Power of 2 Stroke v 4 Stroke?

ChrisE

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Oct 31, 2002
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I am new to this board so apologise if I am asking something that has been often asked before.<br /><br />I am looking at a new motor and am mystified to read that 2 stroke motors that are rated at the same power as 4 strokes actually give more power.<br /><br />Is this really true and if so why? I can see 4 strokes are often heavier and that will make a difference but beyond that?<br /><br />In the real world what is the likely fuel consumption difference in the 75-90hp class? It matters at $6 a gallon!
 

JB

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Re: Power of 2 Stroke v 4 Stroke?

Hi, Chris.<br /><br />The horsepower rating of an engine is the maximum amount of work it can do at it's top rpm.<br /><br />So, a 70hp 4 stroke and a 70hp 2 stroke, each running at it's top rpm, are pretty much equal.<br /><br />But it is not horsepower that spins the prop. It is torque. Those same two engines running at lower rpm might make different amounts of torque.<br /><br />The typical modern 2 stroke makes a bit more torque at lower rpm than some of the 4 strokes, giving it faster accelleration. Head to head comparisons usually support this opinion.<br /><br />The louder noise that 2 strokes make, and the higher exhaust note both amplify the impression of more thrust. <br /><br />My personal experience is that it takes a stopwatch to measure the modest differences, and the 2 stroke is not a guaranteed "winner". :) <br /><br />Red sky at night. . .<br />JB
 
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DJ

Guest
Re: Power of 2 Stroke v 4 Stroke?

ChrisE,<br /><br />Horsepower is horsepower, and torque is torque.<br /><br />The differences are where these numbers cross each other, on a graph, and how quickly each one of these build and remain constant.<br /><br />The two stroke, by nature, develops torque quicker, thus giving a stronger acceleration. However, torque tends to fall off somewhat quicker in a 2-stroke.<br /><br />The 4-stroke, by nature, tends to build torque a bit more slowly, however it holds it longer-a flatter torque curve. The mid range is strong.<br /><br />As JB, said: (paraphrasing here) torque gets you going and horsepower keeps you going.<br /><br />In a smaller rig, weight is a very important factor. The extra weight of a 4-stroke can diminish its top speed even though the horsepower is the same as the 2-stroke.<br /><br />There are some 4-strokes out there now that rival the weight of 2-stroke counterparts. Particularly<br />the 4's made by Suzuki. These are also marketed as Johnson/Evinrude.
 

AndyL

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Jul 1, 2002
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307
Re: Power of 2 Stroke v 4 Stroke?

I am really glad you started this thread as I looked at replacing my 135 V6 Merc with a 115 $ stroke but was told I would suffer from a huge performance loss. The V6 will shove my boat fully loaded at about 36Knots at WOT and cruise comfortably on the plane (22Knots) at about 3800rpm. I was told that the max at WOT on the 115hp 4 stroke would give me 22Knots. Obviously I would rather have an engine that can cruise easily at this range than one running flat out. The fuel consumption saving would be nothing like the repair bill to keep a 4 stroke merc together when running at a nominal planing speed. Especially when the purchase cost new for new is considerable. In my mind I am staying with the carbed version 2 stroke and weep at the fuel later.
 

AUGIDAWG

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Oct 31, 2002
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Re: Power of 2 Stroke v 4 Stroke?

A Johnson mechanic told me, that a test was done between a 225 Yamaha 4 stroke, and a Johnson 225 2 stroke.<br />They had represtentatives from each company mount and tune their engines, mounted side by side on a boat.<br />The boat was run up to full speed, and one engine shut off and lifted out of the water.<br />I don't remember the numbers, but the 2 stroke was like 5 mph faster.<br />Then they tried to get the boat on a plane using only one engine.<br />The 4 stroke never made it, and the 2 stroke did it and got right back up to the other recorded speed.<br /><br />He also said that the Yamaha Owners manual states, that if you are running twin 4 stroke engines, and one fails, just idle back to port. Do not try to get up on a plane, or engine damage can occour.
 

JB

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Re: Power of 2 Stroke v 4 Stroke?

Hi, Augi. . .<br /><br />Dealer's wrenches are in the habit of telling you what the dealer wants you to hear.<br /><br />I seriously doubt most of what you were told, particularly his tale about Yamaha warning that the engine would be damaged.<br /><br />Do not take that as bashing the Johnnys, they are great engines, but so are the big Yammys, including the 4 strokers.<br /><br />Chris.<br /><br />Horsepopwer vs. torque curves vary greatly from engine to engine. In a 2 stroke, port timing and exhaust tuning make a big difference, as do cam profiles in a 4 stroke.<br /><br />It would be a mistake to generalize any statement about all 4 strokes or all 2 strokes.
 
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DJ

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Re: Power of 2 Stroke v 4 Stroke?

JB,<br /><br />I was indeed "generalizing". True enough the port timing and cam profiles make a huge difference. <br /><br />However, when comparing one engine torque/horsepower map over another engines map, one can get a pretty good idea of how one engine will perform under certain load conditions as compared to another.
 

Hooty

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Oct 2, 2001
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Re: Power of 2 Stroke v 4 Stroke?

Where are all the 4 stroke outboard performance boats?<br /><br />c/6<br /><br />Hooty
 

ob

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Aug 16, 2002
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Re: Power of 2 Stroke v 4 Stroke?

ChrisE, In the horsepower range of 75-90 expect around 40%(give or take) less fuel consumption of four stroke compared to it's two stroke counterpart.Depending on what type of boating you most often do (fishing,pleasure boating or pulling skiers)will dictate whether to look into 2 or four stroke models.Also, initial cost and frequency of maintenance is a consideration in selection process.By design 2 strokes will develop low end torque quicker,hence their use in moto cross applications.However at all out, horsepower is horsepower driving the same load.
 

JB

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Re: Power of 2 Stroke v 4 Stroke?

I don't think we disagree in the end, DJohns.<br /><br />One can certainly compare one model of one make to one model of another, but it is not true that all 2 stroke torque curves will have greater low rpm value than all 4 stroke torque curves.<br /><br />For example, compare the 70ci Yamaha 90 2 stroke to a 106ci JohnnyRude 90 2 stroke or a 119ci Suzi 90 EFI 4 stroke. All are the same "hp" but at modest rpm the torque must look like engines of very different hp ranges. <br /><br />I think we said the same thing from different angles. :)
 

ChrisE

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Re: Power of 2 Stroke v 4 Stroke?

Thank you.<br />I think that the answer is what I hoped. As long as the engine is not marginal at getting the boat on the plane & racing is not my objective I am not likely to find a significant performance difference.<br />I think the fact that I will need to do a lot of work at slow speed favours the 4 stroke with its good low speed economy and engine behaviour.
 

vinney

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Oct 29, 2002
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437
Re: Power of 2 Stroke v 4 Stroke?

The up side of a four stroke is quieter at idle. The down side is they need more maintaince. More rotating parts. On the big engines they lack perfomance. All the big fish boats run two strokes for a reason. You don't see triple for strokes on the back of a fountain. The DEA boats don't run four strokes, and imigration boats that run four engines have two strokes. Nothing turns on like a two stroke. Vinney
 
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DJ

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Re: Power of 2 Stroke v 4 Stroke?

JB,<br /><br />Yep, you're right (as usual). We went about from different angles.<br /><br />Now, throw another variable into the mix-displacement. That falls right in line with your engine comparison.<br /><br />I recall, in the early 70's, that John/Rude always published their torque curves in their procuct literature. Their reasoning, as I was told, is because they had a displacement advantage over their competition and it helped them achieve a better holeshot "feel". This was particularly true of the crossflow V-4's over the comp's. in line 4's.<br /><br />It was my experience, in my waterskiing days, that a V-4 would outpull a similar hp. in line. It didn't hurt either that the V-4 had a larger gearcase that could swing a larger dia. prop.<br /><br />Top end speed was a close match, as would be expected, when running at rated top rpm and horsepower.
 

Ficht Guy

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Sep 8, 2002
Messages
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Re: Power of 2 Stroke v 4 Stroke?

Get the best of both worlds...2 stroke DFI. Great torque and great gas mileage!! I believe Evinrude, Nissan, Tohatsu, and Yamaha make DFI engines. I have heard in some cases that the DFI's actually have less gas consumption that the 4-strokes...in some cases.
 

dmessy

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Re: Power of 2 Stroke v 4 Stroke?

New 2 strokes are very quiet, just a slightly higher pitch noise. I've mistaken newer 2s for 4s during operation.
 

vinney

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Oct 29, 2002
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Re: Power of 2 Stroke v 4 Stroke?

Two strokes forever. Dfi are the best. they gust require a little more care. Vinney
 

vinney

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Re: Power of 2 Stroke v 4 Stroke?

We had a customer last year we took his tired two strokes off an installed a set of 225 four strokes. He called us down to the boat for a cable adjustment. It had to be a cable adjustment. He went from 200 yamies to 225 four stokes. He said it lack top end performance we made sure it was set up correctly. He then gave us four weeks to get them of his boat and get his two strokes back on. We put new 200 back on. When I think of four strokes a think of a pontoon boat a cocktail barge. drinks any one. Vinney
 

Jacques321

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Jul 23, 2001
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Re: Power of 2 Stroke v 4 Stroke?

Different strokes for different Folks !
 

vinney

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Oct 29, 2002
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Re: Power of 2 Stroke v 4 Stroke?

I race a Thundercat with a Hoper racing engine. It smokes and run very, very fast. It,s a two-stroke. V
 

BillP

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Aug 10, 2002
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3,290
Re: Power of 2 Stroke v 4 Stroke?

There is no contest when comparing 2 strokes vs 4 strokes for speed. 2 strokes rule.
 
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