1979 20' ma 140 suzuki

[bluechip]

i am considering re-powering my 79 20' ma seacraft from a 1995 150 2 stroke omc to a 2005 4 stroke 140 suzuki
my kids like to water ski, tube, wake board and knee board with the boat. my question is What will be the difference in pulling power with the suzuki
thanks

[Bushwacker]

As I posted on another thread last Saturday, a 4-stroke motor generally needs about 30-40% more displacement to produce the same torque as a 2-stroke. I believe your 150 has the same 2.7L/158 cu. in. block as my E-TEC. That says the 140 Zuke needs to be at least 158x1.3 = 205 cu. in. to have roughly the same torque that you're use to with the old V-6, but it actually has only a 125 cu. in. powerhead, so it only has about 60% of the displacement it needs for the same torque! I.E., there's no substitute for cubic inches! If you'll PM me with an email address I can send you the torque/hp vs rpm comparison curves for the 150 E-TEC vs the 163 cu. inch 150 Yamaha 4-S. Even correcting for higher gear reduction on the gray motor, the smaller displacement E-TEC still has 23% more PROP torque than the gray motor at 3000 rpm where you're trying to get on plane!

Now the 140 Zuke is a fine economical motor and my favorite 4-stroke because it's relatively light and the chain driven cams and extra gear reduction appeal to my basic mechanical engineering instincts. The extra gear reduction lets it spin a large slow-turning more efficient prop so the 30-40% extra displacement rule might be overstated for it. It's no doubt pretty strong for it's size. However that 150 is a very conservatively rated (and thirsty!) motor with a VERY strong mid-range, and if it's propped so you can turn it up to about 5500 with your normal load, I suspect that Zuke is gonna feel pretty weak in comparison, although you could probably reduce your fuel load by 50%! Now if the V-6 has a lot more power than needed for what you're using, the Zuke might be acceptable. To get 140 hp out of only 125 cu. inches, it's got to have pretty high lift and large duration "hot rod" cams that make power at high rpm and sacrifice low and mid-range torque, so don't know how much of that you can offset with gear reduction. Since you're looking at older motors, you might look at the early 115 or 150 E-TEC's (2007 was the first model year for both.) Wet weights of the 140 Zuke and 150 E-TEC are nearly identical, but the E-TEC will be stronger than your current V-6, quieter than the Zuke at 30 mph and it won't burn that much more gas than the Zuke because you won't have to use so much throttle to get the same speed.

[bluechip]

bushwacker thanks for the reply, i understand completely what your saying, but the suzuki does has more than twice the compression numbers (85 psi omc 210 psi suzuki) which has to account for some power gain.
i no the old saying "there's no replacement for displacement" but the number two power adder is higher compression with that said i hoped somebody with a 20' ma w/140 suzuki would post some performance #'s
thanks in advance bluechip

[Bushwacker]

210 psig is almost certainly a bogus number because that works out to be a compression ratio of 15.3:1, which is way too high, even if you're running 103 octane Av gas with aluminum heads!! I'd see if you can find what the specs are from a Zuke service manual. 85 psig does sound a bit low for the 150, so it may have some stuck rings and need a good decarb. And compression ratio primarily affects BSFC, but has sort of a 2nd order effect on power. And if you have to back off on spark advance to avoid detonation with the high compression ratio, you will lose more power from that than you will gain from high compression. I own an old muscle car and the race block I bought for it turned out to have about 12:1 compression ratio which is way too high for premium pump gas, so I tore it down and installed 9.5:1 pistons in it!

[flyingfrizzle]

Originally Posted by Bushwacker

210 psig is almost certainly a bogus number because that works out to be a compression ratio of 15.3:1, which is way too high, even if you're running 103 octane Av gas with aluminum heads!! I'd see if you can find what the specs are from a Zuke service manual. 85 psig does sound a bit low for the 150, so it may have some stuck rings and need a good decarb. And compression ratio primarily affects BSFC, but has sort of a 2nd order effect on power. And if you have to back off on spark advance to avoid detonation with the high compression ratio, you will lose more power from that than you will gain from high compression. I own an old muscle car and the race block I bought for it turned out to have about 12:1 compression ratio which is way too high for premium pump gas, so I tore it down and installed 9.5:1 pistons in it!

I agree, I have a Camaro with a small block Chevy in it with 14.8 to 1 compression and I have to run at least 112 octane. I usually run 113 oxygenated race fuel which is as good as 114. A stock mercury 200 has 125-130 lbs but we shave the heads down to 34 cc's to run 145-150 lbs but even then that is the absolute limit for 93 octane. Most go easy on the timing to run this much compression. The drag racers with 2.5 drag outboards are only 160ish from the factory and some of them guys cut them down to get over 200 lbs but they are running 112 octane at least!

*EDIT: I was wrong in my post earlier, I did some checking around and found this: "2005 Suzuki 140 hp - Specks: 185 psi to 242 psi with a 14 psi delta maximum between all cylinders" So I guess they are doing it some how with low octane fuel???

[77SceptreOB]

Well said fellas. I think Denny's original analysis is spot on.

[FishStretcher]

I don't see that 210 as a bogus number. Others achieve it on other outboards and the quasi-adiabatic heating during compression could explain it on a tight 4 stroke.

Originally Posted by Bushwacker

210 psig is almost certainly a bogus number because that works out to be a compression ratio of 15.3:1, which is way too high, even if you're running 103 octane Av gas with aluminum heads!! I'd see if you can find what the specs are from a Zuke service manual. 85 psig does sound a bit low for the 150, so it may have some stuck rings and need a good decarb. And compression ratio primarily affects BSFC, but has sort of a 2nd order effect on power. And if you have to back off on spark advance to avoid detonation with the high compression ratio, you will lose more power from that than you will gain from high compression. I own an old muscle car and the race block I bought for it turned out to have about 12:1 compression ratio which is way too high for premium pump gas, so I tore it down and installed 9.5:1 pistons in it!

[CaptLloyd]

I won't even try to comment on the technical side, or the comparison of the 150 2-stroke. But from experience with my kids and friends (12-13 years old), my 140 Suzuki on my 73 20SF pulls them just fine. Solas Amita 4 Blade 13.75X19 Prop.

However, no matter how much I try to squat the boat, the Seacraft hull is too efficient to make a big enough wave to satisfy my son and his buddies for wake surfing.

Lloyd

[Bushwacker]

Originally Posted by FishStretcher

I don't see that 210 as a bogus number. Others achieve it on other outboards and the quasi-adiabatic heating during compression could explain it on a tight 4 stroke.

Fish is thermodynamically correct! Adiabatic heating could explain those high pressures. For the less technically inclined, the term adiabatic means that there is no heat loss in the process. Although some heat is actually transferred to the piston, cylinder and head, at high speed the process happens so fast that there is very little time for much heat transfer to occur. Compression ratio is actually the ratio of volume of the cylinder + combustion chamber to the volume of the combustion chamber, not the ratio of pressures before and after compression that I had used as a "quick and dirty" analysis of the numbers posted. Although the pressure ratio is approximately the same as the volume ratio, when the heat generated by an ideal frictionless reversible (isentropic) compression process is not allowed to escape, there will be an additional increase in pressure, similar to what you would get if you simply increased the temperature of a gas in a small constant volume space equal to the same temperature rise you’d get with from an isentropic compression process. This is why diesel engines are able to generate cylinder temperatures that are well above the flash temperature of diesel oil.

What’s interesting as a practical matter is how Suzuki is able to run those compression pressure (and corresponding temperature) levels on 87 octane fuel without getting some serious detonation. They’ve demonstrated some good engineering practice with the chain cam drive and offset driveshaft/extra gear reduction, so it wouldn’t surprise me if they also used oil jets built into the connecting rods for additional piston cooling and an extraordinary amount of cooling water flow in the cylinder heads, so that significant heat is removed from the cylinder during operation. Although these cooling losses hurt thermodynamic efficiency, they might allow you to run cheap low octane fuel. An additional good (but expensive) practice would be to bore and hone the cylinders with a head plate bolted to the block so the cylinders are perfectly round in the assembled condition, creating a better ring seal which would help generate those high compression numbers. (BMW does this on their production engines and it’s a common hot-rod trick done on blueprinted race engines.) The oil and water cooling don’t exist at cranking speeds, so you’d tend to get the adiabatic heating effect during a compression test, while normal operation would not be adiabatic because of the additional piston and cylinder head cooling. This additional oil and water cooling would also improve durability, and that’s something you’d want in an otherwise highly stressed engine that’s generating 140 hp from only 122 cubic inches!

I think the 140 Zuke just uses a simple port-type EFI system like most cars that only requires about 35 psi in the fuel rails. A DFI system requires either an expensive 600 psi fuel pump like the HPDI motors use, or powerful injectors like the E-TEC that create 600 psi at the nozzle using only a 35 psi fuel pump. As Friz and Blue Heron were implying, you can get a lot of cooling out of that DI fuel spray however, which would allow you to run higher than normal compression. Modern turbo and/or high compression (~12:1) engines now feature DI and this would be the next logical step if Suzuki wanted get even more power out of that block, but you’d pay for it with higher hardware costs!

[FishStretcher]

First- Thanks Bushwacker, for fleshing out my abbreviated post. I hadn't had time to come back and explain what I was thinking. And of course it is still just a guess on my part.

A comment on the Suzuki, insofar that I think that it isn't so unusual in terms of peak performance compared to car engines, although comparing cruise in a car to a boat is a whole different thing.

The DF140A specs look like small car engine specs. 2 liter 16V engines that hit 140hp were not that uncommon as far back as 1990, and those typically had (I think) 9.5-10.1:1 compression ratios, port EFI, knock sensor and catalyst. Sporty cars like the Nissan Sentra SE-R or VW GTI.

I have a suspicion that modern 87 octane might burn just slowly enough that at high speed and WOT, a knock sensor can pull timing to deal with pinging. At cruise, the boat shouldn't be at WOT, so effective compression ratio is lower due to throttling. And jabbing the throttle probably splashes in more fuel for accel and that might also reduce the tendency for pinging because of the cooled fuel rich mixture. This can also be tweaked a little with intake runner design to help/hurt cylinder filing at different engine speeds.

As an aside- a fast burning fuel that has decent "octane" or AKI is probably ideal for a boat. We want max torque at max speed- as this is what the prop needs. In this case, if you can get a very fast burning fuel, you retard timing and make more power. That way you aren't starting combustion while the piston is still coming up. A high tolulene fuel can do this, and the results show up on a dyno almost automatically with a knock sensor equipped engine that has the authority to pull ignition timing. *

But we get E10 most of the time. Or maybe REC 90.

* And you can bet that I will try this on the knock sensor equipped 350 MAG MPI bravo 3 on the 25 to see if I can touch 50MPH. Just to try.