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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Gas > Atomic 4
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  #11  
Old 11-03-2011
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Roger that.

This is the 2nd site Ive been on in as many days where some people just like to argue. Dont answer questions directly . Ive been trying to find the owner of an fx-inshore mainsail and gotten feedback on who to vote for and how to name my children. Know what I mean? Your rundown on proper rebuild was spot on.
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  #12  
Old 11-03-2011
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It already makes 30 Hp at way to much RPM (3600) to have a long life and you burn a ton of fuel compared to going along at 5.5 knots and 1400 to 1500 RPM as we are pushing displacment hulls

The cam thing is pretty much a no go because any more aggressive grind will cause Water reversation which is and ongoing battle in high performance boats and even low performance motors with waterlift mufflers

The crank is NOT all that strong or well supported
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If a dirty bottom slows you down what do you think it does to your boat
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  #13  
Old 11-03-2011
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Quote:
Originally Posted by RobGallagher View Post
Nothing like a tranny with a good rear end.
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Old 11-03-2011
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blj,
I thought about that yesterday but thought I'd wait and see if someone else would take the bait.
If a tranny does not have a good rear end what does he/she have going for them?
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Old 11-03-2011
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Quote:
Originally Posted by tommays View Post
It already makes 30 Hp at way to much RPM (3600) to have a long life and you burn a ton of fuel compared to going along at 5.5 knots and 1400 to 1500 RPM as we are pushing displacment hulls
I agree, that's what I had in mind about the cam grind - possibly lowering the torque peak RPM. I'd imagine the stock cam is just a "bump stick" without a lot of sophisticated design behind it. A sharp cam grinder can really change the performance characteristics of an engine by playing with the lobe angles, the overlap, the advance or retard of the events etc. They can move the torque peak up and down the RPM range, sacrifice power for torque (which is what really matters in a boat) and so forth. It's not always just about "higher and longer" like it is in most high performance cam applications.

Quote:
The cam thing is pretty much a no go because any more aggressive grind will cause Water reversation which is and ongoing battle in high performance boats and even low performance motors with waterlift mufflers
I don't understand this - does it have something to do with the overlap of the intake and exhaust events?

Quote:
The crank is NOT all that strong or well supported
I've never seen the bottom end of an A4 - is it 3 main bearings?
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Old 11-03-2011
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blj,
I thought about that yesterday but thought I'd wait and see if someone else would take the bait.
If a tranny does not have a good rear end what does he/she have going for them?
Since we're talking hot rodding - a rigid front end?
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Only two bearings with four pistons in the middle


You are pretty limited on any marine inboard with wet exaust the A4 is not as bad because most have a good size dry section

Even the new big outboards have to be carefull about shutting down to drift fish in a big swell as water can reach the lower cylinders as the boat bobs up and down
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If a dirty bottom slows you down what do you think it does to your boat
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Last edited by tommays; 11-03-2011 at 07:44 PM.
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Old 11-04-2011
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Quote:
Originally Posted by Arpegecap View Post
Car engines are designed for light load. Pushing a boat under power is like driving a car uphill forever. That's why automotive engines are "marinized" using heavy duty truck engines. I'd stick with a real marine engine myself.
Prop it, gear it, and govern it. You can tweak the requirements into the entire driveline.

Lemme splain:

Electronically govern the RPM to slightly over half of it's max. Use a reduction transmission to cut the revs to a point where a good prop can make use of them. If you add the right prop into the mix, the engine won't be working excessively, but just enough to keep things tight. All you need is enough to push these bad boys up to hull speed into a moderate headwind.... we're not trying to win races here.
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Old 11-04-2011
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Quote:
Originally Posted by SloopJonB:793192

What I'm thinking of is this;

First, stripping it for a rebuild
Next, adding the Moyer stuff - electronic ignition, oil filtration etc.
Next, shaving the cylinder head and block to raise the compression to, say, a whopping 8.5 to 1.
Then, having the mains line honed.
Then, cleaning up the intake and exhaust ports with a carbide burr and die grinder (port & polish). Since it's a flathead, I expect relieving the area between the valves & combustion chamber may be necessary. Also, matching the intake & exhaust openings with their respective manifolds. Also as part of this process, checking and matching the volume of the combustion chambers.
Next, having the rotating assembly carefully balanced.
In other words, basic balancing and blueprinting.
Next, painting the inside of the engine - all non machined surfaces, with Dolphinite to promote rapid oil return to the pan.
Last, possibly having the cam reground to increase its lift & duration a little. A good cam grinder will know how to shape the lobes to maximize low RPM torque

I don't know what any of this means, but it sounds bad Ass.
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  #20  
Old 11-04-2011
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I don't know what any of this means, but it sounds bad Ass.
Yeah, I want make the nickname "Atomic Bomb" really MEAN something.
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