Fuel consumption on a Johnson 88 SPL 90 HP

Spent the day today overhauling the motor on the Vonda Lynn and I'm wondering just how bad the fuel consumption is going to be. It's a 90 HP Johnson 88 SPL 2 stroke V4 from 1988. My friend says he thinks it will burn something like 12 GPH wide open. I'm wondering if this is correct and if anyone has documentation on these motors.

 

Fuel consumption depends on a lot of things, boat weight, type of boat, propeller and how fast you go. WOT is not a good measure. Cruising speed is a better measure. most people cruise at 1/2 to 3/4 throttle. I am not familiar with that engine but I would be willing to bet that it would be significantly less than 12 GPH at 3/4 throttle and even less at about 1/2.

 

I have had a few of those, I would estimate around 5 US gallons per hour at around 3800-4000 rpm, assuming the thing will rev out to say 5200 rpm flat strap. They were not too bad on fuel, really. The 115 and particularly the 140 on the same block, got thirsty.

 

I though 12 GPH seemed a bit much. Thanks for the info!

Still, looking forward to replacing it some day with something more reasonable.

 

They were not a bad old thing, and a strong motor, and whilst my estimate might be a touch on the light side, I generally thought fuel consumption acceptable.

 

I agree with me e . I had an 85 hp v4 and it was a great motor. Very economical at 3/4 throttle on an 18 ft halfcab. Idled almost as good as a 4 stroke.

 

I'm hoping to put a flowmwtwe on it and get an idea what I'm really burning. Be nice if it wasn't as bad as I am afraid of!

 

12 GPH is about right. If the engine is new and tuned up properly it may go as low as 11 GPH.

 

It's mostly a guess on these old motors, but standard estimate is 10% of rated HP at WOT.

So wide open at full RPMs you'd be in the 9 gph range.

At 3/4 throttle it gets much better, but these motors weren't known for being fuel efficient.

I had the same motor, only newer, the estimates were close enough.

Make sure it reaches slightly above the rated RPMs at wot with a load in the boat. If you don't do this they tend to not survive nearly as long.

 

Why does everyone go on about wot fuel burn. Only a fool would drive a motor like that for more than a couple of minutes if that. Drive most outboards sensibly and you can achieve good economy.

 

If you need 30 HP to get the target speed, then you run them WOT. Otherwise, it is more economic to install a smaller engine. There is nothing foolish about running an engine at the rated power. Speed is expensive. The cost is not the most important factor for those that want to go fast.

 

Because for a very long time there wasn't a more accurate way to estimate GPH over a broad range of HP ratings and hull designs.

And people do run around at wot in smaller boats, the motors are designed to run at those rpms for hours at a time. Not that this is good, only that it can, and is common.

That's why 2 stroke outboards run at such low RPMs compared to other 2 stroke motors.

 

I agree with Brendan, some dumb stuff being promoted here, running those old motors continuously at WOT was strictly for the brainless, the fuel burn was ridiculous, and engine life drastically reduced. The old two strokes pre loop-charging, had some some odd characteristics, if they didn't try and get high HP out of low displacement, they were actually better than the loopers, to my observations. Brendan alluded to the 85 hp OMC V4, that motor in the 1632cc model was quite good on fuel at 3/4 throttle, but the 140hp based on that 1632cc block was a petrol bandit, a gas guzzler, and basically across the rev range, as they tried to squeeze extra HP out of it. Similar could be said about the old Mercury "tower of power" 6 cylinder, the bottom of the HP range 90 was not bad at all on fuel, but the 14o was killing you with its thirst. The 90hp engine discussed in this thread is probably the "old" 115, so in the middle. In summary, in the old two strokes, the best loop-charged engines had lower CID for a given HP rating, with the cross-flow engines, you wanted high displacement for the rated HP, to be getting the best fuel economy. All a bit academic now, but there was some virtue in the simplicity and ease of servicing of those old motors, compared to some 4 strokes with servicing that can't be done by the owner.

 

And as a rule of thumb, full throttle burn about twice that at three-quarters of maximum RPM. You can say that pretty accurately with any engine. For a 90hp Yamaha 4-stroke, this chart is instructive.
PERFORMANCE DATA
RPM
MPH
GPH
MPG
1000 3.8 0.4 9.50
1500 5.3 0.8 7.07
2000 6.6 1.1 6.00
2500 7.5 1.8 4.17
3000 9.6 2.8 3.47
3500 15.2 3.2 4.73
4000 22.0 3.7 5.95
4500 26.5 4.6 5.82
5000 30.1 5.9 5.10
5500 33.5 7.9 4.23
6000 36.2 10.3 3.53

Anyone who wants to run consistently at WOT needs their head examined.

 

I find it's better to go with a larger motor throttled back than running a smaller 1 at wot.