Great!!!! Post Bob,
No one said I could spell "stroker" ...I won't even blame my Canadian spell
checker.
Guess we are back to hot rodding!!
Your point about the side and bearing loads is well Taken. The only evidence
I can use is indeed anecdotal as the Camero we used to race went through
many iterations in it's history. One of them was the switch from the Chevy
Pink rods to the 6"carrillos as we ate a set up due to oil pressure loss
under cornering at the track. If my memory serves me correctly, on the dyno
the Original engine made peak torque of 448 ft lbs. at around 6100 RPM with
the 5.7 rods and a Crane TR-242 Roller Tappet cam with a 106 degree lobe
centerline. Under the suggestion of our engine builder to improve
acceleration, out of the corners we tried the 6" Rods expecting more RPM
durability and maybe the side benefit of more torque. In fact the 6" engine
made 485 ft lbs. at around 6200 RPM with the same Heads, Cam, Valve train
Gear, carb. and intake (We Bought new Pistons that were slightly lighter).
In fact the engine actually allowed us to go 1 jet size fatter before power
started dropping off. If anything the rods were heavier than the stock pink
rods and as such should have dropped power (but the pistons were lighter and
I believe they were the same compression). This engine version was far from
the final iteration that we built for the car. Now of course this is just
our experience and it could be that the rings seated better or the file fit
rings were done better but I find it hard to believe that the impact of such
minor things resulted in an increase of +40 ft lbs. I have always kind of
done things my own way so I do not expect others to agree I just try to
understand the dynamics.
As for the smaller tube header making better Torque than the big tube at low
RPM it makes perfect sense as the smaller tube would be more effective at
handling the volume of gasses and scavenging the cylinder of exhaust gasses
at low RPM (below 6000). In fact the smaller tube with higher gas velocity
may get the sucking of intake gasses started during the valve overlap
period. This would allow more burnt gas to exit leaving more room for the
new charge of un-burnt gas. At lower RPM flow velocity is more important to
the dynamics of engine performance in producing torque than flow volume due
to the gas mixing and scavenging advantages. At Higher RPM it seems that
there is a gas velocity threshold that is exceeded and to maintain effective
dynamics you need to increase the volume that the exhaust and/or intake can
handle.
As for theoretical analysis I agree the area under the curve would appear to
be the same, as the curve shape is similar. Our torque curve appeared higher
on the chart at most RPM so in our case while the profile of the curve
seemed similar we actually realized greater area under the curve for our
usable RPM Range at the time (We ran 5500 RPM to 7400 RPM).
Our experience with a 1 jet size increase for max power and the increase in
peak Torque is far from the definitive example as we probably did not follow
ALL of the rules of the scientific method. We may have changed plugs and
plug wires as well as other things like different oil. The experimenting we
did way back still seems to me to be valid but all I can say is that it sold
me and it will be a consideration for which engine I build.
I LOVE this list ..... where else can you feel like you are playing hooky
from work without leaving the office.
Thanks for the response Bob!
Regards,
Tim
-----Original Message-----
From: Bob Palmer [mailto:rpalmer@ames.ucsd.edu]
Sent: Wednesday, March 29, 2000 11:38 AM
To: Ronak, TP (Timothy); 'jpmorrison@erols.com'
Cc: 'Tiger News Group List'
Subject: Re: Strocker Motor
Tim,
The chief Tiger owner "Strocker" [sic] or Stoker motor
exponent in S.
Calif. is Ken Mattice. His 347 motor doesn't seem to have
any more cooling
problems than average, although his cooling system isn't
typical either.
There are intermediate stages of stroking between 302 and
347 that might be
a better compromise. For example, a 330-incher with
5.135-inch rods. The
problem I see with this and other discussions of engine
performance options
though is the lack of a specific goal. Just what are we
trying to achieve
and what are the priorities? BTW, the only replacement for
displacement is
cubic bucks!! More displacement may be a cheaper way to get
more power, but
higher rpms works too. If you build a 289/302 you can safely
run to, say,
8,500 rpm AND let it breath adequately, then you can equal
or beat the peak
power of a 347 stroker with it's inherently lower rev limit.
On the other
hand, the 347 will be a lot more tractable for the street
since it will
work better in the low rpm range. Using a roller cam helps
both motors run
good over a broader rpm range too. I'm looking at an article
describing
building a 302 with the goal of 400 HP. They achieved
388@6,500 with
188@3,500 to give you and idea of the overall curve. That
was with the
Edelbrock Torker manifold. With the performer manifold it
got a peak of
346@6000 and it was 190@3,500. Another point of interest,
comparing 1&5/8
versus 1&3/4 headers, the 1&5/8 made 25 more horses at 4,500
rpm and the
1&3/4 only got 10 more horses at 6,000 rpm.
The Rod-Length-to-Stroke-Ratio subject was a long running
thread on the
List about a year and a half ago. DrMayf, the Sunbeam land
speed record
aspirant and retired aerospace engineer modeled the rod
length problem and
came up with the these five conclusions:
1. Longer rods have lower piston/cylinder side loads, hence
less cylinder
and piston wear.
2. Short rods have a higher axial loading, hence potentially
more bearing wear.
3. Short rods produce higher torque early, less torque later
(in the cycle).
4. Short rods produce a higher peak torque (in the cycle).
5. No matter what the rod length, the area under the torque
curve is
EXACTLY the same.
I would add, as a footnote to conclusion #2, that the
increased axial
loading is going to translate into rods breaking at lower
rpm, which might
be a little counter-intuitive. On the other hand, as DrMayf
points out, the
longer rod is heavier which creates more inertial forces. My
bet would be
on the longer rod for high rpm though; somewhere between 1.7
and 1.9 would
be good. BTW, the SVO 302 uses the longer 5.155 rod versus
5.092 standard
rod, shades of the old 302 and BOSS 302 motors. (Hard to
believe a silly
1/16" added length could be that big a deal. Sure would like
to know just
what those engineers were thinking!)
Theory aside, there is a lot of anecdotal evidence by
individual engine
builders that longer rods make more horsepower. (Although,
on the other
hand, there's a lot of contrary evidence too.) I suppose, as
you suggest,
breathing differences could be a factor. However, it seems
like better
filling of the cylinders would be better served by more time
at BDC since
that's when the sucking is happening. But the situation is
way too
complicated and dynamic for simplistic reasoning. Take for
example, the
effect of bigger header tubes. What about longer dwell at
TDC? It's
possible, but the actual differences in time at both TDC and
BTC, comparing
say 1.5 versus 1.7 rod/stroke ratios are very, very slight.
I'd put all the
concerns about power differences aside and stick with stress
advantages of
the longer rods as the main point.
Hope you're having a great day,
Bob
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