I've been wondering for a while..and haven't found an answer to my question yet. Which is better for power assuming the CR is the same in both cases? 1. Large CC heads w/ domed pistons or 2. Small chamber heads with dished pistons? my 327 I built ended up having 76cc chambers and flat-top pistons and thin gasket to eek outa 9.0:1 CR. I was on a budget and the heads we a great deal, but I always wondered if I should have use smaller chambered heads and dished pistons.
There are alot of variables here. Combustion chamber design,spark plug location,fuel,flame travel,quench,turbulance,swirl,wet flow,etc. For a low compression motor, Flat tops would be my first choice. Second choice,dished.
[ QUOTE ] I've been wondering for a while..and haven't found an answer to my question yet. Which is better for power assuming the CR is the same in both cases? 1. Large CC heads w/ domed pistons or 2. Small chamber heads with dished pistons? my 327 I built ended up having 76cc chambers and flat-top pistons and thin gasket to eek outa 9.0:1 CR. I was on a budget and the heads we a great deal, but I always wondered if I should have use smaller chambered heads and dished pistons. [/ QUOTE ] what kind of compression ratio are we talkin here? 9:1? piston to head clearence will help maximize the quench effect as will the type of piston you use. did you get standard replacement pistons? i have read that those pistons arent too good if you are going for performance, apparently their design tends to get rid of the quench effect. the turbulence effect that happens during the compression stroke atomizes the air/fuel mixture more efficiently and pushes it out of the quench area and into the chamber. i dont think bigger CC chambers with domed pistons or small CC chambers with flat tops will do any good if your quench area isnt optimized, your carb is running too lean or rich, and valvetrain/spark timing is off. might want to check google for a more detailed version... i know there are articles out there as its a pretty common topic.
Dave Vizard, a serious Cevy Guru with lots of dyno and flowbench time to back his opinions, says that small chamber is significantly better in all cases. I think he said difference could be 10-15 HP with the same compression. He once published a back-to-back dyno test of this, using heads with ports as similar as possible and swapping pistons to equalize chamber volume. His book on porting Chevy heads is a BIBLE. He also recommends pistons with a D dish rather than round if compression needs to be lowered, to maintain proper function of the squish area. Some late small block heads seem pretty good sources if the sixties stuff is hard to find, and the aluminum Corvette heads are getting to be cheap. I seem to remember 305 heads run like 57cc, 350 in the low sixties in late 1980's smallblox.
Don't beleive it! All things are NOT equal. The SMALLER the chamber, the BETTER...and it must be WITHIN REASON. I suppose you want an explanation. I'll start with the EASY part. A smaller chamber has much less shrouding of the valve when it opens. A deep chamber keeps plenty in the way of the incoming or ourgoing flow, and also in the way of the fast-moving flame front. Efficiency is key here, and the BEST case scenario is a small chamber, barely larger than the valves themselves, tapered slightly to the deck of the head. The spark plug (between the valves) is included in this, so the resulting shape is like a kidney bean. So, we've got a nice, small, kidney bean-shaped chamber and lots of flat area (head deck) residing in the circular home atop the cylinder bore in our "optimus combusticus" model. Let's take a peek at the piston it'll need to work well. The top of the piston has a gradual dish which replicates the kidney bean shape. This means the flat-top portion of the piston is all quench area when the piston is at TDC. The depth of both the chamber and the dish are what determines compression ratio, but here's a cool point... ..in a teriffically efficient chamber like this, more compression can be supported on lower octane fuel, and less timing lead (ignition advance) is required to fire it. Here's a couple thought on ignition advance, and how it relates to chamber design...the BETTER the chamber, the LESS advance is required to produce optimal power. In checking out the Engine Masters champs this year, none of them were running over 30 degrees advance. One ran a total of 24. That's an efficient chamber! This was on 92 octane pump gas from 2,500-6,500 rpm. The winning 410ci Cleveland Ford made 475 ft/lbs at 2,500 rpm, and 700 hp at 6,500. The chamber displaced a mere 26cc. The dish in the custom Venolia piston was extensive. It boasted a shape that made smaller quench areas on the valve faces and used only the chamber area for the dish. It was a thing of beauty. A tiny (12mm) Ferarri spark plug was fit into the chamber to minimize the plug's effect on flow. The third place engine (a Ford Windsor) used a 10mm plug for the same reason. Smaller chambers are better, but they need to be teamed with the correct piston and quench area design. Coatings help too, as we've discussed previously, and in a street engine (to be run on low octane fuel) it's okay to have the rings down from the piston deck as much as .150-inch. This helps keep detonation away. Deep chambers suck. Just my opinion, I could be wrong... Scotch~!
i might have been getting my termanology jumbled up. bruce pretty much summed it up, its amazing what kind of numbers you can get when all 8 cylinders are working efficiently. scotch - deffinately some impressive numbers! thanks
I've heard that large domes on pistons are horrible on flame front travel. I've also heard that a 4 valve pentaroof chamber head is very good for flame travel, but only if you have enough quench to get some compression without a large dome.
Basically it seems you have to lok at the whole combustion shamber and visualise how the flame front will have to travel. From there it follows that an open chamber, unfettered by piston dome and other obstacles will give better results. Also look to recent motorcycle practice for optimal chamber design. Since motorcycles are not having to hew to emission and fuel economy rules overmuch, they aim more for power than any other consideration. And the hot ticket there is a four or five valve head with a very small included angle on the valves. Included angle is simply the angle between the valves. This will obviously lead to a chamber shaped much like a squished rubber ball. Quench on these heads is to the outside of the chamber, and forces the mixture in towards the plug, which is always centrally located. Cosmo
This is good stuff...I'd say tech-o-matic worthy. Afterall, making power and going fast IS traditional. Anyone care to comment on why the Hemi with its large hemispherical chambers seems to work so well? I'm guessing it has to do with the unshrouding of the valves. Scotch?
One hemi plus is that the port can come fairly close to aiming at the back of the valve rather than having to turn close to 90 degrees as in a wedge--this allows good flow all the way around the valve, whereas other designs usually have a pretty large part of the curtain area doing very little of the flowing. And of course this is directly aided by the unshrouding on the other side of the valve. Note that expensive race heads for Chevies move the port UP, coming closer to a straight shot at the valve.
Sure thing Bass. Hemi heads work well for a few reasons... First- The valves open at an angle, and are greatly unshrouded by the chamber almost immediately upon opening. Secondly, with a centrally-located plug, and a well-designed piston top, a "decent" chamber can be created with the proper engineering. But, Hemi heads are not "optimal"... ..Proof? They don't use "true" Hemi chambers in NASCAR or F1. Why, then, are they THE head of choice in Top Fuel and Funny Car? Volume! The Hemi design separates the intake and exhaust ports from each other by a (relatively) HUGE distance, which means the ports can be large and more importantly, IDENTICAL to each other. Now, you have a monster size port (capable of huge volume) with the capability to flow tremendous amounts of Nitro/Alky (which these engines use in wretched excess, and also the air that carries this fuel) and the ports can be created in an identical fashion to ensure equal feeding to each cylinder. These monster Fuel motors all rely heavily on blowers to get port velocity up- they'd run like poop without the boost, and the huge valves are limited (by the Hemi design) on how much they can open before the intake and exhaust valves touch each other. The corresponding angles on Hemi valves mean they aim toward each other when they're both open, and that limits lift and duration. So, add more boost, add more valve diameter (again, not a problem fitting htem in the head because of the angled valves and ample real estate inside the chamber) and make the ports huge. The "new" Mopar Hemi borrows only the most-basic iof engineering design from it's fatter forefather, and if you lok at the chambers, you'll see they are much smaller, and rely on a less-aggressive piston topdesign for the reasons mentioned above. It's still a good head, but still not in the neighborhood of the LS1/LS6 and/or Ford's latest Mod Motor chambers for the 2V, 3V, or 4V (Cobra) designs. The reference to motorcycle chambers is a good one. They are more like F1/indycar stuff than almost anything we see on the streets. For home builders and most using traditional V-8s as powerplants for the street, decking the head and having custom dish pistons created to correspond is a "best-case" scenario. Anything you do to move closer to this optimal design will help, and avoiding making the chambers deeper is a good, basic design goal to shoot for. Again- the timing tells the tale. A good set of heads/pistons should ALWAYS require LESS timing advance to produce optimal power than what you started with. If the factory stuff ran best at 36-38 degrees total (like most small-block Chevys, for instance), and you've worked the heads over, you should find best power at 32-34 with the improvements. Making a good chamber is always a compromise between better flow and better burn rate, but the latest research shows that a shallow chamber teamed with a minimal diameter corresponding dish (almost regardless of it's depth to adjust for compression ratio) and a decent quench area outside this same dish will result in improved flow and more efficient burn. just my opinion, I could be wrong. Scotch~!
Wow...Bruce, Scotch...you guys rock! While we're on the subject of chamber design, what about Flatheads? Are you saying that the best performing chamber in a Flathead would be a nearly "flat" head, with almost no chamber area, a relieved block, and dished pistons? Or does none of this technology apply because of the "L-head" design? I know that Barney Navarro was a big advocate of pop-up pistons, and his engines always seemed to run well....but what you're saying makes me think "pop-up" pistons are likely a waste of time. Whatta ya think?
I don't believe (I don't have a flowbench or dyno, so this is based on reading the flow gurus and trying to adapt their wisdom to flatheads)that relieving the block is a good approach. I don't think the airflow can turn as rapidly off the seat as it would have to to use that area. I think the relieving needs to happen in the head area between valve and cylinder. Also, the FH Ford has a very shallow valve pocket and a shallow angle of approach from port to valve--the air needs to turn sharply, but can't since it doesn't have a deep pocket area to turn in, like a large-volume Chevy racing head. Yunick calls for at least an inch of depth to allow much turning in the pocket, and it isn't there. I would guess that most flathead intake flow is straight across the back of the valve and out on the cylinder side, with little use being made of the rest of the valve area--but that's my speculation. The researchers say the air can turn only in about 15 degree increments, and that looks to me to mean the area beside the valve opened in relieving isn't worth much. One FH racer said that relieving IS useful in race engines with the longest possible strokes.
That makes a lot of sense Bruce. By bringing the face of the head down and getting rid of its chamber, you would effectively be shrouding the valves. A small chamber is apparently good for unshrouding the valves when they are sitting over the piston, but wouldn't be very useful when the valves are next to the piston. What you said about stroke got me thinking, however. I've read that a long rod used to slow piston speed allows you to run a higher compression ratio on lower octane and less total timing. Scotch, did the Cleveland with the 26cc chambers have a really long rod coupled with the dished pistons and super small chambers?
Actually, it had a really SHORT rod teamed with a really LONG (tall) piston. The piston dish was deep, the rings were far down, and the rod was short. Looked like a Diesel slug.
Awesome info. It's too late for my engine. I wasn't trying for lots of power, I just needed a reliable engine at the time. It is built and done and I'm not pulling it apart. Next time though I'll put this great info to use. So while we are on the topic disect this combustion chamber... MEL V8s it's a wierd one, valves seem to be pretty unshrouded though.
Scotch, That's weird...what was the benefit of that? I'm guessing an increase in piston speed, but I can't figure why the rings being far down the cylinder would be an advantage. Was the short rod to boost total HP without a corresponding torque increase for the competition? I can now see how a really shallow chamber would be a good thing since it un-shrouds the valves..and you can customize your quench area by your piston dish shape. Thanks guys...I learned something new today.
I had been thinking chamber volume since I saw the "Vortec" BBC heads on the SDPC website. They have a chamber 99cc. Too bad they say that they can only be used on 1991 and later BBC's I have not seen anything like that outside of all out race stuff. Msst BBC chambers are around 110-130cc. Another cool thing that illustrates the point of the small chamber design is the C5-R heads that they use on the race vettes. Their chamber is 38cc, and the valve angle was reduced from 15* to 11* Great info throughout this thread!
[ QUOTE ] Awesome info. It's too late for my engine. I wasn't trying for lots of power, I just needed a reliable engine at the time. It is built and done and I'm not pulling it apart. Next time though I'll put this great info to use. So while we are on the topic disect this combustion chamber... MEL V8s it's a wierd one, valves seem to be pretty unshrouded though. [/ QUOTE ] This is a great thread. Mucho information to digest here, thanks guys! This cross section of the Lincoln 430 head is much appreciated. It is the most underrated engine of all time IMO. My take; small tight combustion chambers with shapes that induce a "charge motion" are the most efficient for gasoline applications (especially when emissions are considered). These types of chambers allow the use of flattop or dished pistons. These are inherently lighter and allow the ring pack to be moved "up"closer to the action. This is critical for emissions as there is much less unburned hydrocarbons hanging out around the piston above the top ring. For us hot rodders it means deriving more power from a given amount of fuel. The small chambers promote good flame propagation and thus require less ignition lead. On the other hand a hemi on NITRO is a whole different animal. A top fueler relies on sheer volume and virtual welding rod type ignition energy to make the awsome power that they do. Nitro burns slow and a nitro engine turns realatively slow rpms.The big heavy domed pistons don't hinder its operation as much as a high winding gasoline engine. Further the ring pack is located far down the piston to protect them from the brutal environment of the nitro combustion. If you look at the new IHRA pro stock Boss and hemi engines you will notice that they are not hemi chambers at all. They are "wedge" chambers turned sideways with short direct ports and very small combustion chamber volume to work with the gasoline they are required to run. Oh yeah almost forgot. Valve angles are getting shallower these days to encourage the charge to flow all the way AROUND the valve. In addition this makes for a shallower chamber and eliminates the need for domes to attain higher compression.
The MEL "flatbottom" head, like the 348/409, relies completely on the piston top for a chamber. This isn't all bad, but the shape of the chamber top is as important as the shape of the chamber bottom. Having no chamber in the head at all isn't great either, since controlling the incoming and outgoing flow is critical and careful engineering in the critical areas around the valves and spark plug can really help. The mixture motion (tumble and/or swirl) as the air/fuel charge enters the cylinder is a big deal, and careful choice of angles and curves (angles are beocming more prevalent lately) can truly aid in the fight to get around the valve and keep fuel in suspension. Recent wet flow tests have shown vortices on the ultra-soft chamber edges we used to think were optimal. Harder angles have proven to reduce these vortices greatly, but we already learned how the tips of sharply angled cuts inside the chamber were hot spots when heat is introduced, and these hot spots provide a place for detonation to begin. So, now the push is to find a happy medium between the sharper angles favored by incoming flow and the softer curves favored by a rapidly expanding flame front. Interesting times indeed. On the exhaust side, and particularly within the exhaust port, sharper angles are truly finding favor. The supersonic speed of escaping exhaust gases (seriously- they move faster than the speed of sound) has shown that flatter surfaces and crisper angles can be engineered to increase flow. Expect more aftermarket performance heads to showcase these findings in the near future. Great thread, by the way...Bruce's stuff is excellent and makes me want to do some flathead homework...Maybe once I get my other projects complete, I'll think about that some more. Thanks for the insight Bruce. Scotch~!
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