small valve vs. large valve after porting

is there much flow difference in big vs. small valve heads if both have had the basic porting?

 

How big is the bore size?



Wild guess: the porting makes more difference than the OEM valve sizes.

 

Probably not much driving on the street, actually the small valve head would probably have a little better velocity at lower rpms than the bigger valves giving a little better throttle response. The BBC guys learned a long time ago that the heads with the smaller round ports were a better street build than the big square port heads.


Bob H.

 

Interesting.

I was specifically thinking of the stock 2.000" intake and 1.625" exhaust valves vs. the stage 1 2.125" intake and 1.75" exhaust.

Standard Bore size of 4.3125" up to 0.030 over.

 

I was told going from the standard to Stage 1 valve is ~5 cfm.

 

doesn't seam like a huge advantage for the street vehicle. But maybe if you are looking for every little advantage you can get, all the little stuff adds up.

 

You would never spot the difference on the street.

 

The key word here is porting!
If previous porting work ( even just bowl porting work) was done in error and low lift flow was lost, then stepping up to bigger valves provides the chance by means of the needed valve job to get that lost flow back .
Over the years I have flow test some 12 to 15 sets of heads Of all brands that where home ported and the work done to the short turn shaved 15 cfm off of what the head flowed stock in the low lift range with only a meger gain at higher lifts.

 

This isn't a BBC with small vs. huge ports in relation to the cylinder it's feeding.
The two sizes you are asking about would both be very effective with proper or equivalent porting.
For your goals consider the valve sizes for seat wear and tip height purposes, not flow or in terms of a few hp.
**Comparing CFM between the two and predicting throttle response is an absolutely irrelevant thought...velocity and understanding how engines operate from Delta P and said velocities is more relevant ( a futile discussion usually inappropriate for a forum like this).
You would see increased response with either valve size due to increased velocity.
The port isn't radically being changed as a whole, therefore the average cross sectional areas would not be dramatically increased.
The process of porting speeds up velocities near the valve job and subsequently the rest of the port works better.
If you have issues with wear or inconsistent heights then use the bigger valves.
If you really want to regrind and use your existing valves then do so.
I strongly recommend new valves after more than a few months of hard use in their lifetime.
The difference between the valve sizes would be less than the difference in overlap and other cam specs, installed position, minor timing curve changes, muffler flow differences etc...

 

I recall Denny Manner, the Buick factory engineer for the 455, once told me that, if all things otherwise were equal, a non stage 1 455 powered car would out run a stage 1 powered car stoplight to stoplight.

 

Was Mr. Manner referring to two factory stock engines as equal as practical for a comparison or are you implying that it would still be true given the camshaft and other modifications the OP is suggesting?
Are you also suggesting that if the only thing the OP had changed on his engine was the heads to stage 1 specs would that still translate to the rest of his engine's specs and regardless of porting done to his heads?

 

When dealing with iron heads,I like to start with a small-valve core.You can remove material where needed,for valve job,bowl work,porting,etc.

 

I am simply stating my recollection of a conversation we had 10-15 years ago. At the time I think we were also discussing the obsession of some with installing too big of a cam in a street car such that it sounds bad at idle but the car actually loses performance stoplight to stoplight.

 

Doh! That is exactly how I read it.
Forgive me for the way I stated that.
The reason I illustrated it that way was to caution the OP to consider the changes when applying the anecdote to how his project will end up...not stock.
As cherished and respectable as the conversation should be, I'd hate for the OP to be misdirected by an anecdotal name drop being the high card in the conversation without more technical aspects to consider.
I'd hoped that my post using BBC as an example would help him understand his question better.

 

I understand what you both are getting it. Thanks for clarifying. Posting/texting isnt always the easiest communication whereas a lot of assumptions can be cleared up In a 5 minute phone call. Nevertheless, I have access to a lot of Buick experts now.

 

I am starting a rebuild on a 455 that I want to appear stock, so stock intake and exhaust. Stepping up to stage 1 valves and a heftier cam. other than port matching, is it worth the expense to do much porting. I have access to hogged out exhaust but I don't know how much he wants yet. Worth it?

 

From what I have been reading, and I'm no expert, but creating stage 1 heads isn't the most bang for your buck . Mild porting is supposed to be a better value at around $800 range. As for the exhaust, I've seen charts where the gain is around 8 hp for exhaust porting on a 450ish hp motor. Wish I would have forgotten a zero after the 8......

 

With the 455 and lifts of under .480"it does not pay to do much more then a bowl port job since the size and shape of the bowl totally controls the flow up about .300" lift, which is 62% of that total .480" lift.

A correctly professionaly done bowl porting job will net you 65% of the power gain that a full porting job would get you, this assumes that Heads fully ported had a better then stock flowing Intake Manifold and headers.

Bowl porting Heads while running the stock Intake and Exh Manifolds will get you more power , but with the peak Torque and HP of the motor now taking place at a lower rpm since you are moving more air in and out of the motor sooner, yet have not changed the TOTAL amount of air the motor can pass due to the cork being the stock parts on the Intake and or Exh side!

In terms of Cams the very old rule of Thumb still holds true , that is that added valve lift makes Torque through out the power band , and added duration makes more HP, but at a higher rpm.

Also do not forget that HP numbers are mathematical derived by Torque numbers X rpm!

 

Also, won't a longer duration cam on the exhaust side help with the stock manifolds?

 

It depends where the added Exh duration is.
If it's in the closing side of the lobe where the Exh port / system is already flow saturated, then little gain will be seen.