367 vs 410 Engines Masters

I've enjoyed watching this thread as a recently retired OEM engine engineer (hint: it was not Chrysler!). But I thought I'd share some insight into how the OEM's engineer today's modern engines.

The physics:
Indicated power = friction power + brake power, or
Brake power (what we all care about! ) = indicated power - friction power. Brake power is what comes out at the crankshaft.

Indicated power can be computed from the P-V diagram (pressure-cyl volume trace). We can see the pressure created on the piston both before tdc (negative work due to spark timing lead) and after tdc (positive work due to combustion). We also see the negative work due to blowdown of the exhaust and negative work of the air induction process (pumping work). So we can integrate the P-V trace on the power stroke and get the indicated power. And we can lump everything else that does not come out at the crankshaft into friction power.

Friction power:
A bigger engine breathes deeper (more airflow) so the induction negative power is greater (higher pumping work). It also has more exhaust volume to blow down so again, more negative power. And all those parts rubbing together, valves going up and down, piston rings friction...all of this is the mechanical part of friction power. And you don't get a choice - you have to feed the mechanical friction monster as you raise rpm. In this simplified explanation, we also are lumping thermal losses into friction power. These are the losses due to the high combustion temps heating up parts and shedding to the coolant. Thermal losses can be considered close enough to each other for a stroker vs non-stroker. Obviously a longer stroke engine will have higher friction than a shorter stroke engine hence why they normally power peak at a lower rpm.

Ahhh, BUT that longer stroke inhales more air so it can make lots more indicated power of the piston pushing down on the crank. Even if sucking through the same straw (cyl head), it will inhale more air. If the straw is too small/more restrictive, the negative work just goes up and, since the mechanical friction is increasing with rpm, too small of a straw just moves the power peak even lower.

So the game is, within an engine's usable rpm range, where do we stop getting more brake power when the friction power gets high enough to negate the higher indicated power?

Clearly at lower rpm, the bigger engine has big gains in its lower speed output. But at some rpm, the increase in friction power will exceed the increased power due to airflow and the brake power will no longer be higher. And since friction power is a hyperbolic function (function of rpm squared), the power falls off pretty quick if the breathing up top is not so good.

Ahhh...but what about cam timing? The valve lift and IVC will determine where the longer stroke or shorter stroke breathes more efficiently so any cam chosen will favor one or the other engine. And the blowdown work (power) is a balance of wasting the cyl pressure before bdc (earlier EVO) and a later EVO that causes higher blowdown losses.

So in general, the larger engine will make a significant increase in torque where the rpm is lower and the friction is lower and the power increase at the high end is highly dependent on the breathing ability of the heads and cam. Within typical prod engine constraints, the bigger engine will always win. And if you want to play the "perfect gearing/rpm" setup for a drag race, the one that gets more area under the power curve will always win. And that will always be dependent on launch rpm and shift speeds. Can a lower power engine be perfectly set up to beat a higher powered engine? Theoretically yes but in all practicality, no.

So when we did new engines and looked at stroking them, we would get proportionally higher torque and slightly more power, just as you would expect. And fuel economy typically goes down due to more pumping losses (you only need so much power to push the car on the federal emissions/fuel economy test!) and higher friction.

I simplified this quite a bit but suspect some of you may not have seen this before. Just wanted to provoke some thought on the "why" behind a stroker.

And yep, I love my 408 "tractor motor" in the avatar. That's why I built it when I put this car together. Much broader operation range to get performance out of and don't have to have some theoretically perfect setup to get the most. 6200 rpm shift point suits me just fine. Now if only I had some TF heads instead of those Eddies.....haha

FWIW!