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991 Views 1 Reply 1 Participant Last post by  BlackMx-3
I didn´t want to mess up the post on turboing the 1.9 worse so I will post why the Hemi sucks here... :-D

The Hemi combustion chamber design had such a large chamber volume and large surface area to volume ratio that it´s design led to a very slow burn rate despite having a symmetric layout and a centrally-located spark plug.

It would need so much spark timing advance (up to 45 degrees BTDC!!!) because of the slow burn and a huge piston dome to achieve decent static CR´s which increased even more surface area causing a vicious cycle.

Engine tuners for the hemi would keep on advancing the timing because they saw that they achieved more and more hp gains, as they advanced more and more. Meanwhile cylinder pressures were going through the roof as they advanced more and more. Then suddenly the engine would grenade with the next increase in advance. These engines were twitchy at high rpms (shorter time for flame front travel) and would grenade out of nowhere as the spark timing was advanced just that little bit more.

Most modern builders have come to realize that the smaller pent-roof shape with a piston crown that has the lowest possible area to achieve the desired CR is the best design despite a lower quench area compared to the compact Bowl in Piston (FlatHead) or Wedge chambers.
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ALSO, for the wingboy comment I would like to jabber on some more, as I am bored. lol.

the Chrysler Hemi chamber is the poster child of how not to make a chamber shape these days.
PLEASE don´t get me wrong: it is an ingenius design developed in an era when high octane gasoline was plentiful and the mentality of " bigger and more is better" ruled the day and people´s consciousness.
It´s 1950´s technology at it´s pinnacle (and already had one re-incarnation or comeback in 1970 when the Cuda and Challenger kicked some serious ass). Remember the 50´s was an era of high post-World War 2 optimism with lots of gas around and the Saudis were just figuring out what to do with all this oil they had.
I am not spitting in the face of the incredibly intelligent men who designed this Hemi chamber. On the contrary, I have incredible respect for what they brought to the table. It was an important link to get where we are today. We have learned more. It´s the evolution of combustion chamber design.
The OPEC oil embargo, fuel efficiency, computer era (CAD CAM design technology, ECU´s in passenger cars, and computerized engine cylce analysers), happened in between 1950 and now. We had conservation added to our consciousness. The 80´s IMSA/FIA Endurance and FIA Formula One road racers had to deal with fuel restriction rules to develop more efficient combustion engines. NASCAR was merrily continuing with the old way of thinking.
We learned that big is not always better...especially not bigger chamber volumes ....and we learned why. The Japanese were at the forefront of lean burn technology and fuel efficiency. If you are not going to use exhaust gas recycling (EGR) back into the chamber to increase fuel efficiency , how are you going to do it?...especially when EGR kills performance (hp) and driveability....
Stratified layers of progressively leaner air fuel ratios as the piston fills...this is how. You can add less fuel, use a smaller chamber size, use smaller displacement, use high rpms...to get the performance without big fuel, big ports, big displacement, big chamber volumes, big CR´s...no more big.
Fast burn, lean burn, small chambers, high rpms....this was the new design driven by the climate of conservation.
So we can now run as fast as some of the pony cars but drive a further distance because of better gas mileage... The 1970 426 cid Cuda Hemi is fast but would be parked at the side of the highway or track because it never got to the finish line running out of gas...we would be at the finish line ahead....isn´t that called progress? This is no tortoise and hare story. We can run as fast as the hare and be as patient as the tortoise.
Just look at the automotive history of engine design and combustion theory over the past 30 years and you will see how we got to a pentroof compact design and lean fast burn.
To go back to a big chamber Hemi design these days would be a mistake IMHO...I hear with all this retro craze and making cars look similar to what they did in the 60´s-70´s, when Daimler-Chrysler brought back the Hemi (again)...I hope they update the design (better turbulence filling and less area)...you know that there are some Harley Davidson engine builders still trashing Japanese bike engine designs and promoting the glories of a big Hemi design even today...they still cling to the past with a strong grip oblivious to the change in awareness for the climate that we have today.....

OK I AM DONE, I think its time for bed, lol. I just realized I posted all this under the forced induction forum. lol. stupid me

[ This message was edited by: BlackMx-3 on 27-02-2003 23:49 ]
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