4cyl vs. 3cl vs. 2cyl?

[Silvertip]

Re: 4cyl vs. 3cl vs. 2cyl?

Actually it is not the bore of a cylinder that provides torque, it's the stroke. Remember, an engine uses a crankshaft to convert up and down motion into rotary motion. That crankshaft has rod journals that form a lever. A longer stroke is equal to a longer lever. High torque motors like truck engines tend to have long stroke compared to auto engines that tend to favor large bore short stroke which provides high reving capability even though the displacement may be the same. Long stroke truck engines are not very high reving, nor would you want them to be. Hence you have under-square and over-square engines. How the manufacturer of those engines manages the torque, horsepower and resulting fuel consumption curves depends on what type of service the engine is intended for. One need only look at the various versions of the venerable Chevy small block to see the relationship between bore and stroke. The same block has provided 262, 265, 283, 302, 305, 307, 327, and 350 cubic inches. And in the process there have been a gazillion different torque and HP ratings. Note the similarity of the 302/305/307 but the bore and strokes on those three engines were quite different because those engines appeared at very different times and with very different needs.

 

[jay_merrill]

Re: 4cyl vs. 3cl vs. 2cyl?

Yes and no. Longer stroke does tend to create lots of low rpm torque, but its not the only factor in the equation. You also wouldn't end up comparing under-square and over-square engines in a comparison to one another, as we are discussing this issue. I think the same goes for stroke length in general - each engine would probably have some variation, but not much.

Truck engines do tend to have "more stroke than bore," and 2-stroke outboards tend to have "more bore than stroke." Within each realm, however, you have to think not only in terms of the greater fulcrum created by a longer crank journal, but also in terms of the amount of force applied to that fulcrum. Think in terms of being a kid in school sitting on the low end of a teter-totter. The fulcrum length is fixed and if a skinny kid jumps on the other end, you will not be forced upwards as violently as if a big, fat kid (I can say that - I was a fat kid in school!) jumps on it.

As I understand it, there are a couple of things that determine the force applied on the power stroke. One is compression ratio (which I suppose gets into portions of the volumetric efficiency issue) and another is bore. Somewhere in the mix of those two things, there is a balance between the two. Once again, I think someone who actually designs engines could speak more intelligently towards the issue, but there are some things that are pretty obvious.

As an example of what I am talking about, consider a Detroit Series 60 diesel v. a Detroit DD13 engine. Both are inline 6 cylinder engines. The Series 60 is a 14.0 litre engine with a bore of 106 mm and a stroke of 168 mm. The DD13 is a 12.8 litre engine with a bore of 132 mm and a stroke of 156 mm. In comparing the various figures, notice that the bore of the DD13 is quite a bit larger than that of the Series 60 engine, but the stroke lengths are not that much different.

The DD13 is a much smaller engine in overall displacement, and it has a significantly smaller stroke to bore ratio, so you might expect it to develop less torque. But it doesn't - while the Series 60 engine is capable of putting out about 65 more HP than the DD13, its maximum torque capability is less. The 515 hp version of the Series 60 produces up to 1,550 lb-ft of torque, while the smaller DD13 maxes out at 450 hp and 1,650 lb-ft of torque. Further, if you compare the 445 hp Series 60 with the 450 hp version of the DD13, the series 60 has a flat torque curve beginning at 1,100 rpm & 1,450 lb-ft, while the DD13 powerband also starts at 1,100 rpm but creates about 1,575 lb-ft of torque. The torque curve then continues upward, creating 1,650 lb-ft of torque at 1,200 rpm.

The only other difference between the two motors is that the Series 60 has a slightly lower compression ratio than the DD13, at 17:1 v. 17.3:1. Given the much smaller displacement of the DD13, I doubt that its compression advantage makes the difference in the relative performance characteristics.

 

[mthieme]

Re: 4cyl vs. 3cl vs. 2cyl?

Poor biscatn8...his first post...how the heck is he going fit a truck engine in his skiff!

 

[coolguy147]

Re: 4cyl vs. 3cl vs. 2cyl?

is the guy looking for a two stroke or four stroke?

i would choose evinrude etec if ur looking brand new but the older 40-i believe 55hp 2 inline motors will work too. those r going to be two stroke since u can create 40hp from two cylinders then a four stroke(mercurys) 50hp four cylinder engine.

there honda and suzuki... i heard great stuff and believe in my area or around the usa that honda is more popular but mostly the deiciding factor would be prices and location as in the dealer.

ur mainly not going to be much difference in hole shot top end etc and fuel eco like someone said earlier 40hp is 40hp.


i believe in my area 4700 was a mercury for 40hp think theres was 3 cylinder and 4700 is a estimate depends if u want power trim or electric start etc the goodies


5600 was a evinrude etec. now that stuff is really good in quality tell u that 3 years no maitenence they claim but u know there r little thing u have to do in between. flushing waxing etc general inspection. poursonally i would take it in about ervy 2 years at most.

so the really deciding factor is the dealer location corresponding to ur area price and the weight of motor and maxhp to ur boat etec tend to get a bit heavy 240 pounds for 40-60hp

 

[mthieme]

Re: 4cyl vs. 3cl vs. 2cyl?

I have some old Merc 2 strokes. 40 hp - 151 #, 50 hp - 199 #.
So the 2 strokes are going to be much lighter. This may or may not be an issue for this application.
We don't know biscayn8's budget. I assume he is going used because he doesn't wan to drop $5K (+/-) down (?).
The last two used motors I bought (coincidently the above mentioned) were $400 and $300 respectively. The first needed about $350 and the second, only an impeller kit.
I expect there are enough rigs floating around FL (like here on the Chesapeake) that something decent under $1K could be had.

 

[coolguy147]

Re: 4cyl vs. 3cl vs. 2cyl?

dont know much thing about mercs but i have a 1961 merc500 4 cylinder. idk i guess back in those days i would want a four cylinder motor i guess since its smoother? but how is it ? "smoother"

 

[Silvertip]

Re: 4cyl vs. 3cl vs. 2cyl?

Since we've now thrown diesels into the mix I suspect the engines you reference are turbocharged and likely have very different boost pressures which can vary HP and torque a great deal. Further, the difference in compression ratios can have a significant effect in turbocharged engines just as it does in gas engines. As for bore/stroke relationship, that all plays into the "service" the engine will see. As usual, this discussion got way off track over the the "all else equal" and "number of cylinders" issue. I still contend that if a two, three, and four cylinder engine of exactly the same displacements and design and all other aspects of the outboard are the same, the four banger will not have a hole-shot advantage. One would need to do considerable testing to arrive at the optimum bore/stroke relationship for the twin and three cylinder engines to make it a fair comparison. Engineers spend countless hours working those and other relationships to configure an engine for specific purposes.

 

[rodbolt]

Re: 4cyl vs. 3cl vs. 2cyl?

how is it smother?
as mentioned before,its all about levers,commonly known as either crank pins or rod journals.
its the throw of the crank that converts linear piston motion to rotary crank motion.
for 3 given engines with the same displacement the 4 cylinder will normally out accelerate and idle smother than the twin.
remember outboards dont have gear multipliers(transmissions) they use a fixed gear ratio and the propellor is the govenor.
getting back.
the 4 cylinder 2stroke will have a power impulse every 45* of crankshaft rotation with usually a smaller bore and shorter stroke than a 3cyl wich has a power impulse every 120* and a 2 cyl which is every 180*.
now say the crankshaft speed is 750 RPM, which engine is seeing the higher piston speed and the longer time between power impulses?
remember once a piston fires it tries to accelerate from a dead stop at TDC to a dead stop at Bottom dead center,BDC. however the other pistons coming up are actually trying to slow the crank down while pusing exhaust gas and friction.
so, 4cylinder 2 stroke,4 power impulses per crank revolution,3cyl 2 stroke 3 power impulses,and a 2cylinder 2stroke has 2 power impulses per cylinder.
but if you dont believe me line up two similar hulls,one with a 2cyl 25 jonnyrude and the other with a 3cyl 25 yamaha or suzuki and watch the jonnyrude get wlked like a dog on a leash.
now cost.
its way cheaper to build and maintain 2cylinders over three and three over 4.
and the debate goes on
my choice new would be a 3cyl 2 stroke, used would be the better maintained lowest priced that I could get dealer support for in my area.

 

[coolguy147]

Re: 4cyl vs. 3cl vs. 2cyl?

yes now i understand how it will be smoother. for my prefrence i would choose a older mercury four cylinder inline anyday ovr something if it wasnt for other factors parts etc.

but yes have a two cylinder motor pumping out 40hp would be less maitence.

if ur looking for a brand new motor i would go with evinrude etec but there prices r just so darn outrageous so go a merucy 40hp 3 cylinder four stroke though. believe its less then 5k in my area

 

[Silvertip]

Re: 4cyl vs. 3cl vs. 2cyl?

Rodbolt -- while you are absolutely correct about the number of firing impulses, the more cylinders an engine has (total displacement staying the same as 2 and 3 cylinder motors), each of those pulses produces far less power than the single bang on a two cylinder and still a fair amount less than that of the three cylinder. Couple that with the generally shorter stroke (or bore/stroke ratio) and in the end, things equal out. Do you ride a Harley by any chance? Perhaps a crotch rocket? Ever notice how a Harley can just lug away from a stop with little or no throttle while a multicylinder rocket needs some revs to get away (one must ignore the displacement difference in that particular comparison). Your comparison of the Suzy, Yammy and Johnny, although probably valid in the real world, is not a good comparison in this thread because the "all else equal" is not apparent. Displacements are different as are lower unit gear ratios and props. If someone has torque curves for any of the small/midrange outboards of nearly equal displacements it would make for interesting comparisons. Ideally those charts would be for the engine only, not at the prop. I will end my participation here with an example: A GM marine 4.3 (262 cubic inch) V6 motor develops 226 HP and 268 lb/ft of torque at 4000-5000 rpm. Its bigger brother, the 5.0 (305 cubic inch) V8 delivers 253 HP and 302 lb/ft of torque at the same rpms as the 4.3. I think everyone knows the 4.3 is the same motor as the 5.0 minus two cylinders. The 4.3 has the same stroke but uses the same bore (4 inch) as the 5.7. So in this example, it is very evident that torque closely follows displacement and the number of cylinders the engine has is not as much a factor as the bore/stroke ratio of those cylinders "all else being equal". You can look at those charts at the link below (even if this discussion has turned from outboards to engines in general). My contention here is that IF the 4.3 could be bumped to 5.0 (305 cubic inches) the torque of that engine would be the same or very nearly the same as the 305 V8

http://www.gm.com/experience/technology/gmpowertrain/engines/specialized/marine/marine_engines.jsp

 

[jay_merrill]

Re: 4cyl vs. 3cl vs. 2cyl?

Rodbolt -- while you are absolutely correct about the number of firing impulses, the more cylinders an engine has (total displacement staying the same as 2 and 3 cylinder motors), each of those pulses produces far less power than the single bang on a two cylinder and still a fair amount less than that of the three cylinder.

Exactly. The amount of torque delivered in each power pulse remains the same, it is not multiplied. That being the case, you end up with a constant turning force (torque) at the prop of "X." In the engine with equal overall displacement but fewer cylinders, you have higher displacement per cylinder. That gives you a turning force value of "X" plus some value.

One thing I would say, however, is that there is probably some minor benefit in loss of momentum due to a greater number of power pulses. This would be more a case of minimizing loss of torque, rather than adding to it.

My contention here is that IF the 4.3 could be bumped to 5.0 (305 cubic inches) the torque of that engine would be the same or very nearly the same as the 305 V8

In the case of these specific engines, I think you are right in regard to the "nearly" part, but there is something interesting to be observed. The 4.3 litre engine has 86% of the 5.0 litre engine's displacement, yet if produces just a tad bit under 89% of the 5.0's torque. If you use that 3% increase in torque to extrapolate to the higher displacement engine, you end up with an engine of equal displacement and fewer cylinders producing the same 3% improvement in torque. Once again, this happens in two engines with exactly the same stroke, which means that the mechanical advantage provided by the journal length (fulcrum/lever) is exactly the same. We also know that the engines are substantially the same (same CR, etc.), other than number of cylinders and overall displacement.

In my original discussion on this, I noted that I really didn't know exactly why engines of fewer cylinders tend to produce more torque for a given overall displacement, but that I thought it might be due to larger bores. Having discussed the matter, exchanged ideas and researched the whole thing, I do believe that stroke has a lot to do with the issue, but I also remain of the opinion that bore is a key factor as well.

The reality of the situation is that the relationship between the two is complicated. In the most simple terms, stroke length affects journal dimension, which affects mechanical advantage, but bore determines the amount of force which is multiplied by that mechanical advantage. Just to make life fun, throw in things like the fact that longer strokes tend to create engines with greater internal friction, and you have a very complicated scenario to figure out. What I think we would all agree on, however, is that engines of fewer cylinders but equal overall displacement, have more displacement per individual cylinder.

Another thing that I have noticed in looking at a few power/torque curves, is that a higher stroke to bore ratio tends to create a torque curve that is more flat than with a lower stroke to bore ratio. In the two GM engines, for example, the 5.0's peak torque is at 4,000 rpm, but the curve prior to the peak is relatively flat, and the motor produces nearly the same torque at 3,000 rpm. Conversely, the 4.3 motor develops peak torque at about 4,100 rpm, and the curve prior to the peak is steeper. The peak torque also falls off quicker in this motor. What this tells me me is that the motor that is "more stroked" will have a wider peak torque band, and that it will develop nearly full torque capability sooner.

 

[Silvertip]

Re: 4cyl vs. 3cl vs. 2cyl?

Even though those two engines are as close to being twins as they are, there are still differences. We don't know whether the cam profiles in both of those engines are exactly the same as the 5.7 liter which is really what the 4.3 is bred from. The 4.3 and 5.7 share the same bore and stroke but even there, we don't know about the cam profile (at least I don't without hitting the books). Likewise, comustion chamber volume and hence head design and valve sizes also play a big part in power/torque generation. The 305, even in automotive applications has not been an especially gutsy engine and certainly no match for the 350 primarily because of those factors. It used smaller valves and the heads didn't flow like the 350 versions. Enough of this. I'm outa here.

 

[mthieme]

Re: 4cyl vs. 3cl vs. 2cyl?

I think we lost biscayne8...probably going looking for a sail boat at this point!

 

[jay_merrill]

Re: 4cyl vs. 3cl vs. 2cyl?

Either that or a DD13 powered bass boat! Hmmmmmmmm ..... it would probably have to be no more than about 45' long!

 

[mthieme]

Re: 4cyl vs. 3cl vs. 2cyl?

Either that or a DD13 powered bass boat! Hmmmmmmmm ..... it would probably have to be no more than about 45' long!

LOL
Go with a DD15 and he could use the bass boat as the oil pan !

 

[biscayn8]

Re: 4cyl vs. 3cl vs. 2cyl?

Poor biscatn8...his first post...how the heck is he going fit a truck engine in his skiff!

No kidding! But this has been an interesting discussion. I'm going to keep my eyes open for a decent used 2 stroke. If I find one, I'll grab it- no matter the number of cylinders.

If none can be had, I guess I'll bite the bullet and buy a new one. The Tohatsu/Nissan TLDI is looking pretty good. E-Tecs seem to suffer from a weight/price disadvantage, IMHO.

Nate

 

[mthieme]

Re: 4cyl vs. 3cl vs. 2cyl?

Ah, there you are...wondering if we scared you off or something.
That's always been my technique for acquiring motors - opportunity!
My last find at the end of summer was a 1978 Merc 402. Evidently, someone needed bail money and I scarfed it up for a mere $100. Although it does a few things which haven't cost me a dime - yet.
I've only purchased one new in my life.
Keep your eye open, something will come along.

 

[Silvertip]

Re: 4cyl vs. 3cl vs. 2cyl?

Any engine from any manufacturer built in the last five years or so (two or four stroke) is a pretty good bet -- all cylinders aside.

 

[marquette]

Re: 4cyl vs. 3cl vs. 2cyl?

i've owned a 1994 40hp johnson (2cyl), a 1989 40hp Yamaha (3cyl) and spent a lot of time in a boat with a 45hp classic mercury(4cyl). hands down the mercury would out troll the other 2 and use about a third more gas. the yamaha trolled ok but a little fast. the johnson trolled terrible. they were all on similar boats, 16-17 ft V-hull fishing boats. the johnson and mercury are about the same getting the boat up on plane with the yamaha just a little slower to plane but not enough to worry about. the mercury and yamaha cost more to keep tuned and running right. the johnson was a gas it and go engine. if i understand flats fishing it is basically like bass fishing. WOT to the fishing spot, shut down, WOT to the next spot. if that is true then i would pick 2cyl 2 stroke johnnyrudes everytime. dump the VRO and they are pretty much mantiance free and reasonably priced with T&T.

 

[bman1bpm]

Re: 4cyl vs. 3cl vs. 2cyl?

Get a 2 cyl johnson/evinrude 2 stroke motor. You'll spend your time fishing and not worring about the motor! Low maintenance and bullet proof.

When it comes down to it though, if you find a boat with a motor, take the boat on a water test, if it runs well, and has been maintained then buy it if the price is right, it doesn't matter whats sittin on the back, what matters is the fish you're bringin in.