Intake vs. Exhaust Flow Rates

68COUGAR

Registered User
Can somebody explain to me why intake flow rates are more important than exhaust flow rates?

Why arent exhaust flow given at LEAST as much importance as intake flow? Exhaust has a LOT more to FLOW than intake flows.

Thermo Dynamics say that a bigger exhaust valve should be better, Because it needs to flow more. What am I not seeing?

Confused 68COUGAR
 
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If the exhaust flows around the same as the intake side does on a NA application then reversion starts taking place during overlap, of course. Intake to exhaust flow percentage should stay around 80% unless it is on a forced inducted motor. With a forced inducted motor you can manipulate the flow ratio by the high air speed.

Exhaust isn't what makes power unless it is a restriction point. All it needs to do is get all out at the right time within a certain velocity. What comes before combustion stroke is all what determines most of the hp. From what all i've experienced with on NA applications the closer I got above 80% (78% being most sweet spots) the more power I lost. The more air/oxygen that enters the cylinder at a higher velocity, the higher the force of combustion.

Another reason for the exhaust having a smaller valve is because of previous exhuast strokes from each cylinder will cause a vacuum on the seat of the exhaust valve.
 
Never Heard THIS before

CMac89 said:
Another reason for the exhaust having a smaller valve is because of previous exhuast strokes from each cylinder will cause a vacuum on the seat of the exhaust valve.
Does this apply to All V8's?

Are you saying that the high velosity exhaust will scavage (vacuume), exhaust gasses from other cyclinders on the same bank?

This sounds like Tuned Headers, in the (very short) turn.

68COUGAR
 
It applies to any motor, again, unless it is restricted, but once it is free flowing a vacuum is pulled by velocity of exhaust gasses on the exhaust valve. The problem is just how much vacuum you can get on the exhaust valve after the gasses exit. The length of header can control this alot, but if you run full exhaust then the vacuum is shortened and not eliminated unless it is restricted too much.
 
68COUGAR said:
Can somebody explain to me why intake flow rates are more important than exhaust flow rates?

Why arent exhaust flow given at LEAST as much importance as intake flow? Exhaust has a LOT more to FLOW than intake flows.

Thermo Dynamics say that a bigger exhaust valve should be better, Because it needs to flow more. What am I not seeing?

Confused 68COUGAR


I would have to make an educated guess. The combustion gases which remain in the chamber, while they do impede the next combustion, have far less impact than a reduction in the fresh charge would have on the power generation.

This accounts for larger gains being made in the intake tract, except for initial gains in the overall backpressure in the exhaust and after extreme amounts of work on the intake, where the exhaust side becomes the limiting factor.

The other reason that the exhaust side is more forgiving is that you have immense gas pressure in the cylinder driving the gases out of the cylinder, but relatively mild pressures driving the fresh charge into the cylinder.
 
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