On a NA 2.4L 4 cylinder, would it be wise to perform this? There is only one exhaust valve per cylinder as opposed to 2 on the intake side, hence the exhaust flowing relatively poorly (40mm dia exh valve).
Approximately 1.5-2mm would be removed from the radius of the exhaust port if this is carried out.
I have been told it's not wise to do this...but what are everyone's opinions on the matter?
sssgtr,
If the header pipe is larger than the port this is most likely intentional by the factory.
Is the diameter of the head smaller evenly all the way around or is there a definite "step" usually at the bottom where the header pipe meets the head port throat proper?
If there is a step at the bottom this aids the extraction effect by creating a more rapid expansion of the gasses at that point and thereby a relative vacuum.
If the gap is the same all the way around then this is most likely due to manufacturing convenience where two "already in use" parts were used.
Parts bin engineering in other words and despite the advertising hype ALL car makers do it.
If you do decide to rev up the die grinder make sure there is an adequate amount of metal to go carving into.
Having done this on another engine the results I obtained by this factor alone were about 2 to 3% increase in power measured on an engine dyno.
Useful certainly but in my opinion a lot of work for a miserably small gain and one where perhaps the only benefit is when all other avenues of power increase have been fully explored.
The engine by the way was a Mini Cooper (the original 1960's version) with twin Webers, 731 cam, tuned ignition etc. so it was about 85 / 90% of a full race spec for the time.
Cheers, Pete.
Thanks Pete, but i forgot to mention the car isn't running the std exh manifold but a set of genie extractors.
Would rounding this sharp edge to match the gasket help? (ie instead of matching the port up to 1/2"-1" into the port, chamfer the step instead?)
There certainly is adequate material at the exit of the port.
I've *heard* theories that somewhat of a lip (~1-2mm) is desirable for anti reversion ie helps prevent the gas from swirling backwards toward the port. I imagine this would not be the case for all motors and having never done back to back tests I couldn't confirm it. I also imagine that it would only be good for low engine speeds.
Rust is lighter than carbon fibre.
best grab a copy of vizards books and read them. The anti-reversionary lip/step is best on the top of the post where the highest flow is. But read the book. EX flow should be around 75% of IN flow...more and you will generally lose midrange torque. Beill's book is also very good and is in my local library, so check yours.
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