 600CFM with vacuum secondaries. I think it is safe to assume you are not cranking high rpms. Your needs are the same for someone wanting more out of an RV. Less is always better. Of course, in a boat, a prop has a lot to do with bottom end characteristics too. Everything is a tradeoff.
Pat
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Old 10-31-2008, 08:56 AM
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I just found Some of the Specs on my Engine from the Mercruiser Manual. This might help you guys to know what it is, and what is in it and the numbers.

http://www.karaoke-computers.com/bry....0%20Specs.xps

http://www.karaoke-computers.com/bry...0%20Specs2.xps

The pages will open, and show you in IE the PDF images that I got out of the manual.

Thanks,
Bryan
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Old 10-31-2008, 08:59 AM
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My Rpms will run up to 4800 now, so I am propped right. I have played with the props left and right and currently have a 4 blade 21 pitch, and I have also tried the 23 pitch same brand/4blade prop and it is to much pitch for the low end hole shot, where I need this extra torque.

As stated above somewhere, I have tweaked the setup, now just need to get some more juice out of the Engine, if I can. I am hoping the numbers above in the Spec sheets can give you guys some information.

Bryan
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  #37 (permalink)  
Old 10-31-2008, 09:12 AM
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Quote:
Originally Posted by curtis73
108 LSA is a huge no-no in an I/O, even with that small duration. It doesn't take much intake reversion to suck water back in. Even if it doesn't hydrolock, have you ever seen what happens to an 1800-degree exhaust valve when water hits it

I think you are over estimating the effects of overlap, especially with that little cam, which only has -20 overlap at 0.050" and +28 at advertised (240/248).

Compcam also list two other marine cams (inboard/outboard) on the same page at those cams have 40 and 48 degrees of overlap at advertise duration. 28 isn't even close to be able to suck water all the way up the exhaust pipe then into the manifold then into the head.
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  #38 (permalink)  
Old 10-31-2008, 11:32 AM
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Quote:
Originally Posted by 454C10
I think you are over estimating the effects of overlap, especially with that little cam, which only has -20 overlap at 0.050" and +28 at advertised (240/248).

Compcam also list two other marine cams (inboard/outboard) on the same page at those cams have 40 and 48 degrees of overlap at advertise duration. 28 isn't even close to be able to suck water all the way up the exhaust pipe then into the manifold then into the head.
Ok, I'll buy that on the little cam, but like I said in an earlier post... don't EVER use a cam manufacturer's recommendation on an I/O cam with wet exhaust. Comp in specific has two or three cams (like the ones you list) that have been proven over and over to almost instantly do serious damage to I/O engines because of misleading descriptions. If you call the tech line and ask for a cam application for I/O, not only will the recommend a cam with too much duration (which puts the torque peak higher than planing speed which can cause serious lugging) but they also recommend way too much overlap. If you ask them about water exhaust reversion they think you have a hole in your head. Comp in specific has no clue about marine cams. They don't even know what water reversion is.

So, just because Comp offers them, doesn't mean they're suitable for I/O use. Fine for Jets, not for props.

The other thing that you might be misperceiving is where its getting its water. It doesn't suck water all the way up the exhuast from the hub, it gets it from the riser on the manifold. The water jacket in the riser dumps water directly into the exhaust about 11" after the exhaust valve, and only about 3" past the turn. If it sucks water a few inches back, it will fall down toward the engine with sometimes catastrophic results. Its not about sucking water up from the lake, its about sucking a little bit back a mere 3".

Last edited by curtis73; 10-31-2008 at 11:38 AM.
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  #39 (permalink)  
Old 10-31-2008, 12:09 PM
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bry,
it take a HUGE increase in motor TQ to have a major change in out of the hole wind up performance.....
illustration with typical stock motors:
300 cubes at 2k=approx 255ft/lbs for out of the hole force on the prop
255/2,000lb boat=.1275 ft/lbs per lb of weight

350cubes=310 ft/lbs at 2k
310/2000lbs=.155 ft/lbs per lb of weight

.1275 x 120%=.153,,,,,the 50 cubes gained you 20%+ more prop twisting force to overcome the water drag on the hull and boat weight to wind up quicker and get it up on plane....

I messed with my DD just a bit (I can't do a true marine motor) 4V dual plane and 4V cam gains you 4ft/lbs at 2k and 9ft/lbs at the TQ peak if everything is tweeked with a 8.5CR....

mount a 2 speed powerglide tranny in it for out of the hole TQ with the 305...
255ft/lbs at the flywheel x 2.46 low gear =627 ft/lbs of force on the prop!!!!

or gut every last dead weight pound possible out of the boat to increase the power to weight ratio
255ft/lbs/1500 lb boat weight =.170

or go to a yard and buy a compression tested low miles 350 dirt cheap to use for a long block to build on....

edit add:
re: the merc 4V cam, the majority of the time a stock 4V combo will have a stock higher CR as well....
the added CR helps keep most of the low rpms TQ with the more cam duration for more top end HP....
here's a dyno article example to illustrate what happens to the TQ if you don't increase the CR just for the idea.....
(a stock 2V and a stock 4V cam are pretty close for duration)

http://www.hotrod.com/techarticles/c...son/index.html

Last edited by red65mustang; 10-31-2008 at 01:41 PM.
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  #40 (permalink)  
Old 10-31-2008, 12:26 PM
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how much overlap do you think is tolerable for a marine engine in his setup?

I like the short duration cam (192/200 at 0.050) for 305 cid torque and the slightly narrower lsa for more cylinder pressure (early closing intake valve). Plus the narrower lsa would also help stock smogger heads fill the cylinders better. Finally the longer exhaust duration will help push more air out those marine manifolds.
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  #41 (permalink)  
Old 10-31-2008, 03:29 PM
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Quote:
Originally Posted by 454C10
how much overlap do you think is tolerable for a marine engine in his setup?

I like the short duration cam (192/200 at 0.050) for 305 cid torque and the slightly narrower lsa for more cylinder pressure (early closing intake valve). Plus the narrower lsa would also help stock smogger heads fill the cylinders better. Finally the longer exhaust duration will help push more air out those marine manifolds.
Depending on which manifolds he has, they'll flow tons better than you think. Log style manifolds suck, but newer center-riser exhausts have very large, separated passages with a 4" riser. They act more like 2" shortie headers with a huge collector. Plus the cooling factor of the jacketed manifolds further reduces the volume of exhuast.

Part of the reason for large marine exhausts is so that a backfire or reversion won't fill the whole exhuast with water from the hub. They are designed with a certain volume in mind based on the draft of the hull and the displacement of the engine so that they can't physically suck water in unless they make one complete reverse revolution.

The other reason is that I/O marine engines are designed much like diesels; very little overlap, so scavenging is not as much of an issue. An I/O on plane will actually get a pretty strong suction from the hub, so too much overlap can reduce power; the suction at the hub will actually cause too much scavenging and suck raw intake charge right out with it.

So, while idling and slow cruising, cam selection can be thought of much like an automotive engine that needs to not suck in water. On plane, suction of the hub exhaust can make cam selection a bit more tricky since its acting almost like a "reverse turbo." Too much cam can reduce top end power.

Most stock-cammed I/Os will benefit from keeping the exhaust bellows in place to use that suction at the hub. If you step up one or two cams with a bit more overlap, removing the bellows can help it act more like an automotive application where exhaust scavenging is natural. Many boaters with stock cams note reduced output when they remove the bellows, but with an aftermarket cam they notice increased output because the extra overlap isn't being super-scavenged by the hub exhaust.

On a 305 I'd stick to no more than -9 overlap at .050" on center risers, and more like -11 or -12 on log manifolds. The cam you listed should probably fit that bill. The factory Mercruiser roller cam comes in at -17.5, the flat weighs in at -13. But, the OP is also concerned about low end torque, so I think he can find what he needs while staying far away from too much overlap. We're discussing how much cam you can logically use, and he's in small-duration territory.

So, while I completely agree with your assessment of how the engine itself makes power with overlap, an I/O is a teeny bit different given its unique exhaust configuration. I have one I/O with a comp marine cam and I removed the bellows. It still needs to be designed with reversion in mind, but as far as peak WOT power I have a feeling it might make a few more ponies without it. My father's stock-cammed I/O has the bellows in place and probably benefits from it. Plus, nothing sounds quite like a V8 in a boat with the bellows removed; silky quiet idle, but then it just whines when you open it up.

Last edited by curtis73; 10-31-2008 at 03:40 PM.
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  #42 (permalink)  
Old 10-31-2008, 03:51 PM
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I do have the 4 inch Risers with Straight 4 Inch pipes out the Transom. There is no switchable exhaust and the Thru Prop Y pipe was blocked off when I had purchased the boat. So this is no longer there or available.

Here is a Pic of the engine.



Here are the Exhaust Pipes.



Here is a Picture of the Boat all Shined up.


Just so you can see what I have and the weight is about 2900lbs dry. It has a 50 Gallon Gas tank, and carries lots of beer lol.

Bryan
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  #43 (permalink)  
Old 10-31-2008, 07:03 PM
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Not to state the obvious, but not only is that K&N an explosion waiting to happen, its a ticket and boat confiscation waiting to happen.

Get a USCG spark arrestor on there, man. We want you to stick around in one piece.
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Old 11-03-2008, 10:24 AM
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Curtis,
Thanks for the Advice and watching out for me, but K&N does make USCG aproved Flame Arrestors. This is the one that I have, it states on it that is it aproved by the coast guard and the number.

http://store.summitracing.com/partde...5&autoview=sku

I know about not blowing myself up for sure! This is why I did all the items with USCG aproved items, nothing in my engine compartment, is not Marine grade and USCG approved! I will be here for a while, and that surely would not be the reason why I would not.


Bryan
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Old 11-03-2008, 10:49 AM
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Quote:
Originally Posted by bry21317
Curtis,
Thanks for the Advice and watching out for me, but K&N does make USCG aproved Flame Arrestors. This is the one that I have, it states on it that is it aproved by the coast guard and the number.

http://store.summitracing.com/partde...5&autoview=sku

Bryan
Awesome... I didn't know K&N made those! Thanks for the edumacation
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