# Pontiac Combustion Chamber Differences & Valve2Piston Clearance



## PontiacJim (Dec 29, 2012)

*PONTIAC HEAD DIFFERENCES

46* J312 - 350CI *4X 7H* D143 - 400CI *4X 1H* B163 - 455CI
96CC Press-in Studs 98CC Screw-in Studs 114CC Press-in Studs
1.96"/1.66" Valves 2.11"/1.66" Valves 2.11"/1.66" Valves

Pic #1 - 4X 7H Combustion Chamber
Pic #2 - 4X 1H Combustion Chamber

Pic #3 - How/Where I took my measurements.

Depth of Combustion Chamber measured from flat head surface:
.846" .849" .925"
Valve Lift to be flush with flat head surface - closest edge of valve:
Int. - .251" (1.96" Dia. Valve) Int. - .244" Int. - .347"
Exh. - .312" Exh. - .312" Exh. - .341"

Width of Combustion Chamber at widest point - Horizontal:
4.20" 4.19" 4.21"
(Interestingly enough, the 400 stock bore is 4.12" and 455 stock bore is 4.151" and the measured width of the corresponding heads/chambers are both .07" larger than the bore.)

Width of Combustion Chamber at widest point - Vertical:
3.25" 3.36" 3.40"

Pic #4 - Pistons Compared

On the Left -Depth of Stock 1973 Pontiac 400CI Piston Valve Relief:
.144"-.155" with each relief having a different value.

On the Right - Depth of forged TRW L2262 .030" 400CI Piston Valve Relief:
.178" all four reliefs.

These measurement are intended to answer a couple questions and statements I often see with regards to Pontiac heads.

1st - Do Pontiac combustion chambers have different combustion roof heights?

Yes- Based on my actual numbers, the 350CI was .846", the 400CI .849", and 455CI .925". With a higher combustion roof on the 455, the valve depth as measured from the flat surface is also more. Keep in mind that Pontiac used 3 different valve stem lengths, so the higher roof would use a shorter valve. The RA IV used a longer valve due in part to its lowered combustion roof.

2nd - Can I put 400CI heads on my 389CI?

Several factors come into play, especially the valve relief positions on the piston tops - they don't match. Also note that a 389CI standard bore size is 4.062". Now look at the width numbers on the 400/455 chambers. You can see why you may need bore the block .060" to match the 400CI (4.12") bore and chamfer the top of the bores to clear the valves at high lifts. I would think that the smaller valve heads with screw-in studs, 98CC #46/#4C/#4H could be a better choice if using a stock or .030" over 389CI bore and either limiting valve lift or using pistons with matching 400CI valve reliefs.

I measured how much lift (or valve drop) the Intake Valve had until it struck a flat surface, like a piston top. I got a total travel of .244". So how can this number be used? It allows you to figure out the total amount of lift/drop the Intake Valve has before striking the flat surface of a piston - ie no valve reliefs. If you take .244" and add a .039" head gasket, and then add in a piston deck height (how far the piston top sits down in the bore) of .020", you get a grand total of .303". This is how much lift you could go before striking a piston if it did not have valve reliefs - such as the mismatch of a big valve Pontiac 400 head and 389CI piston. But wait, there is more to this story.

Each piston has valve reliefs cut into them. So take the .303" and add to it the Pontiac 400CI piston's minimum valve relief depth of .144" and you get .447" or .481" with the TRW piston. So with this head and matching piston valve reliefs, you might think the most you could go with cam lift is .447" or .481" before the piston strikes the wide open valve. Throw in that you want .080" clearance between the Intake valve and piston and .100" clearance between the exhaust valve and piston and you are probably scratching your head thinking how do they get lifts of .480" and up without breaking pistons and bending valves. So we are in trouble, right? Not necessarily. Let me try to explain why.

The opening of the intake valve follows the piston as the piston moves down the cylinder on the intake stroke to pull in the air/fuel mixture. From what I read, you want to rotate the crankshaft, using a timing wheel, 10 - 20 degrees After Top Dead Center. The exhaust valve is measure at 10 - 20 degrees Before Top Dead Center. Keep in mind that the valve never reaches its full lift figures until the pistons between .400"-.800" away from TDC.

Establishing the Piston-to Valve distance by a TDC drop test (piston on TDC and valve dropped until it hits the piston) can be a quick, but crude, way to verify enough clearance. There's ample clearance with a 0.400-inch drop (typical of production-based pistons and heads - Pontiac used .406" lifts), a 0.300 or less drop indicates the need to check Piston-to-Valve clearance on the actual build, and with a 0.200 drop this would be barely enough clearance with a 1.5:1 rocker arm ratio and using the suggested clearances of 0.080" intake/0.100" exhaust. Changing to a 1.65 or 1.7 rocker ratio could put the valve smack into the top of the piston due to the increased lift with these rocker ratios. Valve notches could be fly-cut deeper to increase the safety margin if the piston domes have enough thickness, and/or a milder cam can be selected.

I found another article that stated you can use the Pontiac numbers added up from above to determine if you have a safe valve lift. Using the numbers previously added up to get .447" - which could also be called the valve drop. The article said you wanted to use .040" as a clearance number between the valve and piston (valve relief). Using the valve drop of .447" - .040" of clearance = a.407" valve drop. This is basically the same valve lift number Pontiac used on their cams and the same number, .400", that the above paragraph stated was a safe TDC drop test number.

I found and modified a diagram to use as an example in the relationship between the piston and valve lift as the piston is moving up and down in the bore and the valve is opening and closing through the action of the cam lobe. It follows and continues after this post.


----------



## PontiacJim (Dec 29, 2012)

*Part 2 *

Pic #5 *Graph of TDC of piston and Valve Opening & Closing*

I highly modified and changed the graph from its original design so it would be easier to understand - I like simple to understand.

Picture the head placed on the engine block, valves hanging down. In the middle of the graph, TDC (Top Dead Center) is what is represented as the tops of the pistons - three of them. There positioning is based on the Valve Drop method of checking valve-to-piston clearance with the piston moved up the cylinder bore to Top Dead Center (TDC). You would want to do this using a degree wheel or piston stop when degreeing your cam. So you can see 3 different valve drop measures - .200", .300", and .400" - before th valve were to strike the piston.

At the very top of th graph, in color letters, is the cycles of a 4-stroke engine. Looking the bottom of the graph is the position of the piston in relationship to the engine's cycle - TDC (Top Dead Center) or BDC (Bottom Dead Center). These positions of the piston are related to the cycle are at the 180 degree rotations of the crank. The Intake and Compression stroke is 1 complete revolution of the crank, or 360 degrees of rotation, for the 2 strokes/cycles - intake & compression. These are the numbers on top in Blue - 0 to 360 degrees. At the top of the compression stroke, TDC, the spark plug fires and the Power and Exhaust stroke follow. These 2 strokes/cycles, power & exhaust, make 1 complete revolution of the crank, or another 360 degrees of rotation. These are the numbers on top in Red -360 to 0. So with these 4 strokes/cycles of each of the 8 cylinders, the completed 4 cycles rotate a full 720 degrees and then begins again when the Intake Stroke starts all over.

Each cam lobe, Intake or Exhaust, operate during one of the 2 cycles/strokes - Intake & Compression, and Power & Exhaust. Each cycle, as noted above, make 360 degrees to complete and therefore each cam lobe has 360 degrees of rotation. The amount each cam lobe holds the valve open is the duration, or the number of degrees within 360 degrees of cam lobe rotation. The valves have to be closed on the compression stroke so as to squeeze the air/fuel mixture tight and made ready for the spark plug to fire and begin the power stroke.

During the bottom of the power stroke, the exhaust valve opens and is made ready for the exhaust stroke of the piston that pushes the spent gases out the exhaust ports. Just before the exhaust valve is completely closed, the intake valve begins to open. Both exhaust and intake valves will be open at the same time at the piston goes from pushing the spent gases out and goes to pulling another slug of the fresh air/fuel mixture. The time these valve are both open at the same time is called the "Valve Overlap" and is what gives the engine that lumpy idle sound we all like to hear. Cams can have little to a lot of overlap depending on cam specs.

You can see a thin black line just below the Blue & Red duration numbers. This line represents when the factory feels that the intake valves lift off their seats to open and the exhaust valves return to their closed position. The number used by the factory is .006" which is tappet/lifter rise, not valve lift at the valve via the rocker arm. To determine this number, simply multiply the .006" by the rocker arm ratio. .006" x 1.5 = .009" valve lift. .006" x 1.65 = .0099" (almost .010").
The aftermarket cam grinders essentially adopted .050" as the standard tappet/lobe lift as a better means to compare one cam grind to another. So using .050" x 1.5 rocker arm ratio = .075" of valve lift. .050" x 1.65 = .0825" of valve lift.

The graph includes a line representing .050" tappet lift, which is .075" valve lift as noted by the valve lift numbers on the right side of the graph. Below this line are the degrees of duration for each of the cam lobes, intake & exhaust, used by the cam grinder when dialing in the cam - which is different from the "Advertised Duration."

Here is the meat of all this - Piston-to-Valve clearance. Beginning with the power/exhaust stroke, lets' start at red TDC -360 degrees. The spark plug fires at TDC, the explosion pushes the piston down and it reaches the end of its travel/stroke at red BDC -180 degrees. While the piston is going down, the exhaust valve begins to open at around red - 250 degrees. The piston reaches BDC at red -180 degrees, and the exhaust valve has yet to reach its full lift. The exhaust valve does not fully open until the piston is into its upward movement, or exhaust stroke. Following the curved arc which begins at "Exhaust Valve Opens," the valve becomes fully open, .500" valve lift, at about the red -118 degrees After BDC, or ABDC.

Once the exhaust valve reaches its peak lift and while the piston is moving up the cylinder to push the burned air/fuel exhaust gases out past the open exhaust valve, the exhaust valve is now closing. With the piston rising up into the cylinder, it is chasing the closing of the exhaust valve. If the piston were to catch up the closing exhaust valve faster than the valve was closing - the piston would slam into the valve and it could get ugly in a split second with things like bent valves, bent pushrods, valve train damages, piston tops busted or cracked, or worse like an engine destroyed. The Piston-to-Valve clearance can be made safe for street engines, or dangerously close for race engines.

Looking to the right of the black 0 that marks TDC, is a pink bar that runs from top to bottom of the graph. This bar represents the 10-20 degrees Before TDC (BTDC) that the Piston-to-Valve is the closest and is where the greatest chance of contact will be found. Going back to the valve drop method, the first number used is a valve drop of .0400" - the distance the valve will move from its valve seat to the top of the piston at TDC if you let it freely drop onto the piston. You can see the distance between the piston at TDC and the arc of the closing exhaust valve. The valve appears a good safe distance away from that pink bar and the piston represented at TDC.

Next you can see the how the valve moves closer to the pink bar and piston at TDC as the valve reaches .300" lift on its closing cycle. Still appears safe, but...... Then look at the .200" number. The valve at .200" lift appears to be right at the edge of the pink bar and the area where Piston-to-Valve contact will be made. There may not be enough safe clearance, .100", left between the Piston and Exhaust Valve at this point. This is where this clearance will need to be checked using the "clay" method or using a dial indicator and a micrometer to measure all valvetrain and related engine parts.

The same goes for the Intake Valve opening. It begins to open during the exhaust stroke, and BTDC, and when the piston reaches black TDC 0, the piston moves down the cylinder on the intake stroke to pull in a fresh slug of the air/fuel mixture. The downward piston is not chasing the opening of the intake valve. The same principals apply with regards to the clearances between the Piston-to-Valve which is now represented by the light blue bar tha goes from top to bottom on the graph.

What I have hopefully shown is that Pontiac combustion chambers do have differences that need to be measured to ensure your build is assembled to be its best using these measurements, and how it is possible to use an aftermarket camshaft having more lift and duration and various Lobe Separation Angles and not have the valves strike the piston. That the valve clearances are most critical between the 10-20 degrees BTDC for the exhaust valve and 10-20 degrees ATDC for the intake valve. Valve Drop can be a quick check in determining if valve to piston clearances are sufficient or if they need to be measured. However, any good engine assembly should include checking the valve to piston clearances rather than assuming when using anything other than stock or near stock cam specs.

What variables can affect the Piston-to-Valve clearances?

Intake center line (cam position)
Duration
Valve lift
Rocker ratio
Valve diameter
Piston top (domed, dish, or flat)
Shape and angle of piston valve reliefs
Valve angle
Piston deck height
Head-gasket thickness
Valve float at high rpm (loss of spring control)
Pushrod deflection at high rpm
Angle milling heads
Advancing the cam timing decreases intake valve-to-piston clearance and increases exhaust valve-to-piston clearance.
Retarding the cam timing decreases exhaust valve-to-piston clearance and increases intake valve-to-piston clearance.


----------



## PontiacJim (Dec 29, 2012)

Just for comparison for some, here is what a "closed chamber" head looks like. These would be found on 1966 and earlier engines AND the early 1967 big car 400CI "061" head. This is a "670" head and you can see the small hole to the right of the exhaust valve guide that was used for air injection on found on the California cars.

Closed chamber heads do have some advantages, but emissions controls and reducing exhaust emissions out the tail pipe were why heads went to the open chamber design.


----------

