It is all about the ratio of the rod to stroke not the absolute number of the
rod length.
The 3.35 stroke with a 6.125 rod gives a ratio of 1.82 in round numbers. A race
engine
Less than a 3 inch stroke with a 5.7 rod at 1.90. A production passenger car
engine
If you take the extreme. 6.2 rod with a 2.85 stroke you have a 2.175.
I have no clue what this is for...
The other end of the scale is a 3.5 stroke with a 5.7 rod at 1.62 a truck
engine..
A little side line note...
I have run an injected 5.85 rod with a 2.83 stroke and it ran well a ratio of
2.07. But it had EFI and made peak torque at 8900 and peak power at 9900. On the
track we ran it between 8500 and 10500. Even with the EFI it sounded like it was
swapping cylinders below 6500. Big cams big heads long rods and no air speed
below 7500. The reversion was so bad at 3500 rpm I had to double the fuel in the
EFI just to keep it running as most ended up on the ground. It set a record but
was a poor combination in the long run.
Back to what we are talking about..
These are all completely different engines in cylinder head requirements.
When the port minimum restriction and volume are held constant there are some
relationships that can not be avoided because the difference in the ratios
changes the peak air speeds in the runner. A smaller ratio will make more torque
sooner and run out of air sooner as well. this is because the peak airspeed in
the port reaches the critical pressure ratio while the mean airspeed seems
modest. To fix this you add a longer rod but like all good things there is such
a thing as too much. For the sizes of most heads and the ratio of runner to
valve size there is a sweet spot this is generally around 1.8 to 1.9 for most
push rod v-8 engines. If you have to run a carb it is closer to 1.8. the reason
for this is unlike EFI a carb has to have a signal in the form of a delta P in
order to start to meter fuel from the main circuits. With EFI you can stretch
this out to 1.9 and still be fine and gain a little top end. Now if you make the
cylinder head smaller in cross section and volume then you can and should run a
longer rod. This is because you will reach the critical pressure ratio sooner if
you did not change the rod length. Super stock drag race engines have
historically run a very long rod and high ratio because they have to run
production castings that were not intended for racing and are very modest in
both cross section and volume. So it is a combinational thing a 6.2 rod is a
good thing if it makes an appropriate ratio that is suitable for the cylinder
heads you are running either by choice or by rules mandate. When you buy a set
of heads either new or used the first 2 questions that ought to come to mind are
what size engine and what rod ratio were they designed for? The other obvious
one is what rpm range as well.
What you can't do is take the rods out of the winner of D super stock and the
intake off a truck and the heads off a winston cup car and run them together
just because they were all winning engines in their class. They are winners
because of all the other parts around them, not because of some individual
secret.
A case study of a long rod engine.
In stock trim a 302 SBC has a 3 in stroke and a 5.7 rod. The heads are very
modest '492 Chevy castings 2.02 X 1.6 valves and a dual plane aluminum intake.
about 295 hp and 310 ft lbs on pump gas. Rod ratio is 1.9. This engine was
developed for trans am racing and was needed because the rules said you have to
sell them in the car if you want to run one at the track. It was tossed together
from existing parts to fill that need. A 327 block and rods with a 283 crank and
the 327 heads and intake back in '67. GM never assumed it was the correct way to
do it just that it filled a need size wise with no tooling or design costs. They
were pretty gutless if you don't lie to yourself and try to remember them faster
than they were. They needed rpm and lots of it to get going. A good 327 300hp
out of mom's station wagon was in fact a faster setup. With the 3.25 stroke and
5.7 rod it had a ratio of 1.75. Carb worked better made more torque and still
had about the same rev range if it ran the same cam. This was not about the 25
inches at all it was about the geometry.
Now fast forward
Winston cup 355.. 4.125 bore 3.35 stroke and a 6.125 rod.. Rod ratio 1.82.
Halfway between 1.75 and 1.9, Results good power band lots of torque and still
can and will run at high rpm with the right cam. port sizing is similar ratio
wise as is the relative valve sizes to each other. In all a well balanced
combination.
Now to the extreme
300 inch D engine for Bonneville 4.125 bore 2.85 stroke and a 6.2 rod. Has to be
good because it has more of all those good trends. Big bore for good breathing,
short stroke for high rpm and long rods because all the Winston cup cars run
them. Rod ratio 2.175 everything is at max race mode. Torque is 400 ft lbs hp
around 560... RPM range is beyond practical valve gear limits.
Now for the reasons for this.
To start with you have to set some guidelines as to what good is.
Typically a well developed race engine has between 1.5 and 1.65 ft lbs per cubic
inch and a hp rating around 2 per cubic inch for a 90 degree push rod v-8 N/A on
gasoline. The larger the engine the harder it is to get these numbers. On the
dyno if I can't get 1.5 ft lbs I will generally suggest we shut if off and quit
burning gas and making noise and find out what is broken. If nothing is broken
there are 2 things that contribute to these numbers. Cylinder head design and
related air speeds both peak and mean and the angular relationship between the
piston rod and crank. IE mechanical advantage to turn the crank when the
pressure in the cylinder is at a useful level to make some torque. It is after
all torque that we need to make hp is merely a mathematical model as to how fast
we can make it. In order to have HP you have to have the torque.
The level of 'goodness' of the 3 engines in question..
The 327 is most typically raced as 331 a 0.30 overbore to clean up a used
block.. I don't think anyone would question a 495 ft/lb to a 520 ft/lb version
of this with 635 hp..
Power is down a little but the rod ratio is at the bottom of what you might
expect for a race engine..
The Winston Cup engine...
355 cuin.. 575 ft/lbs and 720 hp. not race day anymore but very respectable. rod
ratio 1.82 and heads sized correctly.
the D engine..
ought to make 450 to 495 ft lbs and 600 hp or a tiny bit more..Typically these
engines do with a 3.00 stroke and 5.85 rod but need some RPM to get it done. The
5.85 rod is used often to get the piston weight down rather than a 5.65 or 5.7
rod. GM makes a block so you can run a 5.65 rod and a 3 inch stroke to make
things happy in the ratio department..
Well what happened to our 300 inch to the max setup? Rod ratio is very long
there is no mechanical advantage and the time that we have usable pressure to
convert to torque is short because of the short stroke. The cylinder heads are
18 degree and made for a 355 so the air speed in the ports is very slow and has
no inertia to fill the cylinder. The port speed is so bad in fact you will
probably have more liquid fuel in the floor of the manifold sloshing around
rather that anything that is atomized at all. How slow is slow.. Well we lost
17% mean air speed because of the size difference and 30% peak airspeed because
of the rod ratio increase and displacement decrease... The answer is rpm in
order to get the airspeed back up right.. well if we shoot for the middle and
say we will try to get 20% or the loss back we need 11200 rpm. It now begs the
question with what valve gear that will happen along with what piston weights
etc.. Answer .. it is pretty unlikely to ever happen..
The fix a low deck block a 5.65 rod and fill the heads and intake 15 to 18% in
volume. You now have airspeed and a reasonable rod angularity to convert
pressure into torque..
The lesson..Or the reason to run long rods..
They will
take loading off the cylinder walls ie. high boost endurance engines
make a small cylinder head and manifold seem bigger to the engine by reducing
peak air speeds to less than the critical pressure ratio.
they make the stroke I want to run come up with a 1.7 to 1.9 rod ratio..and my
heads are ported for those airspeeds.
I'll save the cams for another morning..
Dave
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