What is the general consensus on exhaust bowl porting? I've heard people say that it is of no value. It seems to me that if you are going to get a free flowing exhaust (headers, mandrel bent tubes and high flow mufflers) you would want to have smooth exhaust ports too...before the air even gets to any of those parts. Can someone explain to me why that logic isn't true?
That doesn't make much sense. An engine is basically an air pump . Faster in make more power why not faster out . This is why they have different patterns of exhaust lobes vs intake lobes. It's about the ability to push the air out of the cylinder effectively in order to get next air charge in. . I would do both intake and exhaust ports. Runner doesn't need a lot of work just stick to bowl ,guide and short turn.
The cylinder is "blowing down" the pressure from within, as soon as the valve cracks open. To better understand this, think about a boosted engine and that it might even bend exhaust pushrods opening the valve, if against enough pressure. The scavenge effect of headers is more about a sound wave and it's reflections influencing the mass more so than the flow of the gases themselves. If you "reverse engineer" a Nascar header and exhaust valve using header calculations you can see the rpm range they are attempting to scavenge in, and that a major amount of hp can be had from a not-so-big valve or pipe. The port's characteristics along with the cam and headers' relationship with the cylinder need to work well together to have the intended effects. Surface finish of ports have been studied in depth, for decades. The way I understand it is that an increase in power might show up as a result of a very small increase in cross sectional area, if needed. Nothing (debatable term) relevant shows up on most flowbenches mainly because the pressures are much greater in an actual running engine (for an instant) than a typical flowbench can achieve. Think about the methods of seating a tire's bead to the rim and the concept of measuring that.... The other aspect negating polished surface finish is that it doesn't stay that way forever, it carbons up. Whether or not one agrees with the research or theories behind polished exhaust ports, it seems to be well understood that there are much more fruitful ways to making power. There is a ton of researchable math and engineering behind exhaust systems, dating back to the earliest motorcycles and piston aircraft. This is about as simple and concise as I can convey what I've learned. [The valve job, throat and runner have the lowest hanging fruit for gains, as mentioned]
Definitely polish the exhaust side...when people say don't spend the same amount of attention on the exhaust side they mean increasing the port size and Spending time tear dropping the valve guide bosses scavenging should take place in the collector of the header not reverberate in the head Sent from my iPhone using Tapatalk
There was an excellent article posted by someone(LarryGS) I think about a port job Jim Burek did for one of the magazines. I can't seem to locate the thread/article. Does anyone have it handy?
Thanks, Brian. I've read that article and it is an excellent one. I don't have a flow bench so it is just a very mild port job....more of a clean up than anything. Getting rid of all the sharp corners/angles around the valve guides and any obvious gobs of metal. I've already ported the intake side of the heads and was hesitant to do the exhaust side because I've heard so many people say that it is of no value and could possibly hurt more than it could help.
Thanks for all the input everyone. So to be clear: There is a gain to be had by cleaning up the exhaust side of the heads? I'm not polishing or removing any material from the runners....just cleaning up the throat/bowl and valve guide.
I think it helpedfor sure.. But i also have TA springs and bigger Valves... As well as polished bowls Gesendet von meinem D6503 mit Tapatalk
Almost universally speaking, the region an inch above or below the valve job is most sensitive area and usually yields the most productive gains from your efforts. One of the wisest things you can do to help this run on crappy pump gas is to knock the sharp edge down where the freshly surfaced deck meets the combustion chamber. Whether it helps really depends on how close to certain limits you get, but it never hurts. It really should be mandatory for shops to do.
Thanks for posting the link to the article. The one posted here(article copied and then pasted in a post as an image), I'll find it later as I'm on my phone right now, had airflow numbers. I just wanted to point out the large difference between the intake and exhaust numbers. I know they are of different design, but Pontiac's racing heads(known as round port head because of their circular exhaust pots) were very successful because they reduced the difference in flow between intake and exhaust. I'm not sure if there is an optimal design number but most stock heads are a compromise on this issue.
I've always heard that the optimal flow range for a head was around 70-75%. This would be the exhaust flow reading/intake flow reading. Really just a rule of thumb and will vary based on a multitude of factors around runner lengths, chamber design, etc. This would suggest to me that leaving the intakes the same, greater benefit would be gained from what is done to the exhaust side of the head for stock Buick 350 heads. (at 0.400 lift 128/198 = 64.6%) Now obviously, to maximize performance you would want to increase both as intake flow generally is the cap on peak horsepower. Posted a few weeks ago:
Everyone should read the second paragraph of the magazine article, and when they finish, read it again. It's not always about how big you can HOG something out. What you may do is actually HURT performance by killing velocity, and end up with a HOG. Like the article says, the Buick Engineers knew a thing or two about velocity and Torque. Some things are best left alone.
Understandable. That is why I suggest to stick to bowl and guide contour and cast clean up. The area of intake next to exhaust isn't even round or matching the valve contour. Not spending a lot of time in there but alitte work could help a lot. Seen plenty of heads where they spent hours port matching the runners to almost no gains as the valve area was untouched
Actually, the bulk of the posts are spot on. Following the basic advise above and from the article...the potential increase in CFM is based on the port's bad areas being worked on allowing a higher velocity, more stable port, without "hogging" the wrong area. I generally don't use my flow bench for the more rudimentary tasks, such as in the content of the article..maybe a quick verification. You can follow that from the valve seat and work towards the intake manifold face fairly safely. There are long proven, age old formulas to help plan this stuff out if you have access to a flow bench, otherwise everything you're doing will help.
Don't think I'm going to go much further than this. Just going to hit it with some sand paper now and polish it up a bit.
nice, pretty safe to blend the bottom (60*?) angle of the VJ to the port throat, leaving .100" of the 60*(?) exposed. Fairly safe on the flow bench with most ports. I use a 14* included angle carbide and a careful hand. If it isn't perfect, the engine will never know. You'll find that if you caliper that, it will be 88% or so (of the valve OD), depending on the OD of the seat work and how it compares to the valve.
Yes bring it down to the near the valve opening. Clean up and countout near the side that shares exhaust port wall
Got a chance to do some more porting today. There were several large globs of metal on the ceilings of the exhaust runners. Only on one of the heads though. Interesting how different the imperfections were on either head. Also interesting how honest the pictures are...they look much better in person lol. Check out the pictures. I welcome any critiques/suggestions.
