[Diy_efi] Combustion chambers - was The Hunt effect

Geoff Harrison geoff_h
Fri Oct 28 01:13:14 UTC 2005


Adam Wade replied on  Friday, October 28, 2005 6:12 AM


--- Geoff Harrison <geoff_h at smartchat.net.au> wrote:
> In my humble opinion, the first requirement for a 4
> stroke hemi is a true crossflow head, not one where
> the valve stems are parallel to each other
> as in the Aussie 6 cyl Valiant / Chrysler Hemi.

"Well, as I understand it, the combustion chamber shape
in the Chrysler Hemi is much closer to that delivered
by a wedge-shaped chamber roof."

I aggree, but you can't have a true hemi with 2/3rds of the dome 
being almost flat.

"Your above comment reminds me of an engine you would
call a "true hemi", and it actually does a great job
illustrating my point about the combustion chamber
roof shape *in isolation* being a basically useless
thing to specify (of course it will determine valve
size as well as valve and port layout -- or vice versa
-- but I'm specifically talking about the combustion
chamber shape at the moment of combustion).

It was a great engine. In R/T versions held the world record as 
the fastest accellerating
passenger vehicle. (14.4 sec 1/4 mile, on 1970 tyres, 14.1 on 
modern rubber, only one gear
change) for several years.

" The example is the Porsche 911."

Yeah, the Porsche is a better design. Crossflow, OHC, inclined 
valves.


> But it was probably as close to a hemishephical
shape
> as possible with that valve layout.

No true hemi chamber roof design, even with something
like those 911 racing pistons, could meet today's
automotive emission standards, I think.  Swirl,
turbulence, and more complete combustion are the order
of the day, and very different combustion chamber
designs are required to make that happen.

They can approach it. E.G. Ausie Ford 4L L6. Crossflow head. 
OHC, valves inclined about 30 deg off vertical.
Slight squish band on otherwise flat top piston. Slight recess 
for intake valve. Slightly undersquare B/S ratio.
It looks more hemi than a Chrysler, and about 25% more fuel 
efficient.

Swirl / turbulence can be improved by the use of port angle. 
Suzuki 4 cyl DOHC 750 / 850.

One of my anticipated projects, is to take a 4.3 L Chrysler, 
design an inlet manifold to further
enhance low down torque, and add fuel injection.





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