Exhaust Porting?

[KsHighboy]

I am no authority on FE's but have been around them for 35yrs. I love reading about them on this site and learning what helps bring them up to the modern world of technology.  One thing I'm curious about is I dont see much being done to the exhaust side of the heads. It seems all the porting is done to the intake side which I understand it needs attention. But, if the exhaust cant get rid of what's coming in then isnt it wise to make sure the exhaust flows a good percentage of the intake? Keep in mind, I've never ported a set of heads and am ignorant of the topic  but I'm just curious as to what the logic is. Would love to hear some info from you guys with the knowledge.

[cjshaker]

Lots of guys here with much more knowledge than me, but the just of it is this: First, I think most people who take the time and effort to port intakes (and intake manifolds), usually port exhausts also. Second, the exhaust has the benefit of the piston forcing out the spent combustion gasses, so free flow isn't as critical to power production. Third, I think it's harder to make significant gains in the exhaust port because of it's short runner length, extreme angle of exit etc. Fourth, there is only so much you can benefit from on the exhaust side when many FE cars have shock towers that require the exhaust to abruptly turn immediately after the port exit...negating much of the gains that could benefit from a free flowing exhaust port.

Having said that though, there is a recommended percentage ratio target for exhaust to intake flow. Trying to achieve that target ratio is a good thing, if you have the money and/or means to do it.

[TomP]

Almost all FE heads can benefit from cleaning up the lumps and sharp edges in the exhaust ports. Only the High Risers had an exhaust port which didn't really need anything and they still can be helped. Narrow the guide boss, if they are the earlier heads, 65 and older they have some strange lumps in the side walls that can be smoothed off.

[Joe-JDC]

Percentage wise, I always try to get the exhaust at least 70% of the intake flow, and many times it is over 75% without a tube.  I have had the exhaust as high as 88% of intake flow, and it got terrible gas mileage.  The industry standard that is unspoken but measured for reference is usually at least 70%.  I first used a flow bench back in the 1988-1989 time frame.  When I bought my SF-600, I tried to find a standard, and Super Flow salesman recommended a book that I still have to this day giving instructions on flow characteristics, and how to improve a cylinder head.  That book had documentation that verified dyno testing on the 70% rule, so I have always tried to get the exhaust up that far.  A CJ FE head that flows 260 cfm needs at least 182 cfm on the exhaust.  One that flows 340 cfm needs 238 cfm on the exhaust.  Most of the CNC heads available now are close to those figures.  Hand porting is dependent on the skill of the porter and their experience with a particular head.  If you decide to work on an iron FE exhaust port, do not try to get rid of the hump in the bottom/short turn of the exhaust port.  That is a water passage.  It can be blended some, but best to leave it alone and work the sides and roof of the port.  That is where most of the flow is on the FE, anyway.  Joe-JDC

[TomP]

I was unaware excess exhaust flow made for bad gas mileage but that explains the 454 Chevy trucks and their Boeing 747-like fuel consumption. Those things have bigger exhaust ports than intake.

[Barry_R]

Although pretty unlikely on any rational FE combination, if you "over scavenge" the exhaust can pull intake mixture right through the engine during overlap.  This is actually "a thing" on race hemi engines where the valves face each other, and they sometimes shrink or alter the exhaust ports to resist this tendency.

[KsHighboy]

Thank you! All of you have shed some light on this for me. I wasnt meaning that nobody ever touches the exhaust but it just seemed like there was very little attention paid to it. Your explanations make a lot of sense and help my thinking. I was thinking there should be a certain percentage of flow on the exhaust compared to the intake and it seems most heads meet that percentage.  Interesting about the gas mileage and the over scavenging!