Bigger valves require porting?

[dom]

I have been considering installing larger valves when I get my head rebuilt. I was wondering if you donot get the head ported how would a bigger valve help performance. The diameter Of the Port before the seat seems like a limiting factor unless enlarged. Intake valve is 43 mm but the port size is as little as 35 mm. Seems like a bigger valve may just get in the way.

[So Cal Mark]

opening the bowl area will help alot, but just using the larger valves provides a top end increase

[surfingfreeman]

Porting of both intake and exhaust (if adding a header) are said to improve performance and I had it done and notice great results, for a more in depth discussion you can read guy crofts book or join his website and see some of his discussions on the topic. Good luck.
Mike

[BEEK]

i want to expand on what mark said, just by installing big valves and doing nothing else absolutly no change will happen. you will have to open the seat area to even properly fit the big valve. again this is only a very minor increase until bowl work and de shrouding in the head are done. at this point you will notice an improvement. mostly all top end. myself, i cannot do a valve job on a head without fixing casting flaws in the heads with my porting tools. im unsure how much it helps, but it just makes sense.

[vortspeloce]

Here (below) is a photo of a deshrouded 1800 combustion chamber I have been working on. This head has bigger valves (especially intake side) and a 3 angle seat job. This is how I received the head back from the machinist after a new Trojan copper alloy valve seats were inserted to suit bigger intake valves. The gold area is the new seat and you can see the silver band underneath. This band is quite a ridge and as you can see needs to be blended into the port at minimum. I am not sure you could go with bigger valves without doing at least some blending.

[timinator]

Here (below) is a photo of a deshrouded 1800 combustion chamber I have been working on. This head has bigger valves (especially intake side) and a 3 angle seat job. This is how I received the head back from the machinist after a new Trojan copper alloy valve seats were inserted to suit bigger intake valves. The gold area is the new seat and you can see the silver band underneath. This band is quite a ridge and as you can see needs to be blended into the port at minimum. I am not sure you could go with bigger valves without doing at least some blending.

Leave the step on the intake port as is. It doesn't hurt flow and it helps to stop reversion into the intake port. It is an aid to help limit the low rpm bog that many people experience. If you want to radius the exhaust port below the seat go right ahead. Just know if you slip and touch the seat it will require a trip back to the machine shop.

[dom]

I decided to use stock valves. Without porting It does not seem worth it and I don't want to lose low end. Also I was afraid of a bad porting job that wrecked drive ability.

[So Cal Mark]

you can use oversize valves without porting and still gain top end power

[Daniel]

you can use oversize valves without porting and still gain top end power

your not going to make any gains by adding larger valves without porting and resizing the seats.

[BEEK]

im assuming mark is refering to properly installing bigger valves, which , in it self includes opening up of the seats or installing larger ones. and blending the seat job. this is not porting, and will make increases.

[So Cal Mark]

correct on the big valves. We've done it on many cars and experienced nice top end gains without a full port job

[dom]

When is a valve or port too big for a given size motor or is this never the case. A 1.6 motor is much smaller than a 2 litre yet the stock valves are the same. I wonder if bigger valves and porting are more beneficial to the bigger motors and might be too much for the 1.6 which is low on torque and likes to rev already.

[vortspeloce]

Leave the step on the intake port as is. It doesn't hurt flow and it helps to stop reversion into the intake port. It is an aid to help limit the low rpm bog that many people experience. If you want to radius the exhaust port below the seat go right ahead. Just know if you slip and touch the seat it will require a trip back to the machine shop.

I am intending on blending the lip. I have not seen anything to support what your saying about the lip not hurting flow. I am intending on getting this head flow tested, so it might be prudent for me to do a before and after measurement. That said, I am happy to be wrong on this. Please send me some references ("flow benched" would be good) so I can do some more research.

[timinator]

Blend the lip if you like. As I said don't blend the step. The 90deg. cut(tangent to bowl) that establishes the bowl diameter has a distinct depth of cut and should remain as is. If you can't find any reference you have not tried. All of the editions of the Chevrolet Power books give detailed drawings of it. If you plan on running your engine consistently between 6-9k rpm then blend it(radius) from the bottom of the seat into the bowl.

Not all things are found on a flow bench. You might benefit from the fuel shear caused by the lip you want to remove. Honda uses this technique in their performance heads to advantage as due many head porters. You should try the head as is to establish a baseline performance. Then cut the ridge to see if the stopwatch shows an improvement or if drivability is better. Use a MLS head gasket and you can reuse it. If the performance goes down you can always do another head.

[baltobernie]

Not saying he's right ... not saying he's wrong, but a number of engine "modifications" are counter-intuitive. For example, you should "never" port-match the exhaust manifold. The manifold is intentionally larger than the exhaust port. That lip on the head, similar to what you've photographed, helps keep exhaust gas from being sucked back in to the combustion chamber, diluting the charge. The explosion shock wave is more powerful than intake vacuum. "Never" in quotation marks, because, well