Re: What makes a marine engine a MARINE engine
Hmm mayble I can help explain this situation from both sides....
First off let's define reversion, someone step in and correct me if I'm wrong here.
Reversion is when water enters the combustion chamber through the exhaust valve. This can happen in two ways. #1. From the water level being higher than the riser, causing the exhaust manifolds to fill with water, example: beaching you're boat and having you're stern go down. #2.Through the voodoo science* of acoustic and harmonic dynamics, water is drawn into the manifold by a temporary vacuum.
I say voodoo science because unless someone here has a PHD in harmonics and acoustics, you don't understand what exactly is going on in the exhaust system.
Okay 270WIN I want you to visualize something. Think of a mercruiser riser. Think about the how slightly the downward angle is as the water and exhaust merge. Think about how small of a role gravity has in pulling the water down and keeping it from going up-stream into the manifold. The merging point is almost level, in fact at some points, during decel for example, it's leaning in the direction that WOULD cause water to enter the exhaust.
Now 270WIN I want to talk about camshaft specs. Various manufactures have various metrics for rating camshafts.
For example, if a camshaft is rated at 270 degrees of intake duration, at what point (lift) do they consider the valve open?
Some companies rate their cams duration from fully closed to fully closed. Others rate them from a given lift to a given lift.
So if a cam says it has no overlap. It could very well mean that it has no overlap over .150 lift. but may have 50 degrees of over-lap BELOW this lift.
You follow?
Here's another thought for you. you're signature shows that you have a lot of old american V8 experience. Answer me this: What is the science behind the H-pipe?
Why is it if you take a 302 mustang, or a 350 camero with dual exhaust that you will ALWAYS have more power output with an H pipe than without?
American v8 exhaust tuners always reference the "voodoo" science behind the h pipe.
It actually has to do with the firing order of a north american v8. In order to get their engines to run smoother, they have essentially the "wrong" firing order. This leads to all kinds of weird acoustic and harmonic events taking place in the exhaust system. These events also vary greatly with RPMS.
Think about it this way. Does a 302 mustang have a hot cam from the factory? 1988 5.0LX. 190 HP out of a 302? Limited valve-train from the factory? I think we can all agree that as far as cam's go. the 88 stangs wasn't that hot from the factory.
If you had two exhaust systems. One 3" dual independent, and one 2" with an H-pipe. The H-pipe system would make MORE power.
But how can this be? Why does linking the left bank to the right bank make MORE power?
Would it not make more sense to have the BIGGER pipe and hence less back pressure and more flow?
The reason is, the H pipe uses vacuum pulses from one side of the engine to pull exhaust out the other side.
And these vacuum pulses have to travel through 3-4 FEET of exhaust piping and they STILL have an effect.
The water and exhaust merge on a mercruiser engine not even 12 inches down stream from the exhaust valve itself.
If you want more evidence of the "wrong" firing order, consider this.
Italian V8's (Ferrari, etc) DO use the correct firing order. GO lookup the firing order for a small block chev, then go look up the firing order for a ferrari v8.
Now compare two engines with the same dispalcement. Both v8's and listen to the sound of their exhausts.
I understand there are other factors, but there is NOTHING you could do to a 305 or 350 small block chev to make it SOUND like a 5-6 L ferrari motor. The reason is the firing order (Among other things of course)
8 7
6 5
4 3
2 1
Small block Chev Firing order
1-8-4-3-6-5-7-2
RIGHT - LEFT- LEFT - RIGHT -LEFT -RIGHT - RIGHT - LEFT
This firing order is bad for performance and exhaust pulses, but GOOD for balance.
ferrari firing order
1-5-3-7-4-8-2-6
RIGHT - RIGHT - RIGHT - RIGHT - LEFT - LEFT - LEFT - LEFT
As you can imagine, there is a HUGE difference in flow through these systems.
And these differences are all different at different RPMS.
So, once you factor ALL those things in.
The fact that the stock mercruiser exhaust is VERY susceptible to reversion.
The fact that cam manufacturers don't show the TRUE overlap in their specs.
And the fact that the chev small block's exhaust system is extremely turbulent by design.
There is VERY VERY little "wiggle room" in cam design over stock. All the GOOD cam manufactures (summit for example) will have SPECIFIC cam grinds for marine. And they will show WHICH cams are at the harry limit of marine ability.
And a cam profile that doesn't suck up water 99% of the time is no good. Because then 1% of the time, you're stranded and you're toasted you're engine.
Boats need a cam profile that won't suck up water: at Idle, at cruise at wot, at 2000 rpms, at 5000 rpms, in rough water, in calm water, on Mondays, in july... when the captain is wearing a blue shirt.... you get my point.
So people are getting mad at you, like I did, for coming onto the forums and giving what essentially amounts to BAD advise.
I have no doubt that you have a LOT of cam experience, but you need to show some humbleness and some respect to some of the members here. We're fortunate enough to have some veteran stern drive mechanics in here of 20+ years of experience. Some of them who in their younger days have PUT cams that are too hot in marine motors and toasted them.
And THAT is only ONE side of the equation.
I'm not sure if you noticed or not, but there is currently a horsepower and gas mileage war going on in the truck market.
Ford, toyota, chev and dodge have been duking it out for some time to see who can get the best mileage and the most power.
Our boat motors ARE truck motors.
There are MILLIONS of dollars of research being thrown into to cam profiles. If there was some magic cam profile that made MORE power WITHOUT sacrificing torque, economy and revision, WE WOULD ALREADY HAVE IT.
IT would be in the GM marine motors (which are the same as their GM truck motors)
Even modern day gen IV GM marine motors run similar cam profiles. They get their power through better flowing heads, better combustions chambers and variable valve timing.
If you want more mojo from you're boat, go with a GEN IV chev small block.
GM marine has been using them for several years now, they're the only REAL way to get more HP, other than a COMPLETE engine overhull, like a 383 for example, and even then, a lot of those setups require dry exhaust.
