Re: New heads?
 

I thought the cam pretty much dictates at what RPM an engine will peak out at ??? Of course, over the years I have had cam manufactures want to sell me a cam that peaks out at maybe 6400rpm (this of course depends on CID) for a marine engine that will only run around 5500rpm, and not even peak out at the cam's limit. But these days (within the last few years) with the revolutionary low and mid lift air flow numbers now available from aftermarket head manufacture'rs it sounds like a smaller cam can be utilized.

Back in the early 1980's when I began to really get into the offshore performance boating hobby/sport, I can always remember either reading or hearing about not to "over cam an engine". With today's aftermarket heads having better low/mid lift airflow numbers it seems that using a smaller cam is becoming more and more important. The newer head designs are really changing things up and causing quite a stir in the performance industry. One has to really start thinking of the cam as more of the brain and RPM limiter of sorts than a Horse Power maker as the new head designs are really coming alive. At least this is how I am beginning to see it more and more lately. What do you guys think ?

Re: New heads?
 

The new heads are flowing better, but you are not going to make your peak horsepower at the .200 .300 .400, lift numbers, you need airflow. The cam has
always been a rev limiter,best place to find peak hp is on the dyno. In the boat
you can switch props try different rpm's to find your top speed but it still may not be your peak horsepower. Depending on your cubic inch you may not be able to run
the best flowing heads, in a marine application. You need the combination to work together.

Re: New heads?
 

That should be close to your peak HP.

Re: New heads?
 

If you size your chambers properly without going overboard then yes you can pick up a significant power gain. Plus swapping your cast iron heads with aluminum ones will gain you a little freeboard. ;)

Happy New Year
Roby

Re: New heads?
 

Well, I just thought the better flowing aftermarket heads that are available these days make it possible to pinpoint the specs a little better when making a cam selection---which in turn would allow for a smaller cam. Outside of that I agree that there are many other variables to consider with the correct components combination being the key to a good running engine.

To make an example of my point, I have some new AFR cnc ported heads going on a pair of big block Chevy offshore marine application engines to be tested on the dyno this spring and I am going to try and get away with using the smallest hydraulic roller cam I can while still making peak power up to about 5700-5800rpm. I want to make a certain level of HP, but HP is not necessarily my top priority. My priority goals are a smooth as possible idle, a wide/flat torque band, and excellent throttle response and (as I have mentioned) still be able to make peak power up to the 5700-5800rpm level. It should be interesting----we're going to give it a go and see how things turns out.

Re: New heads?
 

Kaama, Great plan! What CR and intake are you going to try?

Re: New heads?
 

[QUOTE=GPM]The new heads are flowing better, but you are not going to make your peak horsepower at the .200 .300 .400, lift numbers, you need airflow.

The reason flow at these lift values DO ultimately make hp is because that is the valve lift achieved when the piston is at max velocity, 75* atdc or thereabouts. Max lift is usually around 105-110* atdc on the intake valve (ICL). The piston is decelerating by the time max lift is attained. These low lift figures are where the inertia building begins in the a/f charge, therefore it controls ultimately how much charge gets packed in.

BT :cool:

Re: New heads?
 

Cam science...

#1: Choose your desired WOT rpm.
#2: Choose yuor bore and stroke.
#3: Choose your fuel octane.
#4: Choose your desired torque curve (since there are a couple of ways to get to the same peak hp).
#5: Choose your induction (single/double carb, TBI, Port injection)
#6: Turbo or supercharged? (if yes, blowthru or pullthru - intercooled? - plumbing routing?)

Now you can start matching your combo.
Intake port volume, length, plenum size,
Combustion chamber style/size/surface area,
Port characteristics, valve size,
Piston crown configuration..

Now you start matching the cam.
When examining the area under the flow curve, does it appear that the intake will have an unfair advantage over the exhaust? You start by matching the lobe to whichever side will be the limiting factor. At the chosen WOT RPM, you look at the acceleration parameters you need to stay within on the valvetrain. Since it is a marine motor at 5500 rpm, you can grind the accel ramps more aggressively than you would normally do so on a 7500rpm motor. But since you will try to use the weakest springs possible, you will also use gentle decel ramps. Open the valves as quickly as possible (without flexing pushrods), yet slow them down a little more gentle. This gentle decel will require your max lift to be higher than it would be with aggressive decel ramps, as the "overshoot" will be higher but more gentle. The valvesprings will need to be matched to this max lift, and careful matching will ensure longer life. Once you got your ramp figured out on the "weak" port, you can do the same thing for the "strong port", but you will either be using less lift or less duration. Depending on your flow curve, one way may be better than the other. Most catalog cams simply "grow" or "shrink" the same lobe.

Once you got your lobes, you can size your springs. Might find out that you can't get the right spring for your lobe and you'll have to back up and regroup. Extra long valves and longer valvesprings are definitely an option. Every .100" in additional length makes the spring smile a great big smile.

You'll want to run the longest springs you can run. You'll want to use the lightest springs that will completely prevent float or bounce at WOT. It's a shame that nobody makes a good rev kit for a BBC.

It just keeps getting weirder and more involved.
But somebody does this for specific race motor combinations, and then we consumers get to reap those benefits because some of these custom lobes work well for us guys too.

But unless you got a custom cam designed for
1) your WOT rpm
2) your heads
3) easy decel

It's a compromise.

But what isn't?

Re: New heads?
 

And some people think it's simple flow numbers.

Re: New heads?
 

Marty,

Thanks! With 121cc combustion chambers and the JE flat top pistons I am using it should be right around 9.4-9.5 comp ratio. I would prefer it being around 9.2, but that's where we're at for now---kind of a long story. I suppose I could send my new heads over to JimV's shop and have him do a chamber port to open them up a little more, but I don't really want to lower the comp ratio by enlarging the combusition chambers. My attempts are to let good airflow and port velocity work MORE for me than trying to rely too much on cam duration.

Anyway, good arguement on the airflow, velocity, and cam specs here. I guess I will leave it where it lays---it's probably all I can do until we dyno my engines (naturally aspirated) with the concept that I tend to lean towards and see what happens.

By the way, this might be a little TMI, but any of the input or info I give here on OSO is pretty much volountary as I choose to do so, and my motives are only in attempt to help/bennefit others. I am no expert, but can only offer what I have experienced in the past 25 years of being around the offshore performance boats and even then it certainly doesn't always make me correct. As far as this airflow and cam thing goes with these newly designed heads like the cnc ported AFR's, etc----at this point it's only a GUESS on my part based on some research I have gotten from some others. I suppose there's two ways to skin a cat, but the dyno will hopefully give us the "skinny" on it all. If the results turn out to be sour grapes, then I will certainly pass that info on and let you all know.

In the mean time, I hope Spashandburn got the info he was looking for.