1968 327 reubuild, vortec heads or not?

[Marshy]

The Vortec heads use a raised ceiling port that the old manifolds won't line up to, even with some grinding on the ceiling of the lower manifold opening can't account for the difference without welding/machining IIRC. Your best best is to call summit and ask them if they have an intake that will fit your heads and have the front oil fill tube. There might be something out there that's easily over looked.

Regarding issuing a 72cc combustion chamber, that's a really good way to go to get your CR down. However, I wouldn't recommend it blindly. Ask your builder to give you the deck clearance and piston relief volume so you can estimate your CR now and with the 72cc heads. You should (ie your builder) needs to know the deck clearance to choose a head gasket anyways so you aren't asking an oddball question anyways.

Did you chose a cam already? I'll review if you did... you might want the smaller heads if you are using a large duration cam. That's where you get into dynamic compression which dictates what octane you need, not necessarily static compression.

[toolboxchev]

Originally Posted by Marshy

The Vortec heads use a raised ceiling port that the old manifolds won't line up to, even with some grinding on the ceiling of the lower manifold opening can't account for the difference without welding/machining IIRC. Your best best is to call summit and ask them if they have an intake that will fit your heads and have the front oil fill tube. There might be something out there that's easily over looked.

Regarding issuing a 72cc combustion chamber, that's a really good way to go to get your CR down. However, I wouldn't recommend it blindly. Ask your builder to give you the deck clearance and piston relief volume so you can estimate your CR now and with the 72cc heads. You should (ie your builder) needs to know the deck clearance to choose a head gasket anyways so you aren't asking an oddball question anyways.

Did you chose a cam already? I'll review if you did... you might want the smaller heads if you are using a large duration cam. That's where you get into dynamic compression which dictates what octane you need, not necessarily static compression.

X2 on that!

[lil hoodlum]

Originally Posted by Marshy

Regarding issuing a 72cc combustion chamber, that's a really good way to go to get your CR down. However, I wouldn't recommend it blindly. Ask your builder to give you the deck clearance and piston relief volume so you can estimate your CR now and with the 72cc heads. You should (ie your builder) needs to know the deck clearance to choose a head gasket anyways so you aren't asking an oddball question anyways.

Did you chose a cam already? I'll review if you did... you might want the smaller heads if you are using a large duration cam. That's where you get into dynamic compression which dictates what octane you need, not necessarily static compression.

Hi Marshy,

I have posted the cam specs in post #5. I'm not really familiar with what all of the specs mean. I know it has to do with the opening and closing of the valves and what not but that is about it.

[lil hoodlum]

So today I was able to stop by the machine shop and speak to the builder about the motor.

I told him my cocerns about having to use the premium gas and that I was wanting to avoid that. I told him about the 72cc combustion chamber heads that I am thinking about using. He plugged the numbers into the computer and said it would bring my compression ratio down to 8.7:1 or he may have said 9.0:1. Anyway he said it would bring the compression ratio down and I should be able to use regular gas and that he still thought it would be a good motor. He said he felt the 72cc heads would be a better choice.

So, my next steps will be to figure out what to do with the Vortec heads. Hopefully Jegs will take them back or will give me credit in exchange for the heads. If they will I will purchase the 72cc ProMaxx heads from them.

Or another option the builder said they would rebuild a set of double hump heads. He said they have plenty in their storage unit to choose from.


I feel better about the new direction the build is going as I really have my heart set on using the oil fill intake. I would be even happier if the double hump heads are pre '69 as they won't have the accessory holes in the head. Yes, I allready have the correct exhuast manifolds and alternator bracketry along with the correct pulleys and short water pump.


I will say though that I thought the motor looked badazz with the Vortec heads bolted on.

[TheGhost]

The "double hump" heads as rule of thumb are 64 CC heads. So, you would be in the same place as the Vortec heads.

Just some thoughts...

A camshaft will affect cylinder pressure not compression ratio.

Static compression ratio is based on fixed mechanical features of the engine:
Cylinder Bore diameter, compression height of piston, combustion chamber size, head gasket thickness, etc..

A short duration high lift cam will build more cylinder pressure than a high lift long duration cam.

A camshaft can effect dynamic compression ratio(cylinder pressure).

So, when you put a bigger cam in an engine, you aren't really giving up compression. You may have lower cranking pressure, and less low speed performance, but sooner or later the extra duration will help get more air into the engine than you would have ever gotten with a smaller cam. Eventually both cylinder pressure and power will go up even if the actual static compression ratio stayed the same.

There is one side efect to this though...detonation, which is based in part on cylinder pressure and engine speed (or the amount of time the fuel has to detonate). The faster an engine is spinning, the more likely there will be a leaner mixture, and the less time it has to detonate. Because of this, an engine with lets say 9:1 compression and a very early intake valve closing may be able to trap most of it's intake charge in the cylinder at a fairly low speed. Since the engine is spinning at a low speed, it won't be able to make as much pressure before the fuel detonates. The result would be an engine that detonates at low speeds and lacks high speed performance because the cam is too short. An engine with the same compression and a later intake closing event will trap less air in the cylinder at low speeds, and wont be as prone to detonation at low speeds. At higher speeds, the engine will intake more air and make more pressure, but since the engine is spinning faster, the fuel has less time to detonate, and detonation is avoided despite there being more pressure.

All this to say...make sure you really take some time to consider your camshaft choice as it relates to static compression and not place to much focus on any one component. Everything has to work together. Otherwise you end up with and engine that sounds great at idle, but doesn't really live up to it's potential. A rough idle can be accomplished by pulling off a vacuum line... :-)