2 Stroke Or 4 Stroke Options

[poly]

hears one for the team, i am in the market to up grade my motor, i have decided to go yamahar for one reason only as there is not a lot difference between brands, yamahar have a much better setup on the tiller handle with all controls at your finger tips, the weight difference between the 2stroke and 4stroke is only 9kg so no real advantage there.
so here is the kicker which is better 2 stroke or 4 stroke

[Spudly]

Smaller motors 2 stroke, Larger (90+) 4 stroke

Most smaller boats I have been in with 4 strokes have been underpowered, which can be fixd with a bigger HP, however there goes your weight and cost!!

Modern 2 strokes are nearly as fuel efficient and I personally think more reliable in smaller applications. Ie, they can cop a bit more punishment.

Yamaha! Agreed, THE BEST, all my motors will be Yamaha, Same as My Bikes!

[SUMOFISHIN]

Poly, if you want more power than you have now try comparing a 40hp two stroke to a 50 hp four stroke as the lack of acceleration power in the 50hp 4 stroke would make it equivalent to the 40hp 2 stroke so do your sums again on the weight difference 50hp 4stroke114kg ::40hp 2 stroke 84 kg making a 30kg difference to achieve the same Torque.

[kkw]

I tried out my 'new' 40hp Merc yesterday. What a buzz. It really powers the 4.2m quintrex at a high speed. I also trolled for 40 mins at just above idle speed, and it didn't foul up the plugs. I think I will need a sandbag up front though, when I go out on my own. My wife says a better option would be to lose some weight
I was really keen to get a 50hp 4 stroke, but I am very happy with the 2 stroke I got, and after an hour plus of running around, used less than 1/4 tank of fuel. The old 25hp ('76 model) would have used 1/2 to 3/4 of a tank.
So poly - I'm with 'The Chadd'. If it is a smaller motor, stick with the 2 stroke.

[poly]

Sumo i asked the bloke about prop size for the two motors and he told me it would be the same for both motors, i do agree that the two stroke should have a better hole shot but over all the top end should be the same as they both have the same revs but i will check it out a bit more

[Fed]

You hear this all the time (2 strokes being more powerful than 4 strokes), it just doesn't make sense.
A 50 HP 2 stroke should be the same as a 50 HP 4 stroke.... end of story.

Judge Judy says, "If it doesn't make sense then it's not true".

Perhaps people are being fooled by the motors developing their power at different rev ranges?

If weight is not a factor and unlimited finances I'd buy the 4 stroke.

[Spudly]

power is calculated as a MAX at the engine,

While they may have the same HP Reading they will not have the same torque! Which give the thrust to drive the boat forward, Thus the reason most 4's have smaller props.

Compare Cars to Diesel's

A HSV 5l V8 Might have Heaps more HP, But you would do better pulling a large load with a diesel with less HP (most semi's only have around 400 or less, where v8 HSV might have more HP but will never pull the load!

Some trucks still runs 2 stroke engines also due to the power curve of the engines!

[Spudly]

Compar bikes also.

A 250 2s has less power but is quicker and can pull harder than a bigger 4s

[Fed]

power is calculated as a MAX at the engine,

Outboards have been rating HP at the prop for the last 20 years.

While they may have the same HP Reading they will not have the same torque! Which give the thrust to drive the boat forward, Thus the reason most 4's have smaller props.

Sure, you can have different props and different gear ratios but at the end of the day 50 HP at one prop equals 50 HP at the other prop.

A HSV 5l V8 Might have Heaps more HP, But you would do better pulling a large load with a diesel with less HP (most semi's only have around 400 or less, where v8 HSV might have more HP but will never pull the load!

500 HP will outpull 400 HP every day of the week.
It's all in the gearing.

A 250 2s has less power but is quicker and can pull harder than a bigger 4s

That's comparing apples to oranges, weight and gearing in different bikes changes everything.

If it doesn't make sense then it's not true

[Spudly]

Outboards have been rating HP at the prop for the last 20 years.
Sure, you can have different props and different gear ratios but at the end of the day 50 HP at one prop equals 50 HP at the other prop.
500 HP will outpull 400 HP every day of the week.
It's all in the gearing.
That's comparing apples to oranges, weight and gearing in different bikes changes everything.

If it doesn't make sense then it's not true

Ok, Ive trid to word it simply and simplified it to much!

My mistake on the engine and prop, however that make no differance, and the gearing also makes no differance to what we are comparing.

My point is that 2 stroke engine have a higher power output than 4 stroke engines with th same rated horsepower.

To start, Horsepower is simply a mesurment from made from torque ie:

Torque x RPM
HP = ------------
5252 (= 33000 foot pounds per min = 1hp , at 5252 HP and Torque are always equel)


Get a print out of the power and tourque curves of the 2 engines and compare them, See how much longer the 2s power curve is at full or near to full power compared to the 4 stroke.

While horsepower ratings may be the same they are simply a meausurment, 2s engines deliver the power (torque) in a differant way to a 4s, The power curve is held in a higher range for much longer simply because a 2 stroke engine makes power (torque) EVERY Cycle where a 4 stroke makes power every other cycle. While there is load on the engines the
2stroke is driving EVERY STROKE, wheras the 4stroke has a rest each cycle.

Large capacity engines improve on this with heavy flywheels and pistons which help to hold momentum, (ie trucks)

I can get MUCH MUCH More technical if you would like but most peeps (thats for you jumpy) would not understand.

a 500hp V8 Holden would get smashed by a 400hp Diesel SIMPLY Because the Holden motor could not hold it engine at full HP Due to th power only peaking for a very small rev range, where the diesel has a much longer peak. (unless you could hold the engine on the same revs for long periods which creates its own problems)

This equates to boats as you are on and off the throttle and thats where the 2stroke wins, UNLESS your in a big boat with Big outboards!

Hopefully Ive comvered this in slightly more detail, im busy and i have rushed my reply so i will re reaad it later and check!

This post has been edited by Rum Dust N Ruckus: Aug 26 2007, 09:41 PM

[SUMOFISHIN]

Poly, if you believe that it doesn't matter just ask the Dealer which one he makes the most money from, The motors both have the same HP BUT there is a lot of difference in torque, as an example if you were going 50klms the two stroke will get on the plane a lot quicker than the 4 and be two kilometers away before the 4 stroke gets you on the plane,;; sure once the four is on the plane it will develop the same power and always be 2 klms behind BUT if you are only going 2 klms GUESS WHAT.

[Spudly]

and dont forget

that that it all comes into effect everytime you are on and off the throttle to slow down for a wave or cause you dropped your beer!

Slower response time!

[poly]

there are other things to consider other than hole shot, yes i do agree that the two stroke will have better acceleration but we are not talking about a race boat, with fishing as it's main roll i would think that the quieter the better and at idle the four stroke will be almost silent fuel should all so be considered, the little 30hp is capable of draining a 25lt tank in a 3hr session so i beleave thease things must be taken into acount, what do u think

[Spudly]

you are upgading from a 30hp 2 stroke to a 40hp 4 stroke????

That changes things, as you should expect similar performance from the two engines,
however like you say, better fuel economy and quieter..

The 40hp 2 stroke will give you better performance but if the 30hp pushes it along fine
then the 40hp 4 stroke should be more than adequate.

Poly, it will also depend on your use of the boat, If your crossing bars or heading outside,
then more power the better. Especially when crossing bars when you might need the quick
respose of a 2 stroke. Though again, I didnt realise you were upgrading to a 40 from a 30
so It shouldnt be a problem..

[poly]

no chad the power is not adequate that's why the up grade in the first place + the need for p t&t as she holds her head up to high when running into a chop.
at 4.1mts i will not be crossing to many bars, well maybe the odd pub bar
that is why i asked the question in the first place, now if i under stand what u are telling me the 40hp 4 stroke will be about the same as a 30hp 2 stroke in performance is that right or can i expect a bit more grunt, thanks :o :o

[Spudly]

Never driven the two of them poly, but i know going from a 25hp 2 stroke to a 30hp 4 stroke was like going back to rowing!

the 30hp struggles to put the 4.1m boat on the plane with 3 medium sized people!
I must say though that it is VERY quiet and great for trolling and fuel efficiant...
But i have been caught in some big chop and it did not have the power to get over the waves when I would have liked..

Have you posted this on the Poly Forum and spoken with anyone with the same boat?

[STEVE.P]

poly want a motor, whats the diference {price} of a 40hp - to a 50 hp 2 stoke if the price is not mutch I would go the 50 reason being you are not pushing the motor , making it work . and if you need to get out of some where in a hurry the power is there . always better to have more than not . and at normal working pace the motor is just huming along.

[Fed]

I'm with you Poly, hole shots, who needs them, certainly not a 4.1M tiller steer being driven by an old fart.
I'd turn the boat into a little centre consol while you have this opportunity of a new motor.
Have you thought about starting again with a whole new boat, there's a lot to said for boats with some shelter from the elements especially if you take your Missus out.

RDnR, do you think the 40hp 4 stroke would have a higher top speed than the 30hp 2 stroke?

[Spudly]

Speed is irrelevant as it is to do with gearing, NOTHING to do with Torque or HP

What I am saying is that the response and power of the two 40hp's Is NOT the same,

And this can be important, NOT for hole shot speed but in the case where you need then quick power
in big chops or when crossing a bar, Better to have it when you need it than not to have it at all..

[Fed]

Calm down mate I'm only asking your opinion.

RDnR, do you think the 40hp 4 stroke would have a higher top speed than the 30hp 2 stroke?