Though I hate to contradict somebody who has obviously tried some of these things, I also can't let stand unchallenged, statements that are contrary to the laws of physics.there is no difference between a TLDI 70 and a TLDI 90 except for the programing in the ECM and a throttle stop. Unfortunately we found that we gained nearly no speed configured as a 90 (though it did match the Suzuki 70) but it sure used fuel like a 90! when configured as a 90, the TLDI motor produces the same torque as a 70, and torque moves these boats.
There are two ways to make HP: torque and speed. With torque in lb-ft and speed in RPM, the equation is:
HP=(TxN)/5252; or
T=(5252xHP)/N
Maximum HP on an outboard is reached very close to the max specified engine operating speed, which not coincidentally is also where the boat reaches max speed. Both these engines spec the same number for max engine speed, 5850RPM. So we can easily back calculate that at 5850RPM, the 90HP is making 81 lb-ft of torque, the 70HP only 63. In practical terms, this means the 90 can turn a bigger and/or higher pitched prop, which can't help but increase the max speed.
This is obvously only an estimate, but it wont be off by more than a few %. If we had torque or HP /speed curves for both motors we could do a more thorough direct comparison.
You say that torque drives these boats, as if they are in some way unique. That's only partially true and theyre not unique. When it comes to comparisons of torque versus HP theyre no different than any other boat. For all boats, higher torque is rougly equivalent to higher acceleration, while higher HP means higher speed.
Bottom line is, if you cant get higher top speed out of a 90 (any 90) than you can out of a 70, you need to get back to work because you're doing something wrong.
I'm not an apologist for 2-strokes, I own one (carbureted) and it is probably the worst decision I made on my original boat purchase.
But in general, they are considerably lighter than equivalent HP four strokes.
The above weight comparison also requires some explanation as to where the figures came from. You can't just make them up.
You started with a weight of 208. The Tohastu website says 206.
A 20 pound propeller made of what, depleted uranium? The 11" aluminum prop on my 50HP Tohatsu, complete weighs no more than five pounds.
You added in 5 lbs for the weight of the oil tank in the Tohatsu, but not the crankcase oil in the Suzuki. Why? Do you have data that says the specified Tohatsu weight is dry but the Suizuki weight is not? Not on any spec sheet I could find.
The published weight for the long shaft Suzuki is seven pounds heavier than the short shaft, but your number for the 2S is fifteen lbs. The difference between the long and short shaft on my M50D2 is three pounds, not fifteen. I understood the difference between the carbed and TLDI 2S was in the induction and fuel systems, but that most of the mechanical parts were identical.
You spec the 50 carbureted two stroke at 160. That's a mistake in the other direction. My carbed 00 Tohastu long shaft, which I believe at the time I bought it was the lightest of any 50HP, be it two-stroke or four, is 195 lbs.
Here's one I'll concede: I'm pretty certain the one area the 2S TLDI/ETEC will never catch the 4S is in noise. The 4S is quiter by a lot, it always has been, and probably always will be. There is nothing about a TLDI 2S that makes it inherently quieter than carbureted 2S.
