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Old 04-23-2012, 05:04 PM
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Porting and Polishing the L31's

I am tempted to NOT port my heads, but I called the local head shop and asked them about pricing just for kicks. Here is what they told me (price is per pair of heads). From my research it appears the best thing to do would be to bowl blend and deburr the combustion chambers, messing with anything else would be detrimental. However, I have not read anything about tapering the valve guides. Maybe I can do a mix and match of work. Any comments?

Stage 1 $300
bowl blend
intake gasket match
taper valve guides

Stage 2 $550
gasket match exhaust
deburr combustion chamber
intake runners

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Old 04-24-2012, 11:08 PM
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Thanks for the reply F'bird! Sounds like my initial thought of leaving them alone and saving my money was a good idea.

I am a DIY'r, and normally I would take that money I would pay someone else and buy the tools to do the job, but considering I have no experience here and I don't want to mess up a good set of heads I would farm this out. I have tried looking up the proper way to port heads, but all I see is some shady work done by Bubba with a grinder or some blurry pictures that say "take some material off, but not too much" and I have no idea *where* to take material off. I still have no idea what a pocket port/bowl blend is, or even what unshrouding the valve means. Maybe there is a good wiki write up on this???
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Old 04-25-2012, 12:50 AM
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Quote:
Originally Posted by Silver Surfer
Thanks for the reply F'bird! Sounds like my initial thought of leaving them alone and saving my money was a good idea.

I am a DIY'r, and normally I would take that money I would pay someone else and buy the tools to do the job, but considering I have no experience here and I don't want to mess up a good set of heads I would farm this out. I have tried looking up the proper way to port heads, but all I see is some shady work done by Bubba with a grinder or some blurry pictures that say "take some material off, but not too much" and I have no idea *where* to take material off. I still have no idea what a pocket port/bowl blend is, or even what unshrouding the valve means. Maybe there is a good wiki write up on this???
There's some photos here showing the lip just below the seat that can be carefully smoothed.
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Old 04-25-2012, 09:26 AM
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Cleaning up a set of vortec castings isn't that hard, just be careful. You can spend $100 or more on supplies to get started though. I port every head I use for my own engines, its just cheap easy work once you get the supplies. Aluminum heads are MUCH easier to do.

Keep in mind its possible to not only improve the quantity of flow but also the quality
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Old 04-25-2012, 11:36 AM
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The nice thing about learning on cast iron is you have no choice but to go relatively slow, it just takes longer to grind iron. get a comfortable place to work, lots of light, a stool. Wear earplugs, a dust mask and gloves. Check work often
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Old 04-25-2012, 12:05 PM
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Quote:
Originally Posted by Silver Surfer
I am tempted to NOT port my heads, but I called the local head shop and asked them about pricing just for kicks. Here is what they told me (price is per pair of heads). From my research it appears the best thing to do would be to bowl blend and deburr the combustion chambers, messing with anything else would be detrimental. However, I have not read anything about tapering the valve guides. Maybe I can do a mix and match of work. Any comments?

Stage 1 $300
bowl blend
intake gasket match
taper valve guides

Stage 2 $550
gasket match exhaust
deburr combustion chamber
intake runners

I could go on for paragraphs, but if you buy a copy of this book How to Build & Modify Chevrolet Small-Block V-8 Cylinder Heads by David Vizard you will find your questions answered, even those questions you don't even yet know you have. This is a top notch book that is very through, easy to read, with clear pictures, drawings, and charts.

The first question you have to ask yourself is where you want the power and how often will you use it. A race versus a competent street engine kind of question. The Vortec already gives up a little torque below about 3200 RPM. Porting the Vortec accentuates the bottom end torque loss against a higher end horsepower gain. This head provides a substantial power gain over previous heads without touching it. Compared to many earlier smog heads a simple replacement with the Vortec will show a 20 to 40 horsepower gain and about 25 ft pounds on the torque peak which will move upward a few hundred RPM. The amount of horsepower gain is very related to the cam timing and lift along with having the DCR (Dynamic Compression Ratio) that matches the cam to keep the DCR around 8.5 to 1. As the cam gets to later and later closing of the intake valve the Static Compression Ratio (SCR) (the one you measure volumes to determine) may need to push up around 10 to 1 to get the needed DCR. The 20 horse gain would represent bolting Vortecs on to replace smog era heads on a mildly cammed truck engine. The 40 horse gain would represent an engine with a cam similar to the Comp XE268 having static compression about 9.5.

Porting the Vortec adds about 10 horses on the top end. This is already a pretty good head so the gains from porting it are not as astronomical as on earlier heads. You can pretty easily crack 400 horses with this but you'll find a lot of tricks need to be done to bridge the last 6 or so horsepower. Blending the backside of the valve, using 1.6 rockers, beehive springs, stuff like that is needed to dial in those last few ponies. This tells you that while it isn't difficult to crack 400 horses with a Vortec topped 350, in service it won't stay there long as wear and deposit build ups begin to appear.

You need to pay a lot of attention to the cooling system, in service the Vortec has proven to be quite crack prone when over heated, so you need to take steps to insure that the conditions leading to overheating problems are minimized if not eliminated. This gets to be a bigger problem as you sup up an engine as the natural temperatures rise as more power is expected from it and the time in which it will get into trouble if something goes wrong with the cooling system is greatly reduced. So there gets to be little space nor time to deal with an error. So while spending your life's savings on hot rod parts set some aside for the cooling system.

Bogie
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Old 04-25-2012, 01:25 PM
How fast is fast enough?
 
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So far we have a max of 232cfm on a port that hasn't even been created yet and a 10hp peak with that same port running on an unknown engine.

The SWAGs are getting more WAG and less S...

Bottom line is that porting can improve a head, by how much is debateable. But starting with a better head for the application is usually the best move in terms of power and total cost.

If you have the time and tools it can be "free power" how much power is there is debatable at best and borders on wild *** guessing. The same can be said for modifying almost any part on an engine though...
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Old 04-25-2012, 08:19 PM
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Quote:
Originally Posted by ap72
So far we have a max of 232cfm on a port that hasn't even been created yet and a 10hp peak with that same port running on an unknown engine.

The SWAGs are getting more WAG and less S...

Bottom line is that porting can improve a head, by how much is debateable. But starting with a better head for the application is usually the best move in terms of power and total cost.

If you have the time and tools it can be "free power" how much power is there is debatable at best and borders on wild *** guessing. The same can be said for modifying almost any part on an engine though...
Actually getting 232 CFM on a L31 Vortec intake port at .5 inch valve opening isn't very difficult.

A little porting in the bowl easily bumps the top end another 10 horses but it continues the existing trend of shaving off lower end torque. I figure that's about what the home porter with no experience will see. I don't expect more from a 300 dollar shop job either. Plus given the L31 Votec's propensity to crack I'm not very excited about carving them deeper. For a street engine that will see a lot of heating and cooling cycles I pretty much limit porting to no deeper into the material than the paste line. If you want to go fast, both GMPP and the aftermarket offer better Vortec type choices that both flow more out of the box and will take a lot more carving in the ports before you hit water.

I'm not impressed with port matching on the manifold side, unless you really know what you're doing and are prepared to take on the intake manifold as well there just isn't enough gain.

The exhaust side needs a similar treatment to blend the seat to pocket walls and clean up the guide. For an L31 there isn't much to be gained gasket matching on this side either.

I fear the deburr of the combustion chamber, not that I'm against knocking off any globs or sharp edges, but and this is where I take an exception with the modified pictures of the Vortec head in Vizard's book I suggested is, by my research, that the smooth aero dynamic, polished chamber he presents undoes a lot of that chamber's purposes. I wouldn't want that done to my heads.

Bogie
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