View Full Version : Why is the Exaust Port Smaller than Intake Port?

12-31-02, 11:30 AM
I see some specs on some heads that the intake is a 2 in diameter and the exaust port is 1.57 in diameter. Why is this? Why are they equal? After all, if exaust is choked, that would be a lack of hoursepower, right?

I don't understand this. Can somebody please explain? Thanks! :D


12-31-02, 06:59 PM
The cool air/fuel mixture is pulled into the cylinders by the vacuum created when the piston is traveling downward, which means that (ideally) the air/fuel mixture is being forced in under no more than atmospheric pressure (14.7 psi). Conversely, the exhaust gases are forced out through the exhaust valve as the piston rises at VERY high pressure, temperature, and velocity, and don't need as large an opening. Cylinder filling is at LOW pressure and velocity, but the cylinder empties under very HIGH pressure and velocity.

12-31-02, 10:09 PM
Okay, I understand now. Thanks again, John! :D


12-31-02, 10:31 PM
I never knew that but it makes sense to me . Thanks John . Cliff

01-05-03, 12:29 PM
You want the exhaust port smaller to increase exhaust velocity to promote cylinder scavaging. At least that's how I understand it. It does limit horsepower a little, but helps with torqe, fromwhat I understand.

01-05-03, 04:14 PM
I would think that it has more to do with the burned exhaust gasses being much less dense than the intake air/fuel mixture, taking much less force to push the exhaust out than to suck the air/fuel mixture in.



01-05-03, 06:11 PM
I thought that it would have something to do with the density, but that was just logical guessing. John knows what he's talking about and I'm taking his word for it. ;) :)


01-07-03, 07:20 PM
The exhaust gas velocity corollary is correct as well, but that effect doesn't come into play until the hot gases are in the exhaust manifold where smooth flow is enabled. As an example, headers on a small-block with 1-1/2" or 1-5/8" primaries will produce better low-end torque than headers with 1-3/4" primaries, due partially to increased gas velocity enhancing the pressure wave development and scavenging capability and reducing unwanted wave reversion at the exhaust port. However, this effect is quite small with a stock exhaust system (pipes and mufflers), as the scavenging effect is almost totally lost due to the restriction; optimized header designs assume an open, low-restriction downstream exhaust system and cam designs that can take advantage of the scavenging properties of the system. Headers show very little actual torque and power improvement when bolted up to a stock exhaust system compared to results with a properly-tuned low-restriction open exhaust system.