An open chamber head with a flat top piston or a closed chamber head with a dish piston. Both at 10.5:1 compression.
Is One Better Than The Other & Why?
- Thread starter srduster340
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And then there's a domed piston...
I think the answer is "it depends" on the precise configuration and flow characteristics. Good engines have been built both ways.
I think the answer is "it depends" on the precise configuration and flow characteristics. Good engines have been built both ways.
33IMP
Well-Known Member
I would take the dished piston with a d-dish to get a quench, which is a lot harder to get with an open chamber head.
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Second one especially if the the piston mirrors the chamber. like in this thread:
318 Wolf in Sheep's Clothing
A closed chamber with a flat top is the best scenario.
Rat Bastid
Dunamis Metron
If you stick the piston out of the block like Chrysler said to do 55 plus years ago it doesn’t make a pinch of **** difference.
In fact, I can argue that the original SBM chamber is better than 90% of the **** that gets palmed off as the latest, greatest thing.
The MOST timing you should need on a SBM is 36, and that engine is probably low on compression and over cammed.
A lot of the guys out there today claiming their chambers will run at 34 (or less) run a far hotter plug than they should.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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The one with a tight Quench.
Besides; how do you get to 10.5 with a flat top and an open chamber? A 360 would need 78cc.
The factory iron open chamber, plus a Fell-Pro, plus 5cc of eyebrows, totals at least 86 cc, and probably more, so your pistons are gonna have to be at least 8cc up. At 4.03 bore, that comes to .039 up.
Where is the top ring?
On fire....... lol.
and still no quench ................
Edit
for clarity;
I'm not saying it can't be done.
I'm asking how would you do it.
You didn't say what engine it's going on, so most likely you would need a custom piston with a lowered ring-pack, which reads..... probably heavier than it needs to be.
and besides that;
the point was
"and still no quench."
Just like the factory hi-compression 340s,
which weighed in at a ffffat IIRC 745 grams with their ffffat tops.
There are better ways to get to 10.5
Besides; how do you get to 10.5 with a flat top and an open chamber? A 360 would need 78cc.
The factory iron open chamber, plus a Fell-Pro, plus 5cc of eyebrows, totals at least 86 cc, and probably more, so your pistons are gonna have to be at least 8cc up. At 4.03 bore, that comes to .039 up.
Where is the top ring?
On fire....... lol.
and still no quench ................
Edit
for clarity;
I'm not saying it can't be done.
I'm asking how would you do it.
You didn't say what engine it's going on, so most likely you would need a custom piston with a lowered ring-pack, which reads..... probably heavier than it needs to be.
and besides that;
the point was
"and still no quench."
Just like the factory hi-compression 340s,
which weighed in at a ffffat IIRC 745 grams with their ffffat tops.
There are better ways to get to 10.5
Last edited:
I have to ask, was that you dyno testing in 1980 that made St Helens blow.
I heard it was caused by engine testing I'm just trying to pinpoint it.
We've had a few earthquakes over here in the past few years.
Chrysler did it. The 340. Remember that one?
Rat Bastid
Dunamis Metron
LOL…nope. I was still street racing and chasing skirts at that age.
MOPAROFFICIAL
Oogliboogli
Think what in part quench is doing... then you'll come to the conclusion that a piston with a dish/D cup that's either zero deck or step headed is best 'depending on chamber'. As said in a round about way..the piston makes more diff than open or closed chamber when mostly the plug side is the same. It's just simpler to flat top and closed chamber the build than it is to fiddle chambers and measure/mill piston quench pads. Weight/ grams of ea. are some other deciding factors in the trade for this or that.
Here's the third scenario. Dome piston with corresponding chamber shape. Think of where the spark plug is located and how far does that ignition of flame front have to travel. The shortest distance from all posable angles created by that ignition is best.
This sucks, piston to the right, flame front travel distance, go over a mountain and hope there wasn't preignition on the other side.,
To me, closed chamber and necessary size of piston dish to achieve the compression ratio targeted.
This sucks, piston to the right, flame front travel distance, go over a mountain and hope there wasn't preignition on the other side.,
To me, closed chamber and necessary size of piston dish to achieve the compression ratio targeted.
Here is a good article on quench from Engine Labs.
www.enginelabs.com
Quench: Unlocking Performance Squished Between the Piston and Head
Quench is the clearance between the piston and cylinder head at TDC. It can affect engine effeciency and performance, if you truly understand it.
www.enginelabs.com
Rat Bastid
Dunamis Metron
Thats a very good article.
The first thing that really stood out was that first picture that was posted showing quench area. While the chamber shape is nice, spark plug location is not that good. And for most 23 degree SBC heads it’s never very good.
My point is even with proper quench (if you can get it and not get the compression ratio higher than needed) some of these chambers have only decent to horrible spark plug locations.
And spark plug location is a HUGE factor in ignition spark angle (timing).
https://www.speednik.com/files/2019...iston-and-head-2019-03-23_15-55-21_659850.jpg
I couldn’t get the picture of the Chevy head from the article to post but you can click on the link to see it.
Compare the spark plug location of both heads and you‘ll notice how high up in the chamber the Chrysler plug is compared to the Chevy chamber.
That high plug placement means less total timing and a completely different curve than the SBC head. Also, you can get a bigger dome in the Chrysler before the flame front is impeded than you can with the SBC, or even the BBC.
As Darin Morgan says, NA you do everything you can to speed up the burn rate because faster is better (to a point) but with a power adder you do whatever you can to SLOW the burn rate down to keep it out of detonation.
My thoughts always were . Close chambered with custom large domed pistons. Get an alcohol carburetor and shift it as high as the RPM's would allow. Then work on the parts that couldn't take the abuse. The last repair should be the hole in the floor from the snubber.
Will there be much difference in efficiency between a 60cc closed chamber head vs a 70cc closed chamber head both with flat top pistons? I know the compression will be different given the same piston cc value, deck height, gasket, etc.
Brian Hafliger
Well-Known Member
Hotter sparkplugs CAN burn more fuel (or burn fuel better so more can be added), so if done right will make more power...within reason.
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Guess that means the more you improve the burn the less timing it needs........
Rat Bastid
Dunamis Metron
Exactly. Within reason.
Rat Bastid
Dunamis Metron
I agree. I’ve seen plugs in engines so hot you could weld with them. One example was a customer we did a 14:1 BBM for (I can’t remember what heads were on that engine but IIRC they were a set of Stage IV heads) and always on the invoice I write down the exact plug to start with. And of course, he didn’t want to dyno the engine.
Somehow, he got to the races (a divisional) and forgot to put spark plugs in it before he left. So he goes through the pits and borrows a set of plugs.
At 14:1 he should have been using a Champion C55 or equivalent. He ended up with an NGK that was the equivalent of a Champion 12!!!!!
Needless to say, he crapped that engine. So I had to leave the shop, drive to his place, grab his spare and drive to the track and help him swap them out.
The upshot is I couldn’t figure out how he killed the engine in two passes. Well, about 100 feet into the second pass.
Luckily, I grabbed some plugs off the shelf before I left the shop. I was screwing them into the back up engine and I asked him where the plugs were out of the dead engine.
He walked over to the garbage can and pulled out those welding electrodes he was using.
He was off by 5 ranges.
Rat Bastid
Dunamis Metron
That’s not bad. The plug is still pretty flat, relative to the deck. The SBM has about the best wedge head plug location.
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