Del Cecchi wrote:



So this is some alloy significantly different from the aluminum alloys
used for conventional pistons?

Not significantly, but to be labeled "high melting point" they'd better
be made to withstand more.




Are you saying that this special oil is necessary to achieve reliability
in etec?

I do not believe this is a requirement, only if you want to commit to
the lower volume setting / longer refill intervals.





OK, here is a calculation. Air has a specific heat of 723 J/kg-degree,
and gasoline has a latent heat of vaporization of 300kJ/kg. So at 15 to
1 you get 20kJ or about 30 degrees celsius, less at leaner mixtures.
That about right? Is that really significant in preventing detonation?
Doesn't seem like much help in cooling the motor to lower the temp by 30
degrees.

The heat of vaporization is not the only critical issue. A two stroke
engine by design has to pass some unburned fuel, as it also must put up
with some exhaust gases in with the new (a trick used in 4 strokes to
lower the combustion temp & oxides of nitrogen for emissions compliance).

2 strokes have always been run richer than "stoichiometricly" required
to provide cooling via unburned fuel.

Again, it's an amazing balancing act that the 2 stroke manufacturers are
attempting in keeping power up, economy up, longevity up, & weight &
failures down. Fast acting processors & high pressure injection systems
are the only way to do it. BRP is in this for the long run.

And saying what the "properly controlled by ECM injectors *should* do
sort of begs the question of what they actually do.

Well, a 14.7:1 A/F Ratio is ideal, but not attainable in practice, as
there is not enough time in direct injection to vaporize the fuel. A 2
stroke will leave behind some exhaust gasses too. The injector must
be able to atomize the fuel into droplets small enough to approach
vapor, but large enough to absorb some of the heat of combustion. What
E-Tec is trying to do is minimize the losses associated with
overlapping strokes in 2 stroke design theory, while using as little
fuel as possible. The chamber design is very critical.

The above is sort of a truism, eh? Droplet size shouldn't matter in heat
absorbtion because it all evaporates as it burns, right?

Not in a 2 stroke.

The trick is to
get it to evaporate so it can be burned at the right time.

Only the part you want burned for power...

How does running an engine at power on hugely lean mixtures stop
detonation??

This is the balancing trick - and hence the other tweaks - the lean
charge must also be enough to complete the cooling process too.

it caused the base heat buildup that starts detonation.


How does a beefed up huge extra drag gearcase stop detonation??

Keeps the Sidewinder/Switzer kids from picking this model to run WOT
all day long up on a jack plate...


Better not put one on your bass boat. Those guys love their jack
plates. And they love WOT.

I just made that up.


Oh. I have heard that some of the etec motors were having lower unit
problems, not that that would have anything that I can see to do with
the fuel induction system .

I agree on that. I just did not see the correlation, unless the E-Tecs,
once broken in, are putting out more than they are rated for or BRP got
poor leg parts from their supplier.



The answer is none do: all those things are a lame (is not an MP3
emulator:-)) attempt to make the engines able to withstand the
inevitable detonation consequences when run lean at power on poorly
atomised low pressure injection with very questionable lubrication.

Thanks for the E-Tec spruik Rob but hey I'm sure you got a lucrative
deal to fit them?? much more dollars than the other brands???

You REALLY don't know me...

but what is it to your advantage to de-spruik something you don't
understand?



Always looking for technical details. The geekier the better.

del



I can see the dilemma and confusion K. Smith is facing, but the engines
do work. The BRP units are an improvement over the OMC offerings,
which did have isolated issues - as there are thousands still in use.



Yes, but there was a significant failure rate among at least the 150/175
Fichts. Is more rugged construction, (presumably) higher pressure
injection, and new combustion chamber/piston shapes sufficient to resolve
them?

We'll have to see. BRP gives you a "free ride" for 3 years.

I have been curious for some time about why the 150/175 was said to have
many more problems than the big blocks. Do you have a theory? Or even
facts?

No facts, but the differences between similar architecture V's are
several when you are dealing w/ different bores/strokes (I'm not sure
which spacing is shared between them) and then the injector volume must
be changed too, it's not like the carby days where you could change the
HP w/ a throttle bore change & an exhaust tuner.

Rob