67 300 HP and 350 HP intake differences?

[Joe R.]

I have a 67 300 HP base engine and I'm looking into ways to increase power while maintaining a fairly stock external appearance (still trying to sort out what to do about "life after Top Flight").

On Duke's advice the heads were pocket ported while being rebuilt earlier this year. I also put in a cam that is roughly mid way between the 300 HP and 350 HP GM cams. Now I'm looking at possible improvements to the intake and exhaust flow. On the intake side, I'm thinking about changing to the 350 HP aluminum intake and "dressing" the engine as a 350 HP. I have a few questions about this that I'm hoping some of you can comment on:

1) Performance-wise, how much better is the 350 HP intake compared to the 300 HP intake?

2) Why does the 350 HP intake use a bypass connection between the water pump and the manifold, while the 300 HP intake (and most aftermarket intakes) do not?

3) Is there any harm in using the 350 HP intake with my existing 300 HP water pump and the bypass fitting on the intake plugged?

Assistance with any or all of these questions would be most appreciated.

[Joe L.]

Re: 67 300 HP and 350 HP intake differences?

Joe----

1) I don't think you'll notice very much difference with the L-79 manifold. There might be a little at higher RPM;

2) I don't know the reason for external versus internal bypass when the different manifolds were used. External bypass disappeared forever for passenger car small blocks after 1967;

3) No, there's no problem, at all, as long as the existing internal bypass is present. The internal bypass is the hole between the waterpump leg and block on the passenger side.

[Joe R.]

Re: 67 300 HP and 350 HP intake differences?

Thanks Joe for the explanation about the bypass. I recall seeing the extra hole when I had my 3782608 water pump off for rebuilding, but I did not really think about its intended function.

Does this mean that the 3859326 water pump casting used on the 350 HP engine did not have the bypass hole, or did it have the external bypass in addition to an internal one? I'm just curious about GM's thinking behind the two different bypass schemes.

On the block side of the water pump interface, did the block have the bypass hole drilled regardless of whether it was a 300 HP or 350 HP application?

The only reason I can think of for using an external bypass is that it might have greater flow capacity, but it seems that would only be relevant at high RPM when the thermostat was still closed.

[Joe L.]

Re: 67 300 HP and 350 HP intake differences?

Joe-----

All small block waterpumps, regardless of whether they had external bypass provisions, also had the hole in the passenger side leg for internal bypass. All small block engine blocks had the internal bypass hole regardless of what the final application was for the block.

However, SERVICE waterpumps sold by GM for years were generally of the type with external bypass provisions (but also, of course, having the internal bypass hole in the passenger side leg). Instructions supplied with the pumps specified that when the pumps were used for external bypass applications, the hole in the right side leg was to be plugged with a dowel pin included with the pump. When used with an application using internal bypass, the external bypass fitting was to be plugged with a threaded pipe plug also supplied with the pump.

In PRODUCTION, I don't know if the hole in the right side waterpump leg was plugged with a dowel or otherwise plugged when the pumps were used for external bypass applications. I've never looked carefully at a known-original pump removed from an external bypass application. I'd be very interested to find out from someone who has made such an observation. I've wondered about this for years but never got around to researching it.

Big blocks all use external bypass and have no provisions for internal bypass that I am aware of.

[Joe R.]

Re: 67 300 HP and 350 HP intake differences?

Thanks Joe. This is very interesting. Maybe there were two schools of thought within GM regarding how to best accomplish the bypass function, and the internal method eventually won out for the small block applications.

It seems to me that the external method has more flow capacity, and also returns the bypass flow to a point within the water jacket system that might result in better flow circulation in the bypass mode. On the other hand, the internal bypass method is clearly less expensive to implement.

[Joe L.]

Re: 67 300 HP and 350 HP intake differences?

Joe-----

As far as external bypass on passenger car, including Corvette, small blocks, it had a very limited "reign". I don't know of any applications with cast iron manifolds that used external bypass. However, there may have been some in the 61-62 period.

Generally, the only small blocks that used external bypass were 62-67 small blocks with aluminum manifolds. Some 1961 Corvette applications with aluminum manifold may have used it, too; I'm not sure about this. Mostly, aluminum manifolds were used for Corvette applications, but there were also some used for L-79's in other Chevrolet cars and 67-69 Z-28 Camaros.

After 1969, external bypass was used for no passenger car small blocks, regardless of intake manifold type. However, I believe that it was used for many MD and HD truck applications with small blocks.

[John H.]

Re: 67 300 HP and 350 HP intake differences?

'67-'68 Camaro Z/28's (302 high-rise aluminum intake) used the external bypass; the '69 Z/28 aluminum intake (one-year-only part) had a threaded hole for the bypass fitting, but it had an internal square-drive plug in it, and no external bypass was used. 1969 was the first year for the new "long" water pump accessory drive system on small-blocks, and none of them used an external bypass.