Are Exh pushrods more stressed even in NA applications?

[plovett]

Would it be advantageous to grind ex valve/seat to maximize low lift flow?

Intake angles for high lift flow?

Is this already a common practice?

I think that would be advantageous, but I am not a head porter or engine builder.

paulie

[3V Performance]

I believe its because most are still chasing flow bench numbers on the exh. Exh ports are HUGE today. Less lift is helping control the rapid blow down.

Like the analogy of the shook up beer can, open it quickly all the way vs just slightly cracking it slowly...

Don't forget that blow down affects the exh pulse ( timing ) at overlap. Most time over looked.

[MadBill]

Yes, it's easy to view any pressure above atmospheric at EVO as 'lost potential work', but a percentage of it returns during overlap in the form of the negative scavenging pulse.

[Stan Weiss]

Unless I have the dynamics of the cylinder pressure decay wrong. If the exhaust valve was not to open they would still be pressure above atmospheric ABDC. (Multiple atmospheres). Also for the last "X" number of degree BDC the crank rod angle is not very favorable to generate much turning force.

Stan

[David Redszus]

Dr. Andrew Randolph of ECR in his NASCAR Engines 101 lecture said best power is made on the exhaust side when the expansion loss is balance to the pumping loss. In the example he gave, it’s about 20hp for each...

That is very important to note.
Assuming that EVO occurs at about 75 deg BBC, most of the exhaust gas mass has escaped by the time the piston reaches BDC. During this blowdown period, the exhaust valve is far from fully open; but it does not need to be since cylinder pressure is still high.

After BDC, the piston must do the work of pumping the remaining burned gas out of the cylinder.

Andy is absolutely correct in that we want the minimum pumping work required balanced against early opening exhaust pressure loss.

The exhaust side behaves completely different than does the intake side.

[miniv8]

We are getting a bit carried away from the original post, as to the mechanical stress differences of intake and exhaust valve operation.

Let me add to the derailment by adding to the discussion of why the lift and opening time is shorter on the exhaust;

I don't think it is.

Whatever the formula looks like, that we use to dictate intake to exhaust valve sizes in the bore, or port flow between the two, it must come down to the lift and timing to balance things out for maximum power. Look at the difference in timing requirements between BBC and BBF which are strictly related to totally different flow curtain needed for the exhaust, to keep up with the intake flow.