MILD THUNDER

iTrader: (3)

I'll try to answer that a little better, based on some research i have done on this topic, for my own reasons.

One reason a longer ex duration may be chosen, is yes, to help out a weak exhaust port, or poor scavenging exhaust.

Another reason, may be to increase upper rpm power.

Advantages to a smaller split, or single pattern, is typically, more low RPM torque is produced. Everyone tends to overlook the fact, that what happens at 2500RPM, is vastly different, to whats happening at 6000RPM, or 7000rpm.

Everything is a time constraint. Its like an engine with a very late closing intake valve (more duration per say). At low rpm, it doesnt do well. There is simply too great of a loss of cyl pressure, due to the valve closing so late. However, when the engine is turning high rpm, the late intake valve closing, comes to life!! Why? because the time for filling, or should I say leakage, is reduced.

The exhaust functions on same concepts. You simply cannot have maximum efficient cylinder blowdown, at 2000rpm, and at 6500rpm, without changing something. At 6000rpm, you need more duration to evacuate the cylinder, than you do at 2000rpm. Besides valve timing, the efficiency of the port itself, and exhaust scavenging, overlap, can have an effect on what dicates the most efficient exhaust blowdown event.

It all boils down to what you want out of the engine, what RPM you are turning, and so on. I think its all about balancing the combo together. Would your engine maybe make a little more power with a larger split at 6000rpm, maybe. Would it make a little more torque down low, with less split? Maybe.

I know you have a 454 build going. Lets say for example, you stuck in a 1985 GM truck 454. its got a small high velocity oval port head, a very early closing intake valve, and very mild valve lift. You dyno that engine, and it makes 420ft lbs of torque, at 1800rpm. Thats actually an impressive torque number for such a slow engine speed. That entire engine, was designed, to tow a trailer, and it works great for that. How would it run in the boat, compared to your 454 with 308 iron eagles, fairly late closing intake valve, and higher lift? It would be a turd compared to it. Unless of course, you planned on pulling a wakeboarder around at 1800RPM all day.

My point is, that truck engines combo , was great for its intended use. Outside of that, it sucks. And your combo, would probably suck at lugging a trailer up a mountain at 1800rpm.. In order for GM to get that truck engine to do what it did, they knew that design, would net big low rpm torque, and chitty high rpm power. Your boat engine, you chose parts that will trade off some low rpm power, for much more upper rpm power. Camshafts operate on the same priniciple being that everything is a compromise, especially when dealing with an old school big block engine design with fixed cam timing.

As far as your split, if your rpm/flow outruns the cam/head combo , than you may see a power loss in the upper rpm, vs a longer split. If it isn't , than you gain nothing, and probably lose some power. Unless you're ready to start swapping cams on the dyno, it would be very hard to say what would be better.

I also think this is why the exhaust system needs to be considered when designing a cam for the combo. A set of short runner marine manfolds, might require an entirely different exhaust lobe, than a ideally spec'd header for the combo. One might say, "Well, I swapped out my GIL manifolds for a true header, and didnt see much power gain", well, maybe they would have, if the cam was redesigned with the exhaust as a consideration.

Same goes for cylinder head mods. IF, you had the absolute perfect cam for your combo, and went in and worked on the heads, changing the low lift airflow characteristics of the cylinder head, that cam is no longer "perfect". While you may see some gains from the head work, in a perfect world, making changes to the camshaft to work with that, would net even greater gains. How someone could "cam" an engine, without cylinder head flow data, is a new one to me.

SB

iTrader: (1)

1 million percent I agree on this.

Burns my azz when this get's no consideration until engine issues arise and then it get's blamed. This should be dealt with off the bat. My trust would be a cam co, person, etc that asks and understands this.

sutphen 30

maybe they should have optimized the cam design for this.,,just poking fun at all of you for the 10-20hp you'll never feel in a boat.

https://www.youtube.com/watch?v=azPKIjxmmdU

Ryan00TJ

iTrader: (8)

I agree. Bob M wanted my AFR'S flow numbers and wanted to know about my exhaust system when designing my last cam. It turned out everything I wanted and more.

1989mach1

I don't known if this is a stupid question but I was thinking of going to a 525 cam in my 500efi's looking for a little more out of them. Would this work

1989mach1

Side note I just did top ends on them so was going to waste until they need to be done again and replace them completely with the aluminum ones. Maybe lol

ICDEDPPL

iTrader: (1)

Dammit, I wasted my time and money with 5" Stainless marine exhaust. I could have just got some 454 log manifolds for 1/3 of the price and lost no horsepower.

SB

iTrader: (1)

Not what I was saying.

TylerBurich

Quote from MT
"Everything is a time constraint. Its like an engine with a very late closing intake valve (more duration per say). At low rpm, it doesnt do well. There is simply too great of a loss of cyl pressure, due to the valve closing so late. However, when the engine is turning high rpm, the late intake valve closing, comes to life!! Why? because the time for filling, or should I say leakage, is reduced."

Is it time for someone to develop an adjustable cam timing solenoid powered system for our ever popular BBC marine builds? This would lessen the need to compromise so much when choosing camshafts around specific parameters.

offshorexcursion

iTrader: (5)

I also agree that its very important, and that's the exact mistake Bob made while working with my initial build.