Are Exh pushrods more stressed even in NA applications?

[BradH]

I've seen some write-ups on how "power adder" builds put more stress on the exhaust pushrods than the intakes because the exhausts have to overcome the additional cylinder pressure to open the valve. Does the same apply to NA applications, even though the cylinder pressure wouldn't be as high? Thx - Brad

[Stan Weiss]

Yes. How much more? What was max cylinder pressure and where ATDC? What was CR? What was exhaust valve size? Where is EVO at?

Stan

[3V Performance]

I've seen some write-ups on how "power adder" builds put more stress on the exhaust pushrods than the intakes because the exhausts have to overcome the additional cylinder pressure to open the valve. Does the same apply to NA applications, even though the cylinder pressure wouldn't be as high? Thx - Brad

Ex valve open well after max cylinder pressure. Boost or N/A. When your fighting exh size for power output then you open the valve sooner during the power stroke ( like 60*-75*-95* etc bbdc. ) It still open's with remaining cylinder pressure present from the power stroke and that's what it fights. The earlier the more.

[MadBill]

A realistic example would be a 1.60" valve (2.0 ²") opening against 100 psi and thus adding 200# to the perhaps 200# seat load plus whatever the inertia force.
Reputedly, for a TF engine the cylinder pressure on a ~2.0" (3.14²") ex. valve is over 1,000 psi at EVO and the valve opens not when the lifter comes off the base circle but when the pressure drops enough to allow the 5/8" PR to straighten out...

[NewbVetteGuy]

A realistic example would be a 1.60" valve (2.0 ²") opening against 100 psi and thus adding 200# to the perhaps 200# seat load plus whatever the inertia force.
Reputedly, for a TF engine the cylinder pressure on a ~2.0" (3.14²") ex. valve is over 1,000 psi at EVO and the valve opens not when the lifter comes off the base circle but when the pressure drops enough to allow the 5/8" PR to straighten out...

Bill,

With the smaller diameter and less weight, and often less lift than the intake in the newer lobe designs, do you find that the two factors end up cancelling each other out, or is the extra pressure at exhaust valve open more of a factor than the lighter exhaust valves and less lift?

(I've often wondered why springs on the intake and exhaust sides are normally identical; before I saw this thread I was thinking that exhaust valve because of the reduction in weight and lift could get away with LESS spring than the intake. So now I'm wondering if when the pressures that the exhaust valve is pushing against are taken into account if it's essentially a "wash" and identical springs between the intake and the exhaust all of a sudden make sense again.)


Adam

[Warp Speed]

Why is the exhaust valve lifted less than the intake?

[Orr89rocz]

Why is the exhaust valve lifted less than the intake?

Higher pressure moving to low pressure, can move more mass thru a given area quicker than on the induction side my guess lol

[MadBill]

Bill,

With the smaller diameter and less weight, and often less lift than the intake in the newer lobe designs, do you find that the two factors end up cancelling each other out,

By cancelling, do you mean because with the same spring and seat load the open force will be less on the lower lift exhaust valve?

or is the extra pressure at exhaust valve open more of a factor than the lighter exhaust valves and less lift?

The cylinder pressure is not in play once the valve is cracked, so it doesn't really factor into spring selection.

(I've often wondered why springs on the intake and exhaust sides are normally identical; before I saw this thread I was thinking that exhaust valve because of the reduction in weight and lift could get away with LESS spring than the intake. So now I'm wondering if when the pressures that the exhaust valve is pushing against are taken into account if it's essentially a "wash" and identical springs between the intake and the exhaust all of a sudden make sense again.)


Adam

The factors would seem to point at less spring requirement for the exhaust valve, except for the fact that ex.valve float is more of a concern than for the intake, so I speculate that equal spring seat load, even with the lesser open load due to the reduced lift, are both a safety factor and a build convenience with no significant downside.
Perhaps CK, DV or others of their ilk will weigh in...

[MadBill]

Why is the exhaust valve lifted less than the intake?

I can't speak to why, but a number of mid range and better software programs I've used all show a fairly flat optimum lift range for the exhaust valve, as opposed to the intake, where diminishing returns continue up to impractical heights.

Catalog cams are a different story, with many speculating that their often greater exhaust lift is due to a ''uni lobe" approach (same family for both), so that the longer exhaust event of a split duration grind automatically gets more lift. Another explanation could be that per my previous post, the exhaust's typically lighter weight allows closer to optimum lift with a given spring, exceeding the practicality-limited intake lift.

[plovett]

Why is the exhaust valve lifted less than the intake?

Because, in general, low lift flow is more important on the exhaust side. In general, high lift flow is more important on the intake side. Yes, a generalization. So it is not so much that you are trying to get lower lift on the exhaust. It is that you are trying to get higher lift on the intake. Ducking!

paulie

[paulzig]

Why is the exhaust valve lifted less than the intake?

I vaguely recall seeing the specs for a cup cam and exhaust lift was more than exhaust.. For the sort of RPM you guys are pulling is it a case of trying to get more area and not having to put an excess of exhaust duration in it?

[Roundybout]

Is the fact that the intake valve is chasing the piston on the way down compared to the piston chasing the exhaust valve on the way up a factor on exhaust valve lift? It’s easier to get air flow out of an engine than in. You’ve only got atmospheric pressure and whatever resonant tuning helping to get air in as opposed to a much higher pressure at exhaust valve opening getting the residue out. Im sure valve overlap comes into play on how much exhaust lift is needed and beneficial to the intake cycle.

[GARY C]

Why is the exhaust valve lifted less than the intake?

In inches or area?
In what application does this change and why?

[Warp Speed]

Why is the exhaust valve lifted less than the intake?

I can't speak to why, but a number of mid range and better software programs I've used all show a fairly flat optimum lift range for the exhaust valve, as opposed to the intake, where diminishing returns continue up to impractical heights.

Catalog cams are a different story, with many speculating that their often greater exhaust lift is due to a ''uni lobe" approach (same family for both), so that the longer exhaust event of a split duration grind automatically gets more lift. Another explanation could be that per my previous post, the exhaust's typically lighter weight allows closer to optimum lift with a given spring, exceeding the practicality-limited intake lift.

Why wouldnt you use the same family of lobes?

[Warp Speed]

Why is the exhaust valve lifted less than the intake?

Because, in general, low lift flow is more important on the exhaust side. In general, high lift flow is more important on the intake side. Yes, a generalization. So it is not so much that you are trying to get lower lift on the exhaust. It is that you are trying to get higher lift on the intake. Ducking!

paulie

Really?!?
Low lift flow is important to the exhaust?
How much does an exhaust flow at initial opening?