Glen-L.com

For you inboard guys (and maybe to a lesser extent outboard guys just for a compairison) What do you think the rpm range is that you use your boat in the most. For example 1500rpm and up, 2000rpm and up, etc, etc. I'm not too concerned about the max rpm but I'm wondering about the lower end of the range. The reason I'm wondering is I'm looking at cams and they are advertised more often then not with an rpm range they work well for. Such as 1500-5000, 2000-5800, 2500-6200, etc, etc. When you guys are towing someone on a tube what rpm are you running at? I know different hulls, motors, props, trannies, etc will impact this number a lot but I'm still interested in knowing what you're running rpm wise for this kind of stuff. For a car this would be an easy exercise for me but I'm having problems doing this for the boat.

Thanks

1500-3500

I run the most between 25 and 30 mph. Which on my diesel is 2500 to 3000 rpm.

Bill

Sweet spot on most 2 stroke outboards is about 3300-3800....I run around 3400

Can't remember where my V-drive was used mostly....maybe 2000-2500

It needed more % overdrive though....would go way beyond the 8000 top on the mechanical tach (I didn't run past about 6000 though except for a couple of passes)

I run 2800-3300

Thanks guys, this has been helpful

From what I have read , it is torque that moves a boat. Of course torque and RPM have a close relationship so if you look at graphs that show this relationship then the max torque for a given RPM would be most efficient.

Every outboard has a "sweet spot" which is often called it's "cruise speed". It's easy to find on older carb models that have mechanical spark advance because it's the point where the motor reaches maximum advance (trigger linkage is fully advanced and won't move anymore even if you increase the throttle) but you still have throttle left - so normally in the 3300-4300'ish rpm range on a 2 stroke outboard.

That "spot" will always give you your "best" mpg (only burn about 1/2 as much gas as full throttle)- regardless of what "speed" that translates into. So might be well suited if you are "cruising", but may too fast/slow to pull a tube or skier etc..

5600-ish

Cam selection is allot more than RPM range. There is no other single aspect of engine building that will have as dramatic of impact on performance... CR being a close second. You need to look at more than RPM range assigned by the grinder.

* Wet exhaust will develop a backpressure higher than the cam was designed for (unless it's marine specific) High overlap, and high lift can be counterproductive depending on exhaust system.

* There is no way to accommodate a cam with no low end in most screw-driving marine designs.

* The cam has to have a flat, or slightly-inclined torque curve, and no big valley between torque, and peak HP, or it will never pull into the top end. The lightswitch powerband of a tall cam is not going to cut it for a marine motor.

There is reaaaaalllyyyyy no way to make an intelligent decision without knowing which hull, what drive, which engine, CR, head specs, intake, carburetor, and just what you want to get out of it. Forgoing that data, usually a cam specified as "RV" or "towing" is ideal for the typical cruiser-type boat. It's way too easy to get greedy, and fall victim to the RPM, and HP ratings, then pick more lumpstick than you need.

Does anybody have info on wet exhaust manifolds that would flow good enough to accept the better cams?
Other than the over-the-transom headers, I havent noticed manifolds that could flow well enough. I still dont see how a log-style manifold can flow very well.

Here's some info from Lightning Performance Marine :

Exhaust Systems

Technical information

Header Performance Data

(boatheaders.com)

____________________
Paul Kane Chelsea, PQ

Building the Glen-L Hot Rod : http://www.boats.chelseacoachworks.com