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Tuning a Built Motor

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I've recently put some forged pistons & rods in my 2.0L EJ thats mounted in the back of my kit car.

What I'm wondering is about tuning. We all know that forged pistons are more ductile and will be more resistant to detonation.

However, from what I understand is that all detonation is bad. As in if you had forged pistons, detonation will still be detrimental to the hydrostatic bearing integrity regardless of what parts you are running.

Wondering if anybody could provide some insights into what tuners actually do differently with a forged motor vs stock cast internals (assuming all other parts equal).

There is no tuning difference. The difference is that the engine reliability is probably increased with better internal components. Perhaps the change in components will allow a real performance boost (like increasing the maximum RPM), or lighter weight components that change the engine's ability to rev quicker.

Your stronger rods may allow an increase in compression ratio or boost if forced induction.

The only tuning difference I could say is that forged engine are noisy, so you might need to adjust the knock threshold (or engine noise) a bit higher. Especially if you are using the Subaru OEM ECU. There’s a write up on the COBB website about this, here’s the link!

https://cobbtuning.atlassian.net/wiki/spaces/PRS/pages/635371588/How+To+Knock+Desensitizing+for+Forged+Engines

One important point, forged engines aren’t immune to knock. They can withstand higher cylinder pressure, but for normal combustion.

Interesting... Thanks for your responses.

So as my car is mostly built for the track, the forged internals will help with reliability for extensive periods. In addition to being able to handle more load/boost.

Does the lighter weight directly translate to rpm change? Let's say my rods+pistons weigh 900g vs the stock at 1000g. Suppose factory redline at 7000. Would a safe starting point be (1000/900) * 7000 = 7700RPM?

Yeah the pistons are definitely louder when cold, although the knock sensor didn't seem to pick it up. I don't really mind as the car is so loud anyways.

It's the complete rotational assembly that it needs to be compared against, so the valvetrain, flywheel, crankshaft and all of the accessories that are driven by the engine. You may find that the gain is a quicker rate of acceleration of the engine, but no gains in the overall rev limit as the other components are the limitations to this.

The maximum engine speed is often determined by one or more of the following: mean piston speed (crankshaft stroke vs RPM), weight of the pistons, strength of the rods (tension from the speed and weight of the pistons), valve springs (can't have valve float), camshaft design (which can dictate the valve spring requirements).

I would not necessarily raise the RPM limit unless you have advice from experienced engine builders using your component combinations. The pro builders know what the limiting factors are and can provide guidance (buy parts from them if you aren't having them build the engines!)

Top safe engine RPM falls into the following categories:

- Valve float

- Piston speed

- Imbalance (over/underbalance)

- Oil Pressure

- Rod ductility/rigidity/toughness

If you want a good write-up for these, I can provide at your request. I've been both a design and testing engineer and can give you as complex an answer as you feel is necessary including calculations if you feel that would help you.

Hi John thank you for that offer, any writeups you have would be greatly appreciated. I'm a mechanical engineering student that's planning on diving deep into engine building for my capstone project.

Fr my current build I think I have most the bases you mentioned covered.

- switched to shimless buckets

- aftermarket valve springs

- forged rods

- oil pressure sensor mounted on heads

- higher volume oil pump for increased clearances

As for balancing, my engine is already sealed up. But for the future, do you know of any ways a person could balance a rotating assembly at home? Thinking maybe attach some bob weights and spin the crank mounted to some weigh scales, which i would log.

Do you think the accuracy of that would provide useful information?

Neeraj,

At home balancing is incredibly difficult to do. Scales are not remotely possible to use. Here's the 2 processes you could actually use:

- Accelerometer:

There are 2 mounting points where the crank will sit. The mounts have bearings that the crank main journals sit on. The mounts are resting on high rate springs and have an accelerometer attached to mount to measure the acceleration. You then need to have some kind of angular pickup to find out WHERE the imbalance is on top of the amount of force. This is more tricky as you want to have at least enough precision to get within 0.1 degrees of the imbalance.

- Strain:

This balancer is similarly built, but it is sitting on a Piezo-electric ceramic element which generates a voltage proportional to its strain. This type is more accurate as it can pick-up extremely small variations with very little noise which is suffered by accelerometers.

Realistically, it's a lot more difficult than you'd think. Properly done, you're looking at a budget of about $1500 (US dollars). Let me know if you have any more specific interests you'd like further discussed. I'm a BSME and Electrical Engineer, so I'm always inclined to help others in the field!

As John said, it isn't the sort of thing one can just knock up in one's garage. Another aspect is the drive to rotate the crankshaft, which has to be able to drive it without introducing any imbalance itself.

gord, surprisingly the power source is the least of someone’s worries when building a balancer. Normally a belt is used to drive the crank to a speed then released. Once it is released there are no longer any imbalances acting on the crank other than its own imbalance. With an accelerometer system, the natural frequency would have to be taken into account and the rotation speed would have to be somewhat significantly far from the natural frequency.

gord, surprisingly the power source is the least of someone’s worries when building a balancer. Normally a belt is used to drive the crank to a speed then released. Once it is released there are no longer any imbalances acting on the crank other than its own imbalance. With an accelerometer system, the natural frequency would have to be taken into account and the rotation speed would have to be somewhat significantly far from the natural frequency.