notches at the top of the cylinders on Pontiac blocks
 

This has probably been covered before but why did Pontiac do this? And if you sleeve a cylinder you lose the notch so do you need to take a die grinder or sanding tube and recut it and if so why?

Remember, there are no dumb questions.

valve shrouding.Tom

So it is only to help the engine breathe and not to prevent the valves from kissing the upper edge of the cylinder wall?

The valves need to have an area completely around the intake valve to have FULL intake flow capability with the fixed bore diameter of some blocks. You could bore the crap out of the block (if the walls allowed it), (they will not), and not need the intake bore chamfers.

The valve diameter/valve angle is one concern with valve to piston and valve to block interference. Low lifts on the cam, not a real problem. Smaller valve diameter like the 1.96 diameter valves, not an issue. 2.11" and 2.15 inch valves and maybe the valve is MAYBE too close to the block. It depends on the camshaft lift and duration and the combination of parts..

A edge of the valve diameter is the closest it comes to the edge of the block in one area. But the flow has to get around that restricted opening therefore the valve chamfers are sometimes needed. Hope this helps. Stock builds probably no issues as
the factory offered some engines with no mods to the block for chamfers.

Tom V.

The 69 350 block was notched. 69 was the year they made the 350HP rated 350 with 48 heads. I understood they needed the notch to run those 2.11 valve 65.5 cc heads.
I ended up putting some 48s on a low mile 2 barrel 350 I had with those notches. Ran great with 2801 cam. Scrapped the block and wish I had not.

Assuming the notches were to improve flow it seems odd the 1970 350 with #11 heads had such large notches. These are low-compression small-valve heads so the notches were unlikely related to performance. Maybe emissions?

I think I read they were ~6cc volume, which helped explain the low CR on top of the large chamber volume (~88 cc?).

I copied this from another thread:

http://forums.maxperformanceinc.com/...1&d=1634564108

If you want to talk about Emissions and how to lower emissions, there are a lot of theories on how to do that. Maybe start a different thread on that topic.

There are always exceptions to any concept.
One manufacturer got into trouble for Lean Burn strategies.
Most think lean burn is the right direction to go UNTIL YOU BLOW THE NOx EMISSIONS
which are caused by lean burn being too lean.

Companies were just trying to figure out emissions in 1970.

Tom V.

Most would say that a chamfer on the intake valve side is there for better flow.
Chamfers on both sides of a block would say, the engineers are doing something else in the combustion strategy.

Thanks Tom

Didn't want to talk about emissions, was just curious about other reasons for notches and the 70 350 was an example.

As you say, it may have had something to do with modifying the combustion process ... but seems an odd decision to spend the money machining a low-CR, low-performance 350 block and remains a curiosity for me.

I saw many product design decisions driven by marketing and/or regulatory requirements so maybe those are also possibilities. For example, maybe they just needed to reduce the CR to keep HP under some goal? They advertised 8.8 CR but it wasn't even close...

But I'm not curious enough to start another thread and likely would add no value for most.

Fine with me. Did the job for 7 years, no desire to go down that road again as a topic.
Maybe Keith Seymore could provide some insight as he was working for GM in the 70s.
And he did work in the Engineering side as well as the Plant side of the process.

Tom V.

Since chamfers where machined in accordance to valve inclination angles used such implementation could only be seen in regards to flow improvement’s because nowadays we know of better places to unshroud chamber’s in regards to just flat out improving emissions.

I can view it no other way!

I have been intrigued by this topic too. Looked for a pattern over the years and it is hard to follow. It seems largely to do with the combustion chamber shape of the heads, there may be multiple reasons. Not sure if the inclination angle was a driver, but it certainly effected the shape of the chamber.

I recall Steve25 flow testing the chamfer on a head maybe last year and it pickup up cfm at .100” lift and below, but IRC didn’t show much up at higher lifts. The later heads had narrower chambers and did away with the chamfers. My 2 bbl and 4 bbl 71 455 blocks I have do not have chamfers. Being that I haven’t seen every block made, I don’t know which blocks had bore chamfers and which didn’t. But the 68-70 blocks have the bore chamfer the same as the photo below with the chamfers higher. It seems largely do to the width of the chamber shape in the head. After 71 the chamfers became less and less common. Sometime in the mid 70s the 350s received chamfer on the exhaust only, which is unique, all the other engines had them in the intake or both. I thought I had a handle on what they did until I noticed exhaust only chamfers. It seems more like the chamfers are to unshroud certain combustion chamber shapes.

Here are some photos. If you notice these are both 1967 blocks xxxx133 castings and one block has the chamfered much lower. Both on the intakes.

The oily photo is from a 75 350, exhaust chamfers only with no valve reliefs. The other 2 are 67 400s.

Did 455s with their larger bore even need them?Tom

That is kind of the question that goes through my mind to Tom. I know I have seen bore chamfers on the 1970 455 in GTOs, and have seen them on the 71 455 HO. So apparently Pontiac thought they needed them. But on my 71 455 4bbl which originated in a Grand Prix SJ 455, it doesn’t have them. I assume that the round ports must have wider chambers than my d port engine. I don’t think they are worth adding if the engine doesn’t have them in most circumstances, especially on the 455 or over bore 400.

Pure speculation- maybe the 70 350 blocks were just a "carry-over" from a late decision to adapt to un-leaded fuel and drop the 4bbl/big-valve 350?

Were the notches in the 69 350 blocks the same size as those in the 70?

From what I have seen the 350s all had the same or very similar bore chamfers from 68-72. I Checked the cc’s once on a 71, seems like it measured in 3-4 cc for both sides.

Interesting to note also that the 67 blocks I posted pictures of the one with lower chamfers like you would see in a 66 389 4bbl, it was the code WW, or a 335 HP 4 speed. But the other photo with the chamfers higher was a WZ 325Hp Firebird engine. Both should have had 670 heads.

Here's a cast iron Termites eye view of how a .030" over 350 Bore looks like with no chamfer.

Here's a photo of a 69 350HO posted by GRX in the thread linked by 400 Lemans:

http://forums.maxperformanceinc.com/...1&d=1634659378

The notches in the 70 350 block look the same to me, so I'm still speculating they were just a carry-over in 1970 without adding value.

Jay, Interesting topic...and, agreed, confusing too. I never noticed the intake chamfers on this '71 455HO WL block.

Dennis

In late 66 and early 67 the factory likely machined a lot of the production 400 blocks with the lower set chamfers since those blocks could also get fitted with the 143 or 142 casting head which was still a 20 degree 389/421 type head or the very short lived 061 casting with the new 14 degree valve inclination angle.

Does anyone know if the 400 blocks fitted with the 061 casting still used the 389/ 421 type piston?