Low end torque - 4 stroke vs. 2 stroke

[strick]

I ran a 250 2 stroke yamaha on my 23 with a bracket then switched to a 250 suzuki 4 stroke. I'd have to say that the suzuki with that bigger prop had more noticeable low end torque. I loved that zuke.

strick

[cdavisdb]

Seems like there are two different torque issues that matter to boaters, (1) hole shot and (2) ability to stay on plane at low speeds. I can see where the fast rev ability of 2 stokes would get you out of the hole faster(maybe that's not torque?), but does that have much effect on torque delivery relatively low in the torque/rpm curve, needed for low speed planing. My experience is only with 2 stroke outboards vs 4 stroke 1/0s, but the i/0s seemed to have far more torque delivery low in the rpm curve and much better low speed planing ability. I never put much attention on hole shot, maybe because divers load so much weight into the boat.

[76Red18]

I would think planing speed would be determined by a combination of hull design, propeller design and weight distribution. You would only need enough torque to move the hull forward to a speed that hydrodynamic forces take over, whether it be a 50 horse or 250 horse motor. It would be interesting to see what that torque number would be on a properly set up SeaCraft hull vs. others. Do any of these new motors have a torque display? Bushwacker showed me some interesting print outs from his E-tech; no torque readings though.

[cdavisdb]

I don't think it is quite that simple in practice. All those things affect planning speed, but when its sloppy, the forces on the boat keep changing rapidly. If you don't have the low end torque to maintain prop rpm, what happens is: its sloppy and you want to plane slowly, so you increase throttle until the boat gets up and planes, then its going to fast for comfort, so you reduce throttle a little to slow down a little, but then the boat falls off a plane and you're going 6 knots again, repeat. If you have sufficient torque, you can hold the boat on a plane at a slow and more comfortable speed.

[workinpr0gress]

If there is enough torque and Bite of the prop.

[Bushwacker]

. . . Bushwacker showed me some interesting print outs from his E-tech; no torque readings though.

The 2006 BRP brochure has plots of WOT torque and HP vs RPM for the 225 E-Tec and the 225 4S Yamaha. The Yammi just barely makes 225 hp at max rpm, while the E-Tec makes about 240 HP in the 4500-5500 rpm range, so yes the Yammi is weak compared to the "225" E-Tec. The E-Tec puts out almost 2X more torque down around 2000 rpm.

The 2007 brochure compared the 150 E-Tec and 150 Yamaha 4S with similar results. The E-Tec put out about 165 HP from about 4500-5500, while the Yamaha made about 155 HP over a much smaller rpm range. The 150 E-Tec also had a huge torque advantage down in the 2000-3000 rpm range, so it's no surprise that it easily won the "tug-of-war" against the Yamaha. I've pulled points off those curves and plugged them into a spreadsheet that I can send to anyone that want's it if you'll PM me your e-mail address.

When you're looking at acceleration, it's the AREA under the torque vs. rpm curve that counts! When comparing an engine with lots of low speed torque and a flat torque curve to a "peaky" engine with higher peak torque but less area under the torque curve, the flat torque curve engine will win every time, all other factors being equal. With the Zukes, they've changed the "other factors" and gone to more gear reduction to increase torque at the prop, allowing them to use a much larger slower turning more efficient prop, which helps to compensate for less torque at the powerhead. Putting a lower pitch prop with more blade area on ANY engine will tend to improve low speed acceleration, although at the expense of maximum boat speed.

The low speed planing ability and torque vs throttle sensitivity Connor mentioned is a little bit different. I think what he's experienced is the difference between a small displacement/high output/high rpm engine vs a MUCH LARGER displacement/low specific output/lower rpm engine. The larger engine will tend to have more torque at ANY throttle setting just because of it's displacement, and will therefore be more stable under conditions of varying load, such as going up and down hill in a big following sea. Denny

[joshmon71]

If anyone on Cape Cod wants a run on a 23 with bracket and a Gen 1 250HP Verado let me know (post April 15 when Cod season opens). The motor is dialed in now after working some items out in 09 (batt ground cable had a crack out of sight causing some weirdness, and new impeller, other minor stuff).


Plenty of torque . With a Rev4 17P blade prop it jumps out of the hole! With a Mirage Plus 3 blade 17P it does not jump out of the hole as fast, but has a better top end speed. It is an amazing motor. Heavy though prob ~50lbs more than other 4S. Hydraulic pumps/reservoir is under the console so that helps.

Came with boat, do like the engine, and since all rigged with Digital throttle, new N2K gauge that networks into MFD, analog digital gauges clip into each other, and hydraulic steering pump/reservoir under the helm sort of committed to Merc moving forward . However I'd be tempted to go Suzuki new for the price. Local Merc dealer while pricey is very good. Thats the other factor besides torque/hp/MPG...whose your dealer and how close and good?

Since with a 4S, forget wrenching more than a few things yourself(oils changed,plugs,etc). Its a modern car engine and needs a laptop to diagnose.