Offset Connecting Rod Bushings?

[Tom Oxner]

Does anyone have a source for the offset bushing for a 930 engine. I need to "stretch" the rods for a little more compression. Thanks,

Tom

[MBruns]

They are a custom item, we can do them at JB Racing, there are several other shops that frequent pelican as well depending where you are.

Mike Bruns JBRacing.com

[Rich Rosenberg]

Ollies in Lake Havasu will install off set bushing in rods. Their number is 928-855-6101.
Rich

[ertech]

what is the advantage ?

[Rich Rosenberg]

An offset bushing allow the piston to be either raised up from the crank to increase the compression or lowered toward the crank to decrease the compression or just to allow for more clearance between the piston and valves.
Rich

[tadd]

Tom: sent you an unrelated pm.

[Henry Schmidt]

Originally Posted by ertech

what is the advantage ?

Although there is a compression change that can be effected, the true benefit to offset bushings is the change to rod length to stroke ratio.
In all but the first short stroke engines and some later racing engines, Porsche has compromised this very important engine dynamic.
When the rod to stroke ratio is shortened the rod angularity becomes too great to overcome comfortably.

If anyone really cares I will elaborate.

[lindy 911]

Also, a "long rod" motor will have a geometric advantage on the combustion stroke. With a longer rod and the pin moved up in the piston two things happen; 1 the piston decelerates smoother as it approaches TDC relieving stresses on the entire rotating assembly and 2 the piston remains at TDC longer and builds better cylinder pressure which makes more power in a NA motor.

We built two identical BB Chevy motors (except for rod length and matching pistons) for a boat that used new cranks, rods and pistons. One was set up with a .250 long rod on a stock dimensioned crank. The other had standard length rods and matching pistons (this was a parts order mistake). All other aspects of the build were identical. The end result was a measurable increase in torque and a noticeable change in throttle response on the long rod motor. Granted, these were 540 CI mills and the difference in a 3.0 litre Porsche might not be as noticeable but it will help. The NASCAR boys have known this trick for a long time and still use it today.

[jpnovak]

nice picture Henry. Side load force is often not discussed with long vs short stroke and rod length to stroke ratio discussions.

[dtw]

Originally Posted by Henry Schmidt

If anyone really cares I will elaborate.

Go for it!

[Henry Schmidt]

Originally Posted by dtw

Go for it!

Here goes:
Connecting Rod vs. Stroke Analysis
The ratio between the connecting rod length and the stroke length of a motor greatly affects the way it performs, and how long it lasts. This ratio (normally represented by “n”) can be calculated as follows:
Ratio “n” = Rod Length ÷ Stroke
The rod’s length is measured (for this purpose) from the center of the piston-pin opening to the center of the big-end bore, not overall. There is a small range of ratios for most conventional piston engines: the rod is between roughly 1.4 and 2.2 times the stroke length. It’s not possible for the rod to be the same length as the stroke, and rods much longer than twice the stroke make the motor very tall, and are not practical for most purposes (although used for racing).
The rod angle must not encourage excessive friction at the cylinder wall and piston skirt. A greater angle (smaller value of “n”) will occur by installing a shorter rod or by increasing the stroke. A reduced angle (larger value of “n”) will occur with a longer rod or a shorter stroke.
If the rod length is decreased, or the stroke is increased, the “n” ratio value becomes smaller. This has several effects. The most obvious is the mechanical effect. Motors with low values of “n” (proportionately short rods or long strokes) typically exhibit the following characteristics (compared to high “n” motors):
physically shorter top-to-bottom & left-to-right (more oil pan, header, and air cleaner clearance)
higher level of vibration
shorter pistons, measured from the pin center to the bottom of the skirt
greater wear on piston skirts and cylinder walls
slightly higher operating temperature & oil temperature due to friction
There are also differences in how the motor breathes:
intake vacuum rises sooner ATDC, allowing bigger carburetors or intake port runner & plenum volumes to be used without loss of response
on the negative side, a small or badly designed port will “run out of breath” sooner
piston motion away from BDC is slower, trapping a higher percentage of cylinder volume, making the motor less sensitive to late intake valve closing (hot cams)
Spark advance is also affected:
earlier timing (more advance) is required, as the chamber volume is larger (piston is farther from TDC) at the same point of rotation
the motor may also be less knock-sensitive, as the chamber volume increases more rapidly ATDC, lowering combustion pressure (this is useful for nitrous & supercharged motors)

Effects of Long Rods
Pro:
Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.
The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.
For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.

Con:
They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.
Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.
To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods.
Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle.

Effects of Short Rods
Pro:
Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.
The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.

Con:
Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.
Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.

[drb930]

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[0396]

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[304065]

Originally Posted by 0396

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What is the name of the bird on this website?

[Schmierung]

How much can a stock con rod be stretched?