i will ask one simple question,what is more important to you,a torque figure at 3800 rpm or top speed the boat can run?when you answer that question the manifold decision is made for you!

What manifold did AFR recommend ? Also how did your pushrods & rocker arms work out ?

Valid point, Mike. Honestly, I would like to maximize my part throttle cruising speed, say at 3400 RPM or so. However, that's easier to do with a car that has multiple gears in the tranny. In a boat, I can throw a bigger prop on there to try to take advantage of a big torque number in the mid-range, but then I might not be able to spin that prop at the top end. At what point am I "lugging" the motor? Seems that I read somewhere that you try to prop so your WOT max RPM is about 1000 RPM above the torque peak. Is that correct?

I used to be more worried about a big number for the top speed, but now I realize that someone is always going to be faster, so I'm not as worried about it. I use this boat for a lot of things besides running flat out - it even sees duty slinging the kids around on a tube sometimes.

I also want to be able to hit the diverter occasionally and run it through the prop to quiet it down. Most of the time this will be at lower cruising speeds between 2800-3200 RPM, but I could see where it would be nice to quiet it down at idle too. That's why I asked about a dual plane as a hedge against reversion, since the sequential cylinders in the firing order are not drawing in charge right next to each other in the plenum. Anyone have any experience with this?

I realize I have a lot more compromises in my setup than the average performance enthusiast on this site. That's why I'm spending so much time deciding.

FBC, I'm getting ready to update that thread. Ordered my new pushrods and rockers. Need to find my slip of paper that had the lengths written down on it.

Tony Mamo from AFR thought a marine engine should use the dual plane (Air Gap) based on the fact that the operating range is typically below 5400 RPM. His thinking is that the drag cars are going to take advantage of the single plane's propensity to make more power above 4800 RPM.

GB, I would love to do that. However, dyno time can be in short supply, and those danged GM Performance intake gaskets that fit the "roval" ports on the AFR's are expensive! If there was a way to quickly swap the manifolds without having to scrape off the old gaskets and put new ones on every time, I would do it in a heartbeat. I saw where AFR was releasing a modular two-piece intake manifold made out of phenolic resin. It would allow you to easily swap between a single and dual plane arrangement, and you could swap cams without having to worry about getting into the coolant passages or scraping off old gasket. Sounds like a great idea!

This is an actually easy thread to answer.

Your motor is big enough, and your heads a good enough, where even with a 'smaller cam' and lower operating rpm then say a street strip car , that it will like the single plane.

#2 - The Dart Oval Port Single Plane is one of the few, if not the only, single plane to work as well as high perf dual planes from down low to up stairs.

#3 - as you mentioned, and some don't know, a dual plane will take more carb then a single. Divided plenum see half the carb. Open see's the whole thing.

So.....put the stinkin Dart Oval Port intake with your carb on and worry about other things.

BTW: I'm speaking from experience including on the water and dyno and street and track from what I said above.

Enjoy - sounds like you put together a good combo.

I like the idea of dynoing back to back.

Im curious. We always here about torque down low, being better with a dual plane. We always here about the air gap design, running cooler and making more HP. We always here about the single plane making more power in the upper RPM band.

This brings me to a couple questions.

On a boat engine, that probably sees 150* water temp, if using a oil splash shield under the intake, will the air gap design help alot? I believe where the air gap style dual plane shined, is on a street engine, that sees 200* coolant temps, and smoking hot underhood temps while sitting in traffic on a hot texas day in your Chevy Nova. Whether or not the benefits make a difference in a boat application, is the question.

As for low rpm vs upper rpm power. Lets say you make 550HP at 5400 RPM with a dual plane, and 575HP at 5400 with a single plane. The dual plane might require a 27P prop, while the single plane might require a 28p prop for WOT setup, which is how we all prop our boats. Now, the question is, which setup is more efficient, as far as cruise speed and fuel consumption??

In my opinion, the best scenerio is a manifold design(actually entire engine design), that will make more power everywhere, and not give up anything. .....If you want to prop for 5400RPM WOT, you'll need X amount of HP to turn the prop to 5400RPM. Theres just no getting around that, irregardless of what the engine makes at 3500RPM. All you can do is try to build a stout power curve to get the most power from idle to 5400RPM, without giving anything up at 5400RPM.

I honestly wonder why, Mercury Hi Performace, used a Dart Single plane on their 5200RPM HP500, and their 5200RPM 454/420, 454/450, and 454/460 engines. I would like to think they tested the differences, and had a reason to choose the single plane in those applications.

You guys are thinking too much.

I agree.

What I wanna know is, when are these oso 496 stroker builds gonna hit the dyno. It seems like since 2012, we been trying to figure them out. To stroke or not to stroke, I beam or H beam, small ovals, or small rectangles, dual plane or single plane, crane or scorpion rockers, 750 or 850 holley, 496 manifolds or Gil manifolds, DUI or Merc or MSD?. Budman, Borgie, couple others. I wanna see some dyno numbers! What you guys building over there, some space shuttles? Put a fukin intake on it already and lets go! :party-smiley-004: :gfight:

I was wondering why my head hurts so much. And I thought it was the 9 ambers I had last night.