<VV> 1965-1967 110HP with a/c low compression engine

[Tony Underwood]

At 08:37 AM 10/24/2006, BobHelt at aol.com wrote:
>
>In a message dated 10/24/2006 6:53:00 AM US Mountain Standard Time,
>tonyu at roava.net writes:
>
>IMHO,  those "bowl" shaped open chamber heads were certainly a bad
>design, makes  me wonder why they ever existed in the first
>place.
>
>
>Tony,
>The bowl heads, as you called them, served their purposes.
>
>For the turbo engines, these heads allowed max hp to be developed as  they
>didn't have the restricting squish area.


Then why do engines fitted with the 95 heads develop at least as much 
power (usually more) with less pinging and requiring less ignition advance?


>With the turbo, turbulence was
>sufficient without the squish. In other words, the turbo provided 
>the turbulence  and
>the squish wasn't needed to to that. These heads were only used on the
>1964-66 turbo engines and had a "real"CR of 8.01 which was lower 
>than the 95  heads
>with 8.28 CR.


According to some lab tests in aftermarket research, those bowl heads 
tested out to have appx slightly better than 7-1 
compression.   That's pretty low...   same engine lab was the one 
that determined that the 95hp heads with squish area developed more 
power with less detonation in a turbo engine on a test dyno.

Been there done that, ran an RL engine w/stock heads, then pulled it 
down and built it with 95 heads, SAME cam, a few other mods to let it 
breathe better and used a distributor off a 110 engine with some 
whittling on the weights, and ran around HALF as much initial 
ignition advance.   Fuel mileage went UP and the car ran stronger 
than it ever had.   It was an increase you could feel in the seat of 
one's pants.

Why GM didn't do it this way in the first place remains a question 
I've pondered for some time... unless of course it's assumed that 
they did it to save a nickle or a dime here and there along the way.


>The open chamber heads, used on the AIR engines also served their purposes
>in that they lowered the unburned hydrocarbons present in squish heads.


I understand that, what with the squish area sometimes harboring 
traces of unburned mixture which could throw off smog limits.   By 
the way, has anybody ever actually measured the difference in smog 
output between the bowl heads and 95hp heads in an AIR 
engine...?  Since the AIR system is designed to burn off "missed" 
mixture in the manifolds before it gets out the tail pipe and into 
Aunt Sadie's nostrils or the governor's smogsniffer, wouldn't it also 
burn off that trace of unburned mixture hidden in the corner of the 
chamber under the squish area?    Logic would suggest that with the 
more complete combustion available in the tighter chamber of the 95 
heads, there's actually gonna be less unburned mixture to be 
concerned with in the first place while running these types of heads 
instead of the bowl chamber heads, particularly at higher 
rpm/loads.   Still, I imagine that GM used what they had on hand with 
the tech most easily and economically available to them at the time...


  But using such heads in a turbocharged engine was like shooting 
ones self in the foot.   They are NOT very good heads for any sort of 
forced induction system simply because of the shape of that 
chamber.    Chrysler's hemi engine suffered the same issues, trying 
to light the mixture equally so that it all combusts completely... 
via positioning the plug smack in the middle of the chamber, and in 
hardcore performance applications there were two plugs per chamber, 
both firing together to light the mixture from different sides, speed 
up the flame front etc and improve combustion efficiency.    Corvair 
heads cannot enjoy this sort of twin-plug application so they get lit 
from the edge, flame front has to cross the top of the piston in 
order to burn everything in the chamber, by which time the piston is 
already on its way down.   Power and efficiency is lost.

Bowl chambers have to spread a flame front across the surface of the 
piston from one side to the other and that takes time and at higher 
rpm it's gonna drop off in efficiency more and more as rpm 
increases... may be fine for midrange fuel-burning efficiency with a 
lot of ignition advance to compensate for the unequal flame front but 
not for any sort of performance ap or mileage concern since the fuel 
mixture takes longer to burn (obviously) and at certain throttle/load 
settings and rpm the max cylinder pressure occurs before all the 
mixture charge is fully burned, (which is why that extreme ignition 
advance was required, help get a head-start on burning the mixture) 
resulting in not only inefficient use of the fuel mixture in the 
chamber, but it promotes a tendency to ping as well when the 
incompletely burned charge expands and develops high cylinder 
pressures which further compresses the remaining unburned mixture 
charge which then detonates instead of burning cleanly.

Sorry head design, that.     It's sole purpose was expedience, to 
solve a couple of problems at the expense of 
performance/efficiency.    Such heads have no place on a 'Vair engine 
today unless it's to maintain "matching numbers" status.   Smog 
issues aren't a problem anymore with 'Vairs unless you live in an 
"extreme" area.   On something that's gonna get driven anywhere else, 
there are much better choices in heads to be had.


One more point to consider is that the later 140hp AIR engines did 
NOT use bowl chamber heads.   They had squish areas not unlike the 
95hp heads, with flat top pistons.   They also appeared to meet smog, 
and did not require boatloads of ignition advance nor did they ping 
their brains out and they did get decent fuel mileage.

I sometimes tend to consider that perhaps some of the GM facts and 
figures of the day may not have been entirely accurate... and that 
all was not exactly what it seemed, up front.


Of course, all of this is strictly my own opinion and observation 
(which, as anybody who knows me personally, is suspect ;) ) 
.       However, I'm not trying to dispute anyone's published data, 
figures, specs, etc.   But I bet if GM had it to do over again, 
knowing what we know today, they'd have done it 
different.      Or...  beancounters notwithstanding, perhaps not.   ;)

Lots of things got done in the '60s "in Detroit" that, after the 
fact, sometimes didn't make a whole lot of sense.



tony..