Quote:
Originally Posted by Neil Harvey
Be sure you understand why you need these new rods. Nothing will give more driving pleasure than a light car and lightweight engine internals.
There is a difference between top speed and acceleration. I would take acceleration over top speed. But that's my choice.
Aftermarket rods are typically sold dimensionally, not by application or use. They are usually dimensionally the same as stock, therefore their application is to suit a particular engine model. The same rod is sold for an NA or Turbo engine in some cases.
An 3.0L NA engine or even a 3.6L NA engine will never have an issue with a stock rod. The rod will never see compression numbers to hurt it. Same for the bolts.
But if there is a better part available, for sure use it. But understand why you are using them.
For a NA engine, the typical H beam is overkill. An I beam will work and be lighter. To give a comparison, we build NA engines with displacements over 4.0L with Steel Rods that have a longer CCL and weigh 420 grams without the bolts. We use Titanium bolts in some and steel in others.
Our design parameter is to have parts designed for each application and lightweight that allow the engine to rev quicker.
Budget dictates what is possible too. There is absolutely nothing wrong with your choice or other alternatives sold in the aftermarket. 560g is better than 690g, for sure but understand why you are changing.
Your post states you build motorcycle engines. I assume you are looking for acceleration as well as a higher RPM. Motorcycle internals are a lot smaller and lighter in comparison to car parts. Porsche 3.0L engine parts are some of the heaviest car parts as Porsche built these engine with overkill in mind. There are more important concerns with these engines using high RPM than the stock rods.
If all you are trying to achieve is a higher engine RPM, save your money and use what you have.
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Well put, and knowledgeable post Niel. I know you are well versed in engine building.
I agree on all fronts, and your points have been mulled over for a bit now hence the inner turmoil in my brain.
In the essence of sharing engine building information in public forum I'll just add some points for fun for the group as a whole. I am always on the path to motorsports enlightenment whether it be engine building, chassis design and my favorite, engine calibration. I celebrate the sharing of information with people on that same path.
Cylinder pressure is almost never a cause of failure to a rod, within reason. It is almost always an oiling/oil cushion failure that will cause issues for the rod and subsequently the engine cases sometime
. With the exception of gross calibration issues or fueling errors we never see a cylinder pressure rod failure in max effort motorcycle engines (ie. base timing errors/timing drift, extreme detonation at high boost or high nitrous loads from exuberant tuning and occasional hydra locking of methanol when the ignition system fails to fire with very rich lambdas at high boost and rpm). It never ceases to amazing me the abuse these 400-800hp, 1-1.7 liter four cylinder motors take when I am tuning them on the dyno or the track.
Keeping the rod dimensions as stabile or rigid as possible is the ultimate benefit of a properly engineered aftermarket rod option (also engine case rigidity in the main bearing areas), allowing the engine builder to size bearing clearances to suit their ethos in building, the overall design of the motor and its intended usage.
Overbuilding as a whole is a nice insurance policy sometimes, especially in a race motor.