Thanks for the replies. Is it safe to assume that the scavenging that will occur is effected by exhaust system as much potentially as the exhaust port design/flow? Which has more effect on how the motor performs? It just seems like a great flowing exhaust port will suffer without the right exhaust system, which will have a bearing on the cam selection. Bear with me I have my own opinions and am not saying anyone is wrong, just trying to learn a little!!

Craig,
That is why you flow heads with the intake bolted on for the intake numbers and then with the header/manifold to find the exhaust flow. This is a TRUE representation of what the induction/exhaust is doing so the cam profile can then be produced. And yes, the carb should be on the intake.

Chris

That sounds like a lot more real-world of flow test than the way head mfg's are doing it. So based on the exhaust flow of the head/header (manifold) bolted up on the flow bench the proper exhaust duration and LSA can be determined? I am assuming that the amount of scavenging still has to be estimated based on the flow results and the cam design tweaked to give more or less based on these results? This is the point where I am assuming it really matters if headers are being used or some type of manifold since they do affect scavenging differently.

rmbuilder,
No apologies needed, this is all good information here and thanks for all the input. Jeff

Jeff, Have you ordered your heads yet? Mine should be here in another month.

Tom

Is Crane the only cam out there? I've tried 2 different Comp cams in my 500 ci bbc w/ Merlin vr oval port heads. The current sound awsome @ idle, a bit chopy. But is a little hard on the out drive. It does not like to idle down below 900 rpm in gear. Scince redline is 5000 rpm I feel I,m over camed. So I will be swaping out to the next milder cam witch by the ran superb. Specs. Xtream Marine Comp cam current aprox .560 lift & 236 / 242 @ .050. To much for me. The smaller .540 lift & 226 / 236 @ .050 works great will idle at 700 in gear all day long. Both are ground with 112 deg. c/l. Specs are aprox. I going by memory. Check out Copm Cam web sight.

There is about 1500 grinders across the USA.

Chris

WetteVette got me thinking with his question about whether headers are as important as the exhaust port.

Everything in the air pathway from filter to tailpipe is important according to how much resistance a particular piece of the pathway adds to the total resistance. It makes sense to work on the worst pieces first, but whenever you relieve the pressure at a particular point, you add to the logjam farther downstream.

I use this "bottleneck" diagram to remind me that nothing is more important to total system resistance as the flow restriction presented by the valve and seat. If you can improve this, then you use less cam for an engine that is not going to see high RPM anyway. When I hear Chris say that too much cam with good heads may increase peak HP but also narrow the power band, it makes me think that there is a logjam somewhere else in the pathway. Now maybe it's exhaust manifolds, and the addition of good headers really adds some power and widens the power band again.

Just thinking out loud...

Tomcat,
The diagram is great for putting it all out there! So the question still stands whether some scavenging is a plus on low RPM motors? We all know how scavenging works to benefit a high RPM motor and how even naturally aspirated engines can attain 100%+ VE. On a smaller scale does some scavenging help motors in a lower RPM marine environment? And how much scavenging is due to the headers as compared to the cylinder port? I would think the cam needs to be designed with some consideration to this. My thought has always been that manifolds are poor for scavenging and headers are good. So doesn't the exhaust duration need to be more or less depending on whether headers or manifolds are run? What effect on the HP curve do headers have compared to manifolds on the dyno? The same curve only less with the manifolds, or does the peak HP/RPM change? I still think a lot of boat guys are off with their dynoed motors because of the difference between dyno and marine exhaust run in the boat.

Craig

I have a set of dry "dyno" pipes for the Gil exhaust and plan on running both headers and Gil exhaust on the dyno on the same motor to give us a some input on the header vs marine exhaust question. I might even run the stock HP 500 and the upgraded HP 500 with both exhaust systems. Jeff