combustion chamber question
combustion chamber question recently had my heads ported and polished. the heads i bought were new castings never run with out valves. the chambers on them were real ruff. on older small block chevys people use to polish the chambers in theory of reflecting more heat. head porter said that new school heads do better left ruff like that. anybody have input???
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Joined: Sep 2009
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From: Frozen Tundra, WI
I was under the impression that smooth was better. There will be less carbon build up for sure. I may be stretching it here and this is definitely theory but the burn/flame should be slightly more complete and faster...although this is probably (almost) not measurable. Then again, at 6000+ rpms and high compression ratios we are talking milliseconds that can make a difference.
the guy doing it is very very knowledgeable. he has worked with some bbc guys that all have rails. he really knows his stuff. he said there starting to find that even a ruff surface on the piston is good for hp. iv always heard ruff edges encourages detonation. i thought they might of had it ruff from factory. are my heads just unfinished or are heads gm puts on there cars and trucks the same?
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Joined: Sep 2009
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From: Frozen Tundra, WI
If you have an "as cast" head and the only thing you change is polishing the combustion chamber you will have slightly less compression. This equals slightly less power which may be what this guy is seeing with the bbc rails. I believe you are correct...rough or sharp edges do encourage detonation (hot spots). I suppose if you are not running on the ragged edge of compression (vs octane) then leaving it rough will gain you some power/compression.
One thinking is that it would help a set of ls1 heads because aluminum heat soaks. If the chambers were polished they would act more like cast iron heads and reflect more heat. I'm going to pick my heads up tonight I'll ask a few more question. Not for sure If he didn't do it because he didn't feel like messing with it or he didn't do it because it actually does something to help the motor other then no dropping compression I don't think the difference on tar would even be measurable.
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I doubt he avoided them out of laziness (unless this person was a complete dirtbag, which doesn't sound like the case), there isn't really too much surface area there to work compared to the ports!
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Verdad Gallardo i picked them up today the heads looked good heres a few pics of the heads. wish i had another set of ported heads to compare them to.
on pic is of intake ports one of the exhaust and other of the intake ports valve side.
hard to tell much from the pics though
i guess ill leave them ruff....
on pic is of intake ports one of the exhaust and other of the intake ports valve side.
hard to tell much from the pics though
i guess ill leave them ruff....
yea no problem i have 2 sets of 5.3 heads that hes willing to port if anybody would like a set. he ports for a living he took a lot off the rocker bolt casting that sticks out of the intake. like i said he really knows his stuff. im glad i got them done he said he spend quite a few hours on them i can take some more pics in anybody wants to see any more. think he did a good job?
I'm not a professional but they look very rough to me, especially the intake runners. Most hand ports I see still are gone over with sandpaper rolls to smooth things out some (more on exhaust than intake)
I spent nearly 7 hours each on my 799s doing such smoothing after the carbide work was done.
I spent nearly 7 hours each on my 799s doing such smoothing after the carbide work was done.
there not to bad. he said they need to be ruff. if they were a all out turbo head he would polish them up. the ruff ness stops the pooling up of gas at low rpms. exhaust is polished very smooth. pics are taken with a i phone 4 with the flash on so everything really sticks out. i may go back and go over them with a sanding roll with 80 grit or so.
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Joined: Sep 2009
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From: Frozen Tundra, WI
To put this plainly...that is a crock of @#$%. Roughness is one thing that causes dead air/stall space in a port and that is one cause for fuel to fall out of suspension. Roughness inhibits velocity which is a primary factor in keeping fuel in suspension.
i dont know enough to try to argue a point. just going by what he said. iv always heard not to polish the intake ports smooth and iv heard a lot of other head porters say that so there must be some truth behind it. i think the theroy behind intake being rough is to help atomize the fuel and air mix. if its smooth it dosnt have a chance to mix..... this is how iv understood the whole rough and smooth. im probably wrong though.
Before FI it was beneficial to have some roughness in the intake runners , when FI became the norm the thinking was smooth intake runners were better because of the obvious flow advantage and the fuel injector spray was enough to not only atomize the fuel but help it vaporize , it has to be vaporized to burn. Recently everyone I've talked with about it say they are indeed finding it beneficial to have at least some roughness of the intake runners and of course polish the sh_t out of the exhaust . As far as the combustion chamber is concerned as long as You're intake runners are rough then there would be no need to have rough chambers in fact it is beneficial to have smooth chambers but if You have to remove very much material to achieve that then You might want to cc 1 or more of You're chambers so You know what static compression ratio You have and take that into consideration . And I agree with the earlier poster that those do look a bit rough/unfinished .
Teching In
Joined: May 2011
Posts: 9
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From: Chandler, AZ
Ummm, you have a motor with port injection that has fuel injectors which do a VERY good job of atomizing the fuel. Carbureted engines had relatively "rough" runners in the intake manifold which helped suspend the air/fuel mixture. Any rough spot of "knick" creates a small vertex which slows down airflow. Slow = less velocity.
and the debate is on!
I'm doubting those are actually as rough as they appear in the photos. Could always go over them again with a few cartridges of 80grit.
I'm definitely no expert either, but I just got done going over my 317's myself. I was actually going to ask about the rocker bolt hump, that's why I asked for more angles.... still hard to tell in the photos exactly how much they were taken down, but I don't think I went quite that far. Mine are still visible when you look perfectly straight into the port opening. Not sure if I should knock them down more or not?
I also left much more of that ramp that was next to the intake valve guide too, compared to those.
And I did not open up my exhaust port exits into a big full circle like that. Those look like he basically gasket matched them...? I opened mine up a decent amount, but kept the “D“ shape, but now wondering if I should have just opened them up to size of gasket...?
oh, and did I miss it, which head castings are those?
I'm doubting those are actually as rough as they appear in the photos. Could always go over them again with a few cartridges of 80grit.
I'm definitely no expert either, but I just got done going over my 317's myself. I was actually going to ask about the rocker bolt hump, that's why I asked for more angles.... still hard to tell in the photos exactly how much they were taken down, but I don't think I went quite that far. Mine are still visible when you look perfectly straight into the port opening. Not sure if I should knock them down more or not?
I also left much more of that ramp that was next to the intake valve guide too, compared to those.
And I did not open up my exhaust port exits into a big full circle like that. Those look like he basically gasket matched them...? I opened mine up a decent amount, but kept the “D“ shape, but now wondering if I should have just opened them up to size of gasket...?
oh, and did I miss it, which head castings are those?