I must say that I know a little about "head flowing," or at least I know how useless MASS FLOW is...

You are on the right track but you can only take port velocity so far. Its very true that for part-throttle conditions port velocity just isn't sufficient in the engines we are speaking of because the piston speed is slow and the signal is weak. But when we're talking about power we are talking about wide-open throttle. If you care nothing about performance you go ahead and put 1.0" valves in your 350 with 100cc intake ports, the velocity will be great at part-throttle and the engine will probably accelerate hard from a stop sign with little throttle opening but the engine will be dead at 3000 rpm. You have to realize how destructive choke velocity is to power production, especially when its a few inches away from the valve. If you're trying to rev an engine 1000-1500 rpm past what the mean induction velocity will allow you could be losing 50, 60, maybe even 75 horsepower on high-revving 350. If your velocity is a little slow around your peak power you're giving up potential volumetric efficiency but the lose isn't nearly as detrimental as a velocity "choke" condition...

But the moral is that I am never going to go put 305 heads on a hot street 350 because the peak torque and power will be down from a port that can handle a higher mean velocity or in general have a velocity profile that's better suited to the engine in question. Sure the 305 heads will make better torque at 2000 rpm on a dyno, and at part-throttle but we don't care about these things for a performance ride. That's what gear multiplication is for, getting the vehicles out of these slow spots and into the areas where they are efficient and meant to work.

So just because 400 horsepower is overdone you'd rather have 300? You don't sound like much of a Car Crafter    

GibTg your a legend man!  

Can I understand that? Why do you even need to ask that question, do you really need to feel like you're better than me? By the way, I don't think anyone could understand that because you just got done saying that the intake port is 228 feet. That's some nasty low rpm shockwave tuning!

Everyone knows that the cross-sectional area at the inlet of a C4V head is larger than a C2V head, so the velocity goes down, that's a difficult concept to understand? Could you please tell me the cross-sectional areas of the two heads you're talking about? I'm not a Ford guy, and don't have the numbers measured myself. Do you have any other local velocities from these heads measured? I would believe that the port velocity in an area where the cross-sectional size is more limited (compared to the inlet) would be more relevant to why your engine makes such wonderful power at 7000 rpm.

And just to bring back the other topic. How ironic, but you're speaking to an atheist so I don't want to hear your religious rants. It's tough enough to listen to you trying to show your superiority to me as is...

I personally apologize to "HunterT" if he is still listening. I instigated this argument between "powerdreams" and I, and the subsequent trolling by him stemmed from it.  

Its no surprise that your an atheist! I NEVER SAID I WAS BETTER OR KNOW MORE THAN ANYONE. Gib stop assuming and writing as if you dictate what I can or can't write.I know who backs me up in life. I make no apologies for my beliefs. I am not perfect and your attitude demonstrates that you have a control insecurity issue/complex. The 2v will make more power at 7000rpm than the 4v will AT THAT RPM ,is that so difficult to comprehend? It means on the street ,it will be more dynamic in rpm range. I can see your all about torque.Which is great if all hooks up.In most cases on the street,unless you have the expensive set up at the rear wheels,they are usually traction plagued.Apart from this ,to me anyway the xtra 1000 to 1500 rpm more through the gears,when changing,is a better performance scenario all round.The latest corvette with a 7200 redline is a good example.Anyway end this fued or I'll bless you each day, the "BEAST" wont like that.  

Why are we talking about high RPM's in a 305 thread???    

Actually Gib the valve combo for the 305 that I had in mind,would be a 1.88inlet and 1.6 exh. I probably would'nt go over 1.96inlet for a "hotter"application.I would cocentrate on getting exhaust flow up via porting on the exhaust.A 1" inlet?? It aint a lawn mower.I just would'nt go overboard with the intake port.As a side note ANY(even stock) engine, with a 3000 to 5000rpm stall convertor will rocket just about anything off the line!I know you already know this.Maybe theres someone out there that does'nt.I agree stay away from the tunnel rams.I'd go for a wieand dual plane stealth or performer rpm air gap,a cam with 110 degree LSA and under .500" lift, maybe around .470" at the intake valve,with a 220 deg.dur at.050",3.3 at the diff and a 2200 highstall.  

After reading this thread I have a question, is it more important to have a high intake flow velocity, or a high intake flow volume?  Wouldn't it be more beneficial to have a high intake flow volume so that you get more fuel and air into the combustion chamber.  Isn't that the point of turbo and super chargers, to get more fuel/air into the cylinder?  

Also to the original question I would agree that spending money on the 305 is a waste.  I had a 305 out of a 79 malibu in my monte and even with a good cam, headers, four barrel and good ignition componients, it was a turd.  All it did was use more gas without any results.  I would suggest building another small block, a 327, 350, 383, or 400 because then you can reuse alot of stuff off of the 305 and the swap would be easy.  

CSIROC:
Why are we talking about high RPM's in a 305 thread???  

Because we're arguing!  


Back to my opponent. I sure hope you take more into consideration than inlet velocity when choosing a cylinder head. If indeed a C2V makes more power at 7000 rpm than a C4V (which I find hard to believe) then it isn't ONLY because the inlet velocity is faster. By the way, I kind of wish you would answer my questions rather than just take shots at my beliefs...


To answer "Monte85"...

There is a big difference between having a lot of pressure behind a valve and having atmospheric pressure behind a valve. You have to realize that at high engine speeds there is very little time for the induction system to fill the cylinder and we're asking a lot of those ports! We need to fill the cylinders of our performance engines to more than 100% of their volume in any one cycle! If we increase port volume and the corresponding decrease in velocity is too slow for the engine's need then we lose cylinder filling because the air isn't moving fast enough to take full advantage of the short amount of time the valve is open. Sure, we can increase volume and give more available air to the engine but if the velocity isn't matched to the engine's need then this becomes wasteful (inefficient) and actually is detrimental to power production. Also take into consideration that if the induction area is larger than the engine requires it weakens the signal that the piston creates to the carburetor booster. So now not only is the engine not inducting all of the air that it could but the fuel is probably puddling and not vaporizing as quickly as it potentially could. This requires throwing more and more fuel into the engine. A dyno would show extremely poor brake-specific fuel consumption (BSFC) and volumetric efficiency (VE%). Both signifying that the engine is highly INefficient. The result is poor performance everywhere in the rpm range. The moral is that induction velocity is a highly complex creature. Proper velocity is a happy medium between what the engine's piston speed and induction system can handle.  

It depends on what your after. High intake vol will create more torque and hp given a perfectly matched cam,providing the heads have been flowed!the trade off bieng the engine speed in rpms at which pt the power begins to take effect.High intake flow velocity will commence earlier in the rev range,which is  more desirable on the street,requiring less cam, and a more dynamic rpm range.................... on the subject if your 305 drank more fuel and did nothing you had Way too much Camshaft! Thousands of novice engine builders make this error!The power trip,bigger is better!Not So!Believe it or not overcamming causes hp and torque loss.sure it sounds nasty at idle,but goes backwards performance wise...................... A reputable Chev engine builder advised a client on a specific cam package ,given the weight of that vehicle and all relevant information he provided.What did he do? Thought he knew better,purchased a cam two performance levels up and installed it and was bitterly dissapointed. He went on a public tirade against C.O.M.E(chev offroad and marine) they inturn contacted their affiliate who installed the cam.Everything was re-checked regarding installation.It was all correct.Except that the vehicle owner insisted on the bigger cam. ..............Anyway c.o.m.e ammended to fit the correct smaller cam at no extra charge,to stop his whining.He was gobsmacked at how well his engine went,with the smaller cam suited to his vehicle. He needed to know why. they informed him that the bigger cam was more suited for a lighter vehicle with the same motor..........and that was an off the shelf cam and chip package.I'll bet you had too much cam...............................Rules 1. Unless you know where and how to port a head,Never do it at home. 2.If you can't afford proper head porting and flowing leave them stock std. 3.the motor has to be in good cond.pistons ,rings, bores,bearings etc and valvetrian.If you want good performance gains of fitting a modified cam. Equalised (within reason)compression. and 4.choose a mild cam profile!!!! If your not sure of what that is ,call a reputable cam company. They will be glad to inform you of how much cam (your engine)can take. They should ask you ,what your trans is,what your compression is,what your final drive ratio is and the weight or type of vehicle................................If your 305 can only breathe say .480" for eg. you cant install a .500"lift cam it will be an anticlimax in terms of performance.  

Please keep in mind I really don't know alot about the science of heads and why some are better than others.    
Is the measure of the intake velocity a function of the volume of air that gets into the cylinder, or is it just the speed of the air, no matter how much, that gets into the cylinder?  If it is just the speed, then is the intake velocity more important than the volume of air that can get into the cylinder.  If you reduced the size of the valve, you would increase the velocity of the air going in, that is if you got the same volume of air in the cylinder.  I guess I am just trying to figure out how heads with smaller valves, that have a higher intake velocity, would create more power than heads with bigger valves, that have a lower intake velocity (according to powerdreams) considering all other things equal.  It seems to me that heads with bigger valves would be able to create more power because they can move more air into the cylinder, regardless of the intake velocity.  
I think in my last post I misused the term "intake volume", I meant the volume of air that gets into the cylinder, not the amount available according to the intake, carb, and air cleaner.  

Monte 85, re-read my last post.I answered your question in the first couple of sentences. When talking valve size especially inlets,you have to consider the bore and stroke of the engine.These variables have alot to do with the size of the valves. Also there is a ratio of inlet and exhaust flow that has to be met for a given displacement. Don't quote me here but I'm pretty sure that the exhaust flow has to be around 70% of the inlet flow.Many engine builders actually increase the exhaust valve size to achieve this goal.Especially when an engine has been modified to increase its power rating...........................On forced induction, alot of big hp and torque numbers are being made by leaving the inlet ports standard ,with a larger inlet valve size and the exhaust port and valve maxed out.Exhaust flow is more than just fitting extractors(headers) and is a critical factor when a given performance level is desired.  

monte85:
Please keep in mind I really don't know alot about the science of heads and why some are better than others.    
Is the measure of the intake velocity a function of the volume of air that gets into the cylinder, or is it just the speed of the air, no matter how much, that gets into the cylinder?  If it is just the speed, then is the intake velocity more important than the volume of air that can get into the cylinder.  If you reduced the size of the valve, you would increase the velocity of the air going in, that is if you got the same volume of air in the cylinder.  I guess I am just trying to figure out how heads with smaller valves, that have a higher intake velocity, would create more power than heads with bigger valves, that have a lower intake velocity (according to powerdreams) considering all other things equal.  It seems to me that heads with bigger valves would be able to create more power because they can move more air into the cylinder, regardless of the intake velocity.  
I think in my last post I misused the term "intake volume", I meant the volume of air that gets into the cylinder, not the amount available according to the intake, carb, and air cleaner.

Induction velocity and volume go hand-in-hand. Think about the depression that the piston creates, of course this is dependent on bore size, stroke, connecting rod length, and rpm's. Now keep the "draw" constant and change the intake area (volume). So when atmospheric pressure acts on the intake port the smaller the opening the higher the velocity. So if we increase volume anywhere in the induction system we decrease the average induction velocity.

The maximum velocity for any engine is .5 to .6 Mach or 560 to 670 feet per second. I have no scientific proof for this because it is so well documented. It's just a standard. Obviously many factors effect the exact velocity that an engine "wants." Inlet air temperature, other atmospheric conditions, port shape and efficiency, valve lift, valve size, etcetera, etcetera will have a effect on the exact velocity needed.

The valve is obviously part of the induction system. It is usually the smallest cross-sectional area there, it's the "bugaboo" in the system as OldBogie had stated it a few years ago...

The valve throat creates a cross-sectional area that needs to be sized also according to piston speed and the depression created by the piston. To realize this you need to think of the induction system's cross-sectional area(s) as a whole. In a absolutely perfect world the induction system would be perfectly straight and have the SAME cross-sectional area throughout. Changes in cross-sectional area (and subsequent velocity) in a port are detrimental to power production because accelerating air requires energy (as accelerating anything with mass does). So when atmospheric pressure must speed up or slow down the air around the port's bends, turns, and corners this loses energy that could be used to fill the cylinder. So, ideally the valve throat area should be equal to the area of the the rest of the induction system. Unfortnuately this is completely impractical for many reasons but I will at least give one example...

Valve size also needs to be sized according to the bore size it will be within and the angle it opens into the cylinder. So basically valve size needs to be trimmed by the amount of shrouding present, but that's a whole different story...

Another quirk about the valve throat area is that it can be comparatively smaller than other areas in the induction system because it is basically the end of the system's length. If an area in the intake manifold has a very small cross-sectional area and this area is too fast for the engine design it "messes" with the shockwave tuning within the induction system that is so beneficial to power production. The waves and pulses that traverse the intake tract, that help us go beyond 100% volumetric efficiency are lost and power (and the corresponding volumetric efficiency) drop like a stone...

I hope I'm making this easy enough to understand as it's a very complex subject that almost needs several books to explain. I also do truly hope that someone is learning something from this as well, since I do enjoy writing about it. So maybe "powerdreams," our argument did turn into something useful...  

monte85:
Please keep in mind I really don't know alot about the science of heads and why some are better than others.    
Is the measure of the intake velocity a function of the volume of air that gets into the cylinder, or is it just the speed of the air, no matter how much, that gets into the cylinder?  If it is just the speed, then is the intake velocity more important than the volume of air that can get into the cylinder.  If you reduced the size of the valve, you would increase the velocity of the air going in, that is if you got the same volume of air in the cylinder.  I guess I am just trying to figure out how heads with smaller valves, that have a higher intake velocity, would create more power than heads with bigger valves, that have a lower intake velocity (according to powerdreams) considering all other things equal.  It seems to me that heads with bigger valves would be able to create more power because they can move more air into the cylinder, regardless of the intake velocity.  
I think in my last post I misused the term "intake volume", I meant the volume of air that gets into the cylinder, not the amount available according to the intake, carb, and air cleaner.

Generally, a bigger valve will make more power.  There is a limit to how high you can go...just like there is a limit to how low you can go.

While velocity is important...its not the end all.  The point is not to maximize velocity...if it was...you'd see incredibly small valves with ports shaped like nozzles.  Velocity must remain high enough to fill cylinders at low RPM...but while a 1.78" intake valve (random number) will give you incredibly high velocities...it will choke flow at high RPM (which in this case would probably be around 4000 RPM).  

Its the exact same principle as putting your thumb over the end of a garden hose.  Block most of the outlet and you'll see very high velocities...but you certainly don't want to try and fill a 10 gallon bucket like that...it'll take forever.  On the other hand...you don't want a 4" diameter hose either...the pressure behind it isn't enough to achieve a decent velocity.

Its the same thing with any part of the engine.  Heads will rob power if too big or too small.  Camshafts will rob power if too big or too small.  Intakes will rob power if too big or too small.  Carburetors will rob power if too big or too small.  Headers will rob power if too big or too small.  Its all a balance...one that depends on other variables that must be balanced as well.  

I could ramble on all day about fluid mechanics...velocities vs areas, mass flow, volume flow, friction factors, reynolds numbers, turbulent or laminar flow...heck I could model you up a complete CFD (computational fluid dynamics) analysis on Fluent of an intake port with a valve...but who cares?  Basic principles...don't put a 220 cc intake port on a 302 unless you plan to rev the heck out of the thing and never plan to drive it around town.  Don't install a 150 cc intake port on a 427 unless you plan to always stay below 3500 RPM and never plan to race it.  

Departing from the 305.I misquoted the 351 figures Apologies.The numbers for the 2v are actually 291 feet per second @7000rpm and 216ft/sec for the 4v.To put this in perspective 2V's have more airspeed at 5500rpm(228ft/sec) than 4V's @7000rpm.In real terms this equates to 20ft lbs of torque 1500rpm earlier in the rev range...........................For STREET use the 2V's work better in every way.The valve sizes for this particular engine are 2.05"inlet and 1.715" with 11:1 comp it produced 485hp@ 7000rpm.76cc chambers flat tappet cam 1.73rockers,int- 256deg@.050" .341 lobe lift and exh.266deg@.050" .356"lobe lift(to get valve figures multiply by 1.73}.106 LSA 4.56gears,10.9Et's in a 3308lb car...................................Just in case anyone wanted to know, the heads have a 73% intake to exhaust ratio.Carb is a modified 720cfm DP Holley on a street dominator intake............................ And only 46 cubes bigger than a 305 small block . Back to a more sane power level, Most drivers on the road would have thier hands full with 300hp.............I agree CSIROC it is fun raving on about these subjects! I hope there is enough data here to chew on,oh B4 I forget exhaust flow is 110cfm @.500" and inlet 520hp@.500" lift.  

I absorb more and more of this information everytime it's gone over and 300Hp is only good enough for gas mileage.