I need some schooling, fellas, and I'd request that we keep the discussion general and not brand specific. Need guidance on cylinder head flow vs. lift theory. And please understand that I drive my cars, don't race them, they need reasonable performance, but balanced with economy. They need to be well-behaved in the city and out on the road. So that said... What is the impact of bumping up the intake valve size vs. increasing the valve lift via higher ratio rocker arms? And... What is the difference on performance/economy in increasing cam duration vs. lift? Finally, how do cylinder head modifications work in conjunction with camshaft theory? How do the two work together? I realize these two areas need to work together in balance, but where is there leeway and where is there not? Where do I get the biggest bang for my buck? Can anybody point me in the right direction here? Before making a bunch of changes to my motor just for the purpose of making changes, I'd like to know that I'm making modifications for the right reason. Thanks in advance for the education. My pencil and notebooks are ready. I'm in my chair. Let the schoolin' begin, oh wise teachers of the HAMB.
Hey Phartman, i guess all the wise teachers of the hamb are at the cuise-in gettin a burger. But when they come back they'll ask for more info - a few might want a pic - but the kind of info would be car weght; clutch/auto; cubic inch of the motor; driving style; mfgr of the motor and realistic expectation as far as hp. The reason these factors are important is a motor around 350" could be anything from a 331" bored out hemi. to either a small block ford 351 to a big block ford 351 to a worn out small block chevy and in each of these cases the airflow requirements are different, now factor an automatic or a clutch and the variables just doubled. In a sentence, a street car wants velocity (a race car wants volume) and how the velocity is created would be different in all the engines.
Well...no wise man or guru here, only a Holiday Inn Express occasional guest... These are some difficult questions to answer, in a way, because different engine designs (primarily differences in cylinder head design) react differently. For example, the typical open-chamber BBC head likes a lot of lift vs. duration. For another example, many early SBC cylinder heads have fairly lousy intake port flow, with decent exhaust ports. You adopt specific methods to deal with each situation and budget. However: A) Increasing intake valve size, in most cases, is not the vast improvement that people believe it is....unless the time is taken to do things like open up the port throat correspondingly; "sweep" the chamber edge/wall if needed; adjust the seat & valve angles as necessary, and/or backcut the valve. In some cases, with severely "sunk" valves, a larger valve will "show" improvement simply because the new seat is much higher in the chamber. Higher ratio rocker arms may show more of a gain than just simply replacing the valves with larger ones. You are gaining more of a flow "curtain" or window, more quickly. In the case of the SBC with factory heads, sometimes the best gain is found by using 1.6:1 rockers on the intake side only, and retaining 1.5s on the exhaust. One thing to consider is that by using higher ratio rocker arms, you are also increasing the effective cam duration, which is a hard concept to understand. B) Before I take a stab at addressing this, I have a question: are you familiar with the terms of "lobe separation angle" (LSA), and overlap? C) This is going to be very difficult to answer in general terms. Veerrrry generally: better cylinder heads allow you to use less cam to achieve the same power; or, the same power at a lower rpm. As oj said, at this point, without knowing more about what you expect and what the engine wants in order to achieve it, it's really tough to say too much. A mild street 327 has little in common with a Pro Mod engine, other than that they are both internal combustion and have eight cylinders.
Arrrrggghhh, welp, I guess you're going to drag me down the manufacturer's discussion, which I had hoped to avoid... So before we go there (and I promise I'll give details next) , but generally on flow, volume, etc. what are the merits of making the valve bigger; vs. making it lift up higher; vs. making it stay opened longer? Let me get those concepts in my head, then I promise, I promise I'll share what I'm getting at. And guys, I'm looking for generalizations here. Nobody's under oath or anything like that... And since everybody else has gone out for a burger, it's just a few of us around anyway. And Homespun, yes, I do understand LSA and overlap...as long as you don't grill me too much.
General rule for stock engines: - Increased lift, faster valve opening(higher ratio rockers and/or steeper cam profile, and bigger valves, will(with appropriate associated mods) increase the amount of torque the engine produces. - Longer duration and cam timing changes alter the range at which the engine's torque will be produced(power band), and adjust trade-offs between peak power produced and the width of power band. You can raise the powerband RPM and get more horsepower without increasing torque. You can increase torque and get more horsepower without raising the powerband RPM. Most hot rod applications will benefit from some of both, but the the right blend of the two varies with each situation. Because they want an excessively ragged idle, people often opt for a more radical cam than the engine or application would work best with. On a street engine you run into a trade-off between how radical the cam can be vs excessive valvetrain wear and short spring life.
Al, that's exactly what I was looking for. Now here's what's going on.... I was messing in the shop today, alternating chores between a 223 Ford inline 6 and a 272 Y-block. The 223 is in my '61, but I anticipate an engine swap somewhere down the road. Imagine my surprise when I read in the tech manual that the valve trains use interchangable components between the two motors, specifically the intake valves and the rocker arms. The 223 inliner has a 1.78" intake. The Y-blocks offered either a 1.64, 1.78 or 1.93" intake, depending on the year. The stock rocker for both motors was a 1.43:1, but the high performance rocker for the Y-block was a 1:54:1. Hmmmmm. Got looking at the head on the 223 inliner and determined that there indeed is room to fit the oversized 1.93 intake in the head. Better to do that; or use the high ratio rocker; or both? I need to swap the head out anyway, and while I'm at it, thought about changing the intake or rocker (or both). To make matters more interesting, I went and checked the cam specs on what I am running. The little 223 has a Clay Smith 265 cam. Here are the specs: In. Opens @ 22.5 In. Closes 62.5 Duration at .050 224 degrees In. Valve Life .460 But then I read Based on 1.6 Rocker Ratio. Hmmm. But my rocker ratio is the stock 1.43. So I'm thinking that to maximize the effectiveness of the cam, it is better to get a rocker ratio closer to the 1.6. The high performance 1.54 out of the Y-block will do that. Am I on the right track? Use the Y-block higher lifters and leave the intake valve size alone? Or do both? I'd like to leave the cam alone here. Very happy with how it woke up the motor. Currently with 1.78" valves and the 1:43 ratio rockers the motor is very torquey and a mileage maker, but a little more performance wouldn't hurt my feelings. Top rpm on the motor is 4,000 and the vast majority of my driving is around town. Opinions? Again, thanks guys. Lots of moving parts here, and I've been known to get confused a time or two and make some ugly mistakes.
OK. Burgers...grill..mmm...I'm hungry now. What you need to remember is that virtually EVERY American V-8 with two-valve heads (at least the ones we consider here on the H.A.M.B.) is severely lacking in valve area. This is one reason why you see some older Ferraris with comparatively small displacement engines that make a lot of power (albeit at a lot of rpm)...most of them have four valve heads with overhead cams (which lightens up the valvetrain, a nice benefit). This is also why the Ford Modular 4.6 four-valve engine can be made to make in excess of 2000 hp (yes, really); it has much more effective valve area than any 302 ever can. Sooo, adding larger valves is a great idea, right? Well, assuming you do all the things I listed above, you are still limited in how big a valve you can get into the chamber. In some cases the cylinder head guy will actually move the valves farther apart, but still, you can't cram ten pounds of valve into a five pound head...so to speak. You are limited also by the cylinder wall, which is one reason why bigger bore engines tend to work better for racing. Lifting the valve higher- works well, up to the limits of the port or the other parts of the system (carb, manifold, etc.) Again you are limited by some things, the piston being one, the guide being another. The max lift thus depends on both physical limits and flow limits; as you can guess, if the port stops flowing past .600 lift, there is no point in tossing a .800 lift cam in there. As a vague general rule, as you increase lift, you have to increase duration (to an extent), because of lifter angles and the relationship of the lifter base to the cam lobe. This is one reason why roller lifters shine; you can get the valve open much faster, keep it open longer, and slam it shut, instead of babying the flat tappet up and down the lobe ramps. This is where, also, increased rocker ratio can help; you are both raising the valve higher and effectively keeping it open longer, but without increasing overlap (much). One thing to remember about lift is that the "mid-range" of the lift is seen twice by the cylinder: once opening, once closing, so a wise man will consider this when he chooses a cam profile. Duration is something of a different animal. Since the fuel/air mixture doesn't move instantly from point A to point B, you have to hold the valve open before you need it, and keep it open longer than you would like. The same holds true for the exhaust. Therefore you get overlap, which in one sense is a bad thing, and in another sense, a good thing, since you can use the exhaust pulse to help draw in the intake charge; actually, the exhaust pulse draws much harder on the charge than the piston ever does. You understand the negative point of overlap; another thing to consider is that to effectively use a lot of overlap, you need to run a higher static compression, which may not work well for the application of the engine, available fuel, etc. For the average street engine, increasing the lift with a better cam design is probably the most "bang for the buck", if chosen properly. Of course you get more duration as a rule. Higher ratio rockers may or may not be feasible; besides the expense, they are also harder on valvesprings, and you have to remember things like piston-to-valve clearance, which just got even smaller. Increasing valve size may or may not be a good choice; it depends on so many things. Dunno if any of this helps. If you want to spend some money, I can give you the names of a couple of books to read that explain all this much better than I ever could.
You won't believe this one.... Ford changed the valve cover design mid-way through the run from '54-64. The early series attached the valve cover with two studs that run up through the cover. On the top of the head, there is a convex crown, the valve cover has a concave rim (on the underside) and easy-peasy, no drips no spills no oil leaks. The later model valve covers had perimeter bolts. I happen to own (what I am told) is the very last finned aluminum valve cover that Offy will ever cast for this motor. It is the early head style. But I have a late model motor. I tried to make the handsome early cover work on the late head, and just got too frustrated dealing with constant oil leaks. I bought a junkyard early motor specifically to pull the early head and use on my late motor so that I could also show off the early Offy valve cover. And while I was at it- never one to leave well enough alone- decided to look at a few mods to the head as long as it needs machining. On the last one I did, the head got shaved .030" and everything else stayed stock. Motor has been terrific. And by-the-way, I'm running all the Clifford goodies: their intake, splitter headers and a Holley 390 carb and early Mallory flattop distributor. It's a nice little setup. Prior to the advent of the 300 inliner, these motors were extensively raced in my part of the world in NASCAR "hobby" classes, so I know that the motor is capable of more. There just aren't many people messing with them. John Mummert (Mr. Y-Block) was the one that got me hooked. I'd like to do a little more with the motor. But sometimes with motors- as with life- ya gotta know when to say "Enough" and quit. Just didn't know if that was the case here, or whether I could push on a little further. I suspect what I stumbled onto in my shop today was what plenty of NASCAR shadetree guys figured out in the late '50s.
It's interesting that the cam was ground with a 1.6 ratio in mind....many profiles are ground with 1.5 ratios because of the SBC. I think you are on the right track by switching to the 1.54s. If you can fit the larger valve without having to notch the cylinder wall for clearance, and your machinist can open the throat with a cutter, I'd switch to the larger valve. You might even look at fitting a larger exhaust valve. It seems to me that I heard somewhere that an early Pontiac intake can be cut down to 1.65 to fit, but I may be off base.
Even if the max port flow is poor, either or both the higher rocker ratio and bigger valve would improve the time vs area of the engine's breathing. Depending on the particulars of your engine and what you do to it, it's possible the bigger valve and higher rocker ratio could increase the low/mid range power of the engine without increasing peak power very much. Still a performance increase, and very street-friendly. Make sure the rocker geometry and fit on the tip of the valve stem are ok with the alternate rocker arm. Make sure you don't get coil bind of the valve springs with the higher ratio rocker. There should be a minimum of about .060" between each spring coil when the valve is fully open. Make sure your casting has enough material to accommodate installation of the bigger valve and to allow the associated port bowl mods. There is a point in most heads where a bigger valve causes so much additional valve shrouding that the increase in performance is minimal. To the extent possible/practical you want to unshroud the valve.
I believe the 1.6 ratio rockers are the Barker units. You can find them on the ever popular auction site from time to time. I used them on both intake and exhaust on an engine i built in 1965. I also used 317 Lincoln valves in it. I really do not remember what the valve size was (it was 45 years ago) I was using a 215 head so the valves as stock may have been smaller than your 223. But I seem to think the 317 valves were .125 bigger than what i had. I would go with both, but with your cam I would clay the pistons to insure sufficient clearance to be safe. Mike.
Is there any benefit to be gained by mismatching the rocker arm on the exhaust side vs. the intake? Use the higher ratio on the intake and the stock rocker on the exhaust? And I notice that there is much more room to bump up the size of the exhaust valve than the intake. Any benefit in bumping up the exhaust valve, but leaving the intake the 1.78 size? I'm just throwing out ideas here, thinking that a gasoline engine is nothing more than a big air pump, and the better the flow the better it ought to run in theory....
Mike, I've seen these units. Thought about buying a set from time to time when they come up on Ebay, but never pulled the trigger. Might have to do that next time they come up. Pete
phartman, If you have a few spare dollars, or a book store handy, please find a copy of Dad's book: "How to Build Max Performance Pontiac V8s". http://www.amazon.com/How-Build-Per...=sr_1_1?s=books&ie=UTF8&qid=1312114092&sr=1-1 He does very good job explaining cylinder head flow and why on street cars you need high flow at low and mid lift and why you need smaller ports and good exhaust systems. He also takes you through a lot of cam science in simple terms, like others have done here. Please understand none of us are getting rich from this, just wanted you to have another handy source of really good, "experience proven" information. Tom Hand
Very common trick (or used to be) in racing where there is a "cam" rule. Didn't say anything about valve lift... For reasons not worth delving into (unless you like calculus...), the higher rocker ratio opens the valve slightly faster, making the cam seem a little more aggressive in addition to the extra lift. A (very) general rule is that the port area should be 85-90% of the valve area, so here is where it gets really brand specific as to the benefit of larger valves, and which port the larger valve would help most. It is very easy to over-port most cylinder heads, unless they are being used on a huge engine or turning a gazillion rpm. Too much port = no port velocity, lousy fuel mixture and low torque numbers. At some point (peak torque rpm), the head combination can no longer "fill" the cylinder with air on the intake stroke. As rpm increases, efficiency falls off, BUT the number of power pulses per second is rising; so our definition of horsepower is still going up. Beyond that point you just need a bigger and bigger hole to pass air through, so the larger port is a benefit. Being a Ford guy, you may be familiar with the Boss 302 / 351 Cleveland heads? Classic example of too much port. Great for racing on the 302...turning 9000 rpm. Useless on the street...
I would recommend reading David Vizard's book "How to Build Horsepower". Although he uses specific engine examples in the book the intent is to show how to make horsepower in any engine. There is a great deal of information on cylinder heads and cam timing; far more that could reasonably be put in a posting. What I particularly like about Vizard's book is that he has tested his theories and includes the results in the book. There are also chapters on intake, exhaust, ignition and supercharging.
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