1979 20' ma 140 suzuki

[Blue_Heron]

Originally Posted by flyingfrizzle

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???

Interesting. Are the Suzuki 4 strokes direct injected?

The whole problem with compression is knock caused by pre-ignition, right? If you time the injection of the fuel charge so it coincides with the spark timing, you won't have pre-ignition. Maybe that's how they're doing it.

Dave

[CaptLloyd]

Hi Mark,

My top speed is 40 MPH, and at cruise (about 25 MPH) I get close to 5 MPG. Of course this is with a light load. Once I start dragging the kids around, you can throw those numbers out the window

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.

[bluechip]

hi guys,
thanks for all the interest and technical insight.
i looked into it as far as the dealer, i know his rep is impeccable. so i new the compression numbers where accurate. he says the yam 150 and many other 4 stroke compression numbers are that high.
they also do a leak down test which is a static test and the motor is tight (did not get the specific percentage of leakage). the only other point is that the compression test is done at cranking speed, so i am not sure if Adriatic (sp) heating is causing the higher compression at that rpm

thanks Lloyd that is what i was hoping for: 40 top, 5 mpg at cruise
thanks to all
mark

[flyingfrizzle]

Originally Posted by bluechip

hi guys,
thanks for all the interest and technical insight.
i looked into it as far as the dealer, i know his rep is impeccable. so i new the compression numbers where accurate. he says the yam 150 and many other 4 stroke compression numbers are that high.
they also do a leak down test which is a static test and the motor is tight (did not get the specific percentage of leakage). the only other point is that the compression test is done at cranking speed, so i am not sure if Adriatic (sp) heating is causing the higher compression at that rpm

thanks Lloyd that is what i was hoping for: 40 top, 5 mpg at cruise
thanks to all
mark

The leak down test is "King" as far as a more accurate test, When you get your motor get a good leak down tester (around $100 for a good one) and record the base % loss and check it every few months or seasons. When you start to see a slack cylinder or get a loss of 10% more than the recorded base numbers you wrote down when you got the motor you know to look for an issue then before it becomes a failure. Works great on 2 strokes to determine when to re-ring the pistons but can help determine problems on the 4 strokes too.

[bluechip]

first time out boat performed great
24 mph @4000 rpm 3 adults full tank of fuel
37 mph@5600 rpm 3 adults full tank of fuel
41 mph@5700 rpm 1 adult " "
excellent acceleration
overall very happy, boat floats high, motor is quiet, smooth, and economical

[loon]

Bluechip, thanks for your #s. What is your prop?

[bluechip]

13x19 sst