I'm building a 354, using KB290 std 392 pistons (.160 dome). Given the hemi's lazy combustion chamber, I am trying to introduce some turbulence into the chamber so I can run a little more compression.... So.. on a wedge, squish is easy.. Just mock it up w/clay, measure the clearance, deck the block to get the .035-.040 clearance... On the hemi, I am planning on mocking it up w/clay, and measuring the clearance between the chamber and the dome.. Here is where I'm not sure how to proceed.. Since the dome and combustion chamber does'nt meet each other in a horizontal plane... 1. Does the .035-.040 clearance still apply 2. How much do I deck the block to get a .010 decrease in "squish" between the dome and chamber....What's the formula?
hmmm, and here I thought domed pistons created turbulence...the angle of your valves and the angle on your domed pistons should be equal, so I'd think a .010 deck would give you .010 less clearance...I'm often wrong...
Exactly, that's why Chrysler's engineers chose to do it in the '50's rather than go with a wedge like GM did. Also, it was well known in the '50's and '60's that if you put a domed piston in a Hemi to raise the compression ratio, you ruined the efficiency of the combustion chamber, so you had to weigh the gain of one -vs- the other.
From an airflow standpoint, it is very efficient, the central location of the plug also creates an even flamefront... however the design does'nt impart any turbulence at TDC.. wich makes it less efficient from a combustion stand point.... Take a look at Chryslers "new" Hemi, wich isn't a true hemi at all.. It has a double "heart" style chamber turned sideways with two squish areas in the front and rear of the combustion chamber....Great marketing! Not a Hemi...
A hemispherical chamber is done for more than just airflow. At the same flame speed in a true hemispherical chamber the flame will envelope more volume than in any other type of chamber. The problem is that you are limited to two valves in a true hemispherical chamber. Hence the new "hemi" which is actually a pent-roof chamber. It is a compromise between the two, almost as quick burning as the true hemi but with the benefit of more than two valves. Also with the valves in a line you can run overhead cams. That is why almost every modern engine is a pent-roof design (with a couple of exceptions). With a true hemi the only way to get 4 valves is a radial head which gets complicated rocker geometry although it has been done. I think you will run into valve/piston clearance issues doing what you are trying to do. And I don't think that squish is the problem. Port design has a lot to do with how the flame propogates in the cylinder. I think that early hemis have bad port design. You could try to put a radius into the intake port to get a little more swirl which would probably help, but that would be a lot of work.
I'm pretty sure that It is some percentage less than the removed amount because the piston is moving vertically and the areas that relate to squish are approaching each other at an angle ( jesus, that sounds vague)...
It is my thought that ideally every engine would have a flat-top piston because it distributes the force evenly across the top of the piston. Although that is debatable. A diesel uses different piston shapes to help the diffussion burning of the air/fuel mixture. It is quite different from combustion in a standard gas engine. Domes in a gas engine are pretty much solely for increased CR, and not for combustion efficiency, although than can be shaped for squish/quench.
Yes. I don't think it would make any noticable difference. Port work would help although I don't think there is enough material in the heads to completely reshape the port, so you would have to do some filling/brazing, which is a lot of work.
Here's my 2 cents, Hemi's are very restiant to detonation,they can be run .5 to 1 point higher compression ratio than a wedge on the same octaine. Combustion camber turbuluace isn't an issue , its rolling the hell out of the air fuel mix across the chamber on the compression stroke.On the new Pro Stock Hemi Heads the valves are almost straight up and down to get a smaller chamber for the 15 to1 compression ratio needed for that type of motor. latter dave
I would reconsider using hypereutectic pistons. I don't mean to reopen this debate, but I've seen some of these where the ring grooves literally busted to pieces, and that wasn't even on a high hp car. Even the guys at KB admit the pistons are very brittle and don't stand up to detonation or hard shock. I'd go with cast before I used hypereutectic.
Huh.. This is the first I've heard of it... I've had real good luck w/hypereutectics to this point, even in high horsepower applications.. Of course that experience is almost all w/ big block chevy's......I have also seen some Hypers turned into 5-ring pistons, One from a low rpm nitrous hit, others from piston rings siezing in the bore.. Oh well..... It's kind of academic now... Blocks bored, assembly is balanced... Might be an expensive experiment... Testing, Testing....
hypereutectics break like glass when exposed to detonation so if you use em dont risk being on the edge with compression or timing.Ive stayed away from them and used cast or forged,actually like cast pistons but keep the revs lower.
the debate the last couple of years with blown alcohol cars has been big dome, big combustion chamber versus small dome small combustion chamber. My theory always was that the big dome allowed more compression pressure "push" down because there was a larger area to push on. but the small chamber guy s always said that the larger piston restricted flow enough to be detrimental to power even with more piston area. either way hemis are the most effiecent design as far as airflow. you could add devices in the intake port to induce swirl but that would slow down airflow and only aid tourque output. isnt the Hemi a tourque monster anyway? just my opinion for what its worth
Hemi's have very little swirl, but have massive tumble, so you still have pleanty of mixture motion in the cylinder
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