2 - Stroke Power bands

[Splash]

Hi Team,

Coudl not sleep last night, so I popped onto my fav website (Ausfish Of course ;D ) and read about power bands (narrow RPM range) in modern 2 stroke engines vs consistent (across broader RPM range) power bands for 4 strokes.

I would'nt mind some discussion be had on this topic - relating to pros and cons of having narrow optimum power bands as opposed to broader optimum power bands....


THanks

SPlash8-)

[FNQCairns]

Splash power bands are engineered into motors, they can be engineered into 4 strokes even. Many try and relate 2 st bikes which are on/off throttle engines to marine 2 st bu tthe comparison is weak, the marine 2 st is designed for constant throttle settings and for there to be torque at all rpms available.
In the past I have modified marine 2st engines to have a powerband with port changes amongst other changes, if you ever drive a marine 2st that has has a powerband and proped like a fishing boat you will see what a powerband really feels like on a boat.

Our 2 st marine engines don't really have one in the traditional sense.

cheers fnq

[Noelm]

if you look at most Manufacturers Torque graphs, you will see that most 2 strokes have a very steep "spike" whereas as 4 stroke has more of a "flat top" curve, remember we are not talking anything about how much or anything else, just the shape, so if (say) a 2 stroke has max spike at 4,000 RPM, but has bugger all (by comparison) ot 3,500, if you drop below the "peak" for whatever reason, like following Sea and climbing the back of a Wave, the Motor drops off it's "happy spot" and will bog down, whereas a 4 stroke can drop a ouple of hundred RPM at the same speed and still be on the "flat top" so will continue to push on, now some of this means bugger all if the Boat has good Power and can maintain speed anyway, but is really noticable if the Hull has just enough HP, and some throttle adjustments are necessary, now I fully agree that Motors can be built to have all sorts of curves and power bands and so on, but the fact is most manufacturers will try for absolute Max and sacrifice some "low down" to get more out of what they have, even if it is for Marketing purposes, most new tech 2 strokes will run really well at a constant 4,500 plus RPM, and indeed well over that figure, but above that, they all start to drink fuel at a very high rate, the difference between (say) 4,000 RPM and 5,500 is almost double (2 or 4 stroke) so that is it in a nutshell, it can be changed if "they" think it will benefit sales/use but most will not, now this is just my view on it all, and indeed it is also my own personal observation on the two operating principals, I have owned both, used both and do not have a real preference of either, if I could get a 4 stroke at the right price, then I would, an Etec, Opti at the right price, then I would, end of story ,any Dealers out there that need a tester, just let me know, I am looking at a repower very soon!!! and do not care about brand.

[Grunter71]

Off topic, but, whereabouts do you get hold of manufacturers torque graphs? I have not had any luck finding them on manufacturers websites.

[Noelm]

mostly I had some from some advertising "blurb" from a Boatshow about 2 years ago, but I guess/think they must be available somewhere still, unless they are trying to hide something, most manufacturers have all that sort of stuff (modified to suit their brand of course) available on Brochures and so on, will have a look about and see what I can dig up.

[Splash]

great stuff!

[Noelm]

If you have a uick look here http://continuouswave.com/whaler/reference/graphics/torqueGraphETEC.jpg it shows the Etec curve as very sharp to about 3,000 RPM, quite a bit lower thn in the past (it used to be about 4,200Approx) but the 4 stroke is very flat (and much lower) but we are not doing any real bigger/better/more comparison here, just a simple shape comparison, I will look for a much more detailed one as well, but it can be seen, if your Etec drops a few hundred RPM below 3,000 (slow cruise??) you are bogged!

[Noelm]

also midly intersting is the way they both die after about 5,000, in past models (especially the Fichts) the curve was almost a vertical line from about 3,000 to 4,000RPM below that was stuff all Torque, the difference these days SEEMS to be slightly less, but still relevant (I think anyway)

[Outsider1]

Continuous Wave are blocking that link unfortunately Noelm>:(

Cheers

Dave

[Splash]

If you have a uick look here http://continuouswave.com/whaler/reference/graphics/torqueGraphETEC.jpg it shows the Etec curve as very sharp to about 3,000 RPM, quite a bit lower thn in the past (it used to be about 4,200Approx) but the 4 stroke is very flat (and much lower) but we are not doing any real bigger/better/more comparison here, just a simple shape comparison, I will look for a much more detailed one as well, but it can be seen, if your Etec drops a few hundred RPM below 3,000 (slow cruise??) you are bogged!

Amazing!

This would have to be a drawback owning an ETEC - would it not?

(no intention of starting WW3 here BTW...);)

SPlash

[Noelm]

not necessarily, it has a lot more power, but it is a sharp climb, but if the RPM's are kept up, then it is perfectly OK and in some ways even better, BUT it is a not a so desirable "feature" if you are (say) in tough conditions, then a "clean" plane is difficult to maintain (maybe) but in say a Ski Boat or more normal conditions, it will be perhaps superior than a 4 stroke, and at slightly higher RPM (that a 2 stroke loves) it will be a better package, but certainly not a clear winner on either types, so the "war" will be a forever event, mines better/bigger/faster/quieter than yours!

[Noelm]

Outsider, I have just had another look and it seems OK.

[Outsider1]

This is what I get when I click on the link.

Dave

[Outsider1]

I have got around it by amending the link to take me to their Graphics directory first. Thanks again for the link.

Cheers

Dave

[Outsider1]

The E-Tec torque chart is certainly much more peaky, but the difference in Torque surprises me as well. I would have thought it might have 10/15% more peak torque, but that chart suggests it is upwards of +25%.

Dave

[Noelm]

I guess it is not all that startling, and may even be provided by one of the Manufacturers, but it does sort of show how the power drops off rapidly in the Etec, even though it would seem they have changed the Engine design/Computer or something to drop the RPM down to about 3,000 instead of the "older" 4,000 plus they used to be, the fact still remains from my original "observation" that the two strokes do not "cruise" as good as a 4 (maybe) the older Carby 2 stroke is almost a "Mount Everest" Spike, and either side it is a slug.

[Splash]

not necessarily, it has a lot more power, but it is a sharp climb, but if the RPM's are kept up, then it is perfectly OK and in some ways even better, BUT it is a not a so desirable "feature" if you are (say) in tough conditions, then a "clean" plane is difficult to maintain (maybe) but in say a Ski Boat or more normal conditions, it will be perhaps superior than a 4 stroke, and at slightly higher RPM (that a 2 stroke loves) it will be a better package, but certainly not a clear winner on either types, so the "war" will be a forever event, mines better/bigger/faster/quieter than yours!

Thanks Noelm.

For the extra odd $17K, I would expect yours to be better, bigger, faster, quieter than my $0K donk 8-) .

Thanks for your input :) .

Splash

[Hamish73]

As mentioned before , the power delivery of an engine is decided by the engineer. Any engine, be it 2 or 4 stroke, will be most efficient at a certain RPM. For 4 strokes, this is generally goverened by the port velocity, or the speed at which the air travels through the cylinder head. This is why manufacturers have developed variable valve timing in an attempt to widen this 'high efficiency area'. By reducing valve lift at lower RPM's torque can be increaced. It is also why a pro stock drag car engine will probably make less power than your average car at 5000rpm. They dont come alive until much higher RPM.

2 strokes are also controlled by the size of their ports. The port timing on a 2 stroke is fixed (as the port is literally a hole in the cylinder) with the exception of the exhaust port timing on power valve equipped engines.

[Jabba_]

2 strokes are also controlled by the size of their ports. The port timing on a 2 stroke is fixed (as the port is literally a hole in the cylinder) with the exception of the exhaust port timing on power valve equipped engines.

And that Power Valve, I believe was introduced into 2st by BRP Via the 125cc Rotax Max Go Kart motor..... They are now using the Power Valve in there new E-tec Ski-Doo's, Which they now call a R.A.V.E valve.... http://www.ski-doo.com/brphtml/skidooenginetech/en/Index.htm.... I am positive it is only a matter off a few years before we see these Rave valves used in the Evinrude...

[Hamish73]

And that Power Valve, I believe was introduced into 2st by BRP Via the 125cc Rotax Max Go Kart motor..... They are now using the Power Valve in there new E-tec Ski-Doo's, Which they now call a R.A.V.E valve.... http://www.ski-doo.com/brphtml/skidooenginetech/en/Index.htm.... I am positive it is only a matter off a few years before we see these Rave valves used in the Evinrude...
they have been in motorbikes since the 80's that I know of, possibly before.

[FNQCairns]

The only reason we have not seen them in our marine 2 strokes is becauase being a constant throttle engine application and stuck in history with such a low grade of 2 st oil (tcw2/3) coking of the valves was an insurmountable problem.
Now with some marine manufacturers opting for quality 2 st oils in marine applications a power valve of sorts may be on the cards one day not too long away, will make for awesome performance and a tractor of a marine 2 stroke if it does happen.

cheers fnq

[Hamish73]

The only reason we have not seen them in our marine 2 strokes is becauase being a constant throttle engine application and stuck in history with such a low grade of 2 st oil (tcw2/3) coking of the valves was an insurmountable problem.
Now with some marine manufacturers opting for quality 2 st oils in marine applications a power valve of sorts may be on the cards one day not too long away, will make for awesome performance and a tractor of a marine 2 stroke if it does happen.

cheers fnq
good point FNQ..
Out of curiosity, how much is the top of the range boat 2T oil?
The stuff I run in my dirt bike retails for about $100/4 litres (and leaves the PV and piston skirt clean as a whistle) , but based on a ride every 2nd weekend that lasts for about 10 months.

[Outsider1]

For Evinrude XD100 it is $55 to $75 for a US Gallon (3.78 litres) Hamish depending on who has it on special, so similar sort of pricing.

Cheers

Dave