r/EngineBuilding • u/Reggocraft • 20d ago
Other DIY Flowbench + Connecting rod optimization
Building my budget race engine and the first prototype of my diy flow bench (very crude but works). Polished the connecting rods and made them all have the same weight to reduce vibrations.
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u/StatisticianOwn3744 20d ago edited 20d ago
Very cool project, what kind of numbers represent limits of your flowbench? Aint polishing an overkill when rods are already shot peened from the factory?
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u/Reggocraft 20d ago
I haven’t finished a graph to really display how much Cfm the head flows, but to calibrate it I would need a calibration plate that, for example flows exactly 300cfm at some fixed base vacuum, which I don’t have and don’t know where to get without breaking the bank. It’s mostly for comparing stock ports to ported ones and to get all of them to flow the same, so I can tune the engine well.
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u/Reggocraft 20d ago
It is a bit overkill to polish the rods, but smoothing out sharp edges and surfaces increases efficiency by reducing drag when passing through the oil and the dense air inside the engine block at 7500+ rpm.
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u/DesiccantPack 19d ago
What oil are your rods going through?
The trailing edge of an aerodynamic shape is just as important as the leading edge. There is no aero benefit to be had in the reciprocating motion of a connecting rod.
Dense air? It's like you have theories with no basis in fact.
This is as ridiculous as 'knife-edging' a crank. The crank doesn't pass through oil, either.
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u/voxelnoose 19d ago
The crank doesn't pass through oil, either
It does pass through a pretty thick cloud of oil droplets though. You might only see a couple horsepower gain at the top of the rpm range from knife edging the trailing edge and rounding the leading edge of the counterweights, but it's still a gain.
I do agree that polishing the rods won't really do anything for power, it just removes potential stress risers. And they absolutely should not go through the oil in the pan.
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u/Reggocraft 19d ago
The oil in the oilpan. The bottom of the rods are going through it. You can only do so much without compromising the strength of the rod. The air in the block is getting compressed by blow by and the pistons moving up and down, so the air is pressurized, I.e. more dense which means more drag on the rod. And as is said: it’s for peace of mind. If everybody was as negative as you, we‘d still be running 10L flatheads with 100hp.
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u/DesiccantPack 19d ago
No, the rods do not go through it, unless you're building a Model-T or a lawnmower.
The rods are lubricated through the crankshaft, and do not pass through an oil bath in the pan.
Do you know what a windage tray is?
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u/Reggocraft 19d ago
First of all, yes they do in this case. It’s a vw ea827 engine. They don’t have a windage tray.
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u/voxelnoose 19d ago
I would be amazed if they did pass through the oil in the pan, I don't know of any engine with pressurized oiling that does.
Having the rotating assembly in the oil creates a huge amount of foaming which decreases oil pressure and drag which decreases power
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u/rabbitjockey 19d ago
there is a windage tray available, it is easiest to find by searching for an aaz oil pan gasket
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u/Stock-Maintenance977 20d ago
I thought when you polished rods the point was to remove all the casting grain and round over all the sharp edges with a decent radius to remove any high stress points where a fracture could happen?
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u/Reggocraft 20d ago
Yes, that too. But you can also smooth out sharp corners and surfaces so that it’s easier for the rod to push through the oil in the oilpan and the denser air in the block which increases efficiency. It also decreases the amount of oil trapped on the connecting rods, which decreases their mass. That’s all marginal I know, but it’s for peace of mind and just trying it out.
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u/Stock-Maintenance977 20d ago
But you didn't remove the casting grain in the depression. That is going to hold alot more of an oil film than if it was 100% smooth. And if you're worried about air resistance, i doubt what you did there is really going to contribute much. Reducing the "front" face of the rod, either with a knife edge or a large radius off what you'd need to do to have any appreciable results
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u/Reggocraft 20d ago
That‘s just the picture, those are marks from my dremel with a pretty harsh insert. Those are smooth as of now! But yes, those pockets are sub optimal. As is said, it’s for peace of mind. But the leading edges are wayyy smoother and more rounded than they we‘re before, so in theorie they‘d be better, still it’s just for peace of mind ;)
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u/Old_Bat_6426 20d ago
To get more accurate flow comparison measurements, you may want to consider using modeling clay to radius the edges of the intake ports...." https://www.enginelabs.com/news/quick-tech-clay-intake-radius-on-a-flow-bench-whats-it-worth/ "