Breazlan

iTrader: (1)

How does installing larger valves make more power? Wouldn't the throat area under the valve have to be opened up to take advantage of a larger valve?

sofakingdom

iTrader: (1)

Right.

On the other hand, you can only gain just so much, without enlarging the valves.

The 2 things go hand in hand. Just "hogging out" the heads doesn't do much good if more stuff cant squeeze through the valve itself, and increasing the valves does no good if they're not the limit to the flow.

It's usually "assumed", when installing larger valves, that the throats are opened up. However, it's certainly not necessary. Anybody can just jam a cutter in there and cut larger seats without doing the other. Happens all the time. Cheap eBay heads and the like are usually like that.

You get NO MORE THAN what you pay for. If someone is charging less, and that's their main selling point, then odds are, they're giving you less. However it doesn't always work the other way; just because someone is charging more, doesn't always mean they're giving you more.

Breazlan

iTrader: (1)

I have been trying to make a little sense of what you often hear said about heads and valve sizes. To me it would make the most sense to use the smallest valve possible to fill the opening to the port. I thought I was missing some physical theory involved in this matter.

leeperryracing

I kinda get your drift....slightly smaller valves, more air velocity so more of a useful flow, right?

AlkyIROC

Even with flow, larger valves can only offer so much. Inhale then blow through a straw. It takes a long time to completely exhale. Now inhale and blow through a cardboard tube. You'll quickly run out of air.

In a high rpm engine, you need to get the air into the cylinders quickly. Through a small opening, air flow velocity needs to increase to get the same amount of air into the cylinders. The piston on it's down stroke is pulling the air in. That's a constant. A bigger valve and opening will allow more air to be drawn into the cylinder in the same time frame.

Larger valves simply increase the area. It's like exhaust pipes. Having 2-1/4" dual exhaust can move more air than a single 3" exhaust just because of the area of the pipe.

Multi valve heads work the same way. Two smaller valves have a bigger opening than one large valve. Since multi valve heads are expensive, the next best choice is to just use larger valves.

More air into the engine means more fuel can be burned. More fuel burned means more power.

Sonix

iTrader: (1)

Yea, more valves (or more area for flow) gives more potential to have more high RPM power.

The only thing I'd like to add is, yes, smaller valves give better mixing at low RPM. Every DOHC motor i've ever driven has been SLOW AS SIN at low RPM. Like, pathetically so. And not just because they're usually smaller motors, they just flow too much to have good velocity.

AtomicTruck

One of the things a bigger valve does is allow more air flow at a lower lifts. One of the keys to low-end torque is high flow at low lifts (measure air flow at lower lifts). Air flow at max lift is just a small part of the whole situation.

Depending on the head... the valve seat is the bottle neck for most of the lift. Then near max lift the bowl becomes the bottle neck. The valve spends more time in the lower lift areas than at max lift. All this depends on the cam as well but you get the idea. The valve is going up and down for more time than it sits at the top. This is why roller cams are good. They allow a faster ramp and thus have more lift longer, even if it has the same max lift as a flat tappet (this depends on the mfg and cam profile)

Velocity is one of those tricky things. You have to remember that it is caused by a vacuum (not like a garden hose). You can't just put in a bottle neck that causes high velocity in one area... good atomization doesn't work that way... good velocity needs to be somewhat consistent throughout the port. Herein lies the the head selection trade off, big port for higher rpm torque or smaller port for lower rpm torque...

Some of the issues with many multivalve and smaller displacement engines is cam timing. They don't start making power till 3k rpm because the cams are retarded (no pun intended).

Vader

Excellent description. Since a valve spends most of it's "open" time in the mid-lift ranges, flow at peak lift is less meaningfull than most people understand. Most of the flow allowed by a valve is due to the amount of perimeter area, not necessarily due to lifting a small valve a longer distance. Increase the perimeter, and the potential for flow increases. As the Duke of the Davenport explained, having all that valve perimeter are is useless if the ports leading to it are restricted, or the installation is perfomed poorly, or if the engine is operated in a range which makes it unnecessary.

For example, a typical 1.84" intake valve has a perimeter of 5-3/4". Increase that valve diameter only 0.100" to 1.94" (a 5% increase in diameter) and the perimeter area increases to 6-1/16". Increase it 0.180" to a 2.02" diameter (a common size) and the perimeter area increases to nearly 6-3/8". That's quite a bit more flow area for a small diameter increase.

The velocity is more important in a street driven, torque engine, thus the reason you'll see smallish 1.84" valves even in 350s in some trucks, and 1.72" valves in some 305s. For higher RPM flow, you'll usually see a lot larger valves, even at restricted lifts. Many lift-rule racing classes will use stupid-big valves to make flow at RPM to support horsepower.

Breazlan

iTrader: (1)

My thinking was, lets say, the diameter near to the valve seat is 1.50". Do you want to plug that area with a 1.7" valve or a 2.1" valve?

Damon

The valves can't be tought of as separate from the port. They are part of the port, basically. The last thing the air has to flow past before entering the cylinder. A typical SBC head with, say a typical 1.94" intake valve already has about as much valve as that stock port can handle. Going larger rarely helps much unless the port (and valve seat and chamber- they're important, too) is worked to match.

Getting some basic pocket porting along with a performance 3-angle valve job will almost always pay much larger dividends on a stock head than installing larger valves will.

Vortec heads are a common comparison point vs. older heads. They use the same 1.94" intake valve size that many older heads use, yet they can outflow older heads by 40-50CFM at peak AND clobber them throughout the mid-lift range as well. They are a VERY good design for a production line head- and they did it without using valves that are any larger than older heads. Then look at later LS-series heads (yeah, it's apples and oranges, but worth pointing out anyway) that can put up absolutely STUNNING flow numbers, especially when ported, using not much larger 2.00" valves. The port, valve seat and chamber work is where the magic happens.

Fullsizewagon

Yes, you would gain from opening up the throat area, but not by just "boring" it up, getting sharp corners that creates turbulence. The 3 angle valve job is a step towards ensuring that there's no sharp edges near the seat that will create turbulence. Flow area is important but turbulence hurts flow "more than anything else". It's thus very important to not get the "short turn radius" too small when porting the runner just before the bowl, which is the sharpest corner the air encounters.

AtomicTruck

The part of the seat that is the smallest diameter is that size because that is what works with the valve (not really in a stock head but close). A lot of work can be done to this area to enhace flow as already described. The point I want to get at is that if you put a larger valve in, you have to hog-out this area to make the valve fit properly.

I port my own heads and have a builder flow test them... I am not an expert but I have done enough of them to make them better instead of worse (which is very easy to do!). Each head has different characteristics and needs carving in different places, but uniformly you can gain some flow by smoothing out casting lines, slimming down the valve guide in the bowl, and working on the short radius curve. You don't want a glass polished surface on the intake! And you don't want to spend too much time on the floor of the exhaust ports. You can spend over 100 hours reading info on porting if you are good with Google.

As previously mentioned, the valve is just one piece in a system of parts that must match to make the engine do what you want it to.

As for the newer heads, very small changes in the shapes and curves in the ports make a huge difference in flow characteristics. Some changes are so slight you have to look close and compare, side by side, with another head to find them. And the newest heads (cathedral ports) are pretty much untouchable unless you are expert. It is wierd what those engineers come up with to make things work.