Intake vs. exhaust valve diameter

[TFB1]

Originally Posted By: Robenstein

Well too big of ports and intake valves can actually kill power if poorly matched to the cam and intended driving. In the old days for instance, the street Ford guys with a 351 Cleveland preferred the 2v heads to the 4v. Same went for the low and medium rise heads being preferred over the high rise and tunnel port FE heads. At some point big valves and ports really only matter for high RPM power. But broadly speaking, you are correct as long as the rest of the design and components mesh well as a combo.

Yeah and the '70 Boss 302 had smaller intake valves than the '69, nothing more gear won't fix... Myself, far prefered the 4v Cleveland heads...

 

[splinter]

Originally Posted By: Phishin

...Any and all ambient atmospheric pressure is irrelevant...

Really? Even Betty Crocker has modified her instructions for "irrelevant" ambient atmospheric pressure. Like my airplane.

 

[Shannow]

Originally Posted By: splinter

Originally Posted By: Phishin

...Any and all ambient atmospheric pressure is irrelevant...

Really?

A pearler, isn't it. It's only the ambient that drives the air in, air flowing from higher pressure to lower pressure, through the filter, intake tract, valve etc.

 

[userfriendly]

One would imagine then, better cylinder filling would be achieved with the largest port and valve combination that will fit the cylinder diameter. Splayed and canted valves can be larger because they angle away from the cylinder wall. Why then would large valves and ports make an engine a dog as mentioned?

 

[Shannow]

LOL, yes, I get you. There's inertial and pressure wave stuff that happens, and does that last little bit of jamming the air/fuel in at exactly the right time depending on runner length, RPM, and cam timing. But I'll still suggest that unless ambient was higher than cylinder pressure, there'd be no flow to get the momentum up for the inertia affect to kick in. (An aside, but the cleveland was pretty widely used in Oz, 2 V and 4V...the knowing people put a gasket with a "tongue" to remove the excess exhaust port volume (and the dogleg in the port), with a smaller header pipe to keep velocity and impulse up. BTW, what do you reckon on the dynamics of this one ? Allegedly a 10,000RPM 144 Ford (knowing the builder was John Bennet, could well be true)...

Look at all the fuel running OUT of the carbs...

 

[meep]

inertia plays an outside role in this. at 3000 rpm, assuming the intake valve is open for 100% of the intake stroke, it has 0.01 seconds to gulp in air motivated by now more than 14.8psi, at sea level, unboosted. conversely, for the exhaust stroke, approximately 100psi is present at the bottom of the stroke when the exh valve opens. Pressure drops, but the capability is there to maintain or even increase it should there be restrictions. http://performancetrends.com/Definitions/Images/Cylinder-Pressure-Lrg.gif Crude Example. find a coke bottle. glass one. seal your lips to it. Try to force air into it, to build pressure. pretty hard, right? Air is being pretty rigid. imagine that's exhaust. the piston has a positive force to get it out. Now, equalize the sorry little bit of pressure you could get into that bottle. Use your tongue to pull a vacuum. seems like you can get several cycles, and none are as hard as a single puff to build pressure. Air is much less motivated under a vacuum, it fights back less. The piston has less influence on its behavior, it gives. m

 

[meep]

double post

 

[ironman_gq]

Originally Posted By: userfriendly

One would imagine then, better cylinder filling would be achieved with the largest port and valve combination that will fit the cylinder diameter. Splayed and canted valves can be larger because they angle away from the cylinder wall. Why then would large valves and ports make an engine a dog as mentioned?

Biggest issue was port velocity and fuel drop out. Valve and port size is a balancing act between flow and velocity, a big port will flow a lot of air at a low velocity which makes the right conditions for atomized fuel to drop out of the air stream or the droplets to combine and grow. It also makes for poor cylinder evacuation and filling when the intake air is lazy. At higher RPM's big ports and valves can provide enough velocity to work properly. Direct injection might be a bit of a game changer in some ways along with variable valve timing but on a basic level big valves make big power at high rpm and are lazy at low rpm.

 

[userfriendly]

You busted me ironman with the direct injection game changer. Suddenly valve overlap becomes less of an issue with DI, as the fuel can be injected during the compression stroke and will not flow across the valves and into the exhaust. Port velocity should only be necessary to achieve VE above 100%. How can we increase intake port velocity without making the port smaller? Back in the day.... Oh here we go (Grumpy), wave induced fuel standoff, wet flow and air going two ways through the intake ports, carburetors or injection stacks. The trouble with sonic tuning, is you have to take the good with the bad. Well... at least back in the day you did. I need a little help here, can anyone explain wet flow? Was it caused by supersonic velocities on the intake side?

 

[Shannow]

I think you are getting that this bloke needs more work on his headers. CD carbs and all.

 

[userfriendly]

Well, now that we know about gravity waves and black holes, he should wrap those headers to keep the heat in and exhaust velocity up. Exhaust scavenging is necessary on top fuel engines to prevent hydraulic lock-up. Both blown and injected fuel engines typically have less camshaft exhaust duration than intake (true with some turbo cam designs too) and far less exhaust duration than their naturally aspirated cousins. Some builders of very large cid BBC engines are sinking their 1.94" exhaust valves "to keep the port from taking off too quickly". Is camshaft exhaust duration tied more to rpm than any other single factor? If we look at a typical V8 engine with a 1.6" exhaust valve, the seat contact is much smaller, and the smallest od of the seat may be 1.3-1.4". That is a very small hole for a 4" cylinder to exhaust and scavenge through.

 

[DoubleWasp]

Major issue on marine engines too. They'll happily drink the water being ejected through their exhaust if cam has enough overlap.

 

[userfriendly]

First we have to agree on exhaust velocity before making an attempt to understanding the scavenging event during valve overlap. We have sonic waves, heat waves, light waves and now gravity waves all travelling at different speeds in the exhaust pipe. In addition, good old fashioned inertia of the expanding gasses. If we have a 45 degree window of opportunity for scavenging, at 6,000 rpm, that is only .0025 of one second, doable at 3,000 fps. Now double the rpm to 12,000 or triple it to 18,000 rpm. If the exhaust cycle is time dependent, then an engine operating at 4,000 rpm @ 100% VE would require 1/3 the valve open duration of the same engine operating at 12,000 rpm @ 100% VE.

 

[mjoekingz28]

Exhaust velocity. I forget the exact wording, but I think I read in a magazine that high speed air flow was for high horse high rpm. Exhaust pressure, was for high torque low-rpm situations. Maybe this is the idea behind vtec, vvt, exup valves and the like

 

[userfriendly]

If we use the example of the 302 Boss posted a few pages back, piston movement alone cannot generate the port velocity necessary for ram-tuning. Which is achieving VE over 100%. Try this experiment: Remove the sparkplug from your lawnmower engine at tdc of the compression stroke. Replace the sparkplug and pull the cord against the vacuum generated by the piston travelling down the cylinder on the power stroke. If you could crank the engine over until the piston was at bdc, the vacuum in the cylinder would be very high. (Or pressure very low, take your pick.) The difference in pressure of the intake manifold and cylinder results in port velocity as long as the valve is opened very quickly to a lift that is unrestrictive. Large intake valves lifted quickly and a little late is required to get high velocity from a large port.