Thread: SBC 350 build

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Haha. That sounds like a solid way of thinking. I read little article and the boys at Hot Rod magazine used vortec heads and used a set of COMP Cam's beehive springs and ran .490 lift. (they say they are good for .560) If i understand correctly, I can put on the beehive springs and retainers and be good for a .560 lift cam. I was thinking I would do this since the heads are already on the motor it would be a decent way to make power and save some serious scratch. Of course, I would check the casting numbers on the heads before I did it.
Link to the article:
http://www.hotrod.com/techarticles/e...ild/index.html
Check page 2
Link to springs:
http://www.summitracing.com/parts/CCA-26915-16/
Link to retainers:
http://www.summitracing.com/parts/CCA-787-16/

Originally Posted by Bowtiepickups

Haha. That sounds like a solid way of thinking. I read little article and the boys at Hot Rod magazine used vortec heads and used a set of COMP Cam's beehive springs and ran .490 lift. (they say they are good for .560) If i understand correctly, I can put on the beehive springs and retainers and be good for a .560 lift cam. I was thinking I would do this since the heads are already on the motor it would be a decent way to make power and save some serious scratch. Of course, I would check the casting numbers on the heads before I did it.
Link to the article:
http://www.hotrod.com/techarticles/e...ild/index.html
Check page 2
Link to springs:
http://www.summitracing.com/parts/CCA-26915-16/
Link to retainers:
http://www.summitracing.com/parts/CCA-787-16/

That could be a plan. Parallel springs such as the Cranes I linked really need a spiral-wound damper spring to cancel harmonics in my opinion, whereas beehive springs do not, so my choice in what you have presented here would be the Comp springs.

Notice that the builders used a cam ground on a 108 degree lobe separation angle and had some horribly low intake manifold vacuum because of it. Generally speaking, the motor will make more power the tighter you go with the LSA, but vacuum stinks for operating power brakes and other vacuum-operated accessories. You'll also note that they mentioned a reserve vacuum can or an electric vacuum pump. If you need additional vacuum, don't waste your money on a reserve can. Either pop for the pump or install a hydroboost system like was used on some Cadillacs, trucks and such. Works great with the wildest cam because the brakes are not dependent on vacuum to operate. I don't care what anyone else says, you need a minimum of 16 in hg to operate power brakes.

You can still make good power for street/strip and make a little more vacuum by choosing a cam ground on a 112 degree LSA and keeping the duration reasonable. You keep the duration reasonable by using a reasonable static compression ratio. Understand this: YOU CANNOT INTELLIGENTLY CHOOSE ANY CAMSHAFT FOR ANY MOTOR UNTIL YOU ABSOLUTELY, POSITIVELY KNOW THE STATIC COMPRESSION RATIO BY MEASURING COMPONENTS YOURSELF OR HAVING SOMEONE ELSE MEASURE THEM FOR YOU.
You need 5 values:
1. Cylinder volume in cc's
2. Combustion chamber volume in cc's
3. Piston deck height volume in cc's (the volume between the piston crown and the deck of the block with the piston at top dead center)
4. Piston crown volume in cc's (a true flat top piston will have about 6-7 cc's in the eyebrows that are cut into the crown for valve clearance. Otherwise, for a dished or domed piston, the mfg will usually publish the volume). Most of the stock-type 350 pistons have a recess that isn't actually called a dish, but it is a depression across the crown, with a very thin ring of material sitting up a little higher around the bore. That little thin ring of material is insufficient for generating a good squish and you should be very careful with compression ratio and cam if you use that type of piston. The very best type to use in a street/strip build in my opinion is a D-cup dished piston. These provide a generous shelf of material that comes up against the underside of the cylinder head to generate a very good squish. If you build any kind of reasonable static compression ratio into the motor without using a good squish, there is a good possibility that the motor will detonate on pump gas.
5. Head gasket volume in cc's
If you want me to go through the entire routine and teach you how to find the exact SCR, just ask.
The camshaft will normally be the last component chosen, after everything else is nailed down for the build.

If you do end up using L31 heads, don't let anyone talk you into increasing the valve sizes. Stock, they are 1.94"/1.50" and will do a fine job on a 350. Any cutting done on an already thin-wall casting is just asking for trouble.

Thanks for that post. It cleared up a few things for me. I will definitely get you to help me go through calculating everything when I have it tore apart and it's closer to re-build time. Which is greatly appreciated. I have no desire to increase the size of the valves. So would you recommend I get some new pistons instead of the stock ones? Would they have to be something with a D-Cup dish? Or would flat tops due alright too? Also, what would your input be on rockers? Would roller rockers be worth it for me? Thanks again for your input, it seems to be helping my understanding a lot.

Originally Posted by Bowtiepickups

Thanks for that post. It cleared up a few things for me. I will definitely get you to help me go through calculating everything when I have it tore apart and it's closer to re-build time. Which is greatly appreciated. I have no desire to increase the size of the valves. So would you recommend I get some new pistons instead of the stock ones? Something with a D-Cup dish? Also, what would your input be on rockers? Would roller rockers be worth it for me? Thanks again for your input, it seems to be helping my understanding a lot.

Let's find out what pistons are in the motor first. Another thing that I didn't mention about pistons, the compression height is the distance from the centerline of the wrist pin to the top of the crown. On a 350, this dimension, stock, is 1.560", so that when you add the 1.560" to the rod length of 5.703" and add to that the stroke radius of 1.740" (half the 3.480" stroke), you get a total "stack" height of 9.003". Since the block is normally ~9.025" give or take a few thousandths, you can see that with the piston at top dead center, the piston deck height (distance from the piston crown to the block deck) is 0.022" in this case. Now, when these motors were put together at the factory, they used an embossed steel shim head gasket. It has been a long time since I measured the thickness of one, but I seem to remember somewhere around 0.020"/0.022". OK, so if the piston is 0.022" down in the bore and the gasket is ~0.022" thick, you can see that there will be 0.044" of clearance between the piston crown and the underside of the cylinder head with the piston at top dead center. This is the squish dimension in this case, 0.044". Since most builders will agree that 0.035" to 0.045" is ideal on a small block, we are right there for this example.

Now, that was using a piston with a stock compression height of 1.560". There are also pistons made for a 350 with a 1.540" compression height. These are usually, but not always, cheapo cast pistons with an ultra-cheap price. They're called "Rebuilder Specials". The reason these are made is so that the rebuilder of the motor can take a 0.020" cut on the block decks to clean them up and still have the same crown to deck dimension (piston deck height) as the motor had stock. The problems begin when Henry Hotrod is looking for pistons and sees these really inexpensive pistons for sale. He has no idea about squish and how it affects his motor and is totally unaware that there is any such thing as piston deck height and squish. All he sees is the cheap price of a new set of pistons, so he springs for them. He also has no plans to deck the block. So, he ends up with the piston sitting 0.020" deeper in the bore at top dead center and has eliminated any possibility of generating a good squish. He bolts the motor together and is dismayed when it will not run on his regular grade of pump gas without pinging (detonation). Desparate to stop the detonation, he pulls all the ignition advance out of the motor and now it won't pull the hat off his head. Or maybe he just lives with the detonation and the motor finally throws in the towel with several cracked pistons.

I have seen this scenario come to life time after time after time in the years I have been on these forums.

The very best combination you can put together is a motor that uses absolutely flat-top pistons with just some shallow eyebrows for valve head clearance. Problem with this is that you have to use a cylinder head that has chamber volumes that will produce the proper static compression ratio for the fuel you plan to use. For instance, if you have L31 heads, the chambers are ~64 cc's. If you used flat-top pistons with those heads, your static compression ratio would be around 10.3:1, which would be over the line for using pump gas with iron heads. With aluminum heads and a very tight squish of 0.035" and the proper cam that would close the intake valve at the proper time, you might get away with it. But with iron heads, you'd have to run so much camshaft that the motor wouldn't be any pleasure to drive on the street and I don't think that's what you're looking for.

So, the answer is dished pistons to lower the static compression ratio so that you can use less cam and run pump gas. Using a 12cc dish with 64cc heads would result in a 9.6:1 static compression ratio. Here is an example of such a D-cup piston from Keith Black. Notice in the photo on the left side, that there is a generous flat area on the crown. This is the flat area that comes up to the underside of the cylinder head to generate squish across the chamber. When the fuel/air mixture is squished or "jetted" across the chamber, the turbulence homogenizes the mixture and eliminates any over-rich or over-lean conditions in the chamber, making a more complete burn that will result in maximum power and minimum chance of detonation. The tighter you run the squish, the less octane you can run in the motor.
http://www.kb-silvolite.com/kb_car/p...tails&P_id=155
With such a 9.6:1 combination, I might use a camshaft like this one, or something close to it...
http://www.summitracing.com/parts/CCA-12-420-8/

Now, if a fellow moved to an 18cc D-cup instead of a 12cc,
http://www.kb-silvolite.com/kb_car/p...tails&P_id=154
then the static compression ratio would be somewhere around 9.1:1 and you might use a camshaft such as this one....
http://www.summitracing.com/parts/CCA-12-410-8/

It's all about the combination. You have to choose pieces that will like each other and make max power for the combination.