Head technology

[chargd72]

Ok guys, I've already posted a couple of post about 302 cast heads and porting and such, but this pertains mostly to valve size.  I read the article below and it sounds as if it has some valid points.  I was originally going to go with 1.88 intake and 1.60 exhaust.  But this suggests to keep the 1.78 intake and only increase the exhuast size.  

There ahas been a lot of people saying "don't kill velocity by going bigger."  Then there's the crowd that says "velocity doesn't = HP."  So I leave it here, where I trust the people and the info they give.  So tell me what you think.  Stay with 1.78?  Or increase to 1.88?  I'm really trying to do my research to squeeze as much HP and TQ as possible out of these things (for street use).

Here is the article, highlighted key points:


I'll mainly be talking about the 318 as they are more common and more available. But the same will apply to both engines. For most people, they are looking for good pickup for the street and in this case you would need to stay with the 318 casting heads as the 340/360 heads have too much port volume in them for the bore size of the 318 - even at .030 or .060 overbore.

I know that some are going to say that they never had any problems with the 340/360 heads on their 318, and you more than likely didn't. But you left some HP and TQ on the table. The smaller ports in the 318 heads from 67-90 will have more port velocity than the larger heads and they will use less fuel, and nowadays who couldn't use more fuel mileage?

So, my suggestion would be to use the smaller intake valve that comes in the heads (1.78) and use the larger exhaust valve out of the 340/360 heads (1.60). What this will do is give you more HP from the high velocity intake, and more TQ from the larger exhaust size valve. Most 318 engines won't ever need more than 136 cc's of port volume. Most stock castings are 120/122 in port cc's and the 302's are 118/120 from the factory. Now by opening up the bowl and gasket matching the heads they will gain about 6-8 cc's, depending on how much work you are willing to do. Heads in this state should make about 210/215 cfm's with a good valve job. This is enough to get over 450HP out of the engine. With the right components cam, intake, and carb. and exhaust, of course..

Another thing that most people have done is to add the 1.88 intake valve to the 318 heads and this only hurts the flow at low lifts and port velocity. Basically the port becomes less efficient, and we all know that the more efficient a engine is the more HP and TQ it will produce. The reason that I don't suggest the larger intake valve is the shrouding effect of the head and cylinder wall. Let alone, the drop of flow and low speed operation of the engine. Basically the throttle response won't be there.

Anytime a port becomes larger, it will require more fuel to stay efficient. Another thing that has a direct effect on the port size is the rod length, what this does is determine the piston speed. The faster the piston speed, the larger the port will need to be, as there will be less suction time for filling the cylinder. This is the reason the big block Chevys need a port with 290 cc's as their piston speed is very fast due to the short length of the rod (6.135). And a big block Mopar needs a smaller port in the 175/180 cc range due to it's rod being 6.768.

With heads in the 125 cc range and flow in the 215 cfm range, this will satisfy most all 273/318 engines for street/strip use. With heads of this type they will be able to turn 6,000+ RPM's on the small engine with ease. The only reasons that they won't will be due to under carburetion, under intake'd, under cam'd or not enough exhaust flow. Obviously springs will have a direct effect also but I included this with the cam as most will change the springs when they change the cam. This pretty much gives a general dos and don'ts for the small engines.

[Challenger340]

Where did you find this article ?
Who wrote it ?

The writer may have been applying "generalizations", for easier explanation of concepts ? I dunno ?
however,
Maximum Piston Speed, is a product of "Stroke", NOT Rod Length.

Rod Length, determines "How Fast" the Piston "Acceleration" is, to that Maximum Speed at mid-cylinder, from a dead stop at TDC or BDC.

What the article generalizes, for whatever reason, "Explanatory" maybe ?, is that the Longer the Rod Length in relationship to the Stroke, the Slower the "acceleration" of the Piston away from TDC, TO Maximum Speed, which is reached at Mid-Stroke, hence a smaller Port can be used by the Mopar versus Cheb Comparison ?

Port Velocity is generally "Torque", as related to the better mixture of A/F maintained by the Higher Port Speed.

Port "Flow" is H.P., pure & simple.

While I agree with a good portion of the Context, in which the Article is written, in regards to the Importance of Velocity in the smaller Engines,
I would disagree with the "One Size Fits All" Generalizations ?
Too many variables that are present in many "Street/Strip" 318 engines, that the above, just would NOT apply as "Better" for all applications ?

IMO
I prefer the 1.88 in the 302 Head, with a Chamber Unshroud, and complete Bowl Port to "Bell Shaped".
Extends the Peak Torque another 300rpm, and Peak Power !

[chargd72]

The author was someone who owned a racing shop that I was linked to by A-bodies forum.  I can't find the link again, I was in the depths of searching and only saved the part I posted. 

It was hard for me to accept because most people, like yourself, recommend the larger (1.88) size.  But I read a lot of testimonials saying 'keep small ports on 318s.'  But screw it.  I'm going enlarge to feed that little SOB.  Thanks for your input.     

I just wish everyone could agree on one thing that worked best   

[elacruze]

There is no substitute for valve size, period. Bigger is better.

Now that I've made such a contentious statement, let's examine. Port cross-section relates directly to velocity. So, bigger ports get moving later and slower (Boss Fords anyone?). Higher velocity keeps the air column moving via inertia, and packs more into the cylinder before the intake valve closes the port. So why ever have big ports? If the port is too small for the displacement and speed of the cylinder, the air column can reach a supersonic speed limit. That's the end of it's capacity, so the port must be sized to keep the velocity as high as possible without approaching the speed limit. This is why you see modern engines making enormous power without having ports like a Boss 429 or Square Port BBC heads. It's also why when you ask your cylinder head guy for a "Race Port Job" He talks you out of it, because he knows that you don't need the same ports on your street-driven 440 as the guy with the 600" Dragster.

The important distinction here is to recognize that *Valves* are not *Ports*.

Larger valves open a bigger hole so more air can get through them, obviously.  That's good, as long as the cross-section of the port behind the valve doesn't become large enough to lose the velocity of the smaller port. Shaping the bowl under the valve head is critical to maintain the best velocity and still take advantage of the increased valve size.
Some argue that if the port only supports X flow at Y velocity, then a valve larger than Z diameter is wasted, or even flows less due to shrouding. Shrouding is usually worse with larger valves, however remember two things; First, for every 1mm you increase valve diameter you increase circumference by ~3mm. This additional 'curtain area' begins flowing at approximately .050" lift-note that this is where cam manufacturers measure the effective duration-so even though the valve size may not contribute to flow at .400"+ lift, it contributes mightily at low lifts, giving the same flow effect as having a smaller valve open higher sooner. The effect is the same as having a cam with higher ramp speed (e.g. roller vs flat tappet)

None of this addresses the actual aerodynamics of the port shape itself, only the effect of a larger valve. Also, every engine/head design is different so the relationship between the stock port, valve seat, cylinder wall, combustion chamber, and companion valve is different-this is why only the flow bench can know you're going in the right direction.

I don't own a flow bench but I've spent lots of time with a few friends who do, and we've built some very strange combinations which just murdered everybody's expectations. One common feature of every one of these (naturally aspirated, no power adders) was as large an intake valve as could be fit without constraining something else.
I'm going with the 2.02" intake in my 302 heads, fitting them myself and blending them in by feel; just because I want to build my 318 the 'dirty old way' and see how it turns out.

I'd be interested to hear from flow bench operators, about their experience with big valves in unported/lightly ported heads.

[chargd72]

Once again, great points.  I think most people associate getting larger valves with hogging out the ports (as you mentioned). 

Here was my original plan and I think you just confirmed it.  Get the larger valves and bell shape the bowls appropriately while leaving the intake runners untouched (minus any cleanup, of course).  However, I do have an air gap manifold which my heads are already port matched to.  So my air/fuel mixture will go from a relatively large plenum, down the larger manifold runners, into the smaller head runners.  My only question is would that increase velocity or decrease velocity?  Help or hurt perfomance?   

[elacruze]

Weeeellllllllll.....I dunno.

Port dynamics is another long subject, and I can only speculate.

On the surface it would seem that the velocity would speed up in the head, and you may lose a little overall because of the additional manifold volume. But it's a lot more complicated than that. Any time you have an abrupt change in cross-section, you cause reflected pressure waves. So you may get something really weird like a pulsing port at a certain RPM, giving a terrible stumble or lean/rich spot. You may get resonance in the manifold only between the plenum and head. Beyond that, the shape and length of the manifold runners can't be ignored. On my Sp2p 318 manifold the runners are small, long, and have relatively sharp turns. They have less volume, so the velocity is up higher through the entire port structure. If I was going to use the big port manifold on the small port heads, I'd have a look at matching the two with epoxy up the length of the manifold runner, tapering consistently up the runner as far as I could. That avoids sharp changes in cross section and actually improves velocity. Without a flow bench you can't know if any of this is helping, but I can't imagine how you could make the combination worse by tapering than by leaving that big cross section change, even if you smooth the entry.