My 350 build...Advice needed!

[nextboat1]

Hi all!
My purpose for starting this topic is two fold. I would like to chronicle my 350 build and gain knowledge from of all my fellow IBOATers. To start, the engine is being built for my 1998 Rinker 232 Captiva Bowrider. It is equipped with an Alpha Gen II drive (1.47) and weighs in at just shy of 4100lbs. The engine I plan to use is a late 70's or early 80's (been awhile since I pulled the casting #) 350 four bolt main block. I originally had this engine built for another of my projects, but it has sat on the stand since and is ready to find a more suitable home. The crank is stock and has been machined and balanced with new bearings. It has a double roller timing set, and .030 over speed pro hypereutectic flat top pistons with four valve reliefs for a total of 5cc's. The freeze plugs are brass. I am curious as to what cam and heads to run on this bottom end set up. The cam currently installed is way to aggressive for a marine application. My goal is a modest 325 horsepower or so, as I do not want to push my alpha drive too hard. I would also like to be able to run on 87 octane gas, as that is what is available around here at the local marinas. My plan is to build a solid reliable engine, rather than a high horsepower money sucker. I realize that the flat tops will limit my options, but would rather not change them out if not needed. Any advice will be greatly appreciated.

 

[alldodge]

What cam do you have in it now?

 

[Fishermark]

While I do not have a specific recommendation for the cam - a stock cam from a truck is basically what you want. Something that builds torque in the low rpm range.

The heads are easy - go with Vortec Heads from GM. Pace Chevy and others sell them pretty inexpensive. Then get an intake to match. A marine Edlebrock carb and you are good to go!

 

[nextboat1]

The current cam is a comp cam 12-523-5 circle track cam, as the engine was orginally being built for a pure street stock car. I thought about vortec heads, but with the flat top pistons my compression would be too high to run on 87.

 

[Tail_Gunner]

That cam is a flat tappet???? Use the vortec head they can be used well to about 9/1,set you quench to about .040....a good cam would be a comp cam xm270hr @ 112lsa..that's about 6 degrees of overlap @ .050...ls6 springs...there that should start the conversation off a bit...

By the way the useable powerband begin's at about 1400 /1600 rpm that's the point where the bow just begin's to create at pressure wave with a 1.5 ratio that's about 7 mph...There the pots stirred well/

 

[nextboat1]

Its a mechanical .470 lift rule flat tappet cam...I used an online static compression ratio calculator and came up with around a 10.15:1 compression ratio with my current pistons, 64cc vortec heads and a .040 quench. I think I would have to use a set of heads with larger chamber volumes in order to get he compression down enough to run 87 octane. I looked at the cam you reccomended and thought that it may be a good fit as well. Any other thoughts on heads other than vortecs?

 

[keith2k455]

For the heads, maybe you should call up someone like Summit or Edelbrock and discuss with them. I'd bet discussion with one of the technical people from these outfits could help you. If you don't like Summit or Edelbrock, try Jegs or anyone else.

I also thought alphas were recommended for less than 300hp, your numbers put you a little over this if I recall what I have seen correctly.

 

[alldodge]

So your pistons come up equal to the deck height, you have no space?

 

[nextboat1]

Sorry All dodge, i was typing while at work and apparently not thinking. The block was decked. Piston to deck height is at .015so compression would be a bit lower

 

[gm280]

Call Comp Cams and talk with one of their very knowledgeable techs. They will absolutely be the best folks to guide you what you need for your cam choice AND all the other parts needed as well. And Comp Cams are extremely good cams with equally great reputation. JMHO!

 

[alldodge]

Sorry All dodge, i was typing while at work and apparently not thinking. The block was decked. Piston to deck height is at .015so compression would be a bit lower

With that, your compression is coming out to 9.9 using a standard .041 gasket your quench is at 56, bit high. Decreasing the head gasket thickness will reduce quench but compression will go up. I see no way around getting dished pistons and have the piston height increased.

 

[nextboat1]

I had a thought...what if I had the block decked to say .005" piston to deck height, put on a set of aftermarket cast iron 72cc heads, and ran a .041" head gasket. That would put me at about a 9.39 compression ratio with a .046" quench. I know the compression would still be a bit high, but much more managable. Any thoughts?

Thanks guys for all your help so far!

 

[alldodge]

Your get closer, but since your working on the edge you also need to add in the oil film of .003, with that it will bring you to .049. That said there isn't any pistons which don't have variances, so need to check height of individual pistons with a micrometer.

Decking the block to .005 also brings in another variable as to the intake gasket thickness.

Assuming steel rods and a .005 deck height, your going to need to make sure the pistons have clearance for stretch in higher rpm so they don't come into contact with head gasket or head, doesn't give you much room

 

[Bondo]

I had a thought...what if I had the block decked to say .005" piston to deck height,

Ayuh,.... If yer gonna tear it down to deck it,...
Deck it to 0.00"
Install 12cc or 18cc inverted dome pistons, 'n Vortec heads,....

Vortec heads, with matchin' pistons can run nearly 10:1 without detonation, 'n make awesome power,....

 

[flipbro]

Bondo has a great suggestion that would build real good torque. But if you want to run the 72cc heads no need to tear the block down to deck it. Just run a thinner head gasket. The tighter the quench the more resistants to detonation try to aim for .042. Then a little head work and a nice cam would also produce good results

 

[DeepBlue2010]

Does "inverted dome" mean dish pistons?

 

[Bondo]

Does "inverted dome" mean dish pistons?

Ayuh,... The dish mirrors the head, for quench,.... a D dish,....

 

[NHGuy]

I have been doing the same research for a 383. My Fomula 223LS is over 4000 lbs too. It's 350 isn't enough for higher speeds.
I bought a never used short block last month thinking I would do d dish pistons and normal heads. But in my research I have found most heart chamber heads are vortec efficient.

Go to uempistons.com and learn to use their static compression calculator. Don't put too much value in their dynamic calculator, it uses an assumption at the end that makes it less precise.

Dynamic (actual running) compression calculates using intake valve closing to piston top dead center. It's real.

For a more precise dynamic value here's the formula...

• Divide the intake duration by 2
• Add the results to the lobe separation angle (LSA)
• Subtract any ground-in advance
• Subtract 180

Now take this image and work out your compression to octane

For our needs I ended up printing the page and using a straight edge to extend the graph to the left and down.


My boat has a 140 degree thermostat and runs around 150, so I calculated the 160 to 150 degree realm to leave a cushion.

Boats run at maximum load, but cool temps. So you'll want a low ish maximum advance.

You'll be smart to final tune it with a dynamometer run. Not only do you get the cool stuff of a torque and hp graph. But more importantly, you set the air fuel ratio and timing to maximize power and eliminate detonation.

I also have flat tops in my short block.

I have found those or d dish pistons with a large chamber head, .040 quench distance, and a dynamic compression at or below 7.5 to one are going to be my goals. If I can't get it below 7.5 I might have to run mid grade or premium. But that's less money out of pocket than a set of pistons.
Check out 72 or 76cc heads for the flat tops, and get that piston to head quench distance down to .040 or .045. It pays off with efficient combustion and cooling.

I am eyeing Dart aluminum heads and maybe a Comp cam. If there's enough money left I will get performance exhaust and a custom cam from Bob Madera. He's a major guru for this stuff. He can be contacted via offshore only. He can sell you the heads and stuff at competitive prices. I'd give him a call once you have some of this stuff straight in your thoughts.


Sorry for all the detail, but it seems to all matter.

Read up on quench, maybe at hotrodders.com or jalopy journal, or offshoreonly. Good stuff for a cold winters night "bench racing".

 

[alldodge]

Agree Bob is the guy when it comes to cams, and will discuss at length your issues. You can find his website and contact info by looking up Marine Kinetics in Rochester NY.

As for aluminum heads they are great and can be easily modified if needed, but they are not good for longevity unless in a closed cooling system.

 

[NHGuy]

Oops, I went back and edited. That description I called the dynamic compression was the actually the intake closing point. Plug that in to the appropriate spot in a dynamic compression calculation and voila. You get real numbers.