351c heads open or closed chamber for a 408 cleavor?

what do you think? i found a set of 1971 4v open chambered heads already magna fluxed for $200! i want to do a cleavor stroker whats better open or closed? thanks!

 

I was going to be using my original 1972 Q Code open chamber heads on my 393C Stroker with dished pistons for a 9 to 1 compression ratio. I wanted to keep the engine in the pump gas catagory and flat tops raised it too much. My understanding is that the closed chamber heads are much more efficient than the open ones. My engine builder agreed 100%. A set of close chamber heads from a Boss 351 Stang became available so I bought them. With the forged Keith Black dish pistions the compression is now right around 10 to 1.

 

would you be able to get 10:1 or 11:1 or even 12:1 out of open chamber heads? and i might do a low 100hp shot of nitrous some day but haven't really looked in to it thanks for the info!

 

i totally understand what open and closed means i just was wondering if there are different applications for each. it will mostly be a street car that sees the track every once in a while.

 

I originally checked on using flat top pistons with my open chamber heads but my compression would have been too high to use pump gas. I believe it was somewhere up in that 11 to 1 range. Stroking just seems to breed compression. When you check on the stroker kit, it will have a chart showing you what the final compression will be using different combinations. My combo might be different than what you are going to be using so you can't really directly compare.

Decking your block, shaving your heads, dome pistons, head gasket thickness are also options to gain compression - lots of combos

 

11:1 would be beautiful i'm just looking to get a pony per cube i believe with a good cam and by following volumetric efficiency i should be able to do it

 

open chamber heads will spark knock a lot easier than the closed chamber ones plus you can get more compression with flat tops
a 351c with flat tops and closed chamber heads will be around 10.5:1 you can still run pump gas and make a lot more power than the open chamber heads JMO

brad

 

it seems like a dome piston would fill the open chamber more and create more compression? why do the flat tops work so well

 

Using dome pistons will certainly give you more compression but how much can you handle? Different stroker combinations will give you quite a bit of compression using flat tops. I can't even imagine how much compression you could create with domes in a stroker. Maybe too much.

 

Cause its a stroker. More displacement being crammed into the combustion chamber in the head. Domes would push the numbers way up.

 

Flat tops give a flat surface for the explosion in the chamber to push against. Domes do not. So a bit more power is gained as the pistons are moving in a straighter line down the bore. The reason open chambered 351C heads are more prone to detonation is there's no flat surface surrounding the chamber for flat topped pistons to "quench"(squishing the explosions out) preigniton explosions (also known as dieseling or spontaneous combustion) These multiple explosions colliding against each other are the sounds you hear when an engine is "pinging" So instead of having one big explosion pushing the piston down, you have multiple explosions pushing against each other, cancelling out their thrust. The chamber on the open heads is deep enough so that no off the shelf pistons will create quench areas in them. The same result can happen with a flat topped piston in a quench chambered head if the piston at TDC is too far down the bore to accomplish quench (as in building a 400M with off the shelf pistons, all fall far short of the mark being around a full eighth of an inch down the hole at TDC. So what you're shooting for is quench heads with a flat top or a reverse dome piston (not a dish, but a mirror image of the chamber above) that will yeild the compression ratio you're looking for. I built a 400M a few years back with open chambered heads and 351C flat tops. The ratio was only 9.7 to 1, but even though the pistons were flat tops, they fell too far down the hole at TDC and the open heads, even though milled .030 (block was decked .010 too) created no quench.Sum bitch wanted only 93 octane gas with that ratio, where my 10.25 to 1 390 their heads are quench chambered) would run on 89. Hope this clarifies the picture for you.

 

Great explanation I had a less detailed explanation given to me just before I decided to change over to the closed chamber heads.

After reading this, maybe me calling my pistons "dish" have been wrong all along. The speed shop ordered the pistons for me and I only saw them once but I recall the machining done to the tops of the pistons was not across the entire face but more in line to somewhat mirror a closed chambered head.

 

That's exactly how a dished piston should be designed...

Flat tops, more than anything, allow for maximum exhaust scavenging in the cylinder. I would run flat tops with the closed chambers and have a cam ground to make it work. Less ignition advance is good.

Quench area, as already said, it VITAL to engines. Too much or too little will cause problems. Here's a good link to some info about quench distance:

http://www.kb-silvolite.com/article.php?action=read&A_id=35

A good quench distance will require less ignition advance and, without getting into lots of detail, the engine is actually more mechanically efficient.

_____

There is a distinct difference between pre-ignition (which you can't hear) and detonation (spark knock/pinging). These terms are erroneously used in many cases...let's try to clear this up a bit. Detonation is where the air/fuel mixture, POST SPARK, is ignited by the extreme pressure and heat that the first explosion, ignited by the spark plug, has caused. This creates multiple flame fronts and very high cylinder pressure spikes (that are sometimes destructive). The pinging sound is caused by the structure of the engine, the actual metal, absorbing this energy (not colliding flame fronts). This is what knock sensors tune in on because it's somewhere in the neighborhood of 6.4k Hz. It can be destructive, but engines commonly endure light to moderate, occasional, detonation. Overheating, broken ring lands, broken spark plug porcelain, and other symptoms are sometimes seen when detonation occurs.

Pre-Ignition is where the air/fuel mixture is ignited, PRE SPARK, for any reason. Common causes are hot spots in the chamber, overheated spark plugs, carbon deposits, etc (something that acts like a glow plug!). Visualize this, after the piston has drawn in the air/fuel mixture and the intake valve closes, something causes the mixture to ignite (it's much easier before compression to ignite something). Due to the inevitable compression stroke, you are trying to compress this air/fuel mixture that just EXPLODED. There isn't a sharp pressure spike like we see (or hear) with detonation, rather there is a very long period of time (the compression stroke) where this expanding explosion is getting compressed into a tiny combustion chamber (oh my). This puts EXTREME pressure on the top of the piston/rod/crank/bearings, cylinder walls, heads. I'm sure you can guess what happens next...


Melted spark plugs, melted electrodes on spark plugs, and holes in pistons are very common "signs" of pre-ignition.

I think that's about everything in a nutshell. Please let me know if I misspoke anywhere, I'll fix it.

-Clint

 

All very informative, but I don't think 100% accurate. Your definition of detonation is something very hard to prove. Once the plug ignites the mixture, just shy of TDC, there's very little chance of another explosion happening to counteract it. Unless in the case of a too rich mixture that extinguished the flame and it later reignites, for whatever reason. And the difference between it and preignition is another thing. The two are nearly one and the same. Better choices in building an engine (heads and pistons) plus running the correct octane fuel and spark advance are the ways to counteract both.

 

It's very common and it has been proven. There's a great deal of SAE documents about detecting and controlling both detonation and pre-ignition.

You are right that optimizing the engine parts will reduce the chances of these occurring.